After a PCB has been designed, fabricated, and assembled, the last step before delivery to the customer is in-circuit testing & programming.  And like much of the rest of engineering, “Designing for Test” (DFT) has specific rules that engineers must follow.

To learn more about common DFT mistakes, we spoke to Joe Selvick and Duncan Lowder at FixturFab.

Mistake one: Forgetting to make test points available on a PCB.

Engineers have a tough job and sometimes they forget little details.  One mistake Joe and Duncan mentioned was that engineers will sometimes forget to include test points for a particular net.  In the case of leadless parts such as BGAs and QFNs, there’s quite obviously nothing that can be done other than to respin the board.  But even with leaded parts, it’s never a good idea to target a spring-loaded test pin at a soldered joint.  The test pin could slip between pins and cause a short, or get bent on a steep solder fillet.

Tip #1:  Design a 1.27 mm diameter (0.050”) test pad for every net that you need to test.

Mistake two:  Placing test points too close together

There are a variety of spring-loaded test pins available on the market, but there is a lower size limit beyond which it’s not possible to place them side by side without the risk of them shorting out.  That limit is 1.27 mm (0.050”) on-center spacing for 50 mil diameter probes.  If you place test points closer than this, it’s difficult to make a test fixture that can hold the probes without the risk of shorting.  And probes smaller than 0.050” diameter have shorter lifespans.  Remember, manufacturing has variability, and the performance of a component on day 1 is far from its performance on day 100.  Give yourself a bit of wiggle room and design up from this lower limit whenever possible.

Tip #2:  Place test pads at least 50 mils apart (on-center spacing) for 50 mil probes.  100 mil spacing is preferred.

Mistake Three:  Not including mounting/tooling holes in the PCB

Fixtured test probes are designed to touch test points on a PCB.  But the PCB needs to be accurately positioned prior to making contact to prevent short-circuits or damaging components.  That is typically done with two alignment holes in your PCB that will intercept guide pins in the test fixture.  The alignment hole diameter is typically a plated through-hole with a diameter of 3.0 (+ 0.05, – 0.00) mm.  Place the holes far apart from each other, but keep them at least 3 mm / ⅛” from the edge of the PCB.

Tip #3:  Include at least 2 3mm alignment holes in your PCB to accurately locate it above the test fixture.

Mistake Four:  Placing test points on both sides of a PCB when they’ll fit on one side.

Engineers will sometimes place test points on both sides of their PCB when they don’t really need to.  This drastically increases the cost of the complexity of the test fixture and unnecessarily increases cost.

Tip #4:  Place test points on a single side of the PCB whenever possible.

Summary

Every test fixture designer will have slightly different requirements.  Check with your fixture designer for their exact specifications.  If you’d like to learn more, see this list of requirements from FixturFab. Download here.