Introduction: SMD Testing Jig
SMD's are great once you get used to them, but their teeny tiny size makes them difficult to test.
I started harvesting used SMD's from old circuit boards a few years ago. Free components, yay! But then there is the problem of sorting them and finding duds. It's not too hard to test them with a multimeter once you get the trick of holding the device down with the probes, if you don't mind pinging the occasional one across the room.
But I'll tell you what you really want, what you really, really want, is something to hold them down for you. That's where this handy gadget comes in.
It consists of a spring-loaded arm which holds the component down onto a small board that has 3 pads. The arm can move about a little to accommodate 2 or 3 leaded components. Each pad is connected to a colour coded socket to connect a meter or component tester.
SMD transistors, diodes and other semiconductors are marked with codes which bear no resemblance to the actual part number. There are various guides to be found online to look up the various codes, but one SMD mark code can lead to quite a lot of different devices. For this reason I very highly recommend getting a "Hiland" type of component tester to use with this jig, it's a great piece of kit, you can see it in one of the photos revealing the secrets of a mystery transistor.
I built the jig before taking photo's, so I partly dismantled it to get some intermediate pics. Hence drilling etc isn't shown.
It's still little more than a prototype. It would be improved by having a softer end on the foot, a weaker spring, and interchangeable PCB's for different testing scenarios.
- some kind of solid board. I used a small piece of laminated MDF from a scrapped piece of self assembly furniture. I hope to build a better version of this project using something that isn't sensitive to moisture.
- Plastic sticks. I used some rods that came from shoe packaging, and a small roller that came from a printer I dismantled, but use what you can find.
- Steel rod, about 2mm thick and 3cm long.
- Plastic shape with a hole which your stick will fit into. I used the cut-off end of a large (500L) water filter cartridge.
- Some kind of spring. I used a funny shaped one I dismantled from something. A coil spring will work just as well if it is strong enough.
- A piece of 2cm square single sided copperclad board.
- 3 terminals. I used (very cheap) 4mm binding posts with solder tags.
- Double sided foam tape.
Step 1: PCB
The jig relies for it's connection to the device under test (DUT) on a small PCB about 2cm square.
I designed the area where the DUT sits to have very close pads in the central area, where the gap is 0.25mm, which should comfortably fit SC-90 and 0402 devices (ie, really tiny). A few mm away from this area the gap is widened to reduce coupling between the pads, which will already be raised due to the very close area in the middle. For normal testing this shouldn't matter.
The layout is based on a simple bar and two squares which seems to give the best scaling for differently sized devices.
I've provided PDFs of the layout in positive and negative versions. Use the negative if doing photo-resist (recommended) or the positive for toner-transfer.
Due to the simplicity of the design it may be a more viable option for you to try masking off the design with tape as etch resist.
I left the spare copper around the edge of the board to give it a little protection and in case it's useful one day.
With more complex PCB designs you can have more terminals and test more complex devices. The present jig is little more than a prototype however so doesn't have the facility to change the boards.
Step 2: Construction
Make the arm first
- Cut a piece of plastic stick about 5 inches long.
- Flatten a small area on one end
- Where you flattened, drill a hole about 4mm perpendicular to the stick. Don't go all the way through.
- Take the piece you want to use as the foot. Fit it in a drill chuck, and run the drill so you can shape the plastic with a file. Make one end 4mm diameter to go in the hole you drilled, the other end needs to be 2mm or a little less. My piece was a roller from an old printer so the 2mm end was already shaped.
- Glue the foot into the arm. Fasten it securely so it's tight and perpendicular to the arm.
Make the arm holder
I used a piece cut from the end of an old 500L water filter cartridge, but anything you can fit the arm in and drill a cross hole will do. Try old soap dispenser pumps for suitable parts.
- Enlarge the hole which the arm will fit into, if needed. It still needs to be a tight fit.
- Measure the diameter of the pin you are going to use as a pivot. Mine was 2mm.
- Drill a 2mm (or whatever) hole across the piece, towards the stoutest end.
- Fit the pin through the hole
Make the supports
- Measure the width of the arm holder and one plastic stick, add them together. This gives the spacing for the centres of the support holes in the base. Use the arm as a guide to get the position for them as a pair and mark the positions. A little too far apart is ok.
- Take a drill bit NO MORE THAN 0.5mm bigger than the diameter of the plastic stick. A little bit of movement is good, but you don't want it sloppy.
- If you can't find a suitably over-sized drill bit, just make it a tight fit.
- Drill the holes as deep as you can. Make the depths equal. Don't go right through the base!
- Fit a plastic stick into one of the holes.
- Fit the arm into it's holder and position it so the foot is 2 or 3mm above the base, and the arm is roughly horizontal. Make sure the steel pin is horizontal and the foot is vertical.
- Mark the plastic stick at the height of the steel pin.
- Drill a 2mm (or whatever) hole in the plastic stick where you marked, and cut it above the hole. Cut and drill a second piece to match the first.
- Fit the supports to the arm assembly
- Fit the supports in their holes.
- You should end up with the foot about 2cm from the front of the base with the foot vertical and the steel pin horizontal.
- The foot should be able to move side to side a few mm, and back/forth about 1mm. If it can't, that's fine, you just have to be extra careful positioning the PCB.
- Pack / pad / cut / file / drill as needed.
Prepare and fit the spring
- I used a strange L shaped spring I dismantled from something because it can press the top of the arm. I had to bend it a bit to make it fit. You could use a normal coiled tension spring, so long as you pre-tension it a little. You may have to open out a loop of the spring to fit the arm through if you do this.
- Don't side-mount the spring on the arm as this will twist it. It needs to pull the top, bottom, or both sides equally (you could side mount two springs)
- Choose a position for the spring so it's pulling the arm down with enough force to hold a component securely down. I don't have an actual value for this, so use your judgement. Mine is about 250 grams as measured with a luggage scale, but it should probably be more gentle than this.
Fit the terminals
- Pick a spot you like where the terminals will go. Mark 3 spots. Make them about an inch apart.
- Measure the mounting posts of the terminals and drill 3 holes to fit.
- Counter-bore the backs of the holes so the mounting nuts and solder terminals can be hidden inside. Leave room to get your favourite nut-tightening tool in.
- NOTE: If you have to make the counter-bores with a wood-working bit, like I did, drill them first, like I didn't. That way you won't end up with a horrible mess like I did. Then you can use the indent from the spur of the bit as the centre for the proper hole. Pilot drill from underneath then drill properly from the top.
- Drill holes for the wires so they go from near the terminals at the top, to just inside the counter-bores underneath.
- Assuming you use terminals with 2 nuts:
- For each terminal, remove nuts and solder tag. If there's no solder tag you will need to obtain or make one. Crimp tags may be too bulky.
- Fit the terminal in the hole, with any plastic rings, washers etc, fit the first nut underneath and tighten it.
- Solder a wire (matching colour preferred) to the tag, and thread it through the small hole from underneath. Bend the tag as needed to fit in the counterbore.
- Secure the tag in place with the second nut.
Fit the PCB
- Stick some double sided sticky foam tape to the back of the board. You probably need two pieces, so align them with the middle of the board and cut around.
- Peel the backing off the tape.
- Position the board very, very carefully so the natural resting place of the arm is in the centre. The wide pad goes furthest from you, the two smaller pads at the front.
- Stick the board down.
- The pads are numbered 1 to 3, anti-clockwise, starting at bottom left
- Trim the 3 wires so that they reach the corners of the pads with a little slack. Strip 1.5 to 2mm at the ends, tin, and solder in place. I suggest connecting the terminals so they go in numerical order of the pads.
- Cut a bit of flat plastic - I used a strip cut from the carrying handle from a large cardboard box. Drill 2 holes in it far enough apart for the wires to go between.
- Find a spot where you would like the clamp to go, drill screw holes and fasten it down over the wires. Keep the wires flat, not crossed.
- Apart from this one spot, the wires stay apart to minimise capacitance between them.
Step 3: Using the Jig
Please refer to photos for how to use the jig.
3 lead devices such as the SOT23 transistor and the preset resistor sit nicely on the pads, though I did find the preset to be slightly problematic and needed to move it slightly between measurements. The pads are close enough you should be able test SC-90 packages with no problem.
2 lead devices can go between any 2 pads. 0603 components are shown and the pads should be close enough to test 0402 packages. Being able to move the arm about a little bit proved very useful here.
The best way to use the jig would be with a Hiland type component tester, which are cheaply available in kit form (get it from Banggood) and a superb addition to any electronics workshop. As you can see from the photo, the C1L under test is an NPN with hfe of 390. Looking up this marking gives a possibility of it being CMPT6429 or KSA1623-L. Knowing the gain makes it much more likely to be the somewhat more pedestrian KSA1623-L.