We have seen this issue before and have found 3 main root causes:

1. Insufficient paste deposition.
2. Coplanarity -- If the pins are barely touching the pad the paste may
evenly distribute on the pad making it appear as if there is no solder.
3. The paste actually wicked right up the leg of the component. We
believe the board solderability was an issue so the paste took the path
of least resistance.

-Carrie

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ingemar Hernefjord
(KC/EMW)
Sent: Tuesday, March 01, 2005 2:42 AM
To: [log in to unmask]
Subject: Re: [TN] Disapearing solder

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??

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------