That is my first time asking an assembly house to produce 200 units of a PCB (and not the usual 3 or 5 PCBs). The assembly house came back to me saying that the testability of the board was bad and that they need to have 1.2 mm pads on the bottom side of the PBC for all nets… They require such large pads because beds of nails are a much more economical option than flying probe for 200-400 units since they can do it in-house.
The PCB we are talking about :
- 4-layer PCB (a few power supplies, a battery charger, connectors to screen and other accessories)
- only surface-mount components
- all components on top, 60 euros/PCB (bare PCB + assembly) for 200 units.
- we are targeting 200 units at first, and probably 400 units total. (It is already revision 2 of the same board).
- consumer product
I'm a bit suprised by this request from the assembly house. Using large pads or adding vias on all nets seem to be extreme to me. I'm also a bit surprised not to have noticed such testing pads on other products (if that's even possible to fit them sometimes?).
My questions are :
- Is testing of all nets required for consumer products? Is it common practice? I was more thinking about some higher level tests (if the screen works, if all buttons works, if the battery charger delivers the good amount of current, etc.).
- Is adding 1.2 mm pad on ALL nets common practice for PCB?
- is it normal that the assembly house increases the assembly cost if they don't do the bed of nails? The cost of the bed of nails is almost as much as burning (fully) 40 PCBs…
- I may be a bit naive there but they seem to say that they want to test all nets in order to check if resistors/capacitors/others are good. Is it so common that an assembly house mixed up components on a few boards?
Best Answer
Putting in those pads first time round is called DFM (Design For Manufacture). And it sounds like they're giving you a good price on the bed of nails.
Either get this flow working (it's perfectly normal) or ask your next fab house if they have a flying probe tester in house.
One way to economise is to use bed of nails ONLY for nets that don't already have connectors attached, and to break out test cables from those connectors to their ATE.
I do this for my own projects for small runs, where I don't mind adding the time spent plugging/unplugging connectors to the test time. It can save money on the fixture. but it's a pain and makes the test more expensive. You can ask them if this is an option, but I wouldn't be surprised if they refuse.
Yes there are other test options : as well as flying probe, there are 2-sided versions of bed of nails which saves the need to bring all nodes out to the test side ... but you don't want to know the price!