Bed of nails?

[ngf74]

I am designing a board that will use a bed of nails for testing. The test house claims they need a surface mount pad. Would a via filled with conductive epoxy work? I am having to communicate to the test house through a third party, so it is hard to pick their brains.

[Tom]

All test facility engineers will always tell you that you need (or prefer) a "Surface Mount Pad" to probe to.

In most double sided SMT boards this is impossible. Here are some of the reasons from an engineering perspective.

1. Every net must have a test probe from the "Bottom Side". Many traces are routed on the "Top Side" from one SMT pin to another. In order for this net to be testable, you need to drop a via to the bottom side and then from that via attach a trace to a SMT Test Pad. Engineers do not like this concept because adding a via to a trace that doesn't require one for solvability introduces Inductance. Then when you stub off the via to the SMT pad, this causes an antenna (which will cause noise).

2. The best technique is to have a trace routed on the Bottom Side and place a SMT Pad in a convenient place directly on the trace (no stub). However, many traces start on the Top Side and route on inner layers and never make it to the Bottom Side.

3. Obviously you cannot place a test point pad underneath a Bottom Side component. Since most of the PCB traces are not routed on the Bottom Side, you will be forced to add extra vias somewhere along your trace to avoid the Bottom Side components. Then you'll have to stub off this via to add a SMT Pad. Once again causing unwanted inductance and creating an antenna. If you do this to a couple of traces it's not so bad, but doing it to every trace will give the engineer a nightmare.

4. I would definitely get the engineer involved in the test process since he/she will be the one that has to debug the proto-type. Tell the engineer what the test shop requirements are and mention what you have to do to meet the requirements. The engineer will probably be against what you have to do to meet those requirements.

5. Bottom line is that you will have to simply use your vias and through hole pins as your test points and the Test Shop will have to live with that. Plugging the vias with Dupont Silver Epoxy will help but it will also increase you board cost.

Good Luck!

[ngf74]

Thanks Tom. Now I just have to get my engineer to push the test house.

[robert Tarzwell]

Your testing company is giving themself a easy target , and a wrong answer. My company had three types of testers and all could test a via (open) just a easy as a smt pad. I suggest you inform the testing company that thats they way the boards will be and make the board as you require it . Note the via must not be coverd with soldermask and be at least 10 mils in diamiter.