Estimated reading time: 2 minutes

Trouble in Your Tank: Primary Imaging for Pattern Plating, Part 1

It is the job of the PCB process engineer to ensure that a quality circuit is delivered. This process starts with sound mechanics of the imaging system that include surface cleanliness and resist lamination parameters.

Introduction

You’ve heard the expression “garbage in, garbage out.” This also applies to anything related the manufacturing of printed circuit boards. More specifically the subject of primary imaging for pattern plating comes to mind. There are several key process steps that must be properly executed in order to insure that the electroplated copper trace conforms to dimensions that were intended. This also means that the plated trace is uniform in shape. Of course the electroplating process itself plays a significant role in the quest to obtain a uniform trace

The plated trace should be as free as possible from pitting, undercut, plated copper overhang (plating of metal up over the primary resist) and ragged edges. Again, this all starts with a properly controlled imaging process. In a future column this author will discuss electroplating of copper in more detail.

Primary Imaging

There are several process steps in imaging:

• Resist Lamination
• Exposure
• Development

In the process of lamination, the dry film resist is applied onto a base material, typically a copper clad dielectric (for innerlayer imaging) or an outerlayer surface of the printed circuit board just prior to pattern plating. In the case of pattern plating, the resist laminated onto to a surface that has either been processed with conventional electroless copper or with one of several alternatives (carbon based, conductive polymer, etc.).

The lamination step is designed to achieve intimate contact between the resist and the copper surface as a protective coating against plating or etching chemical attack. By utilizing heat and pressure, contact between the resist and the copper is achieved. Essentially the dry film resist flows and is able to conform to the surface. This is explained as follows: “Flow is achieved by lowering the resist viscosity and applying a pressure differential for a certain amount of time. Viscosity is then lowered by heating the resist. A pressure differential can be created by pneumatic, hydraulic, or mechanical (springs) pressure on the lamination hot rolls of a hot roll laminator or by applying a vacuum to a vacuum laminator.”

Click here for the full column.

This column originally appeared in the May 2016 issue of The PCB Magazine.

For the part 2 of this column, click here.