Friday, March 6, 2009

Make Cheap (almost free) SMD Probe/Tweezers

This home-made surface mount device (SMD) tweezers/probe is made from scrap PCB material and plugs into a DVM or capacitance/inductance meter for easy measurement of tiny SMD components (Of course you still need the appropriate meter). You’ll find these far easier to use with tiny parts than regular DVM probes, and much cheaper than meters with tweezers built in.

1) The tweezers are made from 1/16" thick copper-clad PC board material. Single-sided board will do, but I used 2-sided here. Cut a piece to 4" by 3/4". and scribe a diagonal line, not to the corners, but to points about 1/8" in from the corners on the short edges. I blued this piece to show the line. It helps to have a shear, but you can also cut PCB material by repeatedly scribing a deep groove with a hobby knife and breaking off the board. If you do this, make a groove on both sides to get a cleaner edge.

2) Cut the board along the diagonal. Use a hobby knife and a metal ruler or other straight-edge guide to cut out copper to make a trace along the long edge of each piece. Orient the cuts to mirror each other. Then also cut two 1/4" by 3/8" copper-clad rectangles for spacers between the two blades.

TRICKS FOR CUTTING THE GROOVES: Xacto-type blade will break off at the tip while cutting the copper because you have to press down pretty hard, but that's fine. Keep using the blade even after the tip breaks off. When it dulls, just break more off the tip with a needle-nose pliers (WEAR EYE PROTECTION) to get back to a sharp edge. Sometimes I snap off the blade tip before I even start so there are no surprises while cutting. You can save old blades for this because most get dull only near the tip. To get a clean open groove in the copper, cut once to get a line, then cut a a few times with the blade angled to one side, then a few more with the blade angled the other way. The angles make a clean beveled edge on the copper.

3) Solder the small rectangles to one of the blades. Be sure that they NOT connect to the thin trace along the edge.

4) Line up the second blade as well as you can, and solder that to the spacers, again being sure to not short the edge traces. Drill a hole through the assembly. We'll use these holes for lead strain relief.

5) Squeeze the tweezer ends together and rub the tip on a piece of emery cloth over a flat surface to grind the ends to an even and tapered point. It should only take a bit of rubbing to get the point so that you have no trouble picking up an 0402 resistor. You can also use emery cloth to smooth the cut sides of the blades as well. If you have one, you can use a belt sander for this but use a very light touch because they remove a lot of material in a hurry.

6) Solder on some test leads with banana plugs and you are ready to go. Using color coded leads makes it a bit easier to check diodes.

1 comment: