RE>BGA Solder joint cracking                 2/17/96

Roger,

Two additional things to consider are packaging materials (i.e., ceramic vs. plastic) and size.  Large ceramic BGAs tend to exhibit this solder joint cracking phenomenon more than smaller plastic ones -- mainly for the reasons that others have shared here.  How big is large?  We hear numbers of 35 mm on edge.  Above this value, a shift to "columns" from "balls" is needed.

With that said, we've been using IBM's ceramic BGAs (50 mil pitch, 25mm device) for over two years (without any additional support) on our standard products without any  solder joint failures.  We make sure that we follow IBM's prescription for solder paste volume (4600-10000 cubic mils), and we did a pretty thorough study of their reliability data prior to adopting this package.  

We have found that the solder joints on these devices are susceptible to cracking in high vibration environments.  For our ruggedized versions, we underfill the devices with an epoxy made by Dexter Hysol.  It's a really good material from the standpoint of increasing reliability, but it's a real bear to work with (storage in dry ice and it can't be repaired).  We've been trying to find alternative materials, but no luck as of yet.

Regards,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA