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The classic English summer: Grey skies and rain, but the crowd of eager delegates who gathered at the Wolfson School of Mechanical and Manufacturing Engineering at Loughborough University in the UK were there to learn about state-of-the-art metal finishing processes for PCB manufacture-not to escape the weather. The Institute of Circuit Technology joined forces with the Innovative Electronics Manufacturing Research Centre (IEMRC) for a Summer Seminar on Plating Technology, introduced by IEMRC Coordinator Dr. Darren Cadman and ICT Technical Director, Bill Wilkie.
Dave Wayness from Rohm and Haas explained, in detail, how the cost and performance of copper electroplating processes could be optimised for different applications, with a sage reminder that technologies could be easily over-sold and that it was important in choosing the right process to understand not only what it did well, but also what it did not do well. The days of one-process-fits-all are long past and even different geographical regions now require significantly different functionality to support the design characteristics and end-user requirements of their particular market sectors. Wayness reviewed many permutations: vertical or horizontal processing; DC or pulse rectification; panel, semi-panel or pattern plating; conformal plating or microvia filling; and soluble or insoluble anodes, and described how Rohm and Haas has developed a series of copper plating chemistries to achieve specific objectives in high aspect ratio through-plating and via filling-defining "difficulty factors" and "stability indices" to gauge their effectiveness and using a mathematical model to calculate the cost savings achieved using the appropriate process variant for the job.
Jean Rasmussen, from Cookson, described the technical and environmental benefits of direct metallisation using intrinsically conductive polymers to selectively activate resin and glass without contamination of copper surfaces. Cookson's process works by first depositing an initiator layer of manganese dioxide which then acts as an oxidative initiator for the in-situ polymerisation of ethylene-dioxy-thiophene in the presence of an organic sulphonic acid. The conductive polymer is mechanically and thermally stable, and in high-aspect-ratio through-holes, coverage with acid copper is complete and void-free within 60 to 90 seconds. The process has significant environmental advantages when compared with electroless copper. The process contains no formaldehyde or chelating agents, waste treatment is simple and large savings can be made in water consumption, making it particularly attractive to Asian fabricators for whom water is, generally, a very expensive commodity.
Andy Cobley, a sonochemistry specialist from Coventry University, reviewed studies of the physical and chemical effects of ultrasound in liquid media, and their potential applications in metal finishing, an area where, traditionally, ultrasonics had been used only for cleaning purposes. Explaining the principles of acoustic cavitation, theoretically capable of generating enormously high pressures and temperatures at the point of collapse of microscopic bubbles, and the phenomenon of microjetting, when bubbles collapse asymmetrically at a surface, he demonstrated how increases in limiting current density and current efficiency could be achieved in electroplating as a consequence of disruption of the diffusion layer. In electroless copper plating, the use of ultrasonics at the catalyst stage gave a significant increase in the deposition rate of copper-the rate increased further if ultrasonic agitation was applied in the copper bath as well. This effect could possibly be exploited in the development of formaldehyde-free electroless copper chemistry.
Martin Bunce, from MacDermid, described recent advances in electroless copper process technology. MacDermid's entire sequence has been designed to specifically maximise the reliability of inner-layer connections, and begins with an organo-silane conditioner, which is adsorbed only on non-conductive surfaces. A tin-palladium catalyst reacts with the conditioned surfaces, whilst depositing very little on copper. The accelerator is unusual in that, instead of dissolving-away the stannous tin, it oxidised stannous to stannic and left it in place as a shield to the palladium nuclei-resulting in a slow, controlled initiation of electroless copper. The copper chemistry gives a low-stress deposit with fine grain structure and minimal co-deposition of contaminants. Solder shock and interconnection stress testing demonstrated that, even after the most severe thermal abuse of high layer-count test boards, the interconnections between plated-through-hole and inner layers maintained their integrity.
Brian Reid, from Schloetter, gave a practical account of the development of an acid copper electroplating process to satisfy the demands of smaller UK PCB fabricators who wanted a via-fill process that could be used simultaneously for pattern plating, on standard equipment, with soluble anodes. The result is a high-copper, low-acid formulation with three liquid additives, which combine excellent levelling with good ductility and low internal stress. The typical cycle time is two hours at one amp per square decimetre for via filling in pattern plating mode. Although insoluble anodes gave the most consistent results, they are comparatively expensive and soluble copper anodes can be successfully used, provided they are removed from the electrolyte during idle periods.
The rain had almost stopped when delegates left the university campus, although it was rumoured that a mature student, named Noah, was working hard at his CAD system, drawing up plans for an ark...