here's a good description of how tamper mesh works in a chip,
By: Christopher TarnovskyFrom start to finish, we will walk through how a current generation smartcard was successfully compromised. The talk will discuss ev...
same concept applies to PCB - basically it's a web of traces surrounding the real traces in every dimension - The web is made in such a way that you can detect a break, perhaps a break with a bypass, or perhaps even a change in some characteristic, thereby letting the circuit know that some tampering or attempts to circumvent the intended operation of the circuit is occuring. I'm not a big fan of this, and since I have seen it broken many times - I question it's effectiveness. You might find this sort of thing on secure entry id card readers or payment card scanners and perhaps also on some satellite TV decoders.