1. Do you know if the paste quality and print was good and that the correct paste was used on that shift. You can only know this if you have an SPC plan or in a small shop if you are observing the line throughout the day.
2. Do you know that the correct reflow profile was used. If using a few generic reflow profiles did someone perform a temperature profile for the first build to verify the profile?
3. Were the BGA x-rayed to check for correct size and that there isn't a voiding issue. Also check that the person performing the X-ray is properly trained. Some if not all X-ray systems have a histogram under camera calibration to show if your over or under saturation. If seen people adjust the power and contrast to where the BGA voids aren't properly measured only because of lack of proper training. 
4. If a press process or depaneling process is used are there any stress points. 
5. The BGA could have some oxidation on the balls if they were in stock for a long time. This issue is prevelant on discontinued BGAs that the customer has purchased and stored for future builds for spare parts. If the parts are older than one year and your using a No-Clean paste than you are more likely to have a voiding problem than with the stronger OA solder paste formulations. There are more things to check but these are the major ones that shoulds help.
 

-------------- Original message -------------- 
From: Cong He <[log in to unmask]> 

> dear all: 
> 
> I am facing a problem of bga soldering. 
> 
> when the board have just soldered ,it pass all test. 
> But after sometime, maybe for a few weeks, 
> some of the bga balls opened. the function became normal if i press the 
> chip on the top side. 
> 
> It seems that the pcb didn't warp much,it seems even no warp at all. 
> Does any of you know why? And how to improve? 
> 
> 
> 
> 
> 
> 
> Hechong 
> 
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