Electronics > Projects, Designs, and Technical Stuff

Hand soldering exposed pad devices

(1/5) > >>

shadewind:
I want to try to design my own FPGA dev board for an Altera Cyclone series device since some of them are available in QFP packages. Only thing is that most of these (I think) are exposed pad type packages which are hard to hand solder. I have read that you can simply put vias under the pad and solder it from the back side.

Is this a good idea and how many of them and how large should they be? Or should I simply take the plunge and buy a hot air station (I don't want to if I don't have to)?

I have no problems soldering fine pitch devices (at least down to 0.5 mm anyway).

Alex:
Did you mean QFN?

Lance:
QFP = Quad Flat Package



You're thinking of Quad Flat No-Lead


You can't solder QFN packaged ICs by hand with an iron. They need a reflow oven or a hot air tool.

For a QFN package you can get away by tinning the pads and then soldering the device in place. Make sure you use flux. Be careful though, it takes a steady hand to work with these packages.

Here's a tutorial from Curious Inventor: http://store.curiousinventor.com/guide/Surface_Mount_Soldering/QFP
They have a lot of good tutorials and guides.

shadewind:
Nope, I really do mean "QFP with exposed pad" or EQFP which Altera calls it. The pins themselves are no problem, I've done that fine pitch before. The problem is the exposed pad on the bottom of the device which according to Altera must be soldered. It's for grounding purposes, not thermal apparently.

Alex:
Right, that's the Enhanced QFP.

I would not use multiple vias unless the board is really thick or the device is not resting flat on the pcb. You can apply some flux in paste form on the exposed pad of the device and place it. Then some flux on the other side, warm up the via and start applying solder. If the device is nearly flat on the PCB the entire pad will be soldered by surface tension.

About QFNs:

It is possible to solder the variety that has the pads exposed on the side faces of the device with an ordinary soldering iron and a bent conical or chisel tip.

You must elongate the exposed copper areas of the pads enough for the soldering iron tip to have good termal contact with them. Then apply a tiny amount of flux across the side of the device. I would suggest paste flux in this case to avoid flooding the area under the device with flux you cannot remove.

Apply a small amount of solder on a clean tip and just touch the corner formed by the pad and the device side. Surface tension will wet the pad under the device. The device pin will receive heat from the wet pad, flux and possibly directly from the iron forming a nice concave meniscus joint profile.

The whole process should take no more than half a second.

Navigation

[0] Message Index

[#] Next page