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August 2003

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Subject:
From:
Steve Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 13 Aug 2003 16:24:05 EDT
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Hi All!

I'm always posting about my problems, so for a change, I thought I'd post
about one of my successes...

You may recall a while back, I was asking if I should try and do a
double-sided reflow with heavy, 240-pin, Altera QFP's on the bottom...they weigh over a
half ounce because of the heatsink they have in them.

Dave Fish replied with the formula of how to figure out if they would stay
on, and then said if I wanted all the office people to run out on the production
floor when they heard the noise of them dropping off the bottom of the board,
then go ahead and try...hehehe.

What I wound up doing was using some water solubleTechSpray Wondermask W (the
light blue stuff), and corner bonding all the QFP's with it. I also did 3
military style tantalums because I have had them fall off the backside before,
and I also did a PLCC-32. I probably would have been okay with that one, but
just as added insurance I went ahead and bonded that too.

Takes a while to cure because you're putting it on kinda' thick. But it works
GREAT!
You can see pictures of both sides of the board, and the bonding on the
backside as it came out of the oven after the second reflow. Go to:

http://www.stevezeva.homestead.com

and look at double-sided reflow top & bottom.

I tried a couple of different masks, and chose the TechSpray because it
handled the reflow temperatures real well, and washed off very easily. The others
that I tried blistered real bad during reflow, and were a lot tougher to clean
off.

So if you're ever nervous about bottomside parts during a double-sided
reflow, here's some insurance...

-Steve Gregory-

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