Achieving Fine Lines and Spaces, Part 2

Reading time ( words)

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.


Suggested Items

EIPC Winter Conference 2023: Day 1 Review

02/27/2023 | Pete Starkey, I-Connect007
The EIPC Winter Conference returned to the Metropolis of Lyon in eastern France this month. In 2018, the venue was Villeurbanne in the Auvergne-Rhône-Alpes region. Five years later the setting was the Groupama Stadium in Décines-Charpieu, and leaders of the European printed circuit community gathered in expectation of a spectacular programme of 16 presentations, a visit to a nuclear power station, and an invaluable networking opportunity. They weren’t disappointed.

ICT Christmas Seminar 2022: The Caliber of a ‘World Cup’ Event

12/13/2022 | Pete Starkey, I-Connect007
Regardless of the potential distraction of the international football match between England and Wales in the World Cup competition, an enthusiastic crowd of PCB fans gathered in Meriden UK for the Institute of Circuit Technology Christmas Seminar, an eagerly-awaited networking opportunity that included a face-to-face industry welcome event and an outstanding technical programme. Guest speakers highlighted new technology in selective solder nozzles, flexible circuits, industry cooperation, and a greener future by recycling PCBs.

EIPC Summer Conference 2022: Day 2 Review

06/29/2022 | Pete Starkey, I-Connect007
Örebro, Sweden on June 15 brought a bright and early start to Day 2 of the EIPC Summer Conference for those who had enjoyed the previous evening’s networking dinner, but had resisted the temptation to over-indulge or to carry on their long-awaited catch-up conversations with old friends into the small hours. All but a few were in their seats for 9 a.m., awake and attentive for Session 4 of the conference, on the theme of new process technologies, moderated by Martyn Gaudion, CEO of Polar Instruments.

Copyright © 2023 I-Connect007 | IPC Publishing Group Inc. All rights reserved.