failures from creative 'mechanical' use of PCB - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: November 30, 2019, 09:03:16 am
content: so its cool that you can put together nice 'house of cards' construction with PCB's, but I noticed there is ALOT of failures I have to deal with because people decide to skip on cable harnesses (or flatflex) and wing construction.is there even a safe use of this design technique? I thought it was dead by the 90's but I just found it in a fenix flashlight... even daughter boards... perhaps PCB's should not be having kids every revisionI feel like its surgery at this point, 'scrape' off sinew, suck out the blood in the joint, then get a damn scalpel to cut through some fine bit of solder.. then with some prying..I guess the fundamental misconception is that strength from soldering comes from the 'glob' rather then the lap area interface.. so they think 'wow I can pour a bracket in here' rather then its like liquid double sided tapeit seems to be a bit like cutting little squares of double sided tape (which is sticky on the edge), stacking it up to make a bracket out of it?? (not to mention it shrinks and tears off the pad when you try to fix it).maybe if the PCB had copper along the sides............. it would make a bit of sense.. but this is just nonsense.. i need to make some kinda microscopic stint with watchmakers tools to repair it.. if I even manage to make a fake trace out of copper in something the size of a silver dollarnot that pipes are respected either (cough pipe clamp)not that headers are much better either..

author: Rerouter
author_rank: Super Contributor
timestamp: November 30, 2019, 09:46:40 am
content: It comes down to the designers skill, if your making a house of cards method, the proper way is to have routed slots so one card physically intersects another, this way the PCB material does 90% of the support, then some solder on the seam is just to provide that last 10%,In all pcb's I try and treat solder like silly putty, it doesn't move easily short term, but sharp impacts or prolonged stress can make it crack or flow,

author: ebastler
author_rank: Super Contributor
timestamp: November 30, 2019, 03:31:05 pm
content: Quote from: coppercone2 on November 30, 2019, 09:03:16 amI believe I understand which construction technique you are referring to. But I am confused by your subsequent description of the challenges of repairing a broken joint.Wouldn't you just resolder it -- and, if you want the thing to be more stable in the long run, add some mechanical support bracket or such wherever you can find room for it? Or remove most of the solder, then solder in little pieces of wire bent at 90 degrees, to strengthen the joint?But maybe I'm envisioning a different type of PCB joint. Could you share a photo of the offending PCBs please?

author: coppercone2
author_rank: Super Contributor
timestamp: November 30, 2019, 06:26:43 pm
content: its about the size of a tangerine inside of a flashlight head, I was trouble shooting it and discovered~60% of the solder joints on the vertical riser boards cracked. they used slots too.. its just a poor designthe bracket would be like 1.5x1.5mm (tight pcb)

author: mcovington
author_rank: Regular Contributor
timestamp: November 30, 2019, 07:52:14 pm
content: The first post makes a very important point. People think thick globs of solder are like welding. They aren't. Good solder has little tensile strength.

author: coppercone2
author_rank: Super Contributor
timestamp: November 30, 2019, 08:43:59 pm
content: Quote from: mcovington on November 30, 2019, 07:52:14 pmwhen you consider the fact that solder often pops without lifting the trace, which is simply glued on, this really shows how weak solder isdifficult to notice however, i only noticed when I read about braze theory and how soldering is just low temperature brazing, and actually brazed some metal together with a torch.. when you beat on it you really begin to understand (if its so small, you think crazy forces are at work for some reason), vs the maul hammer and viseunless you have a plated through hole or proper coverage SMD, its essentially a bootleg process.. non plated through holes are just not durable..

author: German_EE
author_rank: Super Contributor
timestamp: November 30, 2019, 09:49:42 pm
content: Seen this problem before and it can work if you do it right.Never join two PCBs at the edges, route a hole through one PCB then slot the second one through the hole. You then make four solder joints, one at each corner. The result is a reasonably strong mechanical joint.However..................As pointed out solder is a weak material, it's easy to pull off a small piece from a reel but try doing that with copper wire the same diameter. Also, your failure point will now almost certainly be the epoxy bond between the copper and the FR4, the same bond that has been weakened from the heat of all that soldering.

author: coppercone2
author_rank: Super Contributor
timestamp: December 01, 2019, 12:02:51 am
content: Quote from: German_EE on November 30, 2019, 09:49:42 pmdoes not work as well as you think.. maybe if you make BIG joints. I figure there has to be a solder bridge close to 100% of the joint length to make this work (even if its just dummy ground connection). And on both sides. Not just a few 'tacks'.