PCB007 Magazine

PCB007-July2020

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90 PCB007 MAGAZINE I JUNE 2020 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 add- ed 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 en- hanced 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 pat- tern plating—and it can be a real challenge to achieve an acceptable thickness distribution on surfaces and in plated through-holes. Iso- lated features become high current density ar- eas, relative to larger ground plane areas with corresponding low current density. In high as- Simulation Technology in Acid Copper Pattern Plating Feature by Pete Starkey I-CONNECT007 In the early days of printed circuit manufac- turing, when the only available photoresists were solvent-processed, the choice of copper plating chemistry was broader, and alkaline systems—such as those based on pyrophos- phate copper—were feasible alternatives. Py- rophosphate electrolytes have good throwing power and give excellent physical properties, as well as being less corrosive to plating equip- ment than acid sulphate. But along came aque- ous-processed photoresists, which effectively revolutionised the printed circuit manufactur- ing process. Toxic solvents and expensive stainless steel equipment were no longer necessary, and ma- ny new players were attracted to printed circuit manufacturing. Being alkali-strippable, these aqueous-processed photoresists were essen- tially acid resists; acid copper electroplating became the industry-standard for pattern-plat- ing, and a whole technology evolved around it. Its physical properties were limited, and its

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