USB Doodad 3: Hot-air soldering of TQFP-32 chips

The processor we're using in the USB Doodad is an AVR ATmega328P.  This very capable processor is at the heart of the popular Arduino educational microcontroller platform.

The two most common packages for the ATmega328P are the PDIP-28 and the TQFP-32.

  <Good place to insert picture of PDIP vs TQFP>

Because the whole purpose of the USB Doodad is to use surface mount components, we want to use the TQFP-32 package.

Image courtesy of SparkFun.

Armed with a USB Doodad prototype board that Ross etched, I thought it was time to try soldering this chip.

There are several ways to solder SMT ICs.  A good guide is in this SparkFun SMT tutorial.  The method I used was using solder paste and hot air.

In order to do hot air soldering, I use a "hot air rework station" which I bought for about $80 in China.  I found another one at Ameritronics which looks very nice, nicer than mine.

Ideally, I'd use a syringe of solder paste, but I don't have one.  Instead I used the paste I covered previously.

I applied my solder paste with a jeweller's screwdriver.  Not a very precise way of doing it, but it did the job.  I applied one thin line of solder paste along the line of IC pads, then squished the chip pins into the paste.  Then I ran some liquid flux along the pins on all four sides of the chip.

Next, I used the hot air to melt the solder on each corner of the chip.  That was to fix the chip into place.  Finally, I ran the hot air along each side of the chip in turn.  The combination of pins, solder paste and flux means that when the solder paste melted, it blobbed around each pin.  I didn't get any bridges between pins, but if I had, I'm confident I could have fixed it with some desoldering braid.

My success makes me think that beginners could solder it too, if they had the right equipment (a hot air rework station).  And if we had a board with a solder mask (as we'll surely have for Doodad a little further down the track), I think beginners could do it with a regular iron too, if they had a video to watch showing how it's done.

Well, that's how I soldered a TQFN-32!

USB Doodad: An SMT exercise project

Surface mount soldering doesn't have to be scary!

Way back in the distant past when I learned electronics, components were "through hole". That means that components such as resistors had a wire out each end, and these wires were bent and passed through holes in a circuit board, then soldered.

An example through-hole board.

These days, through hole technology is on the decline. It's becoming harder to get the newer integrated circuits in through-hole form. In its place, more and more people are using "surface mount" technology to build circuits. With SMT, components are often in the form of a tiny block, which is placed on a circuit board, directly in contact with the circuit board tracks, then soldered into place.

An example SMT board.

Many people I've spoken to think that using SMT is harder than through-hole. Sure, many SMT components are very tiny, but it's quite easy to buy larger sized SMT components.

As a project at our hackerspace, Connected Community Hackerspace, Ross McKenzie and I have been working on a project which shows people that doing SMT assembly is not as hard as people think.  We have deliberately used the largest size of SMT components to make assembly as easy as possible.

The board has a USB connector at one end, a small expansion connector at the other, and a row of 16 software controlled LEDs in the middle.

The primary purpose of the board is to act as a practical exercise in surface mount assembly.  The secondary purpose is to show how to use the V-USB software USB stack to make a USB device.  The third purpose is to give the Doodad some post-tutorial value, in a number of intentionally frivolous ways:

• Persistence-of-vision toy. Wave it around or put it on the wheels of your bike. Enjoy making groovy flowing messages in the dark.
• Secure password keystore.
• Log-data-to-serial-flash data logger, where samples could be retrieved over USB at some later time.
• “Messages waiting” / “processor load” / “build progress” indicator.
• Multimedia keys (if your keyboard doesn’t have them).
• Novelty breathalyser.
• Countdown timer for car parking reminder.

Our estimate for the cost of this board is about AUD30, so it's well within hobbyist reach.

We plan to release the design for this project under an open licence, possibly the TAPR OHL, which means that others can copy this design.

The project has been designed to use a single sided PCB.  Although this makes the job of designing and routing the board traces much more difficult, being single sided means people can make this board at home.

For applications which require the doodad to operate independently of a PC, we plan to add a small Li-Ion battery and management chip to the reverse of the board.

More information about the project can be found in the USB Doodad Google doc.

Ross and I are making rapid progress, and I'll post more project updates here soon.