Agree, we can't explain everything, might be someone out there...
however, sometimes thinking hard works. We had something like yours once,
couldn't see what became of the solder for one or two legs. In our case,
the gremlin's family name was De Wetting and he had a cousin named G Van
Wicking. So, when the solder didn't wet appropriately on such a leg, the
adjacent leg had the impudence to frankly steal solder from the neighbour.
We did not see at first, that these criminal legs were fat and prosperous,
while the nearby legs looked thin and awkwardly naked.
I would dip in the solder pot to check out what.

my two farthings

Ingemar Hernefjord
Ericsson Microwave Systems

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]
Sent: den 28 februari 2005 20:21
To: [log in to unmask]
Subject: [TN] Disapearing solder


Has anyone experienced solder paste disappearing in a convection oven?

We have experienced several "insufficient solder" (no solder) defects on
smt gull wing leads (typically fine pitch but not always).  The last
controlled lot, produced 3 of 10 boards with defects.  Defects: One lead
on each of 2 boards, and 3 leads on the last board.  Component types are
QFP208, TSOP48 and QFP100

Additional information:

1) The paste (6 - 7mils thick) is verified on the pads just prior to
entering the reflow oven.  The paste is a SN63 RMA.
2) Solderablity of the component leads is acceptable
3) Solderability of the board is acceptable
4) There are vias attached to every pad but the solder is not going into
the vias.
5) The defective joints exhibit a small fillet of flux around the leads
with no solder.  It appears as though solder was never printed on the
pad.
6) The missing solder is not on the lead, pad or adjacent component
leads or pads.  It's just gone.
7) Defects found on 11 boards over 3 different lots (months apart).
8) Minute solder balls are evident on the board but may have been from
paste print process.
9) The lead is in contact with the pad.

Theories:

1) Paste outgassing??
2) Gremlins in the oven stealing the paste??

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