PCB007 Magazine

PCB-Feb2018

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72 PCB007 MAGAZINE I FEBRUARY 2018 Surface Preparation and Cleaning, Part 1 Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Introduction Surface preparation and cleaning are es- sential aspects of metal finishing and print- ed circuit board fabrication. The printed cir- cuit board fabricator has several processes that fit the broad category of cleaning and surface preparation in its toolbox. It is critical that the engineer carefully evaluates these methods and processes to determine the most effective way to optimize yields. Overview In general, surface preparation is done to assure good adhesion of metal, dielectric, photoresist, or soldermask to the prepared surface, although avoiding excessive adhesion could also be the object. Take the example of surface preparation before dry photoresist lamination: • Failure to achieve good adhesion in a print-and-etch process will cause etchant attack under the resist and ultimately create an "open" defect • Failure to achieve good adhesion in a plating process will cause tin/lead under- plating, ultimately leading to "shorting" defects (shorts) • Failure to achieve good release of unexposed resist during development can cause etch retardation in a print-and-etch process, ultimately leading to shorts • Failure to achieve good release of unexposed resist during development can cause poor adhesion of the plated copper to the copper base ("copper-to- copper peelers") in a plating process • Failure to achieve good release of exposed resist in a print-and etch process on innerlayers can inhibit the formation of multilayer bonder on such a copper surface • Failure to achieve good release of exposed resist in a plating process can cause etch retardation • Failure to remove residues including chromates, organic soils (including resin spots) will adversely affect innerlayer bonding and plating quality Figure 1 depicts an example of both an open and short due to improper surface prepara- tion leading to poor adhesion. Figure 2 shows a schematic of poor adhesion leading to resist lifting. As a result, the poor adhesion leads to etching away of copper. Figure 1: Short circuit (left) and open circuit (right) due to insufficient resist adhesion.

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