Authored By:
Jeffrey Len Yung Kwuan, Leon Rao, Evan Yip, Wisdom Qu, Jonas Sjoberg
Indium Corporation of Asia Pacific
Singapore
Summary
The growth of the Internet of Things (IoT) has greatly increased miniaturization development in packaging and board level assembly. As the industry is moving to smaller and finer pitches such as 008004, 0.3mm CSP, and BGA, screen printing becomes one of the critical processes to produce a good quality surface mount assembly. It has been widely accepted that 50–70% of SMT defects come from printing applications. There are many variables that will affect the quality of printing such as machine set up, solder paste handling and storage, stencil quality, stencil aperture design, printing parameters, and others. In this paper, we will evaluate the impact of stencil quality statistically through MiniTab software by comparing the printing performance of 0.35mm pitch and 01005 pads from different stencil suppliers.
Conclusions
The following is the summary from this study:
All EP and nano-coated stencils, except for stencil E, were able to perform good printing performance with Ppk >1.67 on SQ0.20mm and C0.22mm pad.
With a C0.22mm pad, stencil D achieved high transfer efficiency on the EP and nano-coated stencil. One-way ANOVA analysis showed that there was a significant mean difference on EP stencils, except for stencils B and C while all nano-coated stencils had significant differences with each other except for stencil B with stencil G and stencil A with stencil C.
With the SQ0.20mm pad, stencil D achieved high transfer efficiency on the EP stencil and stencil F on nano-coated stencil. One-way ANOVA analysis showed that all EP stencils were significantly different from one another. All nano-coated stencils are significant different with each other except for stencil C with F and stencil B with G.
Impact of stencil quality on printing is greater as area ratio reduced to 0.59
Nano-coated stencils provided lesser volume variation compare to EP stencil.
Based on this study, we can conclude that different stencils will provide different printing performances. There are a few possibilities that could contribute to the differences, such as the accuracy of laser cutting aperture dimensions, consistency of foil thickness, smoothness of the aperture side wall, nanomaterial, and the method by which the nano-coating material is applied. Verification of the factors is beyond the scope of this study, and will be part of the consideration for future work.
Initially Published in the SMTA Proceedings
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