Strength of a solder joint vs how oversized the hole/slot is? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 29, 2025, 11:09:34 am
content: I am looking for information about strength of a solder joint vs how oversized the hole/slot is. "If you oversize it by X mm then it will become Y percent weaker"There are two common scenarios where, for the sake of supporting several types of parts from different sources, that I'm looking to oversize features on:Square pins in round holes (misc. connector clones, terminal blocks, etc.)Blades in a slot (DC barrel jack and similar)Has anyone ever encountered a failure of a improperly sized through-hole solder joint?

author: IconicPCB
author_rank: Super Contributor
timestamp: April 29, 2025, 11:28:59 am
content: Sure ...Major problem is not with oversize opening, especially so in case of s need to effect a repair. the opening should be at least a few thou oversize in order not to damage hole wall plating while removing a component pin. Oversize holes are not going to result in reduced strength solder joint, cooling rate of the solder joint is more likely to have a deleterious effect on joint strength.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 29, 2025, 03:58:44 pm
content: Short answer is it's probably not worth wasting a brain cell on. More important is cleanliness and age. There's a reason SMT parts are packed the way they are and have a use-by date. As above, process matters a lot too.

author: incf
author_rank: Frequent Contributor
timestamp: April 29, 2025, 04:12:40 pm
content: Seems quite reasonable.But suppose a 0.1" type male pin (25 mil square) was somehow held and soldered into a 2mm or even a 3mm round hole (normal hole is ~1mm). Would there be any noticeable differences in strength or defect rate from the solder joint alone? Sounds like maybe the answer could possibly be "not really"I'm not sure I fully understand the cooling rate comment. Unless the part is literally a big heat-sink or something. I guess the worry is that they fail to melt all of the solder.I also suppose the solder will just fall out of the hole when molten if the gap is big enough.How big does the gap have to be for the solder to fail to bridge across?

author: Niklas
author_rank: Frequent Contributor
timestamp: April 29, 2025, 06:53:51 pm
content: Too small or too large gap between. The hole wall and the component pin can cause issues with solder fill percentage with wave soldering. Less solder would be more prone to fail.I general you would like to avoid mechanical stress on solder joints as it eventually will cause fatigue cracks.

author: coppercone2
author_rank: Super Contributor
timestamp: April 29, 2025, 08:56:58 pm
content: For silver braze you get a sharp strength peak at 4 to 5 thousandths or so.To a degree you can pick an alloy that has a peak strength in the gap size you are filling, theoretically.Practically you want to consider1) is solder fluid enough to flush into a tiny gap with the desired flux2) can you use a solder pump on it to suck out the solder to do a repair3) can you fit a part. It helps if you have a part former, for common packages you can get pliers that form the leads to high precision, or bend chips leads to fit nicelyIIRC if you actually look at the curves the PCB are basically totally out of spec with how people make them for bond strength (massive wasted potential in both strength and resistance)It is a very important consideration for proper brazing, if you want to meet a tensile strength requirement. Its controlled to such a degree that your fixture accounts for materials expansion from heat to get the correct spacing.Usually people use over-kill in this situation because you will find that doing things right with solder or braze isnot practicalor at least it gets so much more complicated then you thought it would be with your skills and equipment.here it is for silver. Find me a curve for eutetic solder pleaseHere is a random graph that might be relatedand you might ask, what if its not perfectly true surfacesSo your blades in a hole are basically rubbish quality (as far as optimal goes, because of stress.. but it might be perfectly reliable.. I am say as far as solder theory), way undersize pins in a hole are probably weak but its evenly distributed, if its centered. Keep in mind the VIA is its own insanity as far as a 'copper structure' goes. The solder joint is mad strong compared to the what the via might be.A via is a grown pipe that is some how adhered to fiber glass. Vias that have multiple 'flanges' between PCB layers are somewhat strong. Two layer vias on 1.6mm board... if its to be a via (through connection, not for part), nasa actually wants you to solder a wire (like a S shape) through the via to anchor it to the traces, because if it only has two flanges, it can shear out and then you have a copper pipe rattling around in a electrical assembly in a rocket... the bond that it has to the fiberglass, and the 'overgrowth' of copper that bonds it to the very thin thickness of the PCB copper is too weak to be reliable for what they need, so you end up with a extra wire. It ends up being treated like a tree, deep branched roots is more trust worthy.weak and needs reinforcementgood enoughSo even if you can optimize the solder joint, you will find that it might not be the 'weak link' in the failure chain of a pcb. But its important to know for all metal assemblies like pressure vessels, pipes, complex structures, etc.... because regardless of all the welding, brazing, etc you can do... a simple solder joint might be appealing for many structural reasons, but it has to be carefully considered

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 29, 2025, 11:52:04 pm
content: The graphs that the second copper cone posted look very much like the graphs I've seen of glue and epoxy. Peak strenght is with a quite thin, but not "too thin" layer, and after the peak strength diminishes with glue / solder thickness.But for solder, you don't even want to get close to these limits. Solder has creep which is worse with bigger thickness, and it also tends to crack over time on large fills. Leaded solder also behaves differently then lead-free, and there are probably more differences in details between solder alloys.

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2025, 02:40:56 am
content: NASA probably published a paper on this. If the clearance is too small, then lower compliance causes failure when temperature coefficients of expansion do not match. If the clearance is too large, then reliability is also compromised although I am not sure why, but solder is mechanically weak. I have seen examples of the later where the solder cracked around the pin within the too large hole, and square pins do seem to make it worse.

author: coppercone2
author_rank: Super Contributor
timestamp: April 30, 2025, 03:21:29 am
content: What kind of failure though. A delamination failure or a crack in the bond line (from tight fit up with bad temperature coefficient) ?Like does it have something to do with the bond to the fiberglass or the solder bond? Or a break of copper near the trace via interface?What I noticed is that sometimes the larger parts, when you desolder them (i.e. close fit up in same size hole as small parts) the PTH comes off on the part lead when you try to desolder it.... it looks like the via breaks free from the fiberglass, and breaks the trace to via connection, but its still connected to the part

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2025, 04:13:15 am
content: Quote from: coppercone2 on April 30, 2025, 03:21:29 amFor the too large hole case, the solder itself usually fails about midline between the pin or lead and inside of the plated through hole. The exception is when the solder either does not wet the surface of the pin or pad, or when embrittlement occurs from dissolving gold or something into the solder.QuoteIf the pad comes off of the board, then that is a different problem. Pads with a plated through hole are much stronger.

author: coppercone2
author_rank: Super Contributor
timestamp: April 30, 2025, 04:52:26 am
content: No I mean for the opposite case, where the bond is unreliable because of close tolerance.I was wondering if the too tight case can lead to pad failure. I get the feeling there is nothing "soft" in this case, so instead of the substantial amount of relatively soft solder absorbing some force (maybe from thermal expansion or mechanical shock) with deformation, it yanks on the glue, the trace, and so on. I assume when there is a substantial amount of soft solder, it acts like a cushion of sorts.Also, what I understand is that in the tight case, you might get inter metallic laden solder. They say this give solders strength, but it also makes it brittle. If there is a "deep" amount of solder near the interface, the intermetallic diffusion should have a gradient, so its confined to the perimeter of the 'doughnut', and the bulk of it is still soft. But if its shallow (very thin bond line), the intermetallic might make up the bulk of the solder... in that case all the once soft solder in the joint is contaminated into I guess a harder brittler solder, if the material being soldered diffuses into the chosen solder alloy.My logic is that usually if there is a chain of failure points, and you strengthen softest part (but also get rid of a cushion?), something else usually breaks.does it crack the joint, or destroy the pad? Stiff things usually break differently then soft things. I think that... possibly 100%+ increase in the bonds tensile strength must change things up.I mean for a solder joint where you try to get something like a 0.002 inch or better fit (just enough to allow for flow and wetting to occur). potent silver braze is no problem... it flows like water, solder is mud in comparison IMO, I am not sure how tight you can make a solder joint on a PCB. But I need to say, a soldered copper pipe fitting is pretty tight, alot tighter then a typical PCB lead.... and they do work well (and worked better with lead solder before drinking water regulations). So I find it odd that a PCB has a big problem with tight fits.For bad fit up with pipes, you need to use a different solder (its usually described in terms of gap filling), like nickel containing "bridge-it" solder. With a bigger gap, you want the bulk of the solder to have higher tensile strength, and also don't want the solder to wet too easily to accumulate in unwanted areas... nickel additive apparently solves the strength problem, not sure if it is related to flow control too. And if you handle the bridge-it solder, it bends alot worse... you can feel that its a stronger wire, its difficult to take off the spool, and so on.I think a pipe fitting is similar in concept to a PTH, other then the fact that its glued into a fiberglass, and thin, so I can't wrap my head around exactly why a close fit up is bad for the joint. It also makes me think that intermetallic diffusion can't be too much of a problem, because pipes are often roasted real good, held way longer then you would keep the solder fluid for a PCB joint, and work pretty much OK.. and since its liquid longer (and probobly frequently over heated), there should be a much greater amount of diffusion occurring in copper plumbing soldering then in PCB soldering. Reasonable "molten solder" times for a pipe would be considered totally ludicrous for a hand soldered PCB solder joint.... I bet the tight joints made in pipes are very heavy on diffused copper compared to PCB's (reflow might be another story, but then its usually SMT). A PCB hand soldered PTH solder joint is made and solidified within maybe... 2 seconds?

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2025, 12:02:31 pm
content: Quote from: coppercone2 on April 30, 2025, 04:52:26 amGold produces the intermetallic with tin that is the problem. Gold plated parts intended for soldering with tin solders have a very thin layer of gold which dissolves completely into the solder leaving no gold interface.If the clearance was tight enough that there was not enough solder to dilute the gold sufficiently, then the solder would become brittle. This could be tested against a version of the part which lacks the gold plating. I have seen failed joints where this might have been what happened.In the past the procedure for soldering to heavy gold plating was to remove the gold plating before soldering. One way to do this is to apply solder to completely dissolve the gold, remove the contaminated solder, and then solder the joint normally.