SparkFun Forums

[winston]

I've done some surface mount soldering - things like SOT23 (a little bit fiddly due to the lack of mass), SOIC (so easy!), SSOP (once lined up, easy using the don't-care-if-I-brige-it then soak up the excess solder with braid)

I have a project I want to add Ethernet to - I was planning to use the Silicon Labs CP2200 all-in-one Ethernet chip, which looks easy to use. However, my favorite supplier seems to have stopped selling the CP2200, which came in a TQFP (which looks no harder to solder than an SSOP). Instead they now only sell the CP2201, the same chip but in a pin-less QFN. Indeed, I can't find the CP2200 anywhere, only the CP2201.

Is the only practical method for QFN to reflow? Or has any brave soul hand soldered one of these buggers?

Anyone tried reflowing on a home etched PCB (i.e. no solder mask)?

[leon_heller]

I've attached a couple of QFPs to home-made PCBs using a soldering iron, it wasn't all that difficult. Having a stereo microscope helped.

Leon

[inventore123]

I've nedever soldered a QFP, but it seems that these packages, even if the don't have pins, they have pads on the bottom, AND on the side of the package. Someone that have soldered them with a soldering iron can confirm that it is possible to solder the pads on the side to the printed circuit?

[bigglez]

Instead they now only sell the CP2201, the same chip but in a pin-less QFN. Indeed, I can't find the CP2200 anywhere, only the CP2201.

Is the only practical method for QFN to reflow? Or has any brave soul hand soldered one of these buggers?

Greetings Winston,

Here's a very good technical brief for the QFN package.

I've done hot air soldering on the QFP (that you mentioned) and
on leadless LEDs which have an underside pad with side pad
similar to a QFN contact area.

What I found was that excessive solder can cause the device to
'float' (a problem for LEDs as the dome is then off axis). The
amount of paste solder required to get good contact without
excess is miniscule!

Good luck, please report your findings!

Comments Welcome!

[winston]

@leon: Did you do anything to stick the chip prior to soldering, so it didn't skitter away?

For SOT-23 devices, I've used a tiny drop of Superglue (which gives enough time to get the device placed before the glue sets), which makes the actual mechanics of soldering the pins easy. I've also used a very fine tipped screwdriver, clamped in some 'helping hands' apply weight to the top of a SOT-23 package for the same purpose (with the advantage that it gets rid of the stench of hot superglue, and if you screw up, you get to have another try).

[leon_heller]

I hold it down whilst I tack down two opposite corners. I then drag solder all the pads with plenty of gel flux. I put a large via in the centre of the central pad, and feed solder in with a soldering iron. I use Metcal equipment. which helps a lot.

Leon

[deviant]

I hold it down whilst I tack down two opposite corners. I then drag solder all the pads with plenty of gel flux. I put a large via in the centre of the central pad, and feed solder in with a soldering iron. I use Metcal equipment. which helps a lot.

Leon

Yeah, I tried drag soldering using a hoof tip and it works wonders . Use a decent amount of flux and it should be fine.

[phalanx]

My usual approach is to pre-solder one pad on the board, line up the part, and then reflow the one pin with my iron. This holds the part in place while I drag solder the other sides before returning to the initial side I tacked down.

-Bill

[NleahciM]

I've done QFNs by hand. Not too terrible. Couple recommendations: put a large via under the center pad. Solder opposite pins first, then use the standard drag method to finish up the rest. Finish soldering all outside pins before touching the center pad. Lots of flux, fairly fine chisel tip. Make sure your pads stick out about 1mm from the edge of the chip. If they're alot smaller than that they'll be harder to solder to. Take your time with placement.

[silic0re]

Hi all,

Sorry to drag up this old thread, but I have a couple quick questions! I've made footprints for two leadless QFNs (TPS63001 [3x3mm, ~12 pins], and TPS65131 [4x4mm, 24pins]), and checked printouts of the footprints with real devices to make sure they're about right. I've also made the pins extra long to have a little more room for soldering -- they extend about 1.0mm from the edge of the devices.

I'm really not sure how I'm going to solder these things, but was thinking of dabbing on some solder paste to the footprint and going the toaster reflow route. Has anyone tried this?

While I've soldered plenty of small SMD's, I don't have much experience (nor success) with leadless devices. I once tried a SCP1000 barometric pressure sensor, whose pads are actually completely under the device and unexposed to to the sides. I designed a footprint that had much longer legs (about 3mm) so that one could put some paste on the footprint then reflow it after the part was on, but this didn't have very much success at all.

I think hot air would be ideal, but I don't have the equipment -- just a normal decent soldering iron, and patience. Still, I fear a repeat of my past leadless attempt, and think the toaster oven might be a viable alternative. Any thoughts or success stories?

thanks!

[leon_heller]

As said previously, drag soldering works very well.

Leon

[emf]

I've done a few QFNs and DFNs in a toaster oven (most were .5mm). I apply a small amount of regular solder to each of the pads, checking with a magnifying glass to make sure that they're all about the same height so the chip will sit level. Then I clean it, apply a thin coat of flux, sit the chip on, and pop it in the oven.

I've done three SCP1000s this way, two worked on the first try. The third had shorts (bad alignment, I guess), so I had to throw it back in the oven and try to pluck it off the board while it was hot. I did, but I dropped it into the bottom of the oven and you wouldn't believe how well poppy seeds stick to flux. Cleaned everything off and it worked on the second try.

I've never had the guts to try solder paste under a leadless device. It takes me a million tries to get the part aligned properly, and all the while I'd be pushing the paste around under the part. I'll have to try drag-soldering one of these days.

[silic0re]

That's actually an excellent idea! I hadn't thought of pre-applying paste to the pins themselves, melting it, then aligning the part on the pad and popping it in the oven. I had worried, like you, that after pasting the pads then aligning it that the paste would be everywhere, and all the pins would be bridged. Excellent idea!

[phalanx]

Reflowing the board twice isn't the best approach if you are starting with solder paste (which is ground up solder mixed with flux). My guess is you need to work on your reflow profile. You have to give the board adequate time at a moderate heat to allow the component, solder, and pads to come up to temperature. You then do a quick excursion into the reflow temperature range until the part self aligns (pretty cool to watch) and then you cool it off.

-Bill

[winston]

Drifting a bit off the thread's topic, how big of a chip will self aligning work for? I've been doing some hot air (with a hot air gun) reflowing with solder paste recently and I've found that 0603s, small SOIC devices and the like self align very nicely. But they are all small devices with pins wide enough apart that there's solder mask between the pins.

I've not had such luck with 100 pin TQFPs or LQFPs, or even 32 pin TSSOP. I have to put them on properly lined up (or, if I discover halfway though heating that they are not, give them a gentle prod while the solder is molten to encourage them to go in place). Is this likely just due to the rather imprecise temperature increase when using a hot air gun rather than an oven?