Predict Lead Free Solder Joint Life from Microstructure Evolution
The aim of this project was to predict the remaining life of LF solder joints by analyzing the joint's microstructure after the board has been in service for several years where some fatigue damage has occured but the joint has not yet cracked.Test vehicles, as well as field returns, were micro sectioned and analyzed for structural change. Observed structural changes were correlated to failure and field life.
Lead Free reliability is still controversial because no unversally accepted acceleration model exists. Complex factors such as dwell time, pre-conditioning, temperature ramp rate have a significant impact on simulated life time testing and not enough field data, especially on non-consumer products is available.
A potential method to measure the remaining life time in a solder joint is to build a relationship between crack growth length/area and life Nf%; but this presents many problems: crack routes propagation is more irregular in SAC than tin lead and different packages have different crack development rates.
If there do exist the expected relationship about ATC test sample, then we move to
-to get typical LF products with a few years field working (such as cell phone and other consumer electronics, with or without failure);
-metallurgical analysis to see if exists any damage indication the same as ATC
-to analysis the solder joint microstructure and then get the remained life