Reducing stress on through-hole resistor solder joints - Page 1
Forum section: electronics / general

author: Mark
author_rank: Frequent Contributor
timestamp: February 20, 2019, 01:04:44 pm
content: Hello all,I have a small assembly with two PCBs forming a sandwich. There are rigid components that mount between the PCBs and keep them 17.5mm apart (these parts are bolted to the PCBs).There are some through-hole resistors that are also mounted between the PCB sandwich (a resistor has one lead connected to each PCB).At the moment there are plated through-holes for the resistors but I'm worried about stresses on the solder joints as the parts expand and contract with temperature. The resistor body is 10mm, so there is only a little of the lead to add a kink for stress relief.The resistor leads are long enough that they could go through one unsoldered hole and be looped back through a plated hole and soldered there. The loop would allow a bit of springiness for stress relief.Any better ways of doing this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2019, 01:19:15 pm
content: So, effectively cordwood construction, at least for the component in question? What kind of stress are you expecting? Thermal? Flex? Tension/compression?Tim

author: Ian.M
author_rank: Super Contributor
timestamp: February 20, 2019, 01:25:19 pm
content: You could preform a 5mm loop in one lead and still keep the total body + loop length under 17mm.

author: SMTech
author_rank: Frequent Contributor
timestamp: February 20, 2019, 02:19:08 pm
content: Well that sounds like zero fun to assemble. However from the description it doesn't sound like you should really have too much to worry about IMHOIf your boards have plated vias, that would certainly help create a nice strong fillet. A small kink or loop like Ian.M suggests would also certainly give any movement some leeway. Even slightly offsetting the apertures on one side from the matching ones might help with some stresses as you'd have more lead, or at the crudest level, if you have the space make all resistor axes offset from the aperture which at least would be something you could make a tool for.

author: Mark
author_rank: Frequent Contributor
timestamp: February 20, 2019, 02:36:52 pm
content: Thanks for the ideas everyone. I will go with a kink in the resistor leads for this rev, and experiment with offset PTHs for the next rev. There are only 3 resistors so assembly isn't too difficult. The main concern was for expansion/contraction of the rigid components during thermal cycles but that will probably be minuscule as the parts are ceramic.

author: jmelson
author_rank: Super Contributor
timestamp: February 20, 2019, 03:41:59 pm
content: Full cordwood construction was used in the CDC7600 computer, as well as the Gemini computer. They did not worry about these stresses. Figure out how many thousandths of an inch (.01 mm) the parts will expand, and see how much stress you have to apply to the PC board to deflect it that much. Unless these parts are getting red hot, the expansion and stress is going to be VERY small.PC board material is fairly flexible.Jon

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2019, 04:33:50 am
content: It might matter for hot running power resistors but not for small signal devices.One easy thing to do to help is to use a better solder which is more resistant to cracking.

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 21, 2019, 12:36:12 pm
content: Can you offsetthe holes for the resistors so they are mounted diagonally between the PCB's?That could help to relieve stress, especially if the resistor is bent to come straight out of one PCB before bending diagonally or parallel to the PCBs then bending straight i to the second PCB.So rather than bending both legs down to mount into one PCB, one leg is bent up to reach the second PCB.

author: Mark
author_rank: Frequent Contributor
timestamp: February 21, 2019, 01:46:16 pm
content: Quote from: TERRA Operative on February 21, 2019, 12:36:12 pmThat is what SMTech suggested above and I think that would work, so I'll try it for the next rev.I didn't know this had a name (Cordwood construction). Lots of interesting looking old PCBs out there, they must have been a nightmare to assemble and repair.Thanks again everyone.

author: jmelson
author_rank: Super Contributor
timestamp: February 21, 2019, 09:17:17 pm
content: Quote from: Mark on February 21, 2019, 01:46:16 pmThe Gemini modules were potted, never to be repaired. I doubt the CDC 7600 modules were repaired, either, just replaced when they failed.There is also "semi-cordwood", where there is only one PC board, and all the axial-lead components have one lead bent over 180 degrees. They actually made components for this in Japan, with one lead having a paint coating. See the inside of any Japanese transistor radio from the 1970's or so.Jon