Howard

As David pointed out, the question is, how does the test correspond to the real live. I believe there are 2 important points:

- The temperature range is not very extreme for an accelerated test. I assume that your customer extended the range to speed up degradation and this is the idea behind accelerated testing. If we believe that the Coffin- Manson relation holds you can estimate the lifetime in service from the given cycle with an exponent of 1.7- 2. With other words, doubling the temperature range cracks down the number of cycles to failure roughly to a third to a quarter. This means that damaged solder joints after 3500 cycles in a test are not bad per se. One has to judge what this means for the application. In this case a temperature swing of 50°C in service will cause the solder joints to look as they do now after 12'000 cycles.

- An important point is the term thermal shock. If the equipment doesn't see any thermal shock in its live the test is meaningless because:
a) Solder deforms with mainly 2 deformation mechanisms: Grain Boundary Sliding (GBS) and Dislocation Climb (DC)
b) Fast deformation with high stress activates DC, slow deformation GBS. What is fast and slow depends on the temperature.
c) Each deformation mechanism has its own degradation characteristic
d) In real live considerable amounts of DC in a thermal cycle are very rare
e) Thermal shock activates DC

- Thirdly it has to be mentioned that a crack on the surface of through hole solder joints doesn't mean that the joint has failed. In TH technology the damage develops form the surface of the joints and propagates along the pin. We must live with the fact that nothing exists forever. Solderjoints do degrade the question is if they life long enough to ensure a save mission.


Best regards

Guenter

EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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