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18 SMT Magazine • December 2017 From a solder supplier's perspective, one of the key challenges is the ever-shrinking solder deposit, and the challenges that it introduc- es to the printing process. "When we got into the business 20 years ago, the smallest compo- nent that you saw was an 0603 in practice; 0402 was still theoretical. Now, we're talking about 08005s, sub-miniature and below the visual threshold. This not only causes significant chal- lenges to the printing process, but downstream processes as well," explains O'Neill. "As it relates to print defects, the area ratio or the thickness of the stencil as it re- lates to the size of the hole in the stencil, is a criti- cal feature now. So, we are con- fronted with aper- tures that are as tall as they are wide, and get- ting solder paste out of that aperture consistent- ly and repeatedly is becoming more challenging because the ratios are becoming inverted. The 0.66 area ratio was considered the finest area ra- tio that was practical in a production environ- ment; now, I would say a 0.5 area ratio is be- coming more commonplace due to advance- ments both in solder paste printing technology, as well as complementary technologies, includ- ing hardware, stencil coatings, etc. Everybody is moving forward and giving assemblers more tools in their toolbox to address this fine print challenge. Matt's situation with flex inherent- ly produces more variability than somebody assembling rigid circuit boards might have, so he's got an even taller order in addressing con- sistency in his process." O'Neill says consistency and overall volume are the two most important variables that they are focusing on to help assemblers improve transfer efficiency. "Not only do we want to get as much solder paste through that aperture as we can, we want to get it through repeatedly as we possibly can," he says. In order to do that, the solder paste rheol- ogies are evolving. Typically, as the viscosity gets lower, solder pastes become creamier. "It doesn't take a genius to see how a solder paste that is more fluid and less viscous is going to have an easier time getting through the aper- ture hole," says O'Neill. "The problem is that you now have to have a paste that, when the stencil is removed, the deposit maintains its shape because it would bridge and slump and cause other problems. We have to look at those two things together. Lowering viscosity alone, while it may improve transfer efficiency, won't necessarily improve consistency. Pad design, board design—there's a host of variables that all go into it. Board support being a big one—as the flex guys know better than anyone. That's getting it through the aperture. Then getting it through the apertures consistently, the advent of solder paste inspection (SPI) equipment has really given the assembler the ability to mea- sure every single deposit on that board, which is really mission critical now that we're getting to the visual threshold in many cases of compo- nentry. SPI gives us the ability to analyze in real time every deposit made on a circuit board, and that closes the feedback loop so that the pro- cess engineers and supervisors can make sure the print process is well within the defined pa- rameters." "The SPI is a very important tool not just for the assembler, but also for solder paste manu- facturers because it's really the ability to mea- sure your production output, and you absolute- ly need that, in my mind, in the modern appli- cation. If you're not measuring it, you don't re- ally know what you're doing. Especially as you get down into these area ratios at 0.66 and be- low. Matt brought it up: Mesh size is another way to improve transfer efficiency. Everything in solder paste R&D is a quid pro quo. The more activity I introduce to a solder paste, for exam- ple, the more reactive it becomes, which typ- ically adversely affects open time transfer effi- ciency. The solder paste is working on itself as much as it's working on the components that you're trying to solder. When I say it's a quid pro quo, if I increase the activity, I have to ex- EQUIPMENT MATTERS IN SOLDER PASTE PRINTING Tim O'Neill

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