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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 20 — Oct. 7, 2013
  • pp: 23851–23865
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Study of heating capacity of focused IR light soldering systems

C. Anguiano, M. Félix, A. Medel, M. Bravo, D. Salazar, and H. Márquez  »View Author Affiliations


Optics Express, Vol. 21, Issue 20, pp. 23851-23865 (2013)
http://dx.doi.org/10.1364/OE.21.023851


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Abstract

An experimental study about four optical setups used for developing a Focused IR Light Soldering System (FILSS) for Surface Mount Technology (SMT) lead-free electronic devices specifically for Ball Grid Arrays (BGA) is presented. An analysis of irradiance and infrared thermography at BGA surface is presented, as well as heat transfer by radiation and conduction process from the surface of the BGA to the solder balls. The results of this work show that the heating provided by our proposed optical setups, measured at the BGA under soldering process, meets the high temperature and uniform thermal distribution requirements, which are defined by the reflow solder method for SMT devices.

© 2013 Optical Society of America

1. Introduction

The consumer electronic products industry demands miniaturization of integrated circuits (IC’s) with capability for running high speed and complex applications, as well as low manufacturing cost. This has been a trend over the last decades and still remains with the high volume consumer demand on portable and mobile electronic products. SMT, along with very large scale of integration technology (VLSI), have a very important role in the development of new electronic products. One of the main technological challenges for SMT and VLSI is the soldering process of IC’s, due to connectors solder joints distributions are extremely compact and inaccessible [1

1. R. Strauss, SMT Soldering Handbook (Newnes, 1998) Chap. 1.

10

10. A. Raj and T. Latha, VLSI Design (PHI, 2008), pp. 1–4.

].

The most commonly used heat transfer mechanisms in the reflow soldering method for SMT devices are convection and conduction, which are contact methods and are provided by a combination of an electric resistor heating element and a fan that provides mass air flow. Another non-contact heat transfer method is radiation, which is provided by an IR light source [11

11. H. G. Sy, P. Arulvanan, and P. A. Collier, “Rework and reliability of QFP and BGA lead-free assemblies,” in Proceedings of Conference on Electronics Packaging Technology, (Institute of Electrical and Electronics Engineers, Singapore,2002), 194–199. [CrossRef]

15

15. N.-C. Lee, Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies (Newnes, 2002), Chap. 4.

]. Infrared technology has also been used for soldering SMT devices, providing a reliable technique for electronic products assembly [16

16. R. H. Gibbs and D. J. Lowrie, Infra-red rework station, patent number: 4,843,216 (1989)

18

18. M. Félix, C. Anguiano, A. Medel, M. Bravo, D. Salazar, H. Márquez, and J. Chacon, “Infrared thermography of integrated circuits heated by focused IR light soldering system,” in Conference of Latin America Optics and Photonics, (Optical Society of America, 2012), paper LTC4.3.

]. This technique is based on radiation absorption of the packaging materials used to encapsulate the metal-semiconductor IC’s, which generates heating in the body of the whole IC, transferring heat to the solder joints of the SMT device required to be soldered. IR technology has a high capacity for achieving temperatures required to melt different solder alloys with a suitable temperature distribution control, which currently represents an option for lead-free soldering in the manufacturing process required for consumer electronics regulated by WEEE/RoHS [19

19. E. Bradley, C. Handwerker, J. Bath, R. Parker, and R. Gedney, Lead-Free Electronics (John Wiley & Sons Inc., 2007), Chap. 1,5 and 10.

21

21. T. Novak and F. Steiner, “Influence of intermetallic compounds growth on properties of lead-free solder joints,” in Proceedings of IEEE on International Spring Seminar on Electronics Technology, (Institute of Electrical and Electronics Engineers, Bad Aussee, 2012), 213–217. [CrossRef]

].

The reflow soldering technique for SMT devices is usually applied simultaneously to several components placed on the surfaces of PCB’s, by means of using a multi-stage temperature controlled oven and a conveyor [1

1. R. Strauss, SMT Soldering Handbook (Newnes, 1998) Chap. 1.

, 15

15. N.-C. Lee, Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies (Newnes, 2002), Chap. 4.

]. Besides this system, a rework station is also required at the electronic products manufacturing facilities, in order to have the ability to solder or remove single SMT components. For the case of small weight micro BGA’s, removing and re-soldering the component in a user-friendly and cost effective way is quite difficult. For the case of hot air flow systems, the mass air momentum increases the risk of producing misalignment, unevenly and non-uniform temperature distribution over the micro SMT device. In principle, these last common problems can be surpassed by means of non-contact IR soldering method, but a uniform heat distribution across the entire surface of the SMT component is required. Non-uniform heating, on the other hand, would cause the component to unevenly drop or tilt towards the side or corner that has premature reached reflow. For this last reason, an adequate analysis of temperature distributions produced by diverse IR soldering optical systems is worthwhile. However, there are some preliminary results of an IR light rework soldering system which performance for temperature control and uniformity is appropriate over the area of the BGA; nonetheless there is not enough information about important properties as irradiance and infrared thermography [17

17. C. Anguiano, M. Félix, A. Medel, D. Salazar, and H. Márquez, “Heating capacity analysis of a focused infrared light soldering system,” in Proceedings of Conference on IEEE Industrial Electronics Society, (Institute of Electrical and Electronics Engineers, Melbourne, 2011), 2136–2140. [CrossRef]

,18

18. M. Félix, C. Anguiano, A. Medel, M. Bravo, D. Salazar, H. Márquez, and J. Chacon, “Infrared thermography of integrated circuits heated by focused IR light soldering system,” in Conference of Latin America Optics and Photonics, (Optical Society of America, 2012), paper LTC4.3.

]. In this paper, an analysis of irradiance and infrared thermography on different optical setups is presented.

2. Optical systems for FILSS

The purpose of the IR light soldering system is to create adequate temperature distribution uniformity at the target SMT device under soldering process; and here is considered that an analysis of irradiance and infrared thermography at the target SMT device is required. Current FILSS equipment are based on some approaches: a) Focused heater based on single or multiple elliptical or parabolic reflectors to produce concentration of IR lamp energy into a spot or well-defined target area, b) Non-focused heater based on array of IR lamps and flat mirrors to produce uniform heat area over the target, and c) Illumination system that consists of a focused heater based on a parabolic reflector and a condenser system to produce irradiation of IR lamp energy into a spot or well-defined target area [1

1. R. Strauss, SMT Soldering Handbook (Newnes, 1998) Chap. 1.

,15

15. N.-C. Lee, Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies (Newnes, 2002), Chap. 4.

,16

16. R. H. Gibbs and D. J. Lowrie, Infra-red rework station, patent number: 4,843,216 (1989)

].

2.1 Basic optical setup

This optical setup is composed by infrared light source, condenser lens, and secondary focusing lens, as shown in Fig. 1(a). The infrared light source used in the proposed FILSS is a Tungsten halogen infrared lamp (QTH). An elliptical shape reflector with gold coating is also integrated to this QTH lamp, used to improve the lamp optical power output [22

22. Product information Bulletin, “64635 HLX Tungsten Halogen Lamp”.

]. The elliptical shape enables light reflection from the first focus where the tungsten filament is located, to a second focus where the optical power output of the lamp is concentrated, as specified in its datasheet [22

22. Product information Bulletin, “64635 HLX Tungsten Halogen Lamp”.

, 23

23. S. Bahaa and T. Malvin, Fundamentals of Photonics (John Wiley & Sons Inc., 1991), Chap. 1.

]. The function of the condenser lens is to collect the beam provided by the QTH lamp and collimate it into the secondary focusing lens. This is achieved by means of using an aspheric lens, which has great properties for light collection and collimation. The secondary focusing lens is a double convex lens; where both surfaces are spherical and have the same radius of curvature, thereby minimizing spherical aberration in situations where the object and image have an equal or near distances from the lens. This element is used to control the output beam spot size at the point where the BGA will be soldered. Both described lenses used in the proposed setups are made of fused silica glass, because this type of glass has an internal transmission window between 290 and 2100 nm, as well as a low thermal expansion coefficient, which reduces the risk of thermal shock at high temperatures [23

23. S. Bahaa and T. Malvin, Fundamentals of Photonics (John Wiley & Sons Inc., 1991), Chap. 1.

26

26. J. Simmons and K. S. Potter, Optical Materials (Academic Press, 2000), pp. 175–176.

].

2.2 Optical setup with ground glass diffuser beam homogenizer

This setup is composed by the same elements of the basic setup, but a ground fused silica diffuser is added between the infrared light source and the condenser lens, at the work distance of the light source, as shown in Fig. 1(b). A ground fused silica light diffuser was used as a beam homogenizer in order to degrade the filament image from the QTH lamp at the output of the setup, which increased the uniformity of the light spot at the BGA [27

27. N. A. Fomin, Speckle Photography for Fluid Mechanics Measurements (Springer-Verlag, 1998), Chap. 5.

].

2.3 Optical setup with a non-imaging homogenizer

In this third optical setup the ground glass diffuser was removed, and a combination of divergent lens, multilens array and field lens were used as a non-imaging beam homogenizer system. The infrared light source and the condenser lens are the same elements used in the basic and second setups. This optical setup is shown in Fig. 1(c). The divergent lens was used to provide slight beam divergence, after passing through the condenser lens in order to have a closer approximation of a collimated beam at the multilens array. A multilens array is a two dimensional array of individual optical elements formed into a single optical element, which is used to transform non-uniform distribution light to a uniform distribution. For this non-imaging homogenizer optical setup, a multilens array divides the incident beam into beam-lets which pass through a field lens (FL) to be overlapped at the homogenization plane, where the BGA to be soldered is located. The homogenization plane (FP) is located at one focal length distance fFP with respect to the field lens [28

28. F. M. Dickey, S. C. Hoswade, and D. L. Shealy, Laser Beam Shapping Applications (Taylor & Francis, 2005), Chap. 8.

]. The dimensions of the beam in the homogenized plane DFT are given by Eq. (1):
DFT=|PLA1fFLfLA1|.
(1)
where PLA1 is the pitch size, which is the distance between the geometrical centers of two consecutive lens elements in the array; fFL is the focal length of the field lens, and fLA1 is the focal length of the multilens array. The multilens array is made in B270 glass, which has very similar properties to fused silica glass [29

29. Technical/ Sheet glasses, “optical-glass-n-bk7-b270-and-others-data-sheet,” http://www.crystran.co.uk/userfiles/files/optical-glass-n-bk7-b270-and-others-data-sheet.pdf

].

2.4 Optical setup based in imaging homogenizer

The fourth optical setup based in an imaging homogenizer system is composed by the same elements of the third setup, but a pair of multilens arrays is added to integrate the imaging homogenizer, as shown in Fig. 1(d). The homogenization plane (FP) is located at one focal length distance fFP with respect to the field lens. The beam spot diameter at the homogenized plane DFT is given by Eq. (2):
DFT=PLA1fFLfLA1fLA2[(fLA1+fLA2)a12].
(2)
where fLA1<a12<fLA1 + fLA2 and a12 is the separation distance between first and second multilens arrays (LA1) and (LA2), respectively.

The first multilens array LA1 divides the incident beam into multiple beam-lets. The second multilens array LA2, in combination with the Field Lens (FL), acts as an array of objective lenses that superimposes the images of each of the beam-lets in the first array into the homogenization plane (FP) [28

28. F. M. Dickey, S. C. Hoswade, and D. L. Shealy, Laser Beam Shapping Applications (Taylor & Francis, 2005), Chap. 8.

].

3. Heat transfer model for the proposed optical setups for FILSS

The first heat transfer mode is by means of delivery infrared radiation towards the object, which begins with the emitted IR radiation from a lamp. As the lamp temperature increases, the heat transfer output increases exponentially to the fourth power. Then the radiated energy is transmitted through the optical setup components, where three radiation energy loss mechanisms in this heat transfer process are involved, which are spectral absorption of the optical glass elements, reflections at the glass-air interfaces, and the throughput property of the optical setups [30

30. R. N. Cox, Reflow Technology Handbook (Research Inc., 1992), Chap. 1.

]. The infrared heat transfer QIR calculation model is provided by Eq. (3),
QIR=V*εs*Γi*ΓOE*αSMT*σ*(Ts4TSMT4).
(3)
where QIR is the heat transfer in W/cm2, V is the geometric view factor of the system (0-1), εs is emissivity of the IR source (0-1), i is the optical elements’ internal transmission (0-1), OE is the cumulative transmission coefficient values at the multiple glass-air interfaces of the optical elements (0-1), αSMT is the absorptivity of the SMT component (0-1), σ is the Stefan-Boltzmann constant (5.67 x 10−12 W/cm2/°K4) and Ts and TSMT are the temperatures of the lamp and SMT component respectively [30

30. R. N. Cox, Reflow Technology Handbook (Research Inc., 1992), Chap. 1.

].

The second heat transfer mode is by means of conduction, which begins at the surface of the SMT component that absorbs the IR radiation guided beam. Then the heat energy is transferred through the BGA bulk packaging material, in order to arrive to the solder joints of them SMT component. The heat energy loss mechanism of this conduction heat transfer process is because a fraction of the heating energy remains in the bulk packaging material of the SMT component, not reaching the solder terminals of the component. Calculation using this model of heat transfer by conduction QC is represented by Eq. (4),
QC=A*k*(T1T2)/ΔX.
(4)
where QC is heat transfer by conduction in W, A is the cross sectional area in cm2, k is thermal conductivity in W/cm-°C, T1 and T2 are the temperatures in °C, at point 1 (top of BGA) and point 2 (bottom of the BGA) of Fig. 2, respectively, and ΔX is the bulk material thickness between points 1 and 2, in cm [30

30. R. N. Cox, Reflow Technology Handbook (Research Inc., 1992), Chap. 1.

].

4. Simulation parameters

Several simulations were executed by means of using the ZemaxTM software, in order to analyze the parameters of spot size and irradiance at the output of the proposed optical setups. These parameters of the light beam emitted by the tungsten-halogen lamp and that arrived at the surface of the BGA were obtained in a simulated detector located at the BGA distance. Table 1

Table 1. Parameters of the components used in the proposed optical setups

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shows the parameters of the simulated elements used in the basic optical setup. This is composed by the infrared light source, condenser, secondary lenses, and detector. The infrared light source was designed with two elements, an elliptical reflector and a filament, specified in the QTH lamp datasheet. The other elements and separation distances used in the simulation were selected from previously results [17

17. C. Anguiano, M. Félix, A. Medel, D. Salazar, and H. Márquez, “Heating capacity analysis of a focused infrared light soldering system,” in Proceedings of Conference on IEEE Industrial Electronics Society, (Institute of Electrical and Electronics Engineers, Melbourne, 2011), 2136–2140. [CrossRef]

,18

18. M. Félix, C. Anguiano, A. Medel, M. Bravo, D. Salazar, H. Márquez, and J. Chacon, “Infrared thermography of integrated circuits heated by focused IR light soldering system,” in Conference of Latin America Optics and Photonics, (Optical Society of America, 2012), paper LTC4.3.

]. A detector was used in the simulation as a BGA SMT device.

In the third and fourth optical setups, the infrared light source, condenser lens and field lens have the same parameters than the first basic optical setup. In both setups, a divergent lens was used to obtain a non-converging angle of the beam incident in a multilens array.

5. Experimental setup

The four proposed optical setups for the FILSS were mounted on an optical table. The Tungsten-Halogen Lamp OSRAM model 64635 HLX, with 150 W of electrical power consumption, was used in all proposed setups, as well as a BGA with 35x35 mm dimensions. The length of the setups varied between 151 and 345.2 mm. The distances between elements for the four optical setups for the FILSS are shown in Table 2

Table 2. Distance between elements of the proposed optical setups

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.

Different lenses from Edmund OpticsTM manufacturer were used for simulations and experiments of the proposed optical setups. For the condenser lens, an uncoated UV-grade fused silica glass aspheric lens model M67-266 was used. The secondary lens was an uncoated UV-grade fused silica double convex lens model M32-982. The divergent lens was an uncoated UV-grade fused silica plane-concave lens model M08-074, while the multi-lens array used was a B270 glass material model M63-230.

Two cameras were used to measure the principal output parameters under study of the proposed optical setups for FILSS. For each optical setup, the output light spot at the location of the BGA was captured by means of a CCD camera which was fixed at a distance of 20 cm, aligned with the axis of each optical setup.

After recording the irradiance image, the CCD camera was removed and the BGA was put in the soldering plane. The beam was applied to the BGA, and by means of a thermography camera ThermaCAM E25 from FlirTM systems, the thermal distribution and heat transfer in the BGA were obtained.

6. Results

The results from this work are divided in three parts: a) irradiance analysis from simulations and by means of the CCD camera, b) thermal distribution analysis in the BGA obtained by means of an IR thermography camera, and c) a heat transfer analysis due to radiation and conduction processes are presented.

6.1 Irradiance analysis

The proposed optical setups for FILSS were simulated using ZemaxTM software. In these simulations, spot size and irradiance were obtained. The obtained output beams spatial distributions are based on Physical Optics Propagation method, which uses diffraction calculations to propagate a wavefront through an optical system surface by surface [31

31. A. Locke, “Exploring Physical Optics Propagation in Zemax”, http://kb-en.radiantzemax.com/KnowledgebaseArticle50227.aspx, October 31, (2005).

]. In order to propagate the beam from one surface to another, either Fresnel diffraction or angular spectrum propagation algorithm is used. The simulation software automatically chooses the algorithm that yields the highest numerical accuracy. The diffraction propagation algorithms yield correct results for any propagation distance, for any arbitrary beam, as well as can account for any surface aperture, including user defined apertures [31

31. A. Locke, “Exploring Physical Optics Propagation in Zemax”, http://kb-en.radiantzemax.com/KnowledgebaseArticle50227.aspx, October 31, (2005).

].

First we present the light distribution and spot size obtained at the output plane of the simulated optical setups for FILSS, which are shown in Fig. 3
Fig. 3 Irradiance at working distance of proposed setups obtained from simulation. (a) Basic optical setup. (b) Optical setup based in a non-imaging homogenizer. (c) Optical setup based with an imaging homogenizer.
. In the basic optical setup, Fig. 3(a), a non-uniform light distribution with an estimated spot diameter of 30 mm is shown. It can be seen that the shape of this light distribution has image information from the IR lamp filament, mainly because there is no homogenizing element in this optical setup. In Fig. 3(b), it can be seen that the filament image has vanished, which demonstrates the homogenizing function of this optical setup. In Fig. 3(c) the optical setup based in an imaging homogenizer is shown. The estimated dimensions of the light spot DFT are 15.37x11.53 mm, which is in agreement with Eq. (2). In this setup the multilens array pair generates superposition of beam-lets with uniform intensity, as can be seen in the center part of Fig. 3(c).The simulation for the optical setup for FILSS with ground glass diffuser beam homogenizer could not be obtained because none of the random surface elements available for the simulation software matched the surface roughness, material and thickness parameters of the diffuser used in the optical setup.

6.2 Irradiance measurements

In order to confirm the theoretically calculated light spot dimension DFT at the BGA location in the proposed optical setups for FILSS, the light spots at the output of the optical setups were acquired by means of a CCD camera. The images that were acquired are shown in Fig. 4
Fig. 4 Experimental irradiance at working distance of the proposed optical setups for FILS. (a) Basic optical setup. (b) Optical setup with ground glass diffuser homogenizer. (c) Optical setup based in a non-imaging homogenizer. (d) Optical setup based in an imaging homogenizer.
.

As can be seen in Figs. 3 and 4, the light spots obtained by means of simulation or experimentally, are very similar in dimensions and spatial distribution, as well as they are in agreement with Eqs. (1) and (2).

As previously commented, the IR light soldering system purpose is to create an adequate temperature distribution at the BGA soldering area. Following the light spots’ irradiance analysis of the proposed optical setups, the infrared thermography analysis at the BGA soldering area is further described.

The main results presented in this paper are analysis of the thermal distributions measured at the surface area of a 35x35 mm BGA, for each of the proposed optical setups for FILSS. In order to obtain these thermal distributions, the reflow soldering temperature profile was applied to the BGA for each of the proposed optical setups for FILSS. The reflow temperature profile must be applied to be sure that the solder process is executed without damage to the BGA SMT device, besides having correct melting of the solder paste. Figure 5
Fig. 5 Typical reflow soldering temperature profile for lead-free Sn-Ag-Cu solder alloy [32].
shows the typical reflow temperature profile, with upper and lower temperature limits used for Sn-Ag-Cu solder alloy [32

32. J. Bath, Lead-Free Soldering (Springer, 2007), Chap. 3 and 5.

].

By means of capturing thermo-graphic images on the surface area of the BGA with an IR thermography camera, the thermal distribution was measured at the peak temperature stage of the reflow soldering profile, in order to analyze the heat transfer and soldering capacity for each of the proposed optical setups for FILSS. The captured thermo-graphic images are shown in Fig. 6
Fig. 6 Thermo-graphic images of BGA at the peak temperature of the reflow soldering profile. (a) Basic optical setup. (b) Optical setup with ground glass diffuser beam homogenizer. (c) Optical setup based in a non-imaging homogenizer. (d) Optical setup based in an imaging homogenizer.
.

The thermo-graphic images analysis gives a peak temperature at the surface of BGA of 241°C, reached in the basic optical setup for FILSS, as can be seen in Fig. 6(a). This peak temperature is enough to melt lead free solder (~232°C for commonly used Sn-Ag-Cu solder alloy), which is a main performance parameter to be obtained in the proposed optical setups for FILSS. The other three proposed optical setups do not reach the required lead free solder melting temperature. However, soldering technique process use a complementary back-heating stage to improve the rework process and prevent printed circuit board (PCB) bending [19

19. E. Bradley, C. Handwerker, J. Bath, R. Parker, and R. Gedney, Lead-Free Electronics (John Wiley & Sons Inc., 2007), Chap. 1,5 and 10.

]. The heating element that provides the back-heating to the BGA can contribute with an additional ΔT ~40-60°C; and the last increase can help to surpass the temperature range required by the IR soldering process [16

16. R. H. Gibbs and D. J. Lowrie, Infra-red rework station, patent number: 4,843,216 (1989)

].

The other performance parameter to be determined for the proposed optical setups for FILSS is the uniformity of the thermal distribution measured at the surface area of the BGA. In order to determine this uniformity parameter, the thermal images for each of the proposed optical setups for FILSS were analyzed by means of plotting a matrix of measured temperature points. The plots that were obtained are shown in Fig. 7
Fig. 7 Thermo-graphic distribution of the proposed optical setups for FILSS. (a) Basic optical setup. (b) Optical setup with ground glass diffuser beam homogenizer. (c) Optical setup based in a non-imaging homogenizer. (d) Optical setup based in an imaging homogenizer.
. In this Fig., a higher uniform temperature with area of diameter of 25 mm is obtained for each optical setup. However, a bigger diameter area with uniform high temperature can be obtained in all the proposed optical setups by means of varying the elements distances and diameters. The statistical data of average and standard deviation for Fig. 7 is shown in Table 3

Table 3. Statistical data obtained from thermal image analysis

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.

The basic optical setup has a peak temperature of 241.2°C with an average temperature of 225.2°C, measured on a matrix of points across the 35 x 35 mm BGA. This peak temperature value meets the required melting temperature for lead free solder, ensuring that all the solder balls below the BGA under soldering process can be melted to be properly soldered. The temperature standard deviation at the measured points of the BGA for this basic optical setup for FILSS is 8.4°C, which meets the reflow soldering profile variation range for the different temperature stages of this soldering method [1

1. R. Strauss, SMT Soldering Handbook (Newnes, 1998) Chap. 1.

, 15

15. N.-C. Lee, Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies (Newnes, 2002), Chap. 4.

]. This confirms that the obtained thermal distribution measured at the high temperature area with diameter of 25 mm, meets the required uniformity parameter. The other three proposed optical setups have improved temperature uniformity parameter, as can be seen in the lower values of standard deviation in Table 3, but the obtained peak temperature at the surface of the BGA doesn’t meet the lead free solder melting temperature. For this reason, it was required an improvement in the setups that could achieve enough temperature to solder lead free BGA solder joints. After analyzing the parameters that can affect the performance of the proposed optical setups for FILSS, there were found three main parameters: glass absorption, Fresnel reflection, and optical setup throughput. We consider that the main contributor is due to the Fresnel reflection coefficient (~4%) at each air-glass interface of the optical elements used for the proposed setups, because these interfaces lacks of anti-reflecting coatings. So we proposed adding anti-reflecting (AR) coatings for the near-IR range to the optical elements of the proposed setups and analyzed the improvement. The result of this analysis is a reduction in the total glass-air interfaces reflection factor for the proposed optical setups, where the reflection percentage obtained without anti-reflecting coating in the optical setups varies between 15% to 34%, while when an anti-reflecting coating NIR II is applied, reduction of the reflection factor is between 2% to 5% for normal incidence angle [33]. With this reduction in the reflection factor, the potential temperature increase for the output of all the proposed optical setups for FILSS is enough to meet the lead free solder melting temperature. The comparative temperatures between optical setups for FILSS with and without AR coatings are shown in Table 4

Table 4. Comparative temperatures between optical setups for FILSS for the addition of AR coatings

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, as well as the temperature increase with AR coatings.

It can be seen that the peak temperature in the basic optical setup using AR coatings exceeds the maximum temperature to solder BGA’s without packaging material damage, which can vary between 232°C and 265°C. This fact can be an advantage for further emerging lead-free solder alloys with higher melting point temperature. The other three proposed optical setups are in the range of allowable temperatures required to melt the solder balls.

6.3 Heat transfer analysis

The heat transfer by infrared radiation was analyzed in order to obtain the beam guiding properties of the proposed optical setups for FILSS, while the conduction heat transfer was analyzed at the BGA. Table 5

Table 5. Heat transfer analysis for the four proposed optical setups for FILSS

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shows the heat transfer analysis for the optical setups for FILSS, with and without AR coatings. The heat transfer analysis by IR radiation is in agreement with Eq. (3), where ɛs = 0.35 for the QTH lamp [34

34. W. Smith, Modern Optical Enginnering (McGraw-Hill, 2000), pp. 231.

]; i = 0.999 for each optical element of fused silica glass in a spectral transmission window >250nm [26

26. J. Simmons and K. S. Potter, Optical Materials (Academic Press, 2000), pp. 175–176.

], and an average i = 0.993 for B270 glass in a transmission window between 400nm and 2000nm [29

29. Technical/ Sheet glasses, “optical-glass-n-bk7-b270-and-others-data-sheet,” http://www.crystran.co.uk/userfiles/files/optical-glass-n-bk7-b270-and-others-data-sheet.pdf

], which is used in multilens array; OE = 0.92 without AR coating and OE = 0.98 with AR coating (at normal incidence angle) [35

35. P. W. Baumeister, Optical Coating Technology (SPIE Press Monograph, 2004), chap. 1.

]; αSMT = 0.97 for BGA epoxy packaging material [35

35. P. W. Baumeister, Optical Coating Technology (SPIE Press Monograph, 2004), chap. 1.

]. In other hand, the analysis of heat transfer by conduction is in agreement with Eq. (4), across the area A of BGA of 35 mm x 35 mm, where k = 0.2 W/cm2-°K for the BGA epoxy packaging material [36

36. R. R. Tummala, E. J. Rymaszewski, and A. G. Klopfenstein, Microelectronics Packaging Handbook (Springer, 1997), pp. I-322.

], and Δx = 1 mm.

Our radiation and conduction heat transfer physical model requires the condition shown in Eq. (5),
TS>(TSMT=T1)>T2.
(5)
because the absorptivity of the SMT component αSMT, as well as the thermal conductivity k, don’t generate heat energy, representing heat energy loss. The TSMT = T1 condition for the temperature parameters from Eq. (3) and Eq. (4) takes place at the surface of the BGA, which is the point where the heat transfer process switches from radiation to conduction. Regarding of all the multiplication factors of Eq. (3) and Eq. (4), the most influent parameters for the obtained heat transfer values are the temperature differences (TS4 - TSMT4) and (T1 – T2), for Eq. (3) and Eq. (4), respectively. Since all four of the proposed optical setups for FILSS have the condition (TS4 - TSMT4) >> (T1 – T2), the radiation heat transfer value is greater than the conduction heat transfer value for all the proposed optical setups for FILSS, as can be seen in Table 5. Normalization for the 35 mm x 35 mm surface area of the BGA under study was applied to the conduction heat transfer of Eq. (4). After this normalization, the radiation and conduction heat transfer values have the same units of W/cm2.

The variation range in heat transfer by radiation for the proposed optical setups is between 44.2 W/cm2 and 50.49 W/cm2. Heat transfer in this range ensures correct melting of the solder balls of the BGA, without problems as cold solder for lower temperatures, or damaged packaging epoxy material due to higher temperatures [32

32. J. Bath, Lead-Free Soldering (Springer, 2007), Chap. 3 and 5.

].

In Table 5 it can be seen that a higher heat transfer by radiation and conduction is obtained in the basic optical setup with and without AR coating, where few optical elements compose this setup. In the other three optical setups, radiation heat transfer decreases when more optical elements are added. As previously identified, this decrease is mainly due to multiple glass-air interfaces reflections, which consequence is a lower thermal energy arriving at the surface of the BGA. However, it was found an increment in the heat transfer by radiation when AR coatings are added to the optical setups, which enable the last three optical setups to be in correct heat transfer value range for melting solder joints of the BGA.

We consider that the proposed optical setups for FILSS have a highly uniform light distribution that can be used for other applications, which can be mentioned: projector LCD [37

37. N. F. Borrelli, “Efficiency of microlens arrays for projection LCD,” in Proceedings of IEEE on Electronic Components and Technology Conference, (Institute of Electrical and Electronics Engineers,Washington, DC,1994) 338–345. [CrossRef]

], photometers and radiometers calibration [38

38. K. Hauer and A. Höpe, “High-grade uniform light source for radiometric and photometric applications,” MAPAN 24(3), 175–182 (2009). [CrossRef]

], and biomedical applications [39

39. E. Bar-Kochba, S. Govil, J. P. Longtin, A. Gouldstone, and M. D. Frame, “Uniform-intensity, visible light source for in situ imaging,” J. Biomed. Opt. 14(2), 024024 (2009). [CrossRef] [PubMed]

]. The results of our work can provide useful experimental data for these mentioned applications.

7. Conclusions

In this work, irradiance and thermal distribution at the surface area of a BGA SMT device were analyzed for the reaching output beam of four proposed optical setups for focused IR light soldering system (FILSS) which each one has its own specific characteristics as mentioned in this work. The results of irradiance in simulation match with results obtained experimentally at the surface area of the BGA, which validates the deployed simulator as a design tool for optical setups for FILSS. For the thermal distribution at the surface of the BGA, it is observed that all four proposed optical setups present uniform thermal distributions that comply with the reflow solder profile temperature variation requirement. This attribute can be due to temperature evolution on the packaging material of the BGA, which is currently under study. The peak temperature reached at the surface of the BGA for the proposed optical setups for FILSS was 241.2°C, which is enough to melt the lead free solder currently required for mass production electronic products. The dual physical mechanism of radiation and conduction for heat transfer process was measured and analyzed, which provided important heat energy estimation data for the output of optical setups for FILSS, and a good approach to understand the complex mechanism of heat transfer involved in IR light soldering process through the optical setup and the BGA device characteristics. New solder alloys are currently in development for soldering lead free SMT devices, some of which have higher melting point temperatures [40

40. R. I. Rodriguez, D. Ibitayo, and P. O. Quintero, “Thermal stability characterization of the Au–Sn bonding for high-temperature applications,” IEEE Trans. Components, Packaging Manufacturing Technol. 3(4), 549–557 (2013). [CrossRef]

]. We consider that the results of this work can provide very useful information for developing new FILSS, which can reach lead-free solder alloys higher melting point temperatures, as well as providing uniform temperature distribution on smaller or larger area BGA’s that are continuously in development.

References and links

1.

R. Strauss, SMT Soldering Handbook (Newnes, 1998) Chap. 1.

2.

C. Chun-Chi, Y. Li, H. Li, and C. Wang, “Fine pitch BGA solder joint split in SMT process,” in Proceedings of IEEE on International Microsystems, Packaging, Assembly and Circuits Technology Conference, (Institute of Electrical and Electronics Engineers, Taipei, 2009) 602–605. [CrossRef]

3.

T. Kangasvieri, J. Halme, J. Vahakangas, and M. Lahti, “Broadband BGA-Via transitions for reliable RF/microwave LTCC-SiP module packaging,” IEEE Microwave and Wireless Components Letters, (Institute of Electrical and Electronics Engineers), 18(1), 34–36 (2008).

4.

L. Nie, M. Osterman, F. Song, J. Lo, R. Lee, and M. Pecht, “Solder ball attachment assessment of reballed plastic ball grid array packages,” IEEE Transactions on Components and Packaging Technologies, (Institute of Electrical and Electronics Engineers), 901–908 (2009).

5.

P. L. Tu, Y. C. Chan, K. C. Hung, and J. K. L. Lai, “Comparative study of micro-BGA reliability under bending stress,” IEEE Transactions on Advanced Packaging, (Institute of Electrical and Electronics Engineers), 750–756 (2000).

6.

J. Gao, Y. Wu, and H. Ding, “Micro-BGA package reliability and optimization of reflow soldering profile,” in Proceedings of IEEE International Conference on Asian Green Electronics, (Institute of Electrical and Electronics Engineers, China, 2005), 135–139.

7.

Y. L. Tzeng, E. Chen, J. Y. Lai, Y. P. Wang, and C. S. Hsiao, “Stress and thermal characteristic analyses for advanced FCBGA packages,” in Proceedings of IEEE on International Microsystems, Packaging, Assembly and Circuits Technology Conference, (Institute of Electrical and Electronics Engineers, Taiwan, 2006), 1–4. [CrossRef]

8.

S. L. Kanuparthi, J. E. Galloway, and S. McCain, “Impact of heatsink attach loading on FCBGA package thermal performance,” in Proceedings of IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, (Institute of Electrical and Electronics Engineers, San Diego, Ca., 2012), 216–223. [CrossRef]

9.

B. Y. Jung, J. Y. Gim, J. D. Kim, C. H. Lee, M. Jimares, N. Islam, and R. Darveaux, “Study of FCMBGA with low CTE core substrate,” in Proceedings of IEEE on Electronic Components and Technology Conference, (Institute of Electrical and Electronics Engineers, San Diego, Ca., 2009), 301–304.

10.

A. Raj and T. Latha, VLSI Design (PHI, 2008), pp. 1–4.

11.

H. G. Sy, P. Arulvanan, and P. A. Collier, “Rework and reliability of QFP and BGA lead-free assemblies,” in Proceedings of Conference on Electronics Packaging Technology, (Institute of Electrical and Electronics Engineers, Singapore,2002), 194–199. [CrossRef]

12.

K. Dusek, M. Novak, and A. Rudajevova, “Study of the components self aligment in surface mount technology,” in Proceedings of IEEE on Seminar on Electronics Technology, (Institute of Electrical and Electronics Engineers, Bad Aussee, 2012), 197–200.

13.

M. Pecht, Soldering Process and Equipment (John Wiley & Sons Inc., 1993), Chap.4.

14.

N. Heilmann, “A comparison of vapor phase, infrared and hot gas soldering,” in Proceedings of IEEE Electronic Manufacturing Technology Symposium, (Institute of Electrical and Electronics Engineers, Neuilly sur Seine, 1988), 70–72. [CrossRef]

15.

N.-C. Lee, Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies (Newnes, 2002), Chap. 4.

16.

R. H. Gibbs and D. J. Lowrie, Infra-red rework station, patent number: 4,843,216 (1989)

17.

C. Anguiano, M. Félix, A. Medel, D. Salazar, and H. Márquez, “Heating capacity analysis of a focused infrared light soldering system,” in Proceedings of Conference on IEEE Industrial Electronics Society, (Institute of Electrical and Electronics Engineers, Melbourne, 2011), 2136–2140. [CrossRef]

18.

M. Félix, C. Anguiano, A. Medel, M. Bravo, D. Salazar, H. Márquez, and J. Chacon, “Infrared thermography of integrated circuits heated by focused IR light soldering system,” in Conference of Latin America Optics and Photonics, (Optical Society of America, 2012), paper LTC4.3.

19.

E. Bradley, C. Handwerker, J. Bath, R. Parker, and R. Gedney, Lead-Free Electronics (John Wiley & Sons Inc., 2007), Chap. 1,5 and 10.

20.

E. P. Leng, M. Ding, W. T. Ling, N. Amin, A. I. M. Y. Lee, and A. S. M. A. Haseeb, “A study of SnAgNiCo vs Sn3.8AgO. 7Cu C5 lead free solder alloy on mechanical strength of BGA solder joint,” in Proceedings of IEEE on Electronics Packaging Technology Conference, (Institute of Electrical and Electronics Engineers, Singapore, 2008), 588–594. [CrossRef]

21.

T. Novak and F. Steiner, “Influence of intermetallic compounds growth on properties of lead-free solder joints,” in Proceedings of IEEE on International Spring Seminar on Electronics Technology, (Institute of Electrical and Electronics Engineers, Bad Aussee, 2012), 213–217. [CrossRef]

22.

Product information Bulletin, “64635 HLX Tungsten Halogen Lamp”.

23.

S. Bahaa and T. Malvin, Fundamentals of Photonics (John Wiley & Sons Inc., 1991), Chap. 1.

24.

J. Carlton, Frames and Lenses (Slack incorporated, 2000), pp. 68.

25.

C. Pruss, E. Garbusi, and W. Osten, “Testing aspheres,” Opt. Photon. News 19(4), 24–29 (2008). [CrossRef]

26.

J. Simmons and K. S. Potter, Optical Materials (Academic Press, 2000), pp. 175–176.

27.

N. A. Fomin, Speckle Photography for Fluid Mechanics Measurements (Springer-Verlag, 1998), Chap. 5.

28.

F. M. Dickey, S. C. Hoswade, and D. L. Shealy, Laser Beam Shapping Applications (Taylor & Francis, 2005), Chap. 8.

29.

Technical/ Sheet glasses, “optical-glass-n-bk7-b270-and-others-data-sheet,” http://www.crystran.co.uk/userfiles/files/optical-glass-n-bk7-b270-and-others-data-sheet.pdf

30.

R. N. Cox, Reflow Technology Handbook (Research Inc., 1992), Chap. 1.

31.

A. Locke, “Exploring Physical Optics Propagation in Zemax”, http://kb-en.radiantzemax.com/KnowledgebaseArticle50227.aspx, October 31, (2005).

32.

J. Bath, Lead-Free Soldering (Springer, 2007), Chap. 3 and 5.

33.

Edmund Optics, “Anti-Reflection (AR) Coatings,” http://www.edmundoptics.com/technical-resources-center/optics/anti-reflection-coatings/?&pagenum=2#coatingspec.

34.

W. Smith, Modern Optical Enginnering (McGraw-Hill, 2000), pp. 231.

35.

P. W. Baumeister, Optical Coating Technology (SPIE Press Monograph, 2004), chap. 1.

36.

R. R. Tummala, E. J. Rymaszewski, and A. G. Klopfenstein, Microelectronics Packaging Handbook (Springer, 1997), pp. I-322.

37.

N. F. Borrelli, “Efficiency of microlens arrays for projection LCD,” in Proceedings of IEEE on Electronic Components and Technology Conference, (Institute of Electrical and Electronics Engineers,Washington, DC,1994) 338–345. [CrossRef]

38.

K. Hauer and A. Höpe, “High-grade uniform light source for radiometric and photometric applications,” MAPAN 24(3), 175–182 (2009). [CrossRef]

39.

E. Bar-Kochba, S. Govil, J. P. Longtin, A. Gouldstone, and M. D. Frame, “Uniform-intensity, visible light source for in situ imaging,” J. Biomed. Opt. 14(2), 024024 (2009). [CrossRef] [PubMed]

40.

R. I. Rodriguez, D. Ibitayo, and P. O. Quintero, “Thermal stability characterization of the Au–Sn bonding for high-temperature applications,” IEEE Trans. Components, Packaging Manufacturing Technol. 3(4), 549–557 (2013). [CrossRef]

OCIS Codes
(120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
(120.4820) Instrumentation, measurement, and metrology : Optical systems

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: July 17, 2013
Revised Manuscript: September 16, 2013
Manuscript Accepted: September 17, 2013
Published: September 30, 2013

Citation
C. Anguiano, M. Félix, A. Medel, M. Bravo, D. Salazar, and H. Márquez, "Study of heating capacity of focused IR light soldering systems," Opt. Express 21, 23851-23865 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-20-23851


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References

  1. R. Strauss, SMT Soldering Handbook (Newnes, 1998) Chap. 1.
  2. C. Chun-Chi, Y. Li, H. Li, and C. Wang, “Fine pitch BGA solder joint split in SMT process,” in Proceedings of IEEE on International Microsystems, Packaging, Assembly and Circuits Technology Conference, (Institute of Electrical and Electronics Engineers, Taipei, 2009) 602–605. [CrossRef]
  3. T. Kangasvieri, J. Halme, J. Vahakangas, and M. Lahti, “Broadband BGA-Via transitions for reliable RF/microwave LTCC-SiP module packaging,” IEEE Microwave and Wireless Components Letters, (Institute of Electrical and Electronics Engineers), 18(1), 34–36 (2008).
  4. L. Nie, M. Osterman, F. Song, J. Lo, R. Lee, and M. Pecht, “Solder ball attachment assessment of reballed plastic ball grid array packages,” IEEE Transactions on Components and Packaging Technologies, (Institute of Electrical and Electronics Engineers), 901–908 (2009).
  5. P. L. Tu, Y. C. Chan, K. C. Hung, and J. K. L. Lai, “Comparative study of micro-BGA reliability under bending stress,” IEEE Transactions on Advanced Packaging, (Institute of Electrical and Electronics Engineers), 750–756 (2000).
  6. J. Gao, Y. Wu, and H. Ding, “Micro-BGA package reliability and optimization of reflow soldering profile,” in Proceedings of IEEE International Conference on Asian Green Electronics, (Institute of Electrical and Electronics Engineers, China, 2005), 135–139.
  7. Y. L. Tzeng, E. Chen, J. Y. Lai, Y. P. Wang, and C. S. Hsiao, “Stress and thermal characteristic analyses for advanced FCBGA packages,” in Proceedings of IEEE on International Microsystems, Packaging, Assembly and Circuits Technology Conference, (Institute of Electrical and Electronics Engineers, Taiwan, 2006), 1–4. [CrossRef]
  8. S. L. Kanuparthi, J. E. Galloway, and S. McCain, “Impact of heatsink attach loading on FCBGA package thermal performance,” in Proceedings of IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, (Institute of Electrical and Electronics Engineers, San Diego, Ca., 2012), 216–223. [CrossRef]
  9. B. Y. Jung, J. Y. Gim, J. D. Kim, C. H. Lee, M. Jimares, N. Islam, and R. Darveaux, “Study of FCMBGA with low CTE core substrate,” in Proceedings of IEEE on Electronic Components and Technology Conference, (Institute of Electrical and Electronics Engineers, San Diego, Ca., 2009), 301–304.
  10. A. Raj and T. Latha, VLSI Design (PHI, 2008), pp. 1–4.
  11. H. G. Sy, P. Arulvanan, and P. A. Collier, “Rework and reliability of QFP and BGA lead-free assemblies,” in Proceedings of Conference on Electronics Packaging Technology, (Institute of Electrical and Electronics Engineers, Singapore,2002), 194–199. [CrossRef]
  12. K. Dusek, M. Novak, and A. Rudajevova, “Study of the components self aligment in surface mount technology,” in Proceedings of IEEE on Seminar on Electronics Technology, (Institute of Electrical and Electronics Engineers, Bad Aussee, 2012), 197–200.
  13. M. Pecht, Soldering Process and Equipment (John Wiley & Sons Inc., 1993), Chap.4.
  14. N. Heilmann, “A comparison of vapor phase, infrared and hot gas soldering,” in Proceedings of IEEE Electronic Manufacturing Technology Symposium, (Institute of Electrical and Electronics Engineers, Neuilly sur Seine, 1988), 70–72. [CrossRef]
  15. N.-C. Lee, Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies (Newnes, 2002), Chap. 4.
  16. R. H. Gibbs and D. J. Lowrie, Infra-red rework station, patent number: 4,843,216 (1989)
  17. C. Anguiano, M. Félix, A. Medel, D. Salazar, and H. Márquez, “Heating capacity analysis of a focused infrared light soldering system,” in Proceedings of Conference on IEEE Industrial Electronics Society, (Institute of Electrical and Electronics Engineers, Melbourne, 2011), 2136–2140. [CrossRef]
  18. M. Félix, C. Anguiano, A. Medel, M. Bravo, D. Salazar, H. Márquez, and J. Chacon, “Infrared thermography of integrated circuits heated by focused IR light soldering system,” in Conference of Latin America Optics and Photonics, (Optical Society of America, 2012), paper LTC4.3.
  19. E. Bradley, C. Handwerker, J. Bath, R. Parker, and R. Gedney, Lead-Free Electronics (John Wiley & Sons Inc., 2007), Chap. 1,5 and 10.
  20. E. P. Leng, M. Ding, W. T. Ling, N. Amin, A. I. M. Y. Lee, and A. S. M. A. Haseeb, “A study of SnAgNiCo vs Sn3.8AgO. 7Cu C5 lead free solder alloy on mechanical strength of BGA solder joint,” in Proceedings of IEEE on Electronics Packaging Technology Conference, (Institute of Electrical and Electronics Engineers, Singapore, 2008), 588–594. [CrossRef]
  21. T. Novak and F. Steiner, “Influence of intermetallic compounds growth on properties of lead-free solder joints,” in Proceedings of IEEE on International Spring Seminar on Electronics Technology, (Institute of Electrical and Electronics Engineers, Bad Aussee, 2012), 213–217. [CrossRef]
  22. Product information Bulletin, “64635 HLX Tungsten Halogen Lamp”.
  23. S. Bahaa and T. Malvin, Fundamentals of Photonics (John Wiley & Sons Inc., 1991), Chap. 1.
  24. J. Carlton, Frames and Lenses (Slack incorporated, 2000), pp. 68.
  25. C. Pruss, E. Garbusi, and W. Osten, “Testing aspheres,” Opt. Photon. News19(4), 24–29 (2008). [CrossRef]
  26. J. Simmons and K. S. Potter, Optical Materials (Academic Press, 2000), pp. 175–176.
  27. N. A. Fomin, Speckle Photography for Fluid Mechanics Measurements (Springer-Verlag, 1998), Chap. 5.
  28. F. M. Dickey, S. C. Hoswade, and D. L. Shealy, Laser Beam Shapping Applications (Taylor & Francis, 2005), Chap. 8.
  29. Technical/ Sheet glasses, “optical-glass-n-bk7-b270-and-others-data-sheet,” http://www.crystran.co.uk/userfiles/files/optical-glass-n-bk7-b270-and-others-data-sheet.pdf
  30. R. N. Cox, Reflow Technology Handbook (Research Inc., 1992), Chap. 1.
  31. A. Locke, “Exploring Physical Optics Propagation in Zemax”, http://kb-en.radiantzemax.com/KnowledgebaseArticle50227.aspx , October 31, (2005).
  32. J. Bath, Lead-Free Soldering (Springer, 2007), Chap. 3 and 5.
  33. Edmund Optics, “Anti-Reflection (AR) Coatings,” http://www.edmundoptics.com/technical-resources-center/optics/anti-reflection-coatings/?&pagenum=2#coatingspec .
  34. W. Smith, Modern Optical Enginnering (McGraw-Hill, 2000), pp. 231.
  35. P. W. Baumeister, Optical Coating Technology (SPIE Press Monograph, 2004), chap. 1.
  36. R. R. Tummala, E. J. Rymaszewski, and A. G. Klopfenstein, Microelectronics Packaging Handbook (Springer, 1997), pp. I-322.
  37. N. F. Borrelli, “Efficiency of microlens arrays for projection LCD,” in Proceedings of IEEE on Electronic Components and Technology Conference, (Institute of Electrical and Electronics Engineers,Washington, DC,1994) 338–345. [CrossRef]
  38. K. Hauer and A. Höpe, “High-grade uniform light source for radiometric and photometric applications,” MAPAN24(3), 175–182 (2009). [CrossRef]
  39. E. Bar-Kochba, S. Govil, J. P. Longtin, A. Gouldstone, and M. D. Frame, “Uniform-intensity, visible light source for in situ imaging,” J. Biomed. Opt.14(2), 024024 (2009). [CrossRef] [PubMed]
  40. R. I. Rodriguez, D. Ibitayo, and P. O. Quintero, “Thermal stability characterization of the Au–Sn bonding for high-temperature applications,” IEEE Trans. Components, Packaging Manufacturing Technol.3(4), 549–557 (2013). [CrossRef]

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