Let�s be basic. The two most common causes of BGA opens that we talk about here are: 1 Gold thickness and 2 Black pad. Given the thickness of gold that you talking about, it�s reasonable to consider thickness as the first alternative to investigate.
Two other recent threads on SMTnet along similar lines are:
Date: November 07, 2000 09:49 PM Author: Dave F Subject: Re: BGA problem: open after reflow
Date: February 09, 2001 08:05 PM Author: Doug Benefiel Subject: PCB Gold Plating
Depending on the process, immersion gold self-limits at LT 12 u" [Some people say 8u", others say 10"u. And they�re all probably correct. Bottom line � if it�s GT 12 u", it�s not solely immersion. (And therefore will not be called immersion.)] Assemblers that either gold wire bond to gold or have specific gold finger insertion requirements require gold thickness GT 12 u". For these applications, fabricators increase gold thickness over this limit for immersion gold with additional processing. * Electroless gold deposits to 15 to 25 u". * Electroplate gold can�t be any less than 30 u".
Your 12 to 50 u" sounds like a wire bond spec.
Um, customer complaints. That�s bad. So, what you�ve seen is probably just the old tip of the �burg. Tsh, tsh.
Yes, there would be a higher risk of open solder balls after a period of time after the BGA package is mounted on a PCB. There�s a boatload of possible explanations. Examples are: * Intermetallic compounds increase over time, dependent on the materials, their concentrations, temperature, etc. * All solder connections become weaker through the course of time. * Greater exposure to environmental stress.