Saturday, March 13, 2010

Hand soldering QFN

I tried last night to hand solder another 28-pin QFN part.  A QFN part has no leads, but rather copper pads are built right into the package.  And the leads are tiny and closely spaced.  On top of that, many QFN parts have a big ground "paddle" under the part.  This should be soldered down.  Atmel FAQ explains why.

So I'll give you the summary right up front: use reflow soldering for QFN.



Since I can't solder the paddle under the package with my soldering iron, I applied solder to the paddle and to the PCB pad, hoping for some mechanical contact would be "good".  This caused the part to "high center", and it rocked back and forth as I tried to align the part with the solder pads.  So I wicked it down as much as I could.

In the future, I might try putting a via right in the middle of the paddle and use it to conduct heat up to the paddle from below for hand soldering. Watch this Schmartboard video for this technique in action.

Anyway, my next problem was aligning the pads.  In reflow soldering, the viscosity of the molten solder will pull the part into nice alignment.  It's fun to watch the parts suddenly shift when the solder paste melts, and a great way to tell you've reached reflow temperature.  This works really well up to misalignment of 50% of the pad spacing - at that point, the part might move onto the wrong pads. 

Hand soldering, getting that first pin soldered with good alignment all around the package is just trial and error.  The act of touching the pad with my iron tends to pull the part to the iron.  So you need to hold the part down with pressure from your tweezers or something.  I saw an idea on Hackaday on using pliers and a small screwdriver with a rubber band to create a tool to hold down ICs.  It works pretty good!  But rotation of the part vexed me last night.  Eventually, I got misalignment down to 1/4 to 1/2 of the pad width.  Yes, I was satisfied with 1/2 - it was very frustrating.

Next, I applied flux paste all around the part and "drag soldered" the pins.  Look on Youtube for videos on drag soldering.  I use .015 diameter solder and apply a fair amount of solder.

I got quite a few shorts.  One side had just one short, but the worst side had all pins shorted.  I can see shorts using a simple magnifying glass - its pretty obvious when there is the shiny glint of solder from one pin to the next.  You can and should use an ohm meter to verify adjacent pins are not shorted.  What is very hard to do is verifying the pads are soldered down.  So I used plenty of solder.

Removing a short, if there isn't too much solder, can be done just by touching the solder bridge and dragging some  solder away with the iron.  In cases with too much solder for that to work, I used solderwick to pull off excess solder.

I started around the package, removing visible shorts and verifying with the ohm meter they were clear.  It went pretty well for the first two sides.

Unfortunately, on the third side, the ohm meter "lied" to me.  I was trying to clear this one short, and no matter how much solder I removed, the pins were shorted.  Eventually, I gave up and went to bed (4am).

The next morning, with a clearer head, I realized that the two pins I was working on were VCC and GND.  These pins were shorted on the fourth side of the package!  So I had to re-apply a bit of solder to those pins to ensure they were soldered.  That meant getting the flux back out and dragging that side of the part.  Yay, more shorts to clear.

I fairly quickly removed those shorts and the ones on the last side of the package.  Double checked every adjacent pin for shorts.  Everything is clear.  What I don't know is if I damaged the part while I fussed with the VCC GND pins last night.  I should find out fairly quickly when I begin software debug.  Unless the part is "sort of working" and just flakey.  That takes a while to figure out.

Sorry for the lack of photos, but all you'd see is a little tiny IC on a PCB under a 3rd hand magnifier.  I need to get a macro lens for my camera.

2 comments:

SiliconFarmer said...

Later on, I found out that my PCB footprint for this QFN package has the wrong pad pitch.

So it turns out, I must be some sort of QFN soldering master to have gotten the part to have no shorts. Or something.

Piotr Kula said...

HI

You got any more advice on doing this at home? I got QFN40 0.5mm pitch and am paying a ton for companies to do it.