Hi, Mary Jane,
Make sure you use the high-temp tape that looks like standard masking
tape. 3M makes it. It is about $5.00 per roll, and insulates the
components from heat. Do not confuse this with Kapton tape which is much
more expensive but also conducts heat very well, and is thus not a good
choice for masking off adjacent components.
Kapton tape is excellent for taping down a thermocouple to a flat pad or
flat metal surface. Repeated verifications shows this changes the
reading by exactly 1 degree C.
By the way, the only reason those peaks appear if you accidentally touch
surrounding components (not just capacitors) is because there is no flux
on the nearby components. If, during the rework process, you simply add
flux to the adjacent components no peak will be left behind. Adding flux
to nearby components does not hurt anything, you need to wash the entire
assembly anyway.
If it is a no-clean process, that's different. You do not want
additional flux. Your best option is then to tape off the other
components. Solder peaks appearing on the leads of the component you are
trying to solder signifies insufficient fluxing action from the
flux-cored no-clean wire solder. Try going with another no-clean wire
solder or else go to a higher flux content. You can typically order
no-clean wire solder in 1.2%, 2.2%, and 3.0% flux content, or something
like that. I prefer the 2.2%, as it provides good fluxing for most
applications, but does not leave such a large amount of no-clean flux
residue when finished. 1.2% is good for touchup of small leads, such as
SMT fine-pitch components. 3.0% is good for very large leads, but leaves
more residue.
A note of caution: don't jump up and change the no-clean wire solder to
a different brand or flux chemistry unless you are prepared to change
your no-clean solder paste flux as well, otherwise you are leaving mixed
chemistries on the PWB that will not be removed by washing. You don't
wanna do that. Especially if you need to conformal coat the assembly
later.
If you change anything, fully qualify the process and verify you will
not have any problems before you do it in a production environment.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Tremmel
Sent: Thursday, September 29, 2005 6:34 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Protrusions
If you place high temp tape around the QF, it would prevent the
protrusions and also (to some degree) thermally diminish the amount of
heat the adjacent capacitors receive.
My 2 cents
David
-----Original Message-----
From: "TechNet" <[log in to unmask]> on behalf of "Mary Jane
Chism" <[log in to unmask]>
Sent: Thursday, September 29, 2005 9:20 PM
To: "[log in to unmask]" <[log in to unmask]>
Subject: [TN] Solder Protrusions
We are experiencing icicles (solder protrusions) as the results of
rework being performed on a flatpak. The process is hot air removal of
the flat pak and hand soldering for replacement. In the process of hand
soldering, the operators are touching some of the surface mount
capacitors around the flat pak causing solder protrusions off of the
solder fillet of the capacitors.
In IPC-A-610, it states as long as the solder protrusions do not violate
height requirement and electrical clearance, they would be acceptable.
However, I feel the objective of performing rework is that you only
perform the operation of soldering without putting any thermal stress on
the rest of the components.
I realize this is hard for anyone to actually answer this, but my
question is, if you can answer this, would you:
1. Re-address your rework process and also get rid of the icicles?
2. Re-address your rework process but let the icicles go?
3. Don't do the rework?
4. Get different rework operators to perform the rework?
Your comments will be very much appreciated.
Thanks.
Mary Jane Chism
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