Stencil evaluation of ultra fine pitch solder paste printing process

Abstract

For the ultra fine pitch applications, solder paste printing acts as one of the most critical step in the SMT assembly process. A review of literature indicates that solder paste printing contributes around 60% of the total process defects. One of the key factors for the performance of solder paste deposition is stencil fabrication quality. Other important factors include stencil aperture design, paste printing parameters and stainless steel raw material. The incoming inspection of the stencil typically includes frame appearance examination and tension force measurement. Inspection for individual aperture under microscope is not feasible due to the large size of stencil. Related researches also shows that stencil aperture and wall roughness would impact solder paste deposition quality. This study uses measuring tools combined with statistical methods to investigate the effects of laser cutting taper angle and speed on the stencil quality. The stencil quality is defined by the amount of stainless steel residues after laser cutting and the roundness of stencil aperture for a circular aperture design. The solder paste inspection data was analyzed to investigate the influences of electro-polishing and stencil area ratio on the transfer ratio during solder paste deposition. Weighted ranking was adopted to indicate the variation in solder paste deposits for 01005 and 0.4/0.3mm pitch CSP pads due to stencil fabrication process. This study aims at determining the optimal parameters for stencil fabrication and corresponding performance in solder paste deposition. Results shows that the taper angle and laser speed have significant impacts to the amount of stainless steel residue and the roundness on laser entry side. Through EDS analysis, the residue material generated in laser cutting process is organic and it can't be effectively removed by laser electro-polishing process. Stencil printing quality will be affected by laser machine stability. If the laser fabrication process can be well controlled, 60% transfer ratio of solder deposition can be achieved with area ration of 0.5. Stencil thickness of 0.08mm might not be feasible for 01005 components. This is due to that the tension force of 0.1mm thick stencil is higher than that of 0.08mm stencil. Also, the anti-scrape ability of 0.1mm thick stencil is superior to that of 0.08mm. It is critical to meet the solder volume requirements for SMT connector and BGA components with coplanarity spec, of 0.1mm. Considering the consistency in solder deposition for 01005 components, the round stencil aperture is superior to the rectangle aperture. For the rectangle aperture of connector and gull-wing components, performance of vertical printing (with respect to the direction of stencil aperture) is superior to that of horizontal printing.