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Oxide Alternatives to Enhance LPI Adhesion to Copper

Introduction

The printed wiring board industry has experienced issues with liquid photoimageable solder masks under various conditions. LPI breakdown or lifting near the copper-mask interface typifies a common defect seen when fabricators institute electroless-nickel immersion gold (ENIG) as a final finish. The aggressive nature of the ENIG process is a particular nuisance for some aqueous-based LPIs. Simply scrubbing the copper surface prior to soldermask application is often not an effective adhesion promotion mechanism for LPI and ENIG. (Please note that not all LPIs exhibit this problem.) A number of factors contribute to mask adhesion issues including acrylate content of the LPI, degree of cross-linking, mask thickness, and adhesion strength of the mask to the surface. Regardless, surface topography plays a unique role in enhancing the adhesion of the LPI. Before exploring surface topography further, it is important to understand outside influences such as ENIG and its effect on adhesion.

Influence of Plating Processes on LPI Adhesion

The electroless-nickel immersion gold plating process places significant stress on the liquid soldermask’s adhesion to the circuit board surface. Generally, there are several things the fabricator can do to ensure proper solder mask adhesion. Of course, proper surface prep is one of them. However, these other critical success factors are important (and will be presented in a future column):

• Thoroughly pre-cleaning of the substrate;
• Ink layer thickness;
• Complete pre-drying of the LPISM;
• Exposure energy;
• Correct adjustment of the developing parameters; and
• Control of the corresponding final curing conditions.

Surface Prep of Copper Prior to Soldermask

In Figure 1, the surface copper of the PWB was prepared with aluminum oxide. Even though the surface roughness appears sufficient, the fabricator experienced issues with LPI breakdown after ENIG. Figure 2 shows the mask peeling from the surface due to marginal adhesion. Most likely, even with less than adequate surface preparation, most surface finishes would not have had such an adverse effect as ENIG.

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Editor's Note: This column originally appeared in the October 2014 issue of The PCB Magazine.