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In an interview with SMT Magazine, Watson Tseng, General Manager of Shenmao America Inc., talks about the challenges in solder paste printing, key parameters and best practices to consider, and variables involved when it comes to selecting the appropriate solder paste for the printing process.
Stephen Las Marias: What are the biggest challenges that your customers face when it comes to solder paste printing?
Watson Tseng: Many of our customers are facing greater challenges assembling smaller and smaller components onto their PCBs and flex circuits. Due to the decreased size, the stencil aperture has to be much smaller than before, for example 0.25 mm or less. It increases the challenges of solder paste printing—the area ratio (AR) becomes smaller and the transfer rate goes down. Getting a good deposition of solder paste becomes more and more difficult. Some customers increase the printing speed to get higher output rate. Some are printing at 120 or 150 mm/s or even faster. A good solder paste designed for high-speed printing is necessary for this application. Otherwise, there will be a lot of shorts or slump failures found after stencil printing.
Las Marias: From your perspective, how do the tighter tolerances and even narrower PCB lines and spacing impact the solder paste printing process?
Tseng: Yes, they do. Especially when you have both large and small components on your board. It’s a huge challenge to do stencil printing for this kind of build. A good stencil design, a good solder paste, a precise stencil printer is required to achieve the best result.
Las Marias: What about the solder pastes? What are the variables involved when it comes to selecting the appropriate solder paste for the printing process?
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 important. 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 printing 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.
Editor's Note: This article originally appeared in the June 2016 issue of SMT Magazine.