Authored By:
Young K. Song and Vanja Bukva
Teledyne DALSA Inc.
Waterloo, ON, Canada
Summary
As a surface finish for PCBs, Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) was selected over Electroless Nickel/Immersion Gold (ENIG) for CMOS image sensor applications with both surface mount technology (SMT)and gold ball bonding processes in mind based on the research available on-line. Challenges in the wire bonding process on ENEPIG with regards to bondability and other plating related issues are summarized.
Gold ball bonding with 25um diameter wire was performed. Printed circuit boards (PCBs) were surface mounted prior to the wire bonding process with Pb-free solder paste with water soluble organic acid (OA) flux. The standard gold ball bonding process (ball / stitch bonds) was attempted during process development and pre-production stages, but this process was not stable enough for volume production due to variation in bondability within one batch and between PCB batches. This resulted in the standard gold ball bonding process being changed to stand-off-stitch bonding (SSB) or the ball-stitch-on-ball (BSOB) bonding process, in order to achieve gold ball bonding successfully on PCBs with an ENEPIG finish for volume production.
Another area of concern was pad metal lifting (PML) experienced on some PCBs, and PCB batches, where the palladium (Pd) layer was completely separated from nickel (Ni) either during wire bonding or during sample destructive wire pull tests, indicating potential failures in the remainder of the batch. Evaluation of failed PCBs was performed using cross-section analysis, X-Ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM)/Energy Dispersive x-ray Spectroscopy (EDS), which identified process issues, such as inclusions, or hyper corrosion which caused either localized or complete separation of the Pd from Ni layer.
Through extensive investigation, using 8D and Kepner-Tregoe problem solving methods, solutions to the problem were discovered in the majority of cases, even though the exact root cause remained unclear due to multiple PCB manufacturing variables being changed at the same time.
Conclusions
The summary with regards to gold ball bonding process and pad metal lift discussed in this paper is as follows:
Gold ball bonding
- The wire bonding evaluation in this study indicates that a ENEPIG plated surface finish can be compatible with the standard gold ball bonding process on bare boards. However, variation in wire bondability of populated boards within and between batches makes the application of the standard gold ball bonding process or the security bump process difficult for volume production. Contamination on bonding pads during the SMT process seems to be the main factor for the variation in wire bondability of populated boards.
- The SSB process enables the gold ball bonding process on ENEPIG plated PCBs for volume production applications.
Pad metal lift
- PMLs were observed during the wire bonding process and/or wire pull test from multiple ENEPIG PCB samples.
- An overactive Ni bath resulting in compromised Ni deposit is a potential factor in PML on wire bonding pads.
- PMLs were observed with and without the evidence of hyper corrosion of Ni. PMLs due to hyper corrosion of Ni might be related to Au and Pd thickness, and/or Ni MTOs with multiple other contributing factors.
- Inclusions and hyper corrosion in the Ni layer in the wire bonding area that are at the smaller scale (approx. 5 um) could result in reliable re-wire bonding in the same pad.
- A tape test as per IPC-TM-650 is not an effective method to screen for PML on wire bonding boards.
Initially Published in the IPC Proceedings
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