Hi Mary

Well, I assume you don't want to find week joints but make a qualification test. Just in case you look for the former I try to answer your question with two paragraphs:
- Finding week joints: In my opinion impossible non destructive and destructive testing doesn't make any sense. The only way to ensure a reliable production is to control  the solder joints ( destructively) from time to time on samples and to find the reason for any unwanted changes in the structure of the joints.
- Qualification test: Tin based solder has the habit to deform with creep. This means, if you apply a constant load you have an ongoing deformation until the load is removed. This is important for testing since the high time dependency limits the possibility of test acceleration. You need to give the solder joint the time to relax. Why this is important? Degradation in creep is strain dependent. The more strain solder sees in each mechanical cycle the less cycles it survives. This is not linear. The relationship that describes this is Coffin-Mansion law: Nf.e exp(n )= C, Nf= number of cycles to failure , e= plastic strain per cycle, C= Constant, n=1.7( in our experiments).
The way to estimate the lifetime of your product would be:
Do a thermal cycling test such as:  -20 / 120 °C with 4 °C per minute, dwell times 10 min at 120°C, 30 min at -20 °C
Find Nf for that cycle.
Calculate: Nf (test)/Nf (reality)= (dT reality / dT test ) exp 1.7, dT= temperature swing

Vibration test is only sensible if you have vibrations in your application. But designing an accelerated vibration test is a difficult task and there is not enough space here to go into it.

Generally the design of tests is not easy at all. However, if one keeps the time dependency of the deformation of solder in mind the worst mistakes can be avoided.

Have a great day

Guenter

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

8600 Duebendorf
Switzerland

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

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