PCB Test Fixture

About: Hiking, Woodworking, PCB design using Eagle, Writing Software for MacOS and AVR, 3D Design using Fusion 360

Most of the boards I design have through hole pads for a 6 pin ICSP header, and some have through hole pads for a serial header. In many cases neither headers are used after the board is programmed. This test fixture is used in place of a permanent pin header.

This is a 3D printed version of a test fixture I found on AliExpress with a few modifications. The AliExpress version is about $10 US and this DIY 3D version costs about a dollar.

https://www.aliexpress.com/item/Programmer-Module-...

Parts:

3D printed base and lever

(1/2) 2x8cm Prototype PCB

(3) M3 nuts

(1) M3x20 button head screw (ISO7380)

(2) M3x12 button head screws (ISO7380)

(1) nylon cable tie less than 3mm wide

(1) 9mm x 20mm compression spring (wire thickness 0.6mm)

(?) Test Probe Pogo Pins, P75-E2 or P75-E3 for standard pin header through holes, or whatever your requirements are. In any case the pins used should have a 1mm diameter to properly fit the prototype PCB.

(?) Wire and connectors (varies depending on the pin configuration/use)

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Step 1: Print the 3D Parts

The STL files are available on Thingiverse: https://www.thingiverse.com/thing:3592328

Settings for both Base.stl and Lever.stl :

- Material: PLA

- Layer Height: 0.2mm

- Infill Density: 20%

- Support: Touching Build Plate

- Build Plate Adhesion: Skirt

Designed using Autodesk Fusion 360

Step 2: Cut and Drill the Prototype PCB

The fixture requires two PCBs, an upper and lower board.

Cut the PCBs by mounting them in a vise and deeply scoring both sides as shown in the photos. The length of the PCB depends on your pin configuration. The connection to the lever requires/uses 4 rows of holes. The rows after the 4th row are available for probe pins.

The 1 x 6 pin version has 6 rows (or if you’re starting at the end of an uncut prototype board, 4 rows.) The 2x3 pin version uses 8 rows.

Once deeply scored, snap the board while it’s still in the vise. Remove the cut pieces and sand the rough edges smooth. I used 220 grit sandpaper mounted to a block of wood.

Using a 3mm drill bit, enlarge the two outside holes on the 3rd row. Make sure the drill stays centered in the original hole.

Step 3: Assemble the Fixture Without the Spring

Mount the upper and lower PCBs to the lever using two M3 x 12mm screws and nuts. Insert the pogo probe pins into the PCB in the desired pattern.

Optional: To make the pins easier to insert, I slightly enlarged the holes using a 1.1mm PCB drill bit. This removes the through hole plating (which you don’t need anyway.)

Flip the lever upside down to keep the loose pins from falling out. With the lever upside down, mount the lever to the base using an M3 x 20mm screw and nut.

Step 4: Solder the Probe Pins in Place

With the fixture right-side up and the lever parallel to the base, tap the pins so that they just touch the base. The pin heads should extend about 1mm above the top PCB. Solder the pins in place. Remove the lever from the base, flip it over and solder the pins to the lower PCB. Optionally remove the flux from the solder joints using PCB cleaner. Reattach the lever to the base, but this time include the spring.

Step 5: Attach the Wire Harness

Strip and pre-tin the end of the wire harness that will be soldered to the fixture PCB. I trimmed the tinned ends to about 1.5mm. Feed the wire harness through the 5mm hole in the lever. Solder the wires to the appropriate pins (no solder required if both sides are already tinned.) Feed a nylon cable tie through the strain relief holes and tighten the tie around the wire harness.

Done!

Step 6: Part Sources

Pogo Pins: https://www.aliexpress.com/item/100pcs-Gold-Plate...

Prototype PCB: https://www.aliexpress.com/item/10PCS-Double-Side...

Springs: https://www.aliexpress.com/item/20pcs-lot-0-6mm-0...

I didn't use the springs noted above. I had an assortment of springs that had a 9mm spring that was about 35mm long. I cut it in half to make two fixtures. I have ordered the 20mm springs pointed to in the URL above.

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    2 Discussions

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    Alex in NZ

    5 months ago

    This is a brilliant hand-held bed-of-nails. So neatly done! Thank you very much for sharing your work.

    1 reply