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Applied Optics

Applied Optics


  • Vol. 28, Iss. 21 — Nov. 1, 1989
  • pp: 4595–4603

Optical absorption aspects of laser soldering for high density interconnects

Michael Greenstein  »View Author Affiliations

Applied Optics, Vol. 28, Issue 21, pp. 4595-4603 (1989)

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In laser soldering, one has direct control over the incident laser pulse energy and duration but only indirect control over the temperature profile of the parts to be soldered. To understand the conversion of incident laser energy into a substrate temperature profile, two processes need to be understood: first, the optical absorption as a function of temperature and, second, the temperature distribution as a function of absorbed energy. The optical absorption aspect is addressed here. A set of total reflectivity measurements for real surfaces of interest is presented along with analytical calculations of the temperature dependence of the optical absorption for both specular and rough surfaces.

© 1989 Optical Society of America

Original Manuscript: November 7, 1988
Published: November 1, 1989

Michael Greenstein, "Optical absorption aspects of laser soldering for high density interconnects," Appl. Opt. 28, 4595-4603 (1989)

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  1. The integrating sphere accessory is the DRTA-9A made by Lapshere, Inc. as an accessory for the Perkin-Elmer Lambda-9 UV-VIS-NIR spectrometer. The actual reflectivity measurements were made at Labsphere. This schematic is a simplified version of a drawing from a Labsphere, Inc. product brochure.
  2. The x-ray analyzer was calibrated by analyzing an NBS Standard Reference Material 1131, which is a 40:60 Sn:Pb standard. The analyzer gave a 38:62 composition and is thus regarded as accurate to ∼2%.
  3. The soldering bath was purchased from Technic, Inc. This is a fluoborate sulfonic acid type system with ∼0.02% occluded carbon deposits.
  4. E. Lish, Lasers Tackle Tough Soldering Problems (Electronic Packaging & Production, June1984), p. 154.
  5. The solder paste is manufactured by Kester Solder, part R-229D, vacuum dried at 80°C for 30 min.
  6. Kapton is a trademark of the Dupont Company. The measurement of the total reflectivity does not give information about the transmission properties of the film. It is assumed in table IV that the polyimide will be backed up with an opaque material so that the effective absorption may be obtained from the reflectivity by assuming that the sum of the absorption and reflectivity is unity.
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  8. The undesirable effects of splattering may be reduced by spatially scanning the laser during the heating period.
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  11. ℱ5(x)=∫0x[(z5ez)/(ez−1)2]dz, J. Ziman, Electrons and Phonons (Oxford U.P., London, 1979), p. 54.
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  13. The Debye temperatures for Sn and Au are both ∼170 K, so that above room temperature T ≥ ϴD and ℱ5(x) converges to ℱ5 = ¼[(ϴD)/T]4.
  14. R. Klein Wassink, Soldering in Electronics (Electrochemical Publications, London, 1984), p. 105.
  15. C. Kent, “The Optical Constants of Liquid Alloys,” Phys. Rev. 14, 459–489 (1919). [CrossRef]
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  18. G. Kaye, T. Laby, Table of Physical and Chemical Constants (Longmans Green, London, 1966).
  19. Although the SnPb system as an alloy is not accurately described by the Bloch formula, the approximation of linear temperature dependence is used here for simplicity.
  20. A. Golovashkin, P. Motulevich, “Optical Properties of Tin at Helium Temperatures,” Sov. Phys. JETP 20, 44–49 (1965).
  21. N. Mott, H. Jones, The Theory of the Properties of Metals and Alloys (Dover, New York, 1958, p. 124.
  22. J. Petrakian et al., “Optical Properties of Liquid Tin Between 0.62 and 3.7 eV,” Phys. Rev. B 21, 3043–3046 (1980). [CrossRef]
  23. R. MacRae, E. Arakawa, M. Williams, “Optical Properties of Vacuum-Evaporated White Tin,” Phys. Rev. 162, 615–620 (1967). [CrossRef]
  24. J. Miller, “Optical Properties of Liquid Metals at High Temperatures,” Philos. Mag. 20, 1115–1132 (1969). [CrossRef]
  25. M. Otter, “Temperature Dependance of the Optical Constants of Heavy Metals,” Z. Phys. 161, 539–549 (1961). [CrossRef]
  26. M. Born, E. Wolf, Principles of Optics (Pergamon, London, 1980), Sec. 13.4.1.
  27. Effectively there are two independent electron gas calculations, one on each side of the melting point.
  28. P. Beckman, A. Spizzichino, The Scattering of Electromagnetic Waves From Rough Surfaces (Artech House, Norwood, MA, 1987).

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