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Novel formaldehyde-free electroless copper for plating on next generation substrates
Keywords: Electroless Copper Plating, Formaldehyde-Free, Printed Circuit Board (PCB)
TITLE: Novel formaldehyde-free electroless copper for plating on next generation substrates ABSTRACT BODY: The deposition of electroless copper films on dielectric substrates and a subsequent galvanic build-up of the copper layer are broadly applied steps in the production of printed circuit boards (PCB). Over the last decades, a huge variety of processes have been developed that allow the processing of all kinds of materials and that are able to meet versatile industry requirements. Nevertheless, current production technologies that are based on the autocatalytic deposition of copper all use formaldehyde as the reducing agent, even though the chemistry is a known danger to human health. The IARC (WHO International Agency for Research on Cancer) has classified the substance as a human carcinogen. The high volatility of formaldehyde generally increases the exposure to the hazard so that critical air concentrations can easily be exceeded. Having these threads in mind it is obvious that the development of environmentally-friendly electroless copper baths has become essential today. Nevertheless, the introduction of green plating chemistry into the market is still challenging due to high industrial standards in terms of performance and cost-efficiency, set by well-established conventional plating products. In the case of electroless copper baths, formaldehyde-free alternatives have to achieve excellent substrate coverage with metal, provide coatings with high conductivity and uniformity and should lead to very good reliability results (e.g. copper to copper adhesion). Moreover, the solution has to deposit a copper layer in a way that delamination failures are avoided on all types of dielectrics. In recent years, the PCB industry has shifted its interest more and more towards very smooth substrate materials with low coefficients of thermal expansion. The materials allow a further miniaturization of circuits and are optimal for the electrical integration of semiconductor units (IC substrates). Unfortunately, the smooth substrate topography typically leads to a limited adhesion of the electroless copper layer which increases the risk of delamination and blister formation. To prevent this, the intrinsic physical properties of the metal film (and therefore the chemical properties of the copper bath) are critical. A key factor is that the metal deposition occurs under the generation of internal tensile stress so that the blister tendency is minimized. Formaldehyde-based plating solutions could successfully be modified to meet this requirement by the implementation of several additives and organic substances. The additives allow the adjustment of the internal stress during the formation of the copper layer. Nevertheless, very little knowledge is available regarding chemical approaches that can be applied when alternative reducing agents are used. The exchange of the reducing agent generally requires the complete redesign of the electroless system, including the correct choice of complexing agents and additives, the readjustment of chemical concentrations and the optimization of physical working parameters. In this work we describe a new type of formaldehyde-free electroless copper bath with advantageous properties that can be used for a broad set of applications and materials, especially for the processing of next-generation substrates. The plating bath has successfully been applied in laboratory and in production-scale environment. The results have been evaluated in detail and were benchmarked against a formaldehyde-containing reference. A characterization of the obtained metal layer has been carried out by different analytical techniques, including microscopy, XRF, SEM, adhesion tests and non-blister performance. Additionally, studies concerning chemical bath stability, throwing power and electrical reliability have been made. Based on the obtained data we believe that the current achievements provide a suitable technology to replace formaldehyde in existing PCB production lines without loss of process performance and thus provide a sustainable “green” alternative to the industry.
Dr. Christian Wendeln, Research Scientist
Atotech Deutschland GmbH
Berlin, Berlin
Germany


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