Achieving Fine Lines and Spaces, Part 2

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Achieving high yields with fine lines and spaces requires a good understanding of surface preparation methods. These include pumice and chemical cleaning. In essence, the surface profile of roughness plays a role in resist adhesion.  

Pumice vs. Chemical Cleaning

As someone who has frequently worked with PCB fabricators on a global basis, I must confess that surface preparation and imaging issues rank quite high on the list of yield loss. In previous columns, I have stressed the basic fundamentals. That is, understand what you are up against (which soils are on the surface, what are the interactions of the process parameters, etc.) in order to optimize yields. As stated previously, in order to minimize distortion and material stretching, mechanical methods of surface preparation are giving way to chemical cleaning. Copper foils and are also trending toward lower thicknesses in order to improve fine-line etching. Secondly, the need to improve impedance control with today’s high-speed electrical requirements is driving the implementation of low profile copper foil. These lower-profile foils are generally of a finer grain size, further impacting the ability to affect a surface topography sufficient to enhance photoresist adhesion. So let’s first review the importance of surface topography and photoresist adhesion.

Critical Surface Profile Parameters

I stated earlier that poor resist adhesion leading to defects such as opens or shorts, peeling and interfacial voids, are related to numerous issues including, surface preparation, characteristics of the copper foil, lamination parameters, and the formulation of the photoresist itself. There are other causes of poor adhesion that are directly attributable to other factors such as developing and exposure. These causes will be explored in a future column. With respect to surface topography, I already stated that the randomness of the copper topography after surface prep is more beneficial than a unidirectional one seen with mechanical scrubbing. Pumice, aluminum oxide, and chemical clean impart the random or multidirectional topography that we are concerned with. While that is a good thing, let’s take a close look at critical profile or topographical parameters.

Read the full column here.

Editor's Note: This column originally appeared in the December 2013 issue of The PCB Magazine.


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