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18 SMT Magazine • June 2016 cil printer. I'd like to see more application of jet paste in the near future, for example in applica- tions of 3D printing. Las Marias: Are there any other technologies out there that you think could shake the solder paste printing process? Tseng: No, in the next 5 to 10 years, stencil printing will still remain the major process. Las Marias: Is there anything we haven't talked about when it comes to the solder paste printing process that you think we should be talking about? Tseng: I'd like to mention the importance the type of stencil and aperture design make. Laser cut stainless steel stencils have been the major technology used in the industry for many years. We believe it will still be the first choice for many engineers. Their use is low cost, fast and accurate. Yes, the cutting edge is not smooth enough compared with electrodeposition sten- cils to get the best and consistent solder paste transfer rate, but paste release is improved by nano-coating the stencils to increase the trans- fer rate and reduce the standard deviation of volume. People have been studying different com- ponents for years to find the best aperture pat- tern so that common SMT process issues can be eliminated such as voids, shorts, solder balls, etc. However, more and more differently shaped components are designed and used nowa- days. Yet, the industry lacks experience and novel approaches to find a better stencil design solution. For example, the void underneath the MOSFET component with large ground pad is still a pain for the engineers. Las Marias: Great. Thank you very much, Watson. Tseng: Thank you. SMT Tseng: Solder paste plays an important role in stencil printing. We need a solder paste with proper physical properties to yield high and uniform transfer rate, good deposition without slump or shorts, and long stencil life even at high speed printing. Selection of solder powder is also very im- portant. Changing from larger powder size to smaller one—for example, from type 3 to type 4.5 or even type 5—helps to get good print- ing performance. But it's not always good. Fine powder solder paste tends to oxidize faster, so that the stencil life and solderability, especially in air atmosphere, are not as good as that of larger solder powder. And it costs more as well. This is a trade-off. Las Marias: Aside from the miniaturization of de- vices, what other industry or market trends are af- fecting the solder paste printing process? Tseng: We have seen the LED industry is using stencil printing solder paste for die bonding. Solder paste is printed with thinner stencil such as 0.04 mm or 0.06 mm. The pattern is simple but the tolerance of height is tight. Again, fine powder solder paste is required in this process. Las Marias: Can you give a list of best practices for solder paste printing? What factors do users need to consider when it comes to solder paste printing? Tseng: A good stencil design, a good solder paste, and a precise stencil printer is the key to achieve best practices for solder paste printing. Las Marias: What about equipment—what can you say about the advantages and disadvantages of solder jet paste printing and screen printing? Tseng: Jet printing suits very well with NPI or low-volume high variety production. It doesn't require a stencil to get the desired solder deposi- tion. It can easily provide adequate amount of solder to large components while the other tiny components on the same board might need only minimal solder. In stencil printing, a specially designed step stencil is usually required to do so. However, the output rate of a solder jet- ting machine prevents it from replacing sten- SOLDER PASTE PRINTING: CHALLENGES AND BEST PRACTICES Watson Tseng

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