No paste needed.....as long as the pads are still hot from the removal process, they can be bumped using a solder iron and coresolder and flux.....the bumps come out pretty much equal in height as long as everything is warm enough......This accomplishes the retinning of the pads and deposition of paste all in one step.....although some will argue that the heights are not exactly the same, in my experience, it has never been enough of a difference that mattered.
Now if the assembly is not still hot, it does make a world of difference.
"Blomberg, Rainer (FL51)" <[log in to unmask]> wrote:
Phil,
How does one apply solder paste to a board before it has cooled down?
Rainer Blomberg
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
Sent: Tuesday, October 04, 2005 11:22 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Protrusions
Hi Mary Jane,
My question is if you are using hot air to remove the flat pack, why aren't
you using the same hot air to resolder the replacement part?
If it were me, I would have the replacement part put back on the board
immediately, before it has cooled down, and thereby reduce the number of
temperature cycles the board has to go through.
The hot air will not give you any peaks to worry about as long as you use a
paste flux to facilitate the wetting of the solder.
You may not have the luxury of doing both operations at the same time, but I
would still opt to use the hot air on a flatpack. Just make sure you bump
the flatpack pads before putting down the part.
Hope this helps....
Phil
Mary Jane Chism wrote:
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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