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The adhesion between electroless copper and a substrate is one of the most important factors in the reliability of thermoplastic printed circuit boards. The purpose of this paper is to investigate the effects of mechanical grinding and acid etching of thermoplastic substrate materials on the adhesion of copper deposited by an electroless copper plating process. The base material of the test substrates was a new high temperature thermoplastic polyphenylene oxide (PPO) compound.

The effects of pre‐treatment on plastic surfaces are analyzed by the following methods: Fourier transform infrared (FTIR), SEM, the Dyne surface energy test and the surface roughness test. The adhesion between electroless copper and thermoplastic substrate is measured with a peel strength test.

The results showed that mechanical grinding of the substrates significantly increased adhesion but the highest adhesion is gained by using an acid etch treatment before electroless plating. These results indicated that adhesion between copper and the substrates was not directly proportional to the roughness and surface energy values.

The conventional sweller/desmear treatment used in a printed circuit board factory for pre‐treating epoxy based laminates prior to electroless plating is not suitable for these PPO compound boards. The copper adhesion is adequate when the substrates are etched with sulphuric acid/chromate solution. In that case the bonding between the metal layer and the plastic surface is stronger than the bondings between the polymer chains of the thermoplastic material. The adhesion mechanism of electroless copper in these mechanically abraded samples is mechanical interlocking of metal particles.

The purpose of this paper is to investigate if copper nanoparticles could be utilized for two types of through hole plating in printed circuit boards, namely: as a catalytic material to initiate the electroless copper deposition process; and as a “conductive” layer which is coherent and conductive enough to allow “direct” electroplating of the through hole. The employment of nanoparticles means that an effective method of dispersion is required and this paper studies the use of mechanical agitation and ultrasound for this purpose.

The paper utilized drilled, copper clad FR4 laminate. The through holes were functionalized using a commercially available “conditioner” before being immersed in a solution of copper nanoparticles which were dispersed using either a magnetic stirrer or ultrasound (40 kHz). When the copper nanoparticles were utilized as a catalytic material for electroless copper plating, the efficacy of the technique was assessed using a standard “backlight” test which allowed the plating coverage of the through holes to be determined. As a control, a standard palladium catalysed electroless copper process was employed. The morphology of the electroless copper deposits was also analysed using scanning electron microscopy. In the “direct plate” approach, after immersion in the copper nanoparticle dispersion, the through holes were electroplated at 3 Adm⁻² for 15 min, sectioned and examined using an optical microscope. The distance that the copper electroplate had penetrated down the through hole was then determined.

The paper has shown that copper nanoparticles can be used as a catalytic material for electroless copper plating. The coverage of the electroless copper in the through hole improves as the copper nanoparticle concentration increases and, at the highest copper nanoparticle concentrations employed, good, but not complete, electroless copper coverage is obtained. Dispersion of the copper nanoparticles using ultrasound is critical to the process. Ultrasonically dispersed copper nanoparticles achieve some limited success as a conductive layer for “direct” electroplating with some penetration of the electroplated deposit into the through hole. However, if mechanical agitation is employed to mix the nanoparticles, no through hole plating obtaines.

The paper has demonstrated the “proof of concept” that copper nanoparticles can be utilized to catalyse the electroless copper process, as well as their potential to replace costly palladium‐based activators. The paper also illustrates the potential for copper nanoparticles to be used as a “direct plate process” and the necessity for using ultrasound for their dispersion in either process.

This, the third in a series of papers, reports an investigation of the semi‐additive process strategies of PWB manufacture. It will show that, although the adoption of a ‘naked’ palladium catalyst is the optimal production strategy in terms of cost, caution must be exercised in its implementation, particularly with regard to material process flow and solder mask type employed. It will be demonstrated that the use of a thin, sacrificial ‘flash’ of electroless copper prior to circuitisation and full build deposition will be required for the liquid photoimageable soldermask (LPISM) approach.

The purpose of this paper is to examine the use of additives in formaldehyde‐free copper‐plating solutions with low reducing agent (RA) concentration to improve the start reaction of electroless copper deposition and to enable a copper‐plating process which is more environmentally friendly.

Different additives were investigated and their influence on the plating reaction and deposition rate was elucidated using several deposition trials.

On palladium‐activated base material, the additives reacted with the palladium and generated additional electrons in the initial phase of the deposition. Thus, the adequate supply of electrons from two sources (RA and additive) permits the deposition of a homogeneous and compact copper layer.

At the present time, formaldehyde is the established RA in the electroless copper metallization process used with plated through‐holes. Because of its environmental impact, there is a need to replace formaldehyde. In this investigation, the more environmentally friendly glyoxylic acid is used as an autocatalytic RA. However, glyoxylic acid is more expensive and causes undesirable side reactions. In order to keep process costs under control, the concentration of glyoxylic acid in the copper bath should be reduced without affecting the quality of the copper deposits.

Additives can compensate for the lower RA concentration, and thus the lack of essential electrons for the copper deposition.

The high cost of copper combined with a world shortage has emphasised the need for economy in its many applications. The manufacture of printed circuits by the etched foil technique produces a wastage of 60–80% of the copper cladding of the boards and although recovery of the copper can be effected, the process is costly and time consuming to an extent that makes it impracticable for the printed circuit manufacturer. With the increasing use of additive processes for printed circuit manufacture and the consequent saving in copper wastage, the use of electroless copper plating has assumed even greater significance than for its application to through hole plating and the advantages gained by freedom from the current density problems associated with electroplating need no stressing.

In the preceding three parts of this series, the authors have extensively reviewed and quantified the special processing sequences required for the ‘additive’ and ‘semi‐additive’ process strategies of PWB manufacture. In this, the fourth part of the series of five, they wish to present a series of full build processes which meet all the interconnect requirements of the 1990s while eliminating the drawbacks traditionally associated with additive processes.

The purpose of this paper is the investigation of the catalytic activities of selected metals in reductant oxidation.

Electrochemical measurements were carried out in order to obtain information about the catalytic activity of copper, silver, nickel, and palladium in the oxidation reactions of formaldehyde and glyoxylic acid.

Metals with high‐catalytic activity for each reductant oxidation can be determined by using cyclic voltammetry. These metals are suitable for an improved activation process in electroless copper deposition. Electrochemical measurements show that palladium does not have the highest catalytic activity of all tested metals. Therefore, alternative and inexpensive metals are explored as catalysts in electroless copper plating reactions.

At the present time, formaldehyde is the established reducing agent in the electroless copper metallization of plated through‐holes. Because of its environmental impact, there is a need to replace formaldehyde. A change in reducing agent can lead to a change in the activation process, because there is no one metal that appears to be a good catalyst for the oxidation of all reducing agents that have been employed for electroless deposition. This paper suggests that further improvement in electroless copper plating processes may be possible with an alternative activation procedure that yields higher catalytic activity.

Based on the results of experiments, the development and optimization of activation processes in electroless copper plating can be advanced.

Data from laboratory and production scale tests are given that show that the efficiency of catalyst adsorption controls the coverage by the electroless copper deposit in plated‐through‐holes in FR‐4 laminate, and that this, in turn, governs the outgassing performance of the finished board. The nature of electroless copper nucleation and growth is discussed and the reasons for the formation of voids in the deposit are identified.

The Geislingen research laboratories of Dr Ing. Max Schlötter have developed Slotoposit, a new and advanced process for the manufacture of high quality PTH circuits, using conventional subtractive techniques. The aim of the research was to remove the hazardous formaldehyde present in most electroless copper systems, to improve hole wall adhesion and process the boards in one plating operation rather than the two (panel and pattern) employed in the traditional system. The following paper describes how a stable electroless nickel has been developed to achieve these ends and to increase productivity significantly by reducing the process steps and times.

Driven by the demand for higher density in electronic packaging, each signal plane of printed wiring board must accommodate more conductors. As a result, conductor width is becoming narrower each year. This chapter reviews some of the important steps of forming finer line conductors in printed wiring boards, such as surface preparation, plating/etching, photo‐exposure, automatic optical inspection, etc.