Siggi

By now we know some things for certain about lead free solder
- They creep 10-100 times slower than SnPb at a given load. How much depends on the temperature.
- The influence of temperature on the creep behaviour is stronger than in SnPb
- In very slow cycles where we can assume that all tested materials see the same strain per cycle we found that the cracks in SnAgXX  solder are approx  1.3 times longer after a given number of cycles than in SnPb.

The difference in the deformation behaviour makes it essential to know the mission profile of the assembly if a test is to be designed. Especially because you want an accelerated test. 
- Thermal cycling will for sure be encountered. Thus a thermal cycling test makes sense.
. Coffin Manson will also do well with lead free solder. As a first order estimate I dare to say that a damage exponent of 2 will be a good choice. The reliability concern here is not the time of service but the number of thermal cycles seen (15 years, daily 1 use = 5500 cycles)
. We test not lower than -20degC because below the creep becomes so slow that you have to wait forever until relaxation due to creep occurs. At -20degC we applied 40 min dwell time.
. Due to what we know of SnPb fast cycling activates deformation mechanisms not relevant in reality. We are still far from a model, but what I know so far is that in lead free solder this behaviour is pronounced since secondary creep occurs which makes the whole thing more complicated (or interesting but that's a question of the point of view). That's the reason why we tested with a temperature gradient of 2deg./min below 0 degC and 4 deg/min above 0 degC.
- As the upper temperature we chose 120degC because of the PCB material where we have a dwell time of 10 minutes.

Brittle fraction might be a concern with lead free solder because, due to the slow creep, higher stress can occur. However, if vibration and bending are not part of the mission profile such a test makes no sense.
If vibration is a problem you find lots of tests standards depending on the mission. Designing a tests is extremely difficult in this field.

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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