Design for test - Resistance test in PCB manufacturing - Page 1
Forum section: electronics / projects

author: fedimakni
author_rank: Regular Contributor
timestamp: May 04, 2025, 12:50:49 pm
content: Hello,When manufacturing a board, I noticed that the manufacturer can make a resistance test between different test points you specify in order to verify if your board match your manufacturing requirements or not.It is a pass/fail test where you provide 2 tables containing test points location and the resistance value between test points you want the manufacturer to verify.My question is how you can estimate the resistance value in order to prepare the table?Some cases are easy such as pull up or pull down resistor value so from the schematic itself you can get, but for example if the NET is connected to an IC and it is sometimes hard to know the internal resistance so how you can figure that out? especially for resistance between power net and ground.Can you share how you do it?I think the easiest way is to manufacture a board confirm that it is working fine and then measure the necessary combination and fill the table, but is there a way to estimate it from the schematic or PCB? (I use altium)Note: I attached an example as a pictureThank you.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: May 05, 2025, 11:51:05 am
content: The resistance test is usually done on an empty board that is, right after they finish fabricating. It is done so that you can catch any fabrication errors, especially in their etching and lamination processes.I have seen it done on a populated board but there's really no point in doing it normally.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 05, 2025, 12:57:17 pm
content: A board manufacturer can carry out a great many different kinds of test on an assembled board.A resistance check between two points that include active (but un-powered) semiconductors is a pretty crude thing to do, as the resistance you measure doesn't really mean anything. Current could be flowing into an IC, through its protection diodes, partially powering up any number of components on the board and leaking through just about any path back to the other test probe.It really is meaningless to say what the apparent resistance should be, since the behaviour of un-powered or partially powered devices is unknown. It can and will vary between manufacturers of otherwise functionally identical parts, as well as from batch to batch, part to part, and over temperature.That said, if you want to know what the resistance is between two points, and you're just looking for gross errors, the best way is just to build a 'golden' (ie. definitively correct) board and then measure it. Pick a tolerance either side (how much?) and there's your low/high limits.I've not used this method in a very long time, though. For picking up assembly errors, AOI is arguably a better first option as it doesn't require test points on every net. Many pick & place machines can test component values as they're placed, spotting errors and faulty parts before they're even soldered. For complex digital boards JTAG is a good way to go.

author: wraper
author_rank: Unknown
timestamp: May 05, 2025, 01:01:16 pm
content: Quote from: BadeBhaiya on May 05, 2025, 11:51:05 amThis is quite common test for populated PCB. It works very well for finding assembly defects.

author: wraper
author_rank: Unknown
timestamp: May 05, 2025, 01:09:02 pm
content: Quote from: AndyC_772 on May 05, 2025, 12:57:17 pmIt means a lot. While you cannot test IC or transistor functionality, in most cases inputs/outputs can be measured as a diode, thus can be tested for proper connections. Heck, this test can reveal more assembly defects than anything else because many assembly defects do not affect functionality outright. If you're not sure how it should measure, you can run a test on reference board and use measured values for production testing.

author: mtwieg
author_rank: Frequent Contributor
timestamp: May 05, 2025, 01:09:32 pm
content: Anecdotally, my company performs flying probe tests on PCBAs (not ICT fixtures). Or rather, we pay a CM to do it for us.The test program is devised entirely by the CM. I have requested several times in the past information on what they are testing, and what the pass/fail criteria are, but have always been told "don't worry about it, they know what they're doing". This includes PCBAs on which I am the sole designer.My best guess is that they take a "known good unit," run it through a bunch of impedance measurements on some subset of nets (no way they're getting 100% coverage, some of these boards have 1000+ components), and adding some +/-% tolerance on that. Personally when I design the other manufacturing tests, I don't assume the flying probe test provides any coverage.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: May 05, 2025, 01:15:59 pm
content: Quote from: wraper on May 05, 2025, 01:01:16 pmThat's true, rogue solder blobs and bridges are found that way, quite important to do when dealing with a HDI board or anything with small pitch size during assembly.To be pedantic, its a "continuity test" at that point, and its done with a very low voltage to avoid damaging the silicon. It may just be my guess but OP is talking about measuring actual resistance of traces on the PC board. If they're talking about measuring the resistance through the pins of ICs, I have no idea what they're trying to do

author: wraper
author_rank: Unknown
timestamp: May 05, 2025, 01:22:33 pm
content: Quote from: BadeBhaiya on May 05, 2025, 01:15:59 pmYou won't damage an IC unless there is too much current or it's a very special case.QuoteOnly means you have no clue how to test semiconductors in circuit.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: May 05, 2025, 02:10:23 pm
content: Quote from: wraper on May 05, 2025, 01:22:33 pmWell i don't deny that, my experience is limited.What can such tests reveal?

author: wraper
author_rank: Unknown
timestamp: May 05, 2025, 02:41:21 pm
content: Quote from: BadeBhaiya on May 05, 2025, 02:10:23 pmFor example with a diode test you can check if diode protected inputs and MOSFET outputs are actually connected to the traces. Also you can check if they're not shorted to adjacent pads, that they don't leak too much to GND/VDD (due to ESD damage for example). High speed inputs may have built in resistors to GND or VDD which you can measure.