A hierarchical evaluation of the solder paste printing process

Solder paste printing is one of the most critical processes in applying Surface Mount Technology. Investigation has shown that PCBA defectives derived out from reflow soldering inspection, in-circuit testing, and function testing up to final quality inspection, are heavily induced by poor solder paste printing. A hierarchy has been developed according to the stages and importance of the set-up of the printing process. The hierarchy consists of six levels: (i) geometry of the stencil openings resulting from fabrication method employed; (ii) solder paste matching; (iii) waiting time effects; (iv) squeegee materials selection; (v) printing machine operation parameter design; and (vi) reflow soldering process parameter design. The mechanisms, responses and factors within each level have been evaluated and optimized. Relationships amongst the levels were formulated to generate the complete set-up. Taguchi's Method was applied to the evaluation of level 5 and level 6, using L 27 and L 18 orthogonal arrays, respectively. ANOVAs were also run to test the significance of the results. The significant control factors were identified and optimized accordingly. The settings and rules of the hierarchy were established. Confirmation trials and follow-up showed that there was up to 25% reduction in the defect rate and 20% saving of the labour cost. The findings of the project illustrate a complete mechanism for material processing evaluation and improvement.