We made some 6 layers PCB boards (4.5" x 4.5") recently. But we have concerns about the quality. When pressing/bending/twisting the boards with very small force by hands, we can hear very obvious sound from inside of the boards, like the sound from potato chips. I checked some other PCB board at hand, some of them also have this kind of sound when twisting it.But they are very slightly, I have to put the boards very close to my ears to hear the sound. Does anyone know if that sound is the sign of poor in quality? Thanks in advance.
Most decent PCB manufacturing places will provide a cutaway of the PCB encased in epoxy, check for layer separation or copper separation. If you're not flexing these PCB's by a few degrees or putting a lot of pressure on them then they could be manufactured wrong. You get what you pay for.