Laser soldered wire bonding on liquid printed and sputtered contact structures on thin-flexes and injection molded devices

Traditional methods of wire bonding do not work well on liquid printed contact layers and soft substrate materials. Laser soldered wire bonding i.e., SB2-WB constitutes a novel approach to this problem as it works without the need for high pressures, temperatures or vibration [1].In this work we qualify different conductive layers of printed, plated, and vapor coated materials containing Au, Cu and Ag with regard to their suitability for forming a stable and reliable interface with a SAC_305 laser jetted solder bump, which is the basis for the laser soldered wire bonding process (SB2-WB). As printing technologies, we have chosen stencil- and screen-printing, alternatively gold was deposited by sputtering and patterned by subsequent lithography and wet-chemical etching. As substrates, we selected Polyethylene-naphthalate (PET) Teonex Q51, Parylene and Glass. Au wire and Ag ribbons were used for forming the wire bonds. Investigations on printed layers have been focused on correlation between paste receipt, printing technology related layer thickness, paste-related post-treatment and compatibility to laser assisted solder ball bumping i.e., SB2-Jet.Using a shear-tester, the mechanical load-capacity of the formed solder joints was measured, and the corresponding fracture modes were inspected using an optical microscope. The characteristic of the interfacial layers, material bulk textures and other metallurgical properties was analysed using SEM-FIB, EDX, X-ray and optical microscopy.Moreover, we demonstrated the forming of laser soldered wire bonds on two demonstrators showing different pad metallization, contact form and substrate bulk material.Finally, the future prospects of intended feasibility studies for laser soldered wire bonding, with structures consisting of paste and ink are highlighted.