# How to design contact pins for testing SMD board? Only optical inspection possible testing technique?

The point 6.56 and the Teensy's testing are examples about testing, also the pictures below. Now suppose you want to do a SMD board to the size of fingerprint or smaller. The earlier video mentioned things such as "automatic optical inspection" about 4.20. I think the Teensy-style physical testing with pressing to some conductive golden pikes is impossible with very small SMD boards -- or production/testing boards should be larger to have some testing holes that are cut out later.

How can you test some very small SMD boards?

In the semiconductor industry these are called DUT (for Device Under Test) or Load boards and they map from the ide pins to the tester standard connection format. They can be fairly expensive because of size and thickness (to ensure stiffness).

However, you don't need most of that. at the center of a DUT board are cantilevered comb like structures that are very stiff but are spaced at the 100 um pad pitch with 50 um tips. These are relatively inexpensive, no more than the cost of a single pogo-pin.

Here is a super fancy, RF matched and reinforced version:

If you look closely you can see a bunch of very fine fingers. Of course you can only use these under a microscope as you will bend the crap out of them if you hand place them.

here is what a old style DUT board looks like. A lot of the design is the interface to the system electronics, which you don't care about.

Originally pogopins were developed to contact the DUT board to the tester.

Here is a picture of wafer probe or probe fingers taken from UP patent 7688097 B2

This is still very fancy. I have seen these "combs" go for only a few \$100's

Here are some picoprobe probecards from GGB industries which I've used.

And finally if you search on probecards you get lots of hits.

For very small boards, you can test before depanelizing. Bring the test points out into the panel, through an inner layer so the depanelization process doesn't tear up the rest of the traces.

Or just use smaller probes. Semiconductors are tested before sawing up the wafer, using machines that are maybe 60 dB more expensive than pogo pins in a circuit board.

• Can you clarify the point "that are maybe 60 dB more expensive than pogo pins in a circuit board"? 60 dB? – hhh May 2 '13 at 2:19
• Why yes, they do sound more expensive. :P – Passerby May 2 '13 at 2:44
• 10log(B/A)=dB (assuming power..) 60dB would be 1,000,000 times more expensive – Kevin H May 2 '13 at 3:52
• +1 for "60 dB more expensive" – Anindo Ghosh May 2 '13 at 3:53
• If money were voltage, that would only be 1000x more expensive. But money is power, so... – user_1818839 May 2 '13 at 8:43

Pogo pins can be found in super small sizes. A quick search shows 0.52mm pitch. That's almost as tight as 0.4mm pitched ICs, grain of rice sized chips.

They can be tested off board, as @Markrages has mentioned. The panel would have test points on the cutoff parts.

Visual (Camera) inspection is also possible, for any single or dual layer board. Anything with internal layers will not be able to use simple camera inspection.

Xray inspection can be done. This is often needed for bga ICs, where you can't inspect the soldering else wise.

But final testing is simply programming and running a test sequence.