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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?

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    \$\begingroup\$ Conductive epoxy won't have the right surface tension properties and may squeeze out and short to the other pins. Solder from the top is correct; use Kapton tape to protect PCB. \$\endgroup\$
    – pjc50
    Jul 7, 2013 at 18:31
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    \$\begingroup\$ There are epoxies that could be used that way...check the technical data sheet for the product you have. But, like pjc50 says, you need to be especially careful of the epoxy volume or you could short the center pad to other pads. \$\endgroup\$
    – The Photon
    Jul 7, 2013 at 20:27

4 Answers 4

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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.

enter image description here

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  • \$\begingroup\$ do you think conductive epoxy will work? \$\endgroup\$
    – Josh Vo
    Jul 7, 2013 at 18:24
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I put a large via in the central pad and feed solder in while applying the soldering tip to the via. This is only suitable for prototypes, of course.

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  • \$\begingroup\$ do you think conductive epoxy will work? \$\endgroup\$
    – Josh Vo
    Jul 7, 2013 at 18:23
  • \$\begingroup\$ No. It isn't intended for that sort of application, AFAIK. \$\endgroup\$ Jul 7, 2013 at 18:39
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If possible place a pad like 0,8 or 1,0 mm diameter then simply hand solder.

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thanks so much for all of your feedbacks. I tried all of them, and the best method turned out to be drill a large whole behind, and put a lot of solder paste (Well, all of the JTAG issues resolved immediately!). Next time, I will carefully put a via there before printing the PCB.

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