Source
r/PCB•
cuicocha

What is the best way to tighten mounting screws so that they are snug enough without damaging soldermask?

I supervise a team of student lab employees; they graduate and leave after a couple years so there's a lot of turnover and loss of knowledge. So, it's really important for me to be able to teach inexperienced people to do good work consistently.

Most recently, an employee overtightened the mounting screws for a batch of PCBs (screw tightness is not really covered by current training). This mounting screws go between two PCBs with a plastic spacer in the middle, and screw into a nut on the bottom side of the bottom board. Overtightening meant that the screw head abraded the soldermask on the top of the top board and the nut abraded the soldermask on the bottom of the bottom board, exposing bare copper (planes and traces) to the metal screws. This caused a variety of interesting behaviors from short circuits that was fun to track down.

How can we avoid this in the future? Do the pros use something like a torque wrench to get the right amount of tightness? Or do you just come up with a rule like "get this as tight as you can while gripping the screwdriver with your fingers only" and accept that different workers will have different amounts of grip strength?

And, in addition to training workers properly, is there a recommended amount of torque to avoid damaging soldermask or other parts of the PCB?

Posted Mar 23, 2023, 11:19 PM UTC

68
toybuilder•

Repeat after me: Soldermask is not insulation.

If it's a design that you can control, the design should have clearance for screw head and bosses so that there is no electrical connections to be concerned about.

If the board is not something you can control, get nylon washers and/or nylon fasteners. Or even metal washers provided they are deburred/smooth.

7
electricfunghi•

Torque wrench + loctite. Depends on screw. For m3, I’ve had good results with 7lb/in. (Going off memory of aircraft). Washer before screw- screw head shouod not be directly hitting pcb. Washer will disperse load. Plastic washer if you have traces too close to hole.

3
cuicochaOP•

Thanks for the advice. How easy is it to take out screws that were screwed in with loctite? We occasionally need to disassemble them for repair.

1
electricfunghi•

It’s not hard thread lock works a bit differently than glue. That’s exactly the case why threadlock exists. There are different strengths of threadlock depending on your application and if it’s not moving thermal cycling much you may not need it

2
13e1ieve•

Use blue type is removable. Red type is more permanent.

1
Impressive_Living_12•

Depends on the case, I use quite often plastic spacers on both sides, thread lock and small torque wrench. You can calculate the clamping force needed or use some recommendation for the materials (fr4, aluminium, so forth). For example if you ask the pcb fab for the compressive force that the material can get recommendations - but most of the time 0.05 - 0.6 NM torque screw driver set will work I guess - as most of the time this was what was recommended to me from some assembly and fab houses when I asked what to get as screwdriver set or as torque needed for a pcb. Hope that helps

1
[deleted]•

[deleted]

1
OldEquation•

  1. Always keep traces clear of the screw heads.

  2. If you need to avoid scoring of the surface use a washer. That’s what washers are for. That goes for anything, not just PCB’s.

  3. Solder mask is not intended to be insulation.

1
Ottos_jacket•

Get a torque wrench, always good to know how to use one.

0