Roger,
2 reason you may not see failures in other assemblies for the "same" part:
1) Lead-plating characteristics will vary not only lot to lot but also
intra-lot...
2) It could be that the other applications don't force the QFP to resonate
at a sub-harmonic and physically "pump" the part which can generate very
high accelerations at the nodes...
Without the side fillets all you have is a lap joint which is sensitive to
peeling which is what you get when you vibrate a suspended mass like a
QFP.... The voids both increase the stress points within the lap joint and
reduce the amount of metal in the joint capable of with standing the peel
forces...
All of this contribute to the lead failures....
However, getting an EMS to develop a process that will give you 100% toe and
heel fillet should eliminate the problem but it will entail non-standard
processing...... A good test to determine if would solve the problem as is
be to hand touchup the QFP on a failed PCA and subject the PCA to
vibration...
Or you could have them put SMT Adhesive in several locations under the body
of the QFP to breakup the resonance modes...
Paul
Paul Edwards
Process Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48
-----Original Message-----
From: Roger Stoops [mailto:[log in to unmask]]
Sent: Thursday, January 06, 2005 12:12 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Joint Failures after Short-term Vibration
Ah, now this is something to think about...
The grainy appearance of the leads is the first thing that drew my
attention. However, these same parts are used on other assemblies with no
apparent problems (but I will ask about potential lead plating/contamination
problems).
And
1) There are micro-voids in the solder joints. According to 610C, up to 25%
voiding of BGA ball to board interface is process indicator but can be
accepted. But our vendor is telling us they are passing anything under 50%
(minus any area of lead not on land) total area , per 610C section 12.2.5.3
(pg. 12-45). Anyone know or have experience with how this relates to non-BGA
joints?
2) Pads are about a mil [.025mm] wider than lead. (Never been a problem
before, but...)
And we are not seeing full height toe fillets; in fact, some toe fillets are
less than 50% of the lead height.
Roger
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Paul Edwards
Sent: Thursday, January 06, 2005 2:49 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Joint Failures after Short-term Vibration
Roger,
It almost sounds like you have a QFP lead plating problem....
The EMS is probably right and these meet the spec...
However for your application IPC is not enough...
The reason the top of the lead is grainy is because that is the reflowed
condition of the QFP plating...
I would guess that
1) If x-rayed you have micro-voids in the solder under the leads
2) Your pads are the same width as the leads on the QFPs for the failing lot
You need to get full height toe fillets on the QFP leads...otherwise they
will fail...
It is possible to do but requires secondary operations...
Paul
Paul Edwards
Process Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48
-----Original Message-----
From: Roger Stoops [mailto:[log in to unmask]]
Sent: Thursday, January 06, 2005 10:19 AM
To: [log in to unmask]
Subject: [TN] Solder Joint Failures after Short-term Vibration
Hello all,
We are having an ongoing problem with solder joint failures on a 144-pin
PQFP processor, after short-term vibration. The board layout and mechanical
mounting apparatus were designed with vibration in mind, and have passed
vibration testing in the past. The same processor is used on a dozen other
assemblies, none of which have this problem.
The solder joints appear to exhibit dewetting, although there is a toe and
heel fillet. The heel fillet is unusually large, and the top of the PQFP
lead has a grainy appearance instead of a matte or shiny appearance. Some
leads can be pushed off the solder joint with very little pressure.
Sn62Pb36Ag2 solder is being used, and our EMS provider continues to claim
that boards meet IPC-A-610 class 2 requirements.
Any ideas as to what may be causing solder joint failures?
Also, any suggestions or recommendations (preferred) for a test house to
provide x-ray, microsection and composition analysis?
TIA & Best Regards,
Roger M. Stoops, CID+
Trimble Navigation Ltd., Dayton, OH, USA
Ph: +01 937.245.5288
Fax: +01 937.233.7511
Oh, and BTW, changing EMS providers is not an option...
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