As I had mentioned is some past posts, we use LCCs regularly, and have been
for almost 20 years. If you deposit enough solder paste (.010" stencil),
you should see approximately .005 - .006" standoff height under the
components, sufficient enough for cleaning if you have the proper equipment.
We started out with RMA flux and vapor degreasing, this worked well. When
we switched to an aqueous flux, we evaluated in-line cleaning equipment to
perform cleaning under LCCs. A 68 pin LCC was used for our testing. Any
equipment which can clean under this could clean anything. So, for your
problem, it seems that you just don't have the proper equipment.
Jim Marsico
Senior Engineer
Production Engineering
AIL/Electronics Systems Group
An EDO Company
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: Phil Crepeau [SMTP:[log in to unmask]]
Sent: Tuesday, March 20, 2001 11:24 AM
To: [log in to unmask]
Subject: Re: [TN] Solder flux under LCCs
hi,
i always thought that lcc's were somebodies idea of a bad joke. i
can't find anybody that would take credit for inventing them.
lcc's mounted flush to the surface of a substrate are bad news. as
you have discovered, you can't clean under them. i guess you are lucky that
you haven't found fractured solder joints yet. your only solution is to get
j-leads attached to your lcc's, or put standoffs under them before you
solder them.
it has been my experience that sems won't permit too much lcc z-axis
standoff without bumping into their cover. it has also been my experience
that it is difficult to apply enough solder paste to get an acceptable
solder joint when standoffs are used to lift the lcc's up of the surface of
the substrate.
i really hate these packages.
phil
-----Original Message-----
From: Paul Baine [mailto:[log in to unmask]]
Sent: Tuesday, March 20, 2001 7:23 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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