Fluxfree solder paste and process for standard SMD components

In this paper, we present the application of the novel fluxfree soldering process for assembly of standard SMD components on a PCB. A standard SAC305 (Sn-3.0Ag-0.5Cu) type 3 powder is mixed with a specific binder in a ratio of 95:5 wt.% to form a solder paste with a printability similar to that of standard SAC305 no clean flux paste. However, the binder contains only organic solvents which evaporates and has no aggressive chemical components. The prepared solder paste is printed using a stencil printer on a PCB having pads with ENIG metallization. After printing, different standard SMD components such as resistors, capacitors (e.g., SMD 0603), amplifiers and transistors (e.g., SOT23) but also bare dies for chip on board technology are assembled onto the PCB using a pick and placer. After the assembly, the PCB is soldered using a fluxfree soldering profile. Occasionally, depending on the reflow furnace, optical inspection revealed visible contamination of the PCB. It was found that due to the evaporation of Sn formate, deposition of Sn residues (brown in color) around the solder interconnect during the cooling cycle can occur. Therefore, the activation process during the soaking phase was modified and additional purging was implemented which effectively helped to avoid the deposition of Sn formate. Using the additional purging before cooling down no visible contamination were observed. Using this novel soldering process, a clean PCB assembly was realized. The soldered standard SMD components were subjected to optical inspection, wetting analysis, meniscus formation, void detection using X-ray and mechanical shear strength measurements. The results showed good component wetting and acceptable solder joints as per IPC J-STD-001. The mechanical shear strength was comparable to that of the standard SAC305 solder paste. The solder interconnects of 1 mm² test chips showed lower void formation compared to standard flux based solder process.