A publication of the National Electronics Manufacturing Center of Excellence February 2004

EMPF Director

Michael D. Frederickson
mfrederickson@aciusa.org


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Ask the EMPF Helpline!

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.

Three samples were provided for investigation. Each sample was examined visually using low magnification. One of the samples was cross-sectioned in accordance with IPC-TM-650- 2.2.1D and examined using optical microscopy, Scanning Electron Microscopy (SEM) and EDS (Energy Dispersive Spectroscopy, also known as EDAX)

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.

"Black Pad" is a defect of ENIG boards that is a separation of solder joints formed to the surface of the electroless nickel underplate. This is commonly attributed to excessive phosphorous in the electroless nickel (i.e. nickel electroless bath out of control). However, any kind of nickel contamination (phosphorous or not) that results in the failures characterized here, are often reported as "black pad" failures.

The presence of 7.2% phosphorous in the nickel on the pad-side (see Figure 4-1) shows the normal concentration of phosphorous in the electroless nickel deposit, and proves this is not a classic "black pad" defect in which the electroless nickel would show an excessive amount of phosphorous. However, the presence of nickel and normal levels of phosphorous on both pad-side (Figure 4-1) and the lead-side (Figure 4-2) indicates that this "black pad" failure was caused by a mechanism other than excessive phosphorous in the electroless nickel bath

Results:
From the initial x-ray analysis, it was determined that the workmanship and solder joint quality of the assembly were within good commercial practice limits.

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.

Recommendation:
It was recommended that PWBs used by the customer be acquired from a more reliable source of ENIG finished boards.

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.


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