Which package easier to solder on home-brew PCBs:
- TSSOP
- QFN
I generally use solder paste and hot-air / hot-plate.
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5 Answers
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For hand soldering I'd go with TSSOP. QFN pretty much requires hot air, whereas you might be able to get away with a soldering iron with a TSSOP. The pitch of QFN can be smaller too, which is more difficult with home-made PCBs, but TSSOP can be small too. Sometimes they have a exposed pad in the center that needs to be grounded, which makes routing more difficult.
One issue with QFN is you have to consider the package lying flat against the board without any gap under the package or between pins. This means you can't use conductive flux since you can't guarantee the flux will be rinsed away under the package. I know this because it actually happened to me. A local manufacturer that specializes in small quantity hand-built boards was new to QFN and didn't think about this. The boards we got back didn't work for various reasons. Eventually I figured out that pins were being shorted together under the package. The resistance was surpringly low, like only a few 100 Ohms in some cases. What a mess. Letting the boards sit in clean water for a few hours helped, but ultimately we had to remove all the QFN packages with our hot air station, clean the mess, then re-solder them with rosin flux. Then the boards worked as expected.
For real professionally fabbed and built boards, there is no issue with QFNs, but for do it yourself situations they can be tricky.
answered Jul 23, 2012 at 17:26
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2\$\begingroup\$ The hot plate method works quite well for QFN's, especially if you do them before anything else. But a microscope is very useful for inspection - I prefer to aim the scope off its base and hold the board in my hand so I can view the edges at an angle. \$\endgroup\$ Commented Aug 28, 2012 at 18:18
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If you're not soldering by hand they should be equally easy to solder. Visual inspection afterwards will be easier for the TSSOP, just like putting probes on the pins during debugging.
On the other hand the QFN has the advantage that there are pins in both X and Y direction. During reflow the surface tension of the liquid solder paste will pull the IC perfectly over the pads, even if positioned a few tenths of a mm off. So the QFN will do this in the two directions, the TSSOP mainly in the length direction only.
If the QFN has a thermal pad they often advice not to apply solder paste over the full pad, but do so in a pattern of smaller dots.
That's because when heated boiling flux may cause gas cavities, which push up the IC so that pins may not properly be soldered. Reducing the amount of paste for the thermal pad avoids this.
answered Jul 23, 2012 at 16:11
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\$\begingroup\$ Until now I avoided QFN as I feared solder bridges. This occasionally happens to me with micro-SOIC and I was not sure how great the risk was with QFN - and how to get rid of them with this package. With micro-SOIC this is easy enough to cure with de-soldering weave. \$\endgroup\$– ARFCommented Jul 23, 2012 at 16:19
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\$\begingroup\$ @Arik - You'll have to learn from experience how much solder you should apply to avoid bridges. Note that the relative amount between different pins is as important as the absolute amount: if one pad has much less solder paste it might not make contact, but only because the other pins lift the IC too high. The small amount in itself is less of a problem if all pads get as little. \$\endgroup\$– stevenvhCommented Jul 23, 2012 at 16:24
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The answer is: TSSOP
A QFN has pads that are underneath the package. If the package is laying flat, you can only barely see them from the side.
A TSSOP's leads are exposed and can be hand soldered with solder wick (and, optionally, some additional flux).
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And Always make sure when Applying the Solder at the bottom GND PAD of the QFN to keep it as small as possible so that the QFN will remain flat on the pad. That help with good solder re-flow and PINS Alignment.
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On the other hand, the long external leads between the package and the board on a TSSOP or TQFP offer a longer lever for misalignment, a bigger surface to incur solder bridges and the center thermal pad (if present, and if your boards pad matches the size of the one on the chip) also helps center it during reflow.
IMHO, it is a toss-up because it is harder to mess up with a QFN, but harder to fix if you do...
