Novel Formaldehyde-Free Electroless Copper for Plating on Next Generation Substrates

Abstract

The deposition of electroless Copper on dielectric substrates and the subsequent electrolytic build-up of a thicker Copper layer are widely used steps within the production of modern Printed Circuit Boards (PCB), and while there have been numerous developments within PCB production, the current manufacturing technologies continue to be reliant on the autocatalytic deposition of Copper from a solution containing formaldehyde as the reducing agent, even though the chemistry is known to pose a risk to human health. Further, as the high volatility of formaldehyde generally increases the exposure to the hazard, it is understood that critical air concentrations can easily be exceeded. With this in mind it is clear that the development of environmental and user friendly electroless Copper baths has become a subject of importance. Nevertheless, the introduction of “green” plating chemistry into the market remains a challenge due to high industrial standards in terms of performance and cost-efficiency, which have been established by the conventional plating products and limit their replacement. In the case of the electroless Copper baths, formaldehyde-free alternatives have to show excellent substrate coverage with metal, provide coatings with high conductivity and uniformity and should lead to very good reliability results. Moreover, the solution, and final Copper layer have to function with the diverse range of dielectric materials that are currently employed. Due to application needs, there has been a shift within PCB design towards the use of very smooth substrate materials with low coefficients of thermal expansion. Such materials offer the opportunity for further miniaturization of circuits and are optimal for adoption within packaged die components (IC substrates). However, smooth substrate topographies typically lead to a limited adhesion of the electroless Copper layer, and increases the risk of delamination or blister formation. To prevent this, the properties of the metal film itself, as well as the chemical properties of the Copper bath, from which it is deposited, are critical, with a key factor being that the deposited layer is generated under internal tensile stress, as this has been shown to be of importance in reducing blister occurrence. While formaldehyde based plating solutions have been modified to satisfy this requirement through the adoption of additives and organic substances, there is still very little experience available regarding chemical approaches utilizing other reducing agents. Changing the reducing agent generally requires a complete redesign of the electroless system, including careful selection of the complexing agents and additives, readjustment of the chemical concentrations and optimization of the baths physical operating conditions. In this work we describe a new type of formaldehyde-free electroless Copper solution suitable for a broad set of applications and materials, and specifically the processing of next-generation substrates. This new plating solution has been successfully applied in both laboratory and production-scale environments, with its performance being evaluated and benchmarked against an existing formaldehyde-containing reference. The obtained metal layer has been characterized through a number of analytical techniques, including microscopy, XRF, SEM, adhesion tests as well as non-blister performance. Based on the data obtained we believe that the newly developed solution utilizing a non-formaldehyde reducing agent provides a suitable technology for PCB production without a loss of process performance, and thus provide a sustainable “green” alternative to the industry.