DESMEAR AND ELECTROLESS PLATING

Desmear and Electroless Plating

Desmear and Electroless Plating

Introduction

At the moment there is a strong increase in the desire for joint bending and joint bending-rigid PCBs (FPCs) due to certain market sectors. The increased technological demands from the latest handheld devices containing cameras and new high resolution screens as well as the newer mobile phones means that there is a surge in the requirement for joint bending-rigid panels and multi-joint bending panels. The need to mass produce these panel types and to reduce the cost of manufacture, as always has driven the development of newer methods of processing. (Schlesinger, 2002, 82)

Technically the materials involved in joint bending / joint bending-rigid board manufacturing generate a large number of issues. One key concern is the large variance of materials in one board build-up as well as the exotic nature of some of the commonly used materials and the inherent issues they raise. We will discuss some of the differences in common materials used as well as common board build-ups and relate this to processing issues (Tuominen, and Kivilahti, 2000, 18-19). We will provide examples for several common joint bending / joint bending-rigid board constructions and will show some of the processing answers to the various issues raised. Lastly we will integrate new developments in processing whilst explaining the benefits each one brings. (Schlesinger, 2002, 82)

Electroless Copper

Although electroless copper has been successfully used for more than three decades, limits on operator exposure to formaldehyde and difficulties in removing the electroless copper from the waste stream caused manufacturers to seek alternatives. Among the deficiencies are (Schlesinger, 2002, 82):

* Use of formaldehyde as reducing agent.

* The process is inherently unstable, requiring stabilizing additives to avoid copper precipitation.

* Environmentally undesirable complexing agents, such as EDTA, are used.

* The large number of process and rinse tanks causes high water consumption.

The electroless copper process consists of four basic operations: cleaning, activation, acceleration, and deposition. An anti-tarnish bath is common after deposition. Virtually all shops purchase a series of proprietary chemistries from a single vendor that are used as the ingredients for the several process baths in the electroless copper process line. Only the micro-etch, its associated sulfuric dip, and the anti-tarnish baths are likely to be non-proprietary chemistries.

Cleaning. The cleaning segment begins with a cleaner-conditioner designed to remove organics and condition (in this case swell) the hole barrels for the subsequent uptake of catalyst, followed by a microetch step. The cleaner-conditioners are typically proprietary formulations, and mostly consist of common alkaline solutions (Särkkä, 2000, 16-19).

A microetch step can be found on the electroless line, oxide line, pattern plate line and with chemical cleaning if that is the cleaning method used. Three chemistry alternatives are available. Sulfuric acid-hydrogen peroxide (consisting of 5% sulfuric acid and 1% to 3% peroxide) is most common, followed by sulfuric acid-potassium (or sodium) persulfate (5% sulfuric, 8 to 16 ounces/ gallon persulfate) and ammonium persulfate. In each case, the microetch bath is followed by a sulfuric acid dip, which serves to remove any ...