How to solder a 32L QFN

Un bel giorno TT_Man digitò:

You can try to pre-solder all the PCB pads, put the chip in place and heat it with hot air. I've done it with some small BGA chip, with good percentages of success.

hat's

TTMan

We have had quite a lot of success with a hot plate (set at150C) a hot air soldering iron and solderpaste manually placed on the PCB pads. You might be able to forgo the hot plate and just use a modified hot air gun and solder paste. I'm not sure quite how you can modify the hot air gun, what you need to do is limit the air flow (to make sure it doesn't blow the component away) and make sure that the air temp is runnning about 220 to 240C.

Having done this before for a friend... The hotplate trick works for these too. The only hard part is getting the solder paste on the pads neatly, since they're so close together. Try an xacto knife to "neaten" up the paste, place the chip, and hotplate it. Do it first, then do the rest of the parts by hand.

Even without a hotplate, it helps if the lands on the pcb extend out from the chip by 20-40 thou. That gives you something for your iron to touch if you need to reheat under the chip.

Another idea we've tried for QNs with belly pads is to put a big via in the middle of the pad, big enough to fit your iron through from the other side. Just don't try to make that land much of a heatsink or you won't be able to actually solder it.

You're right.!!! I just got a sample chip and the lands do run up the side of the chip a few thou....... so that makes it whole lot easier!. I like the big via idea , but the pad size is only 30x10 thou..... And extending the pcb tracks 20 thou will help. Did I miss something about the big vias? Thanks for the answers.

If the pad is that small, you're probably out of luck. The via needs to be big enough for the tip of your iron to fit through (25 thou in my case), or you need to pump enough heat through it to reflow the paste on the other side.

For a pad that small, I'd try the hotplate method. For prototypes, you don't need the pad's paste to be as perfect as a production run (unless you're testing its thermal limits) and if you do, you can have a mylar mask made and still hotplate it. Still extend the lands, though, in case you need to fix up a solder joint manually.

Note also that extending the lands isn't just so you have something to solder *to*. The lands are small enough that they'll conduct heat under the pad also, so if you have paste under the pad, heating the copper land can reflow the paste without actually having to touch the paste or the chip with your iron. That also keeps the solder from sticking to your iron and messing up the chip's placement.

Thanks DJ....... I'm sure that will help a lot of people. It has certainly given me the encouragement to have a go.!

Here's a thought..... I have a large flow solder machine in the garage. Could I use that as my hotplate maybe? What also amazed me was the way the 132 pin chip had its solder paste applied! I would have thought that would have ended up a total mess, but clearly that is not the case.

Well, every home should have one!

The older SMT packages were designed to be flow soldered as well as reflowed, as I understand it. The components are glued down first.

I wonder if it would work with the QFN (with extended lands)?

If his part has a belly (thermal) pad, I don't think using the flow solder machine will work to actually *flow* the solder under there, but it might work to heat the PCB enough (on the other side) to reflow the paste. Assuming you can hold the board steady long enough for the heat to work its way through the board.

Yes, I was ignoring the "thermal" pad. I use the "soldering through a big hole" technique for that.

Could be worth a try, so long as the part is located bang on the pads.....

Yes, this chip does have a big thermal pad underneath.... But the data sheet says this mustn't be connected to anything, least of all ground. They also say that the gnd via connections on their reference design will not contact the belly pad because of the solder mask..... Sounds a bit iffy to me... but perhaps I will cover the vias with the mask as well ( some peeps do this anyway). I can hold the board steady with the PCB carrier that holds the boards for flow soldering.