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E. J. Bergum: Polyclad Laminates, W. Franklin, New Hampshire, USA
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INTRODUCTION
Drum Side Treated Foil -- DSTFoil[sup][Trademark] -- is an advanced copper foil technology invented by the author's company and is currently patent pending. The technology was initially developed in an effort to improve innerlayer yields, simplify processing and improve circuit board reliability. However, a number of other technical and economic benefits are also achievable. This paper will give a brief description of DSTfoil but will deal primarily with actual field experience regarding processing and yield improvements.
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TECHNICAL DESCRIPTION
DSTFoil and standard copper foil are manufactured in essentially the same manner but with two important differences. Both types of foil are made by electroplating copper from a copper sulphate/suphuric acid electrolyte solution on to a highly polished drum, Figure 1. The resulting foil has a shiny drum side (Figure 2(a)) and an inherently rough nodular matte side (electrolyte side). The matter side of standard foil is then treated with an additional copper plating step to increase the surface area and a subsequent layer of zinc or brass to provide a non-reactive corrosion barrier (Figure 2(b)). Finally, a passivation layer is applied to both sides to prevent oxidation during storage and laminate manufacture. DSTFoil differs in that the additional copper plating step, to increase surface area, and the zinc or brass layer are applied to the shiny drum side (Figure 3(a), hence the name Drum Side Treated Foil. A passivation layer is also applied to both sides. This leaves the matte side with a naturally rough, nodular, pure copper surface (Figure 3(b) and a moderately protective passivation layer.
A schematic cross-sectional view also illustrates the differences clearly (Figure 4).
When standard foil is used to manufacture laminate, the treated matte side is placed against the core with the shiny side out (Figure 5(a)). Laminate made with DSTFoil uses the treated drum side against the core, leaving the pure copper matte side out (Figure 5(b)). This results in a naturally micro-rough surface for improved circuit definition.
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PRINTED CIRCUIT BOARD PROCESSING
DSTFoil requires no special processing. In fact, some of the steps in the pre-cleaning process prior to innerlayer imaging can be eliminated (Figure 6). Since the surface is already rough, no mechanical scrubbing, pumice...