A new paper from CALCE, US and TU Dresden, Germany has measured effects of mixed lead-free assembly and of reballing for BGA's, concluding that reballed assemblies were less reliable in temperature cycling.

SAC305 balls were reballed with SnPb balls using three different methods. Their thermal cycling behaviour was compared with non-reballed lead-free BGA's assembled with SAC305 and SnPb paste.

For the same component type reballing method made no difference and the non-reballed SAC305/SnPb mixed assembly products were the same as, or in fact marginally better than, lead-free SAC305.

However, reballed SnPb assemblies were worse.

Microstructure analyses are discussed. Failure was within the bulk solder on the component side in each case showing that the reballing itself was not responsible for crack propagation.

The paper has been published online by the Journal of Electronic Materials - see the reference link below.