Rob,

 My bet is during placement paste is being pressed under the part when the
part is placed.

 For a real eye opener take a board that has components placed but not yet
reflowed. Lift up a few parts and look at the pads. You will see that the
part is completely pressed down to the metal pad through the paste. Paste
goes where paste wants to go, everywhere. Some get squished under the part
and during reflow bingo, you get balls.

 I could be wrong but I bet this is the case.

 You could try the "Home Base" paste aperture design for this part. 

Bob Kondner



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rob Strecker
Sent: Wednesday, January 27, 2010 11:13 AM
To: [log in to unmask]
Subject: [TN] Solder Ball Under Chip Cap

We recently received a call from one of our customers that their boards
are failing Hi-Pot test.  They sent us a picture of a solder ball under
one of the chip caps (package 1825).  Apparently it's present on all 50
boards we built and always in the same place.  We are using leaded paste
for this build and the stencil we received was from the previous
manufacture who could not fix the problem and went to hand soldering the
part (info we got after the fact).  I have inspected the stencil and it
looks fine.  The apertures are a 1:1, so the only thing I can think of
is getting a reduction of some kind on the stencil.  I will send the
picture to Steve to get it posted.

 

Thanks,

 

Rob Strecker 


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