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SMT PCBs subjected to torsion and high G forces

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profdc9:
I am working on a project where a LQFP-100 part is subjected to signficiant G-forces and the PCB itself is twisted.   The PCB is 1.6 mm thick 2-layer FR4.  I am finding that the LQFP-100 pins are becoming detached from their pads and the solder joints are cracking.  Has anyone else had this problem and found a good solution?  I am thinking of using epoxy to glue several layers of a backing to the PCB to stiffen it.    Or perhaps covering the entire chip with epoxy might help, but then if the joints fail they will no longer be accessible.  Is lead-free solder more or less susceptible to cracking than leaded solder?

Domagoj T:
Instead of trying to prevent solder cracking when PCB is bent, why not try to prevent it from bending in the first place by modifying the mounting orientation or adding bracing?

jmelson:

--- Quote from: Domagoj T on May 17, 2020, 06:37:51 pm ---Instead of trying to prevent solder cracking when PCB is bent, why not try to prevent it from bending in the first place by modifying the mounting orientation or adding bracing?

--- End quote ---
Yes, this is right.  it may be needed to attach the PCB to a stiff structural member that is loosely coupled to some outer structure, to act as a stiffener to the board, to prevent flexing.  Flexing a board with high pin-count ICs is just never going to work.  Many systems that have to work in harsh vibration environments use some combination of flexible mounts and stiffeners to protect the PC boards from flexing.  They also work with vibration tables to assure that the board's natural resonant frequency does not coincide with some other structural resonance.  (This latter testing would be hard to do at home.)  The aerospace guys fight this kind of thing all the time.

Jon

jmelson:

--- Quote from: profdc9 on May 17, 2020, 06:20:37 pm ---  Is lead-free solder more or less susceptible to cracking than leaded solder?

--- End quote ---
Pure tin is definitely more fracture-prone than Tin-Lead.  Some newer SAC305 solders are supposed to be close to Tin-Lead in resisting fracture, but still not as good.  Low-temp solders with Bismuth in them seem to be extremely fracture-prone, taking almost no effort to knock components off.

Jon

Gyro:
Note that if the relatively flexible leads of the LQFP are coming loose, then you are probably suffering hidden cracking damage to small rigid components, particularly MLCCs, too.

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