|A publication of the National Electronics Manufacturing Center of Excellence||February 2004|
Michael D. Frederickson
Customer Issue: The EMPF Helpline received a call from an EMS (Electronic Manufacturing Services) provider who had experienced delamination of the gold surface finish from the nickel under layer ENIG (Electroless Nickel - Immersion Gold) finished Printed Wiring Boards (PWB). The boards were from (and limited to) two PWB fabricator date codes. The PWBs had been solder assembled with surface mounted components and this delamination represented "open" failures of the finished Printed Wiring Assemblies (PWAs) during burn-in of the assemblies. The customer wanted to know whether these delaminations of the gold from the nickel could be repaired on currently failed product or avoided in the future.
When an ENIG surface finished PWB undergoes soldering, the solder must adhere to the underlying electroless nickel plate. This is because the immersion gold is so thin that all of the gold dissolves into the solder upon soldering. The solder bonds directly to the electroless nickel.
If that nickel surface is contaminated in some way, the resulting joint will be weak and the locus of failure will be just below the surface of the nickel, leaving both nickel and phosphorous (a normal constituent of electroless nickel) on both the pad and the failed solder joint surface.
The EDAX analysis of the failed solder joint showed the presence of nickel at both the fracture surface of the solder joint and the pad from which the solder joint had lifted (see Figures 4-1 and 4-2) This is a positive indication that contaminated nickel was the cause of the failure.
The optical, SEM, and EDAX analyses showed that the solder joints appeared normal, but the opens were conclusively caused by the delamination of the gold from the underlying nickel layer on the ENIG. EDAX revealed normal electroless nickel phosphorous content, and a locus of failure inside the nickel.
The customer was completely satisfied with this result and the fact that the failures were shown to be related to the board manufacture and not the assembly operation.
Satisfied with these results and recommendation, the customer elected to change PWB vendors to one that has a history of more robust nickel gold finishes.
If you have questions about this article or any of the topics, please contact the EMPF Helpline at (610) 362-1320. A manufacturing expert will be able to offer technical insight and appropriate advice regarding your concerns.
|ACI Technologies, Inc. - - www.aciusa.org - - (610)362-1200|