Not a wire bonding person but I would think many things could contribute to your problem, copper grain direction, poor tear drops to much acrylic adhesive under the wire bonding area creating a soft point and pressure from the bonding unit and you need to be careful where the soldermask ends. Any edge can create problems with flex and thin conductors. www.quickturnflex.com Thank you, Jim Mahoney Quick Turn Flex Circuits, LLC Applications Project Manager P# 603-821-7071 M# 603-305-6250 ** This email and all attachments have been scanned prior to being sent** Confidentiality Notice: This page and any accompanying documents contain information that is confidential, privileged or exempt from disclosure and is intended for the exclusive use of the addressee. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution or use of the contents of this information in any manner is strictly prohibited. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo Sent: Monday, July 27, 2009 9:16 AM To: [log in to unmask] Subject: Re: [TN] Copper trace shear strength Are you sure is enig? Not electroless gold for wirebond pad? -------------------------- Sent using BlackBerry ----- Original Message ----- From: TechNet <[log in to unmask]> To: [log in to unmask] <[log in to unmask]> Sent: Mon Jul 27 04:07:37 2009 Subject: [TN] Copper trace shear strength 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. 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