Dear all,
 
I have a technical question for the "copperheads" out there:
 
I have a Flex-rigid product which has wire bond device attached to the substrate and subsequently wire bonded to ENIG pads which are 100uM across connected to an 80uM trace under sodler resist. The solder resist is SMOBC. This product is experiencing cracking of the pattern to the extent of creating an open circuit between the trace and bonded pad.
 
The FPCB is BBT'd and passed before final inspection, and delivery to the assembler, which then fails function test after wire bond and SMT. The defect can be random pads on the FPCBA, but always a wire bonded pad.
 
Foil used is RA copper foil, panel plated as it's a 3 layer MLB FPCB. I have suggested that due to lack of ENIG under the resist, we have a "weak" junction between the pad and solder resist where the copper is less supported - this is subject to wire bonding pressure, frequency, time and accompanying temperatures.
 
How do I calculate the shear strength of the pattern junction and work out the pressure required to create the crack, given the rigid substrate is FR4 (I should also point out that there appears to be a stress fracture going slightly into the FR4 resin at the crack point seen by microsection.
 
Anyone seen this effect anywhere?


      

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