PCB007 Magazine

PCB-Oct2014

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62 The PCB Magazine • October 2014 by Michael Carano omg eleCtroniC ChemiCals Oxide Alternatives to Enhance LPI Adhesion to Copper C o l u m n trouble iN your taNk Introduction The printed wiring board industry has expe- rienced issues with liquid photoimageable sol- der masks under various conditions. LPI break- down or lifting near the copper-mask interface typifies a common defect seen when fabrica- tors 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 in- cluding 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 en- hancing the adhesion of the LPI. Before explor- ing 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 plat- ing process places significant stress on the liq- uid soldermask's adhesion to the circuit board surface. Generally, there are several things the

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