All

I'm a bit sceptical about applying HALT to any electronic device and assuming that a relationship between test and reality found in one case is the same for others. Especially when it comes to lead free solder this approach is rather speculative. Lead free solder creeps considerably slower than SnPb. This means, that a steep ramp will result in high stress. This stress is very likely to cause damage in the components or brittle fracture in the solder joint. Both are mechanisms that will not occur in reality. On the other hand, as already mentioned earlier, the slow creep rates of lead free solder requires longer dwell   times than SnPb to induce the same plastic strain per cycle. And the crux is that this creep rate is highly temperature depending. This means, if you go in a HAST test to low temperatures, such as -40 °C creep becomes so slow that it ist questionable whether it makes any sense at all to apply a dwell time. A test with 9h dwell might not be an accelerated test any more. On the other side, if the tested equipment is used at +20°C to 80°C in reality creep will take part in the deformation history. A relationship of the test data ro the real world becomes specimen specific. It's a parametrisation fitting the case surveyed which might compare pears with apples but if it gives satisfactory results, why not. However, since this approach is not based on a physical  law, an extrapolation to other applications is dangerous.


Best regards

Guenter






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

8600 Duebendorf
Switzerland

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

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