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82 The PCB Magazine • February 2017 Introduction Metalizing materials such as polyimide used for flexible circuitry provides a significant chal- lenge for process engineers. Conventional elec- troless copper systems often required pre-treat- ments with hazardous chemicals or have a small process window to achieve a uniform coverage without blistering. It all boils down to enhanc- ing the adhesion of the thin film of electroless copper to these smooth surfaces. In addition, in- ternal stress in the copper deposit is a significant factor with respect to adhesion of the plated metal to the substrate. This process of plating on polyimide flexible materials is ver y much POP (plating on plastics). Electroless copper has been adapted for metalization of difficult to plate sub - strates and materials such as polyimide, POP and molded interconnect devices (MID). The author will present MID in a future column. The Challenge of Metalizing Smooth Surfaces One significant concern with metaliizing thin films over substrates is the concept of deposit stress. Hydrogen gas is a by-product of the electroless copper deposition process. Consequently, hydrogen may be incorporated into the thin deposit with a negative influence on internal stress. The hydrogen gas issue has been attributed to blister formation in the cop- per deposit. In addition, a challenge typically encountered on smooth surfaces such as poly- imide-based flexible circuit materials is ensur- ing good adhesion of the electroless copper to the substrate. Adhesion of a thin film deposit to a substrate that lacks sufficient microrough- ness is compromised. To mitigate this issue, one solution is to reduce the internal stress of the copper deposit as much as possible. by Michael Carano RBP CHEMICAL TECHNOLOGY Metalizing Difficult-to-Plate Substrates TROUBLE IN YOUR TANK

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