Are smaller via drill diameters more prone to failure than big ones? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: March 10, 2025, 09:15:12 pm
content: Simple question:On a large 1.6mm thick 4-layer PCB with >1000 signal vias, in general does via drill diameter affect reliability? Imagine a PCB that remains in the field for 20 years.Consider following via drill sizes:0.3mm via drill0.25mm via drill0.2mm via drill0.15mm via drillhow do they compare in terms of failure modes and reliability?Mostly concerned with vibration and corrosion, but other factors like PCB yield from the factory is of interest too if it is significant.*edit: to make it very clear we are talking about the drill size, not the annular ring size

author: coppercone2
author_rank: Super Contributor
timestamp: March 10, 2025, 09:22:08 pm
content: Well I can tell you with electroplating, you need to adjust the copper plating formula substantially for plating holes uniformly, like vias.I believe its safe to say a smaller hole is more likely to have a botched electroplate then a big hole. The chemistry gets precise and complicated when there is a bad aspect ratio (you need additives in the correct amount). Theoretically you can adopt the plating chemistry to work at specific aspect ratios, but IIRC its always worse then trying to plate a flat object near another flat object. Its like modified to work, not modified to be better.For understanding aspect ratio:1) board thickness stays the same2) hole gets narrower3) final hole has a insane aspect ratio (like lawrence of arabia)As for what the manifestations of aspect ratio modified chemistry are (modifying the FLOW is an option but the CHEMISTRY gets modified too!), it might be less even and such. The main concern I believe is strain, because electroplated metals have terrible strain in them, which is why they crack. You tailor the chemistry to reduce strain but I suspect that optimizing the chemistry for a bad aspect ratio is not doing strain optimization any favors (its separate modifications to the bath to handle each problem, multiple changes and additives which can be at odds with each other). You NEED to change the chemistry for large aspect ratio because you want the via to plate nicely internally (uniform cylinder) instead of being a funnel shape or possibly just closingAs for how the strain works1) intrinsic to the crystal structure of the deposited copper (atom-scale)2) some mechanical thing going on with temp-co because the plating result is physically non-uniform i.e has bumps on top and bottom)(macroscopic)And there is other modifiers too, for instance, not only must it be even, but it must be shiny (for RF vias). Thats yet ANOTHER direction you can take to modify the bath. Ideally each addition/change only effects 1 property, but in reality I believe they are at odds with each other. So you do strain, levelness, brightness and probobly morePractically in terms of actual outcomes rather then theory, I have no idea, you would need to find stats, but anyway having some background info makes stats easier to understand.Now there might be a mechanical consideration here too, kind of like how 6-32 tap's are shit and smaller taps are easier to make (4-40 safer then 6-32). If it turns out that a particular hole dimension has some kind of bad mechanical relationship with the factors at play, a smaller hole could be infact more reliable, despite electrochemical difficulties,. I don't know if there is a mechanical consideration or what it would be but it seems plausible.

author: ConKbot
author_rank: Super Contributor
timestamp: March 10, 2025, 10:46:48 pm
content: Look at your board manufacturers specs for IPC 6012 class 2 and class 3 vias. If they say limit it to 8:1 for class 3, skip on the 0.15mm vias though 1.6mm boards. I don't have any papers for you on a cursory search, but reliability is specifically what the class 1/2/3 specs are for.

author: thm_w
author_rank: Super Contributor
timestamp: March 10, 2025, 11:42:31 pm
content: If you are worried about corrosion in the field, then any smallish via can easily corrode if there is: high humidity, high salt content, contamination, etc.I'm sure class 3 IPC will help, but the only real solution would be a waterproof enclosure (with some heater or moisture removal) or conformal coating the board.Vibration I can't see matters, thats more related to heavy components, physical board support, etc.

author: coppercone2
author_rank: Super Contributor
timestamp: March 10, 2025, 11:53:21 pm
content: That also makes sense because if its smaller then you need less reactants to destroy it

author: Sensorcat
author_rank: Regular Contributor
timestamp: March 11, 2025, 12:19:50 am
content: Since the aspect ratio is so important, there are often two types of vias in a 4-layer board: Those that go through the whole board, and those that only connect two adjacent layers that belong to the same prepreg, i.e. one outer layer and the inner layer next to it. If the prepreg-vias are offered by the manufacturer, they help with increasing the effective layout density a lot. Obviously, it takes more steps to manufacture such a PCB, which makes it more expensive. OTOH, the aspect ratio of the such a via is no problem any more, since prepregs are usually <0.2mm thick.

author: thm_w
author_rank: Super Contributor
timestamp: March 11, 2025, 12:26:29 am
content: https://www.circuitinsight.com/pdf/design_construction_affects_reliability_ipc.pdfQuoteAll of those sizes should be easily tented, I have no issues with tented vias, but, I also don't expect stuff to last 20 years. Some concerns here:https://resources.pcb.cadence.com/blog/2024-tented-via-advantages-and-disadvantages

author: bostonman
author_rank: Super Contributor
timestamp: March 11, 2025, 12:52:36 am
content: Correct me if I'm wrong, but believe when I was required to take solder training every two or three years, the NASA soldering video we watched required vias have wire run through them and both ends get soldered to traces in the direction of the traces.Most likely this was in the older days when vias were used less and/or larger ones were used.

author: coppercone2
author_rank: Super Contributor
timestamp: March 11, 2025, 12:54:21 am
content: https://workmanship.nasa.gov/lib/insp/2%20books/links/sections/501%20General%20Requirements.htmlVias in multi-layer PWBs do not require the use of filler wire (Z-wire), nor shall they be solder-filled. No dedicated effort shall be made to remove solder in unused holes.Looks like a aspect ratio problem. Two sided = long hole, multi layer = lots of anchors. The idea that a long high crystal strain copper pipe is going to hold is dubious (the only 'reliable' anchor points are on the top and bottom sheet, usually separated by 1.65mm , but if it has a number of flanges where its grown TO rolled copper (a superior material) it looks OK. I assume it depends on substrrate thickness. Keep In mind, the copper is grown on INK or electrodeposited palladium or other ions that PERCIPITATE on fiberglass. However, the small 'rings' that touch the rolled copper sheets have more of a "electroformed' structure because I think its kind of like having a true SEED crystal instead of some RANDOM palladium atom or METAL LIGAND. It's like gluing to paint vs bare metal. And rather shady paint at that.

author: incf
author_rank: Frequent Contributor
timestamp: March 11, 2025, 12:55:06 am
content: QuoteCan we define these two failure modes?I interpret both as either barrel peeling away from the hole, or it splitting and breaking the connection between layers."interconnection problems" seems vague - maybe it means something specificedit: found it

author: coppercone2
author_rank: Super Contributor
timestamp: March 11, 2025, 01:03:22 am
content: how about barrel pull out where it fails and you get a thin little copper pipe rattling around in the chassis?\I feel like I did alot of repairs on what NASA considers unacceptable

author: coppercone2
author_rank: Super Contributor
timestamp: March 11, 2025, 01:08:03 am
content: I think that weird foil for some candies is electrogrown and that is what a via is, there is a good reason why they are cautious about it. It reminds me a bit of 'perforated paper'.