Designing a board with strict and inconvienient requirements. Currently the boards are breaking frequently – the aim is simply to reduce this frequency as much as possible.. The board is a 100mm square but only supported with standoffs in each corner.
On the board are 20 throughhole 4 pin sensors. Effectively they are analogous to DIP switches.
The PCB experiences a lot of vibration as well as intermittent force (perhaps up to 100N point load) randomly, suddenly applied on a random sensor. Board is FR4 1.6mm. The environment is also humid and moderately corrosive.
I would love to redesign/support the board etc etc to be supported better, not loaded heavily, with a backing plate etc – but this is not an option.
Unsurprisingly, the board traces breaks frequently under this flexing. This generally causes unreliable performance. Electronically the board is simple slow digital signals so noise etc is not a concern.
I'm unsure as to how best to prevent/mitigate the traces breaking. I could either route multiple narrow traces for the same signal, thus my thinking being that stress is more likely to break one and the crack not propagate through to the others, or one wide trace.
I'm also thinking curved traces might be worth a thought and I'm interested in how best to connect the trace to throughhole connection to minimise risk of fracture. Also interested in if using extensive ground planes may help and if the most critical traces should be routed on top or bottom of PCB – so that they are either predominantly in compression (top) or tension (bottom). I have read around flexible PCB design but not got much conclusive information and am unsure if a rigid PCB that flexes is a significantly different case.
Thanks
Best Answer
Tricks we have used with some success: Make your feedthru holes big enough to insert a small piece of bus wire and solder top and bottom. Also solder top and bottom on your sensors if possible. We have seen lots of breakage between traces and the plated through holes and this is pretty effective, especially if you use 2-sided boards. Multi-layer boards are prone to cracking between trace and plating on the plated through hole, so stick with two-sided if you can.
If you must use multi-layer, be sure and watch the heat on your joints. Overheating can stress the joint because the board material expands at a different rate than the copper. The idea is to prevent cracking at the outset.
Trim component leads before you solder; Trimming them afterwards can stress and start a crack which will propagate.
No sharp corners on the traces. Cracking often begins on the corner where a trace runs into a pad outer radius. Fan out the trace to where it matches the pad tangentially.
For long parts like through hole resistors, mount them parallel to the expected flexure so that the leads are not getting stretched and compressed.
If you have leaded parts that mount flush, leave some space so that they are not sitting directly on the board.
Of course, you are probably already mounting your heaviest parts nearest to the mounting holes where they have the most support.
Good luck!