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Metallization and Soldering Issues

I am trying to solder a substrate to a TEC, and the solder on the TEC does not seem to be wetting to the substrate metallization at all. The substrate has, from bottom to top layer metallization: TaN - 50 Ohms/sq, TiW - .03 microns, Au - .5 microns, Ni - 5 microns. For the TEC, the solder is InSnCd - I'm not sure of the exact proportions. This is deposited onto the TEC by solder pot. The solder thickness is 1.5 mils measured from the top of the solder meniscus. During the substrate soldering we have tried using a nitrogen blanket to reduce any oxidation, but it doesn't seem to improve the wetting at all. We are using this particular solder due to the fact that our solder stack-up requires a solder that flows below 118 °C (this is at 93 °C).

Are there any other solders we can use to improve wetting? Or is the problem the substrate metallization?

Laura Hughes
- Newark, California
2001

Old or oxidized nickel does not solder well. You did not mention an activation step or a flux.

James Watts
- Navarre, Florida
2001

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We cannot use flux at this step due to contamination issues. We are reflowing the 93 C solder at 110 °C for a total time of approximately 2 minutes. The solder flows in about 30 seconds, but we maintain the high temperature for 1 minute to encourage thorough heating of all parts prior to scrubbing. The last 30 seconds are a manual scrub, then removal from heating surface. It seems scrubbing is simply "scooting" the solder to the outside edges of the TEC, leaving little if any solder between the substrate and TEC, rather than encouraging a fillet to form.

Should we be heating longer, or at a higher temperature, or is it simply the wrong solder for the metallization?

Laura Hughes
- Newark, California, USA
2001