MakeStuff

There are lots of interesting electronic components nowadays like FPGAs and powerful 32-bit microcontrollers. Unfortunately most have upwards of 100 pins and many are only available in packages like TQFP which are very tricky to design with at home. There are two main reasons for this:

• The layout of the pins usually requires vias to be placed under the chip to allow decoupling capacitors and power routing on the other side of the board, leaving the topside clear for signal fan-out. Unfortunately there is precious little clearance under SMD chips for the through-board conductor to make a good connection with the top-side tracks.
• The lead pitch is tiny, typically 0.5mm.

Today I was able to make my first PCB using a 0.5mm-pitch chip, with vias placed under it. The design was just an intelligent break-out board for the 48-pin SN74ALVC164245 level-shifter. It is available in the 0.635mm-pitch DL package and the 0.5mm-pitch DGG package, which is what I used. I chose this design because the chips are cheap (~£1.50) so it wouldn't have been the end of the world if I failed a few times. The techniques should generalise to bigger, more expensive components.

The following method worked for me; I hope it's useful to someone else.

Layout

If you want to fabricate your latest PCB design at home, you need to start thinking about the implications at the design stage - and that potentially includes modifications to the library components you use. For successful home-made PCBs you need to be very conservative with the pad & track sizes and spacing, particularly where drills are concerned. In Eagle CAD, I used 1.5mm octagons for the via pads, with 0.7mm drill-holes.

Note: In retrospect, it looks like the etchant has made holes uniformly bigger than the 0.7mm specified by the design, so next time I will reduce the CAD drill size to 0.5mm to compensate.

Making the PCB

I used an HP LaserJet P2014 to print the upper and lower layers on to LaserStar film (upper layer mirrored), aligned the two sides using four pieces of blu-tac to hold the film in place (the ink facing inwards), then slid an appropriately-sized piece of double-sided PCB laminate between the sheets and exposed for 150s in an AZ210 double-sided UV box (pre-heated for 120s). Next I developed the board using an SN110 applicator, and etched with ferric chloride solution in a PA210 bubble-tank. Then I used an SN120 applicator to strip off the remaining photoresist, scrubbed the board vigorously with steel wool and then used an SN130 applicator to remove any oxidisation from the copper.

Drilling and Inserting Through-Hole Rivets

To drill the vias I used a Dremel with a 0.7mm bit. In retrospect I think a 0.6mm drill bit would have been better. To make a conductive connection between the top and bottom layers I inserted one of these tiny rivets into each via hole using a Favorit rivet machine from MegaUK.

Next I very carefully soldered both ends of each rivet using lots of flux and trying to avoid large blobs (remember that the chip goes over these vias, and there is only 0.25mm clearance).

Drill the holes and insert the rivets, then solder both sides

Before placing the chip over the vias, I used a multimeter to check the connections between the upper and lower tracks.

Soldering the Chip

Soldering such fine-pitch components is difficult at the best of times, but on a home-made PCB it's harder still. For aligning the chip on the pads I used a USB microscope and a steady hand - no Relentless for me today! For the soldering itself, I used the drag soldering method using a Weller WD1 soldering station and an Cooper NTGW tip.

Align the chip carefully, solder two corners and then drag the iron down each row

Once I finished soldering I cleaned the flux-gunk off with methylated spirits.

The Finished Product

And here it is, the finished product:

You can download the Eagle CAD design files for the board if you're interested:

• testVC.sch
• testVC.brd

This entry was posted on Sunday, July 4th, 2010 at 21:09:43 UTC and is filed under Electronics. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.