PCBA Test Process

[lowderd]

Hello!

I'm currently working on defining the test process for the PCBA's in a medium volume (10K-50K) product. I'm leaning towards the following:

- AOI on every PCBA (duh)
- AXI on every PCBA with BGA's
- Functional Test on every non-trivial board

From talking with the CM, they're pushing to implement ICT on several boards, my biggest issue with this is that the boards are lacking testpoints on a lot of nets, resulting in test coverage of 45%-50% on the PCBA's they would like to implement ICT on, this leads me to think that we should stick with only performing a Functional Test, instead of spending additional capital on ICT fixtures.

My main questions are:
- How useful is ICT if a majority of nets aren't accesible?
- Should I fight to implement a new board revision with additional testpoints to improve ICT coverage?
- Is Functional Test enough for mass production?

Any recommendations or advice is welcome! 

Thanks!

[Nauris]

- AXI on every PCBA with BGA's

Is that really necessary?

- Functional Test on every non-trivial board

Trivial boards can have faults too...

From talking with the CM, they're pushing to implement ICT on several boards, my biggest issue with this is that the boards are lacking testpoints on a lot of nets, resulting in test coverage of 45%-50% on the PCBA's they would like to implement ICT on, this leads me to think that we should stick with only performing a Functional Test, instead of spending additional capital on ICT fixtures.

Functional test + special test firmware to do as much self testing as possible would be my first suggestion. ICT is that kind of solution you can put together without much engineering so contract manufacturer likes to suggest that. It is questionable how much faults it detects that a well tought functional testing doesn't see. I would not use it but of course it depends heavily on what kind of board you are testing...

- Is Functional Test enough for mass production?

For normal commercial/industrial use, sure

[Siwastaja]

Properly designed functional test should and could detect basically everything that the bed-of-nails or probing test would, plus a lot more. After all, if you test each and every feature, what's there left for the user to find faults at?

Neither of these test reliability issues due to poor solder quality, contamination, etc. I guess it's AOI and random xray or destructive sampling for one every 100 units or so for that.

IMHO, testpoint probing ICT is overrated (it's so easy to just suggest such automated, catch-all, off-the-shelf solution), and I believe that a combination of AOI and well-designed functional testing finds the real-world issues much better. Functional testing has the largest scope.

For example, I designed, outsourced the manufacturing, and production tested myself a batch of fairly complex boards with an STM32 MCU and an I2C acccelerometer. That particular oldish STM32 MCU has a notoriously buggy I2C implementation, and the code had all kinds of workarounds to get the prototypes working reliably. You would expect that once the code for such digital MCU peripheral works (and the signal integrity on the prototypes is OK), there couldn't be software-related surprises when just producing more of the same.

But, strangely, in about 20% of the boards, the I2C sensor was nonfunctional. Of course I first suspected the Chinese soldering of the miniatyre LGA part, but nope, it was in software. Adding extra reset-turn-back-on-sequence as defined in errata sheet for another MCU of the same family (not the affected device itself - I always read the errata!), all units were working.

Only functional testing revealed this.