What cause a PCB track to break - Page 1
Forum section: electronics / projects

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 13, 2020, 09:38:29 am
content: I have assembled 1000 pcs of PCB. 6/1000pcs have track broken (bottom layer) as pointed in the attached picture. The pad with track connected to an electrolytic capacitor. There are two other capacitors have the same footprints with not track break.The assembly using wave soldering machine. Temperature 260 C. The board is FR4 1.6mm thickness 1/2 Oz. ENIG surface finish. What could be the cause to avoid track break when go to 300,000 pcs??

author: Renate
author_rank: Super Contributor
timestamp: September 13, 2020, 11:27:03 am
content: Got a microscope view?

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 13, 2020, 11:43:03 am
content: Yes, nothing unusual in the non-soldered boards

author: chris_leyson
author_rank: Super Contributor
timestamp: September 13, 2020, 11:46:39 am
content: Try tear drop pads. I've just discovered that Cadence 17.4 ultra bundle doesn't this feature enabled

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 13, 2020, 11:52:35 am
content: I didn't use tear drops in last project. I will in future. However, I am afraid there is something else that I should know, risking in 300,000pcs is high!

author: Renate
author_rank: Super Contributor
timestamp: September 13, 2020, 01:03:36 pm
content: How about making the drill hole smaller 0.9 mm for the 0.8 lead?It would allow the substrate to accept more of the stress.At least that's my idea.How about a view of a broken one?

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 13, 2020, 01:28:55 pm
content: I couldn't find a PCB with clear view of the break. This one had a break. However it has be reworked.

author: Alti
author_rank: Frequent Contributor
timestamp: September 13, 2020, 02:43:44 pm
content: This is quite a big capacitor.When it is being charged and discharged, there is a current flowing through the tracks.Tracks have their fusing capabilities and it looks like you have exceeded that value somehow.

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 13, 2020, 02:54:41 pm
content: Even if the power supply is capacitive and its current capability is only 17mA?The inrush current in the Power supply has wire wound inrush resistance of 150 ohm.The capacitor is 1000uF UHV1V102MHD. Voltage is 20V.

author: Alti
author_rank: Frequent Contributor
timestamp: September 13, 2020, 03:59:51 pm
content: Quote from: Ashraf Almubarak on September 13, 2020, 02:54:41 pmYes, even if you charge 1000uF 20V capacitor with 17mA (which takes a bit over a second) and then short-circuit it. We are talking about 0.2J and although not 100% of the energy is available externally (due to ESR), it might be enough to heat up Cu track to near melting temperatures (~1000degC). Do the calculations to check if your case is within range.Cu specific heatis 0.38J/gK and we need about 380J/g to reach melting temperatures. So 0.2J/380J/g= 0.5mgIf this mass of Cu gets in between pins of ideal capacitor (ESR=0), it reaches melting temperature. You need more energy if ESR is higher. When Cu resistance is equal to ESR then you can melt half of what was specified, 0.25mg. I do not know the details of your design so it is hard to speculate about the mass of Cu that could be in the short-circuit path. Of course if that discharge takes a long time then some heat is also lost through conduction. But if you do it in <1ms then virtually nothing is lost through conduction.

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 13, 2020, 04:05:26 pm
content: Good point.I never think about that because the discharge is so slow > 2 seconds.The ripple current is so small ~ 5mA.But I will rethink about it again.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 13, 2020, 04:13:28 pm
content: Do these fail during production, or after a long time in the field?Teardrops are usually to make stress from flexing the PCB less, and mostly an issue with connectros and such which have high mechanical stress when repeatedly inserted / removed. Elco's are relatively light components, which would suggest that stress related cracks would not be the most likely cause.That said, such a small track from such a big pad is always a stress riser location, and adding teardrop features would not hurt.If your PCB program does not support teardrops, then an easy workaround is to use a short piece of fat track leaving the pad, and only reduce it to a narrow pad several mm away from the pad. Olimex does this in the Olinuxino A64. You can get the whole project from github, and it's made in KiCad, which is open source so you easily look at how they do it with their PCB's.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 13, 2020, 04:21:32 pm
content: this thread is a food for thought for somebody who havent experienced this before, so googling is in order... there are some hint...this writing (inconclusive) suggests 2 possibilities:A Study in Printed Circuit Board (PCB) Failure Analysis, Part 2-Mechanical stresses (vibration, thermal cycling, board flexure) may have broken a trace that was originally well connected-Insufficient etchback or smear removal (followup after drilling holes in the board) was performed during the PCB manufacturing process, preventing a good bond between the buried traces and the barrelthis writing talks more about thermal stress and expansion resulting cracks:Circuit Boards and Temperaturemaybeteardropscan fix it, maaybe it wont i'm not sure. but the crack line did not occur on transition between via and trace (neck), it happened further away, so whats preventing it from failing at the end of teardrop?imho maybe you can report to your assembler about the situation. maybe too hot wave soldering? so you can ask to reduce it. maybe that area experienced too much bending during that process? so they can provide some support or something etc i'm not sure. in case you decide to just fix the broken unit in house (statistically 300 x 6 = 1800 boards) i suggest you simply scratch few cm long on suspected area and just bodge solder or using jumper wire on it, save time from microscoping finding it and scratch scratch on few tiny areas like a virgin's work, solder is harder to stick to that. if you can respin, beside teardrops, maybe you can try fancy trick or find in the net. my uneducated guess will be like remove solder mask on that trace few cm away from the via so wave soldering can fill that, or make the trace curvy so trace has more copper and geometry to expand and contract. it maybe just a blind faith ymmv.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 13, 2020, 08:06:09 pm
content: Quote from: chris_leyson on September 13, 2020, 11:46:39 amIs that serious? Maybe they moved it around? Is it no longer under route -> gloss -> add fillet?

author: vk6zgo
author_rank: Super Contributor
timestamp: September 13, 2020, 11:10:11 pm
content: Quote from: Renate on September 13, 2020, 01:03:36 pmYou just incurred the hatred of any Technician who has to change a "through hole" component on such a board!Many boards , especially those from EU, are made with holes that "are just big enough" so removing a faulty component will often damage the plated through hole.

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 14, 2020, 07:09:20 am
content: The failure occur immediately after the wave soldering process.I will make tear drops for all TH pads in the next version.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 14, 2020, 03:36:02 pm
content: If they fail immediately, it's obviously not a fatigue related thing (which woud be unlikely with light weight electrolitycs anyway).looks like you'd better put all produced boards under a microscope. I suspect that a lot of the boards which do not fail immediately have half cracked tracks which will propagate in their lifetime.A possible / partial solution may be to adjust the solder mask, so solder is extended over a part of the track.There are dedicated periodicals / magazines / websites about PCB manufacturing. Such a specialized site may be a better place to go to with questions like this.

author: Ashraf Almubarak
author_rank: Contributor
timestamp: September 14, 2020, 05:56:25 pm
content: I am afraid no way to remove the solder mask for produced boards. It is already shipped away and operated. Thankfully, no one reported an issue of this kind.In next batch I will add the tear drop, increase the copper thickness.