Thermal excursion

AF, Buddy: I know someone "set you up" to ask this question, but:

1 What kind of "thermal excursions" are you interested in? Solder cycles, rework cycles, thermal shock, thermal cycling ... 2 What are you talking about boards, components, assemblies ... ? 3 IPC is not interested in writing "thou shall / thou shall not do" standards. Their standards are more like ... after you get done with what ever thermal excursions you want to, the assembly should be ... nice or something like that.

Good luck

Dave F

reply »

AF: Unfortunately, there is no "acceptable number of cycles" either in air-to-air and one liquid-liquid thermocycling, nor interconnect stress testing. There can be a "minimum threshold" that would assure no failures as the result of the assembly processes, but there is further loading (less severe but more cycles) that during operational use in the field. There are just too many design, fabrication, assembly, and end use variables that enter into the reliability of an assembly.

Continuing, Sandia National Laboratories in California conducted two lengthy tests on 6-layer bare FR-4 MLBs, approximately 0.062" thick, with via holes approximately 0.035" diameter. The temperature range was -65�C to 125�C. Test time per cycle was 3-hours for temperature cycling and 42 minutes for thermal shock.

Results of test #1 on bare boards: Failures started occurring on the thermal shock boards at 440 cycles and 373 cycles on temperature cycled MLBs. Once the via holes were filled with solder no more via hole failures were experienced through 7650 thermal shock cycles and 4910 temperature cycles, the end of test sequence.

Results of test #2 on bare boards with two different conformal coatings: One conformal coating, an epoxide type, performed the same as the bare MLBs with unfilled vias. Parylene-C coated MLBs never experienced any via failures of these boards with unfilled vias through 7650 thermal shock and 4910 temperature cycles.

Certainly, the moral in these tests is that seemingly small variations in assembly process, beyond the number of solder cycles, can have large impact on the reliability of assemblies. As additional background, look at "Tg - Glass transition temperature - Mike Naddra 16:06:38 01/20/2000" in the SMTnet archives.

Rough rules of thumb using a closed circuit board course and professional solders (do not try at home) are: � Maximum 5 solder cycles total, starting with HASL as cycle #1. � Maximum 3 rework cycles total. Your results may vary. And that�s the real point.

Good luck & happy soldering

Dave F

reply »