M. T. Homer Reid MIT Home Page
Physics Problems Research teaching My Music About Me Miscellany

Homemade PCBs With Surface-Mount Components

March 31, 2006

From Thomas Gootee and Kenneth Maxon I learned the arts of do-it-yourself PCB making and reflow soldering, finally realizing my long-standing dream of designing with fine-pitch flat-package ICs, 0201 chip inductors, and other electronic components with lilliputian terminals. Although I am still a journeyman apprentice with years to go before mastering the skills of these two Jedi sages, I have progressed far enough to make some workable prototypes. Here's a simple example. I was building a device that used this 16-bit A/D converter, which only comes in a 28-pin SSOP package, and I needed a breakout board on which to mount the part (with bypass capacitors nearby), plus some way of connecting said breakout board to my main board, a standard 100-mil pitch PC board.

First, using the awesome freeware tool PCB, I created the layout for my breakout board, shown here somewhat enlarged:

pcb2.png

Next, following this procedure, I printed the pattern onto Staples picture paper, ironed the paper onto a copper-clad PC board, soaked the complex in hot water, and then scrubbed with a toothbrush to remove all the interstitial bits of white paper from the board, leaving only the pattern inscribed in thick black plastic ink on the board surface. As Thomas Gootee points out in his description of the method, the printer ink sticks so well to the Staples paper that you can really scrub hard with that toothbrush to remove the interstitials without rubbing off any of the desired regions of ink.

Note regarding printing: In my case I couldn't get the picture paper to feed through any of the laserjet printers in our office, so I wound up printing the pattern onto a sheet of ordinary paper and then photocopying that sheet onto the picture paper using our inkjet fax/copy machine. For this purpose I found out (entirely by accident) that the image sticks much better to the glossy side if you first run the picture-paper through the copier upside down, presumably because the first run through warms up the paper and improves the adhesion of ink to the glossy side. In other words, I put the sheet of picture paper in the copier source tray wrong side up, hit the 'copy' button to make a blank copy, then put the picture paper back in the source tray (right side up this time) and copy the image.

After all of that, I dropped the board into a tupperware container filled with ferric chloride etchant. For this small board, with intermittent jiggling using a pair of plastic tongs, it took about ten minutes for all the copper to come off, after which the board looked like this. (Unfortunately, I stupidly forgot to take snapshots of any of the the earlier stages.)

etched board

The discoloration of the pads on the lower half of the board is the residue of some packaging tape I had applied to the board at one point, the reason for which absolutely completely eludes me now. Anyway, the next step is to scrub off the black ink, which takes about two seconds using a paper towel dipped in acetone:

etched board 2

You gotta love those pristine 50-mil-pitch traces. Hooray for Thomas Gootee!

Next I followed this procedure to stuff the board, using the solder paste they recommend (digikey part KE1512-ND). Everything went exactly as advertised, and in about ten minutes I had a mounted chip:

stuffed board 2

Although my digicam can't take a sufficiently high-resolution to show it, the solder paste really does wick onto the metal traces in the reflow (er, toaster) oven, so that even if you glop it on fairly sloppily in the pre-bake phase you may still wind up with no shorts between pins, as, to my great surprise and delight, was the case here.

For the connectors on this particular board I used a hybrid scheme. To make connections to the A/D input pins (on the right side of the photo above) I used a straight 9-pin DIP header (digikey part 929647-03-36-ND, cut to size), soldering the short end to the breakout board and wirewrapping wires to the long end. For the power, ground, and microprocessor interface pins I used a 9-pin right-angle DIP connector (digikey part #WM6336-ND), soldering the short end to the board and inserting the long end into a 9-pin DIP socket (digikey part #929850-01-36-ND) mounted on my PC board. Here's are end and front shots of the final apparatus:

mounted board 1

mounted board 2


Homemade PCBs and Surface-Mount Component Soldering, by Homer Reid
Last Modified: 11/16/16