Screaming Circuits: Sometimes, I just want to build it myself


Sometimes, I just want to build it myself

It should (and does) stand to reason that I'm a big fan of professional assembly. When you need it done right, or need a bunch built, there's no better way to get working PC boards than a solid professional assembly house. Good thing I work for one.

Here's a Freescale KL03 microcontroller in 2 mm x 1.6 mm wafer level chip scale (WLCSP) micro BGA form, soldered by our folks on the manufacturing floor.

KL03 Prof solder closeup

I'm also a big fan of the underlying skills. I like having the practical context of being able to design and build my own boards. I'm personally no where near as good at it as the folks out on our factory floor, nor do I have the same quality of equipment at home. But I do build things, even attempting some of the super small parts. So, today, I thought I'd speculate a bit on how to home-build some of the more complex chips available.

I tried out a Schmartboard with a small QFN, and the same micro BGA pictured above with a home-toaster (reflowster?) oven. I've got a few pictures to show the results.

The Schmartboard is an interesting concept. The boards have channels filled with solder that connect up to the leads on, in this example, a QFN. The idea is that you put the tip of a fine point soldering iron in the channel, and slide it toward the chip,melting the solder and pushing it up to part.

Schmart unpopulated

I tried it out, with some level of success. I got a good fillets on one side of the chip, broke two of the traces, and ended up with a number unconnected.

Schmart soldered

Good fillets here:

Schmart good fillets

Those are some nice fillets and would pass IPC inspection. I need a bit more practices (see earlier photo), but I could see this as a viable way to make a breakout board for a QFN. This particular part has some of the metal connection on the side of the chip. The Schmart board also works on QFNs without that side metal exposure.

Home BGA processing

Getting a bit more ambitions, I designed my own board for a 20 ball 2 mm X 1.6 mm micro BGA. This is one of the parts that just has solder bumps on a piece of the silicon dice.

The professionally soldered image is at the start of this article. Here's my attempt at using a home reflow toaster oven:

KL03 home solder closeup

I hand positioned the parts and ran them through my toaster oven. Not bad, really. Just messy with all of those solder balls. And, the BGA balls didn't really mix with the solder paste all that well. It would likely be serviceable as a hobby project.

All of the little solder balls scattered around show up because I put down too much solder paste. As the paste melts, the flux will out gass a bit, spitting off the little balls of solder. We do see some of those in our shop sometimes, but we remove them when we do.

With more time at home, I could dial my skills in a bit and get somewhat better results, but for anything I hand out at a tradeshow or other event, I'll stick with factory built.

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Duane Benson
Spider bites man
Man turns into werewolf
No other spiders on spider island
No one seems to care


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