Throwing in the towel

Tonight, I had to admit that it is impossible to complete my design by the deadline this Friday.

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.

Hand soldering SMT components

Below is based on an email that describes how I do hand soldering of SMT.  I'll try to add a few photos soon.

I use a temperature controlled iron with a good tip, set to 315C.  A temperature controlled soldering iron with a good tip is KEY!

How to waste 2 weeks

Inspired by a class I took from the author of Extreme SMT Instructable, I decided to set up a SMT reflow soldering system.  Although his hotplate technique works well, it isn't as repeatable as I would like.  Recently, he added a follow-up instructable that adds a PID controller to the hotplate system, which should make the process repeatable.  But I didn't see that until I was already committed to the toaster oven system (parts had been ordered). 

And the hotplate technique still bothers me.  The heat is applied to the insulating FR4 fiberglass, so only convection heat is getting to the solder joints.  The bottom of the PCB tends to discolor slightly, so who knows what this means for long term reliability.

Reading several DIY guides, I became convinced that the way to go was to buy a PID temperature controller and a powerful toaster oven.  Why duplicate something you've seen work when you can try to do it "better"?  (hint: This is especially not a good idea when you are under a tight deadline, like I am right now.)