A New Flex Electroless Nickel Chemistry for Flex ENIG and ENEPIG Applications

Flexible electroless nickel immersion gold (Flex ENIG) processes have been widely utilized for over two decades in flexible PCB fabrication with little modification or upgrade. With increasing demand for flexible electronics, miniaturization, and increasing popularity of the Internet of Things (IoT), flexible PCB fabrication continues to show growth coupled with increasing demand for technology advancement. At the heart of flexible ENIG is a specialized electroless nickel (EN) formulation that delivers a columnar structure, which produces a microcrack fracture mechanism upon bending. This eliminates the risk of crack propagation through to the underlying copper and the risk of open circuits. In this paper, we have demonstrated experimentally, as well as theoretically, how to promote and maintain a columnar EN structure, with contrast to the traditional electroless nickel deposit. Extensive EN structure analysis is supported with MIT bend test data to demonstrate an extraordinary electroless nickel working life, providing fabricators with significant operational benefits and waste savings. Continuing with the environmental, social, and governance (ESG) theme, further waste reductions are proven by elimination of dummy activation and compatibility demonstrated with both traditional cyanide and cyanide-free based immersion gold systems, with a focus on soldering and corrosion performance. As the trend towards miniaturization continually challenges fabrication processes, we evaluate the interactions between activation and electroless nickel chemistry to optimize this synergy, allowing for excellent fine line plating capability. Wirebonding has been another important aspect in recent times for Flex ENIG process. It is common to see Al instead of Au wirebonding for Flex ENIG surface finishes since nickel will diffuse into the gold wire and form a brittle Au-Ni intermetallic. In the case where gold wire is preferred, one way to address this issue is to increase the immersion gold thickness to minimize the nickel diffusion into gold wire. This can create excessive corrosion concerns due to the galvanic displacement nature of the immersion gold process and create another issue to manage. ENEPIG is well known to have reliable Al and Au wirebonding performance, due to the Pd interlayer providing a diffusion barrier. The new Flex EN is fully suitable for the ENEPIG process and a detailed study of Flex ENEPIG process will also be addressed.