Surface preparation and cleaning are essential aspects of metal finishing and PCB fabrication. The PCB fabricator has several processes that fit the broad category of cleaning and surface preparation. However, the organization needs additional studies to enhance the broad portfolio of products for their respective fitness for use in today’s technology. In addition, this exercise will assess gaps in the RBP cleaning and surface preparation segment and make necessary process improvements through bench scale and field application trials.
In general, surface preparation is done to ensure good adhesion of metal, dielectric, photoresist, or solder mask to the prepared surface, although avoiding excessive adhesion could also be the objective. Take the example of surface preparation before dry photoresist lamination, such as failure to:
- Achieve good adhesion in a print-and-etch process, which will cause etchant attack under the resist and ultimately an open defect
- Achieve good adhesion in a plating process, which will cause tin and lead underplating, ultimately leading to shorting defects (shorts)
- Achieve a good release of unexposed resist during development, which can cause etch retardation in a print-and-etch process, ultimately leading to shorts
- Achieve a good release of unexposed resist during development in a plating process, which can cause poor adhesion of the plated copper to the copper base (copper-copper peelers)
- Achieve a good release of exposed resist in a print-and-etch process on inner layers, which can inhibit the formation of a multilayer bonder on such a copper surface
- Achieve a good release of exposed resist in a plating process, which can cause etch retardation
- Remove residues including chromates and organic soils (including resin spots), which will adversely affect inner layer bonding and plating quality
Figure 1 depicts an example of both an open and a short due to improper surface preparation leading to poor adhesion. To fully grasp some of the surface preparation issues, a primer on the copper foil manufacturing process will enhance the reader’s understanding with the composition and topography of standard electrodeposited (ED) and reverse-treated foils (drum-side-treated foils).
This article originally appeared in the December 2018 issue of PCB007 Magazine, click here.