Hi all,

Recently took up prototyping job for one of the customers. The product had a
BGA 196 pin with Ball pitch 1.0mm and ball dia 0.6mm. The vias are NOT in
the pad. They are in a well defined dogbone geometry. Everything was OK till
we observed that the assembly was not working. We did some vision inspection
with optical inspection, light emitted from one side and detected at the
other side indicating no shorts and light getting blocked in between
indicating shorts. This revealed a short. We removed the BGA and replaced
with a new one, this time without any solder paste but some flux. Result:
Visually could see some balls not touching the pad. The most interesting
observation came immediately as we flipped the PCB. There on the bottom
side, the balls were plunging out!

As a first reaction, this pointed out to the masking of vias. However, the
vias ARE masked from the top side. However bottom side was not masked. This
was confirmed when we tried to solder the top of the via with a normal
soldering iron. In the first shot, the vias didn't get soldered.

Now the question is, or the questions are:

1. Inspite of masking, starting from the end of the pad, through the track,
to the via top, is it possible for the solder to flow to the via?? Of
course, it has! But why?
2. Is it possible that a higher preheat temperature could have deteriorated
the solder mask on the track and the top of the via which resulted in the
solder to flow?
3. On the PCB it was clearly visible that some maskings (of course on the
track and the via) were green and the rest (most of them) were brownish.
Even these brownish vias did not get soldered on the first shot when tried
with the iron. But is it possible that these so called brownish tracks and
vias had less mask and hence got deteriorated during the reflow process?
4. Does it help to have the vias masked on the top ring, in the PTH and on
the bottom ring?
5. Any way to plug the vias, now that the PCBs are already made?
6. Still the shorts issue remains open. My guess: the solder flowing from
the pad to the vias could have created a short since pad to pad gap is 1mm
but pad to via gap is only 0.46mm. Is it possible?

Any suggestions will be greatly appreciated.

With warm regards
Vinit Verma

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