We have a systemic fault in our mature product function that appears to be the result of unequal current supply drive between VCC and GND. The TI SN74HC244DW IC is used to interface an a keyboard matrix - columns signaled and rows read. The issue is where the pulses are combined through multiple key presses (simultaneously) to obtain 2nd and 3rd functions - the logic high on the input isn't high enough to switch the 244 to a logic high output whereas on the older devices, this logic level was always over 2.3V. The input level is a sum of the combined outputs of the same IC (column drive signals). The fault popped up mid production run and was identified by having a mix of two date codes - the good ones all being older than 2016 ('bad' device marking 6CCTY1K). Last known good ID is 53CXGFK.
Initial thought was a bad batch - changed out 16 units to MFG code 76AR5JK, but same result. Is there any known change between 2015 and 2016 with either wafer fabrication or package assembly? XRAY didn't reveal anything obvious, but there's plenty of characteristics that aren't visible (like bond wire material).
If there was a change, we'll probably have to have another look at the application with our customer, but again, this is a mature product that we've been manufacturing for over a decade.
Any insights would be appreciated.
Thanks!
Mark Pallant