I need to manually solder an exposed-pad QFP Altera FPGA. This is crucial, since the pad is used for electrical purpose (not for thermal purpose)
I have tried the following but failed. I just put solder paste on the pad, then place the FPGA and solder the pins. After that, I use Weller hot station (http://www.rapidonline.com/catalogueimages/module/M300670P01WL.jpg) to head up the bottom, hope that it can melt the solder paste. But it can only burn and damage my PCB without doing the job.
I have conducive epoxy (that needs to mix a/b with 1:1 ratio). If I use that to stick the FPGA to the pad, will it work?
Best Answer
Why do you heat it up from the bottom? You should use the hot air station from the top. Place the board over a solid surface and raised a bit on something. I use two metal L brackets from the hardware store. Then, using the hot air gun, slowly bring the board up to temperature. Ideally you should follow the profile of the solder paste (example below), but that's a bit hard to do with a hot air gun.
I like to take my time, and work around the edges of the chip package. You can start with your iron set at a fairly low air speed and temperature (240C), and get everything nice and toasty. After a few minutes increase the temperature of your iron (I got to about 350C) and get all the solder flowing.
You'll still likely burn the board some, but with this method it's minimized.