Hi Dave,
Do you have radiographs that you can post (on Steve's web site?) of the shorts?
My first thought was that if the conveyor is not smooth running it may cause the problem.  The excess heat that gets under the BGA through the vias is melting the previously reflowed balls but you then need some reason for the discrete solder volumes to get together. If the conveyor vibrates there may just be enough energy to bounce the BGA and squash the solder.
With the topside vias masked, presumably with solder resist, I assume there is no obvious path for the solder to run together?
What about moisture problems? Are the shorts in the centre of the package? Normally we don't worry about moisture ingress on components on the top side of wave soldering, but if the balls are melting then the package is getting to high temperature as well.
By the way, I think you will find that "Happy Monday" is an oxymoron.
Regards
Eric Dawson
> -----Original Message-----
> From: Dave Chapman [SMTP:[log in to unmask]]
> Sent: Monday, April 28, 2003 3:03 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] BGA
> 
> Happy Monday, I have a BGA 256 Tundra which has no shorts in SMT (we x-rayed
> all 16 boards) When it goes over the wave we get a short or two. Please
> assume the wave is in control.
> We mask the topside of the BGA because there are a lot of vias which when
> they test it would leak and not get pulled down good in the fixture. I need
> the vias on the bottomside under the BGA to be filled with solder for
> testing so I can't mask the bottom. What should I do?
> 
> Dave Chapman
> Manufacturing Engineer
> Circuit Service Inc.
> 
> 
> 
> 
> 
> 
> 
> 

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