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-