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. 

Project stage: 
Project type: 

Idea Information

Background: 

 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.

Problem: 

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.

By Courtesy of Intel:ECTC08_Effect of Board and Package Attributes on Solder Joint
 
So the question is how to predict the full life if you only have a board worked for a few years without any failure or crack?
 
 A possible solution is to measure the degree of recrystallization. For tin lead joints, grain coarsening nearly indicates 15% life passed; but how about lead free?  Can we find similar rules to predict life through recrystallization.
  IPC D279
 
 
 

Definition Information

Goals / Benefits: 

 

1st stage: feasibility verification
 
-to find any metallographic indication of fatigue damage in ATC;
 
-fatigue in solder joint with different stress( different package, different boards)
 
-thermal cycle or FEM to get  Nf
 
-to draw a general relationship between metallographic evolution and consumed life Nf%
 

If there do exist the expected relationship about ATC test sample, then we move to

 

 

 

2nd stage: field return LF boards analysis (with or without failure)

 

-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