Hand soldering exposed pad devices

[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.

[joelby]

If it's a home-etched board, you could drill out a large hole in the centre pad and fill it with solder. I'd highly recommend getting a hot air tool though - they're very cheap on eBay and have changed my life for the better.

[mikeselectricstuff]

Unless you're pulling enough power that you need the pad for heat conductoon, and aren't running at speeds where super-low impedance grounding is essential, you may get away with not soldering the pad at all for development work.
However it isn't hard to do, as long as there are plenty of vias to conduct heat through the PCB, and ideally no resist on the underside of the PCB under the pad, just put some paste on the pad, place the part and tack a couple of pins to gold in position, then heat from the underside with a chunky iron until the paste melts.

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

Not true :

[metalphreak]

He already said its for grounding purposes not for thermal heatsinking. If its a required ground (as in its not electrically connected to other ground pins) then you will have to solder it. A couple of larger size exposed vias underneath the chip should be fine.

[shadewind]

If I wanted to do this properly, what would be the best way? I had an idea where you would put solder paste on the center pad, put the chip down, solder the pins by hand and then reflow with hot air. Would that work?

And if I were to use solder paste only, how could I apply solder paste to the tiny pads for the leads on the QFP chip?

[joelby]

I'd just solder the whole thing with hot air if you have the tool.

It's a bit easier to do with QFNs - I tape some cardboard 'guides' (e.g. leftover carrier tape for chip resistors) on to the board with Kapton tape to align the part correctly in one axis, but this would be more difficult with a QFP where pins would be in the way. On the other hand, proponents of solder surface tension will tell you that you just need to bump the chip to get it into the correct place.

Put a little bit of solder on the centre pad. You don't want to solder the entire pad because once you squash the chip down, it'll distribute over the whole pad and you want it to sit flat without any solder squashing out. Alternatively you could coat the whole pad with solder and then wick it off. You really don't need a lot for it to work - just a nice, even, flat layer.

Using flux, apply a little bit of solder to all of the other pads. Clean and flux again, then put the chip in place and heat with air. Squash down and then nudge into place. Clean, inspect and clean up bridges with a wick if necessary.

I find that if I just apply solder to the top of pins with an iron (imagine if you'd just glued a QFP in place), not enough solder ends up between the pin and the pad. Pre-soldering the pads and squashing down is a lot more satisfactory!

[NiHaoMike]

Design the board to have a plated hole (2mm diameter or so) under the part to allow the pad to be soldered from the bottom.

[shadewind]

I think I'll go with that.

A question about another device... crystal oscillators like this:

http://se.farnell.com/jsp/search/productdetail.jsp?sku=1842144

They shouldn't be too much trouble soldering by hand, right? The pads wrap around into the little notches but I'm wondering if this provides enough surface area or if I really do have to solder on the bottom.

[joelby]

These are no problem to solder with an iron, if you leave sufficient pad area outside the package's body. Again, most of the solder will end up on the side of the part rather than underneath. Hot air or reflow would be much better mechanically, but I've always just done them by hand.

I generally apply solder to one of the pads, put the part on, heat the pad to melt the solder, and push the module down and into place. This way, you'll at least get one pin attached properly.

[Alex]

These are no problem to solder with an iron, if you leave sufficient pad area outside the package's body. Again, most of the solder will end up on the side of the part rather than underneath. Hot air or reflow would be much better mechanically, but I've always just done them by hand.

I generally apply solder to one of the pads, put the part on, heat the pad to melt the solder, and push the module down and into place. This way, you'll at least get one pin attached properly.

I second that comment, you will most likely need to modify the standard land pattern to solder these manually. I have tried with the standard pattern and it is nearly impossible to get enough heat to the pad under the package. What you will end up doing in this case is raising the package by say 0.5mm to get enough solder under it to act as a heat bridge. It's ugly.

[shadewind]

But if I modify the land pattern I'll be okay?

[Leo Bodnar]

Not true :

Looks good but pin 14 has not been reflowed as far as I can see.

[shadewind]

I think I'll simply invest in a hot air station and be done with it

[Trigger]

You don't need solder paste, you can prep the pads with a little bit of solder before hand and then use a hot air gun to reflow it.

I have seen QFNs soldered by hand with the use of no-clean liquid flux to create a wicking action but that's an expert level task and without a hot air gun you only have one shot. 

[shadewind]

If I prep them with regular solder, will I need flux to make it defile properly or will it work anyway?

[Lance]

If I prep them with regular solder, will I need flux to make it defile properly or will it work anyway?

Probably. Solder only works on clean joints.

[joelby]

I usually try to avoid defiling my PCBs, but in any case I don't think you can ever go too wrong with using flux all the time.

[Leo Bodnar]

It's pity we have abandoned halide photography process.

Stick a bare PCB into a well-used fixer solution for a few hours and you have a nice silver coated PCB.