It is a 3cm*4cm*1mm 2 layer PCB (6x6 panel) with a LQFP128 package Microcontroller, a resonator and 2 capacitors.
What happens is: When i recieve the assembled pcbs from the assembly plant they work great. But after a few days at room conditions the pcbs only work when heated to about 80�C. As soon as they cool down they stop working (the Microcontroller does not do its job, as if there is a bad contact somewhere)
I redid the design many times, increasing the vias diameter, making the tracks thicker, covering the vias with the stop layer, making the pads on the LQFP128 footprint longer both inwards and outwards.
What could be the cause for this problem?
*The PCBs are bent a bit towards the bottom layer, so some pins might loose contact when the bottom ground plane shrinks or the pcb substrate expands with moisture?? *The microcontrollers come also in vacuum packaging with lots of warnings about exposure to moisture. But I am very careful when handling them. Could it be that they are so cheap and that the pins are not coated with something and i should use a different more aggressive solder paste?? *I have the microcontrollers programmed in a socket before assembly. Sometimes a pin gets bent. Could it be that this lifts the other pins off the pcb and that the effects are so profound?? *Could it be that the 3v plane on top, ground plane on bottom and the LQFP128 sink too much heat when in the oven?? *Should any special care be taken when cooling the PCBs - like fixing them to prevent them from bending??
Has anyone had a similar problem - what was the cause and the solution?