Faulty PCBs from the factory? Weird issue, and I'm pulling my hair out! - Page 1
Forum section: electronics / general

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: October 12, 2020, 06:55:07 pm
content: I have a rather complex 4 layer PCB (not my design) that handles multiple high voltage DC solenoids (100VDC). Well, the boost is 100VDC to open the solenoids (they are under high pressure), then 24v drive voltage. It is actually remarkably similar to diesel injector circuit behavior, and I wouldn't be surprised if it were modeled/copied from one.Anyway, things have been going along swimmingly, having a near perfect success rate in assembly/testing.All of a sudden, I'm getting odd issues. The circuits use N channel MOSFETS, so there is a boost circuit to bump the 24v up to about 34 volts so it can use N channel MOSFETS as high side switches. For some reason, that boost voltage is sitting about 8 volts low. Hmm, I wonder to myself.I've spent 2 days investigating the PCB, and I've ran across some readings that have me thinking manufacturing flaws in the PCBs.Mind you, this happened mid-lot of PCBS; all from the same order and arriving in the same package in the same shrink-wrapped pack.Since the PCB handles the return (low)side voltage (has sensing circuits for error detection) from the solenoids, I have multiple traces (internally, unfortunately) that run from the connector to a THT TO-220 MOSFET. From that MOSFET, there is a BAV70 diode and some more circuitry to trigger said low side MOSFET.Upon piece by piece disassembly and inspection of the board, I've found that multiple of these return traces have voltages at the low side MOSFET drain pin circuit, even with the power switch off! With the power switch off, the 100v boost is disabled. When I turn the power switch on, the voltage jumps up a bit more, from 0.4 volts all the way up to 5. For reference, a fully functioning example never has more than .003 volts at this pin.Now here is the fun part: I took out all the MOSFETS and their BAV70 diodes, and still get these same voltages! These traces go from the connector (NOT connected on the test jig; the connector slots are just empty) to those low side MOSFET PTHs. There is NO connection to any components.I had two of these PCBS acting the same way before I halted production to investigate.I can't think of anything except manufacturing flaw in some of these boards. I've tested with my Fluke DMM, and I can't find shorts from these traces to any other circuits, but maybe the short only shows itself when powered on due to the higher voltage being all over the board? Since the system runs at 24 volts and I'm only getting 5 volts (max) "shorted" into these circuits, perhaps they got by testing before they were shipped to me, but were always faulty?These PCBs cost me $100 to assemble and throw away if they don't work, so I'd rather not keep throwing good money after bad, so I really need to figure this out.Thoughts?Wasn't sure if this should go in Manufacturing or Repair, but since this is an issue with new PCBs, I hope I picked the right forum.

author: IconicPCB
author_rank: Super Contributor
timestamp: October 12, 2020, 08:27:32 pm
content: Sounds like ionic contamination.Grab a multimeter and check for possible leakage paths between points of anomalous readings and probable sources of excess voltage.Your multimeter ought to be set for either high resistance ( say gigaohm range ) or conductance range.If You have a local bare board testing service capable of cranking up the test voltage up to a hundred volts ask them to test the unpopulated boards prior to populating them.You will have to provide them with the gerbers/drill file in order that a netlist can be extracted.Baking them will not eliminate contamination.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: October 12, 2020, 10:51:26 pm
content: Are you using the same PCB fab house, or is someone new making the PCB's with this batch?If it's a new place here is a story to think about. At my old work we had to switch a PCB fab house for a board which we had been manufacturing. When we assembled the new batch of boards all failed in the same way. Luckily our technician was pretty familiar with the design and was able to pinpoint the fault to a broken track, which was between a VIA and a pad. The VIA was just touching the pad, which was electrically correct as there should have been a connection between the two (even though there was no track between them). When the new PCB fab house saw this they flagged it as an error and broke the connection, without telling us ;^)

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: October 13, 2020, 03:25:04 pm
content: Quote from: ANTALIFE on October 12, 2020, 10:51:26 pmSame fab house, same batch actually!I'm going to assemble a few more, see if I still have issues. I can't figure out what else it could be other than PCB manufacturing flaws.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: October 13, 2020, 11:13:07 pm
content: Quote from: Rat_Patrol on October 13, 2020, 03:25:04 pmAh bugger! If it is ionic contamination like IconicPCB suggested, then see if cleaning the surface of PCBA helps (to try and get rid of surface contaminants if they exist)

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 14, 2020, 12:02:16 am
content: Quote from: IconicPCB on October 12, 2020, 08:27:32 pmsounds like it.. i was making a simple diy pcb and i can make sure there is no short, when connected to mains, a big bang! ironically the proper traces where the current should go was fine, the bang happened on copper pour next to them, and again i was 100% sure there is no connection to the copper (no net) pour, i was using sort of uncertified flux during soldering and then cleaning with alcohol, substrate break down and carbon formation is for real. from thereon i will not take lightly the high voltage, the pcb and clearance and creepage...https://resources.altium.com/p/pcb-leakage-current-and-breakdown-in-high-voltage-design

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: October 15, 2020, 08:21:51 pm
content: I moved on to more boards, and the new ones are working fine.Dunno what was wrong with those few, and it probably isn't worth the time to dig into them further since production is back up and going.Just a follow-up, albeit with little info.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 15, 2020, 09:49:09 pm
content: what if the boards all in the field operating already and this problem comes back?

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: October 15, 2020, 11:19:07 pm
content: Quote from: Rat_Patrol on October 15, 2020, 08:21:51 pmSomething is going on, and you don't know what it is. The problem may appear in future batches.I suggest you keep some of the faulty boards, and preserve relevant information (lab book entries, test results, consignment notes, etc.). That information may help you next time, if there is a next time.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: October 16, 2020, 03:15:25 pm
content: Oh yes, I'm keeping the faulty units for possible further investigation at a later date, if nothing for reference.Every PCB is serialized and tracked, along with production notes, in a database. The failures on these boards showed up immediately on testing. All PCBs must get through stress tests before shipment, so I feel confident what I put out is GTG.

author: Niklas
author_rank: Frequent Contributor
timestamp: October 21, 2020, 09:21:22 pm
content: What is the insulation distance between the plated holes and the surrounding copper on the inner layers? If the distances are small, there might be a possibility that a combination of drill placement tolerance, layer to layer alignment, quality of drilling and copper wicking can cause this type of intermittent issues.