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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.

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.

This paper constitutes a follow‐up to earlier work published in Circuit World, Vol. 17, No. 4. The first paper studied the metallographic structures of two commercially available full build electroless copper foils. Part 2 reports the investigations carried out into the etching characteristics of these copper foils, chemically deposited from full build electroless copper baths designated high elongation (HE) and high strength (HS). It is shown that, although attention should be given to the choice of etchant for these copper foils, they do not exhibit an uncontrollable etch rate, as is often feared.

The purpose of this study is to investigate the effects of zinc dialkyl dithiophophates (ZDDP) and ash-less triphenyl phosphorothionate (TPPT) on hydrogen-free diamond-like carbon (DLC) coatings. For many years, ZDDP have traditionally been used in engine oils as antiwear (AW) and extreme pressure (EP) additives. However, additives containing sulfated ash, phosphorus and sulfur (SAPS) have a detrimental effect on the exhaust after-treatment device found on modern vehicles. Besides the automotive industry, DLC is also used in hydraulic applications where zinc-free and ash-less hydraulic fluids have gradually gained popularity in recent years.

The tribological tests were performed using a disk-on-cylinder tribometer, where the stationary hydrogen-free DLC-coated steel disk formed a line contact with an uncoated rotating steel shaft under lubricated conditions.

It was found that TPPT and ZDDP separately at a concentration of 1.0 wt% increased the amount of friction of the base oil by approximately four times. TPPT appeared more effective than ZDDP in minimizing wear on the DLC-coated surface. Also, primary ZDDP seemed to have a more detrimental effect on the DLC-coated surface compared to a mixture of primary and secondary ZDDP. With regard to surface roughness of the hydrogen-free DLC-coated surface, the values corresponding to a lubricant containing TPPT were lower than those obtained for a lubricant with ZDDP and a lubricant without any additive.

This is the first report on the effects of ZDDP and ash-less TPPT on the tribology of hydrogen-free DLC coatings.

Over the past 20 years there has been a recurrent problem of a purple‐bluish stain appearing on the laminate materials of finished printed circuit boards between the gold‐plated fingers. The origin of this staining has been traced back to the PWB fabrication plant and it is generally accepted to be related to either the gold plating bath and/or the solder stripper chemistry. In this paper the authors report their investigation of this phenomenon and show that, far from being a benign, cosmetic defect, this purple stain poses a potentially serious metallic contamination to the laminate surface of the PWB. The purple colour arises from generation of a colloidal gold chromophore known as the ‘Purple of Cassius,’ which has been known since ancient times and has been in commercial use in the glass and ceramics industry for at least 300 years.