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In the early days of printed circuit manufacturing, when the only available photoresists were solvent-processed, the choice of copper plating chemistry was broader, and alkaline systems—such as those based on pyrophosphate copper—were feasible alternatives. Pyrophosphate electrolytes have good throwing power and give excellent physical properties, as well as being less corrosive to plating equipment than acid sulphate. But along came aqueous-processed photoresists, which effectively revolutionised the printed circuit manufacturing process.
Toxic solvents and expensive stainless steel equipment were no longer necessary, and many new players were attracted to printed circuit manufacturing. Being alkali-strippable, these aqueous-processed photoresists were essentially acid resists; acid copper electroplating became the industry-standard for pattern-plating, and a whole technology evolved around it. Its physical properties were limited, and its throwing power was limited, but it was a cost-effective starting point for development.
The chemists got to work and formulated proprietary systems with optimised copper and sulphuric acid concentrations. They added minute amounts of chlorides together with organic carriers, suppressors, levellers, and brighteners to influence the diffusion layer and improve throwing power, grain structure, and ductility. Development work carries on as enhanced performance for specific applications continues to be demanded: modification of the chemistry, mass transfer and agitation aspects, the mechanical design of the plating cell, the anode configuration, the electrical waveform of what began as a simple direct-current power supply, and numerous other details.
But electrodeposits are notorious for being non-uniform—especially in acid copper pattern plating—and it can be a real challenge to achieve an acceptable thickness distribution on surfaces and in plated through-holes. Isolated features become high current density areas, relative to larger ground plane areas with corresponding low current density. In high aspect-ratio holes, another challenge is to meet the minimum copper thickness requirements in the centre of the hole.
To read this entire article, which appeared in the July 2020 issue of PCB007 Magazine, click here.