Hi, Paul,

Your yellow deposit is almost certainly dried flux, and it does (or can)
become conductive. This is not an uncommon problem with LCC's given their
low ground clearance. Depending on the substrate they're mounted onto and
the working environment they meet, they are also a bit prone to solder
joint cracking. A way of curing both problems is to use water soluble
spacers about 0.005" thick underneath, and lift the component up a bit. You
will have to use a stepped stencil for screening on your solder paste as
well. The extra joint thickness is more reliable for LCC's in my
experience.

Or you can try using a no-clean process, if suitable.

Good luck

Pete Duncan






Paul Baine <[log in to unmask]>@IPC.ORG> on 03/20/2001 11:23:14 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
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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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