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March 2001

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From:
"Cyker, Howard A (Howie)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 20 Mar 2001 11:17:44 -0500
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Paul,

Do you know what the failure mechanism is for these modules?   Are the
solder joints cracked?  One of the more common failure mechanisms for LCCC's
are cracked solder joints due to thermal coefficient of expansion (TCE)
mismatch between the LCCC and the substrate (FR-4?).  Stress induced by
temperature swings encountered in operation (ambient temperature changes,
power cycling, etc.) cause the solder joint to fatigue and fail.  If you are
using no clean solder paste, then the flux should remain benign and not
cause a problem in service.

Howard A. Cyker
Lucent Technologies
New Product Engineering
        Email [log in to unmask]
        Phone 978-960-2964



-----Original Message-----
From: Paul Baine [mailto:[log in to unmask]]
Sent: Tuesday, March 20, 2001 10:23 AM
To: [log in to unmask]
Subject: [TN] Solder flux under LCCs


Hello.  This is the first time I have used IPC TechNet, so please forgive
me if this question has been asked before.

Our company has built some SEM Modules (Standard Electronic Modules) for a
customer who recently reported failures in the field with these Modules.
These types of Modules have Leadless Chip Carrier ICs with castellated
terminations on them.  The solder pads are underneath the IC packages.  As
part of the failure analysis performed by the customer, some of these LCCs
were removed, and they noticed an off-yellow coating on the bottom of the
LCCs which they feel is solder flux.  They theorize that the solder flux
has hardened over time and has become conductive.

Our Production Manager thinks that this problem is caused by the fact that
these LCCs sit flat on the substrate and therefore the solder flux gets
trapped under the LCC during reflow and cannot be cleaned out.  He proposes
two possible solutions:

 - put adhesive under the LCC

 - put a spacer under the LCC which is dissolvable in water (i.e. during
cleaning)

I was wondering if anyone had experienced a similar problem and knew of
an "industry standard" solution, or any solution.  Thank you in advance.

Best regards,


Paul Baine
Q.A. Manager
C-Tech Ltd., Cornwall, Ontario, Canada

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