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86 PCB007 MAGAZINE I AUGUST 2020 Introduction The electroless copper metalization process (also known as plated through-hole or PTH) is a system of multiple process steps with the ex- pressed goal of producing a continuous, void- free, and tightly adherent deposit through the vias and surface of the circuit board. While these various steps in the process all play a role in the successful operation of the process, the subject of this column is on the electroless copper plating solution itself. The discussion will focus on a copper formulation based on copper chloride, EDTA, formaldehyde, and so- dium hydroxide. The Autocatalytic Copper Process Electroless copper plating solutions by their very nature are thermodynamically unstable. There are several side reactions that take place in the course of the everyday operation of the electroless copper plating solution. The stabil- ity of the plating solution requires diligence A Process Engineer's Guide to Electroless Copper on the part of the process engineer. For cop- per ions to be reduced to copper metal, the inherent instability of the electroless copper solution is expected. However, the stability of the solution must be controlled to prevent del- eterious defects, including interconnect defect (ICD), excessive plating thickness, and exces- sive stress of the copper deposit. Deposit stress alone can lead to copper blistering from the hole wall and separating from the internal lay- er copper interface. A few key aspects of stability include the fol- lowing: • Stability will decrease as specific gravity increases. • Stability will decrease as the temperature increases. • Although stability is improved by running at a lower temperature, the rate is sacri- ficed to the point where a medium deposit bath is unattainable. Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY