Estimated reading time: 5 minutes

Trouble in Your Tank: Surface Preparation and Cleaning, Part 1

Introduction

Surface preparation and cleaning are essential aspects of metal finishing and printed circuit board fabrication. The printed circuit 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 underplating, 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-tocopper 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 preparation 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.

Figure 2: Innerlayer showing poor resist adhesion (left) and result after etching (right).

Understanding Surface Preparation Issues

To fully grasp some of the surface preparation issues encountered in the PCB fabrication process, a short primer on the copper foil manufacturing process follows. This will enhance the reader’s understanding of the composition and topography of standard electrodeposited (ED) foils and reverse-treated foils (drum-side treated foils).

Figure 3: Manufacturing of electrodeposited copper foil. (Source: Oak-Mitsui Copper Company)

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