Issue link: https://iconnect007.uberflip.com/i/881969
20 SMT Magazine • October 2017 nate heating mechanism to get that thermal en- ergy that you need in the actual product before you start soldering, or during the soldering op- eration as well. Their previous experiences can plague it a little bit right where they've never had to do this before or they've only done this in a leaded environment. We're now going to a lead-free environment with a couple other cop- per layers in the circuit board or a challenging thermal component, and really getting that in- novation in the soldering development process versus the way they've always done it before is the challenge. Solder Paste Printing vs. Jetting For volume production, assemblers still pre- fer solder paste printing. "The majority of it is screened down," says Nunenkamp. "We have looked at the ability to dispense solder; it's been a finite example that hasn't been needed across the board. I would say right now, we're about 99– 100% screening. At this point, our jetting focus would be for design-specific requirements or possibly in a rapid prototyping environment where stencil cost isn't insurmountable today and the management of stencils isn't significant because the infrastruc- ture's been in place for years. Our stencil supplier is around the cor- ner, the next driveway over, so lo- cale and accessibility to the sten- cils isn't a significant issue for dry versus a jetting need—unless a design or prototyping solution would drive that, and that would have to be really a volume driv- en prototyping effort that from a timing perspective won't allow for stencil creation or things like that where we have enough process development executed and knowledge base developed in our stencil." "We have products that are developed for jetting and it's been a hot topic in the last few years. However, I haven't seen it as quickly ad- opted as I thought it would, and I think it's be- cause it's targeted toward very tiny deposits," explains Sandy-Smith. "In 80% of assemblies, if customers are switching from tin-lead to lead- free, they're not using aperture sizes that are that small, not even less than 12 mil squares. So, you don't find any challenge with sten- cil printing and there isn't a push to move to jetting, but for customers trying to print very small things, I see jetting being considered. The other interesting application for jetting is find- ing jet heads in SPI systems, so that when you find an insufficient deposit, the jetting head in- side the SPI machine can add a little bit of paste where it is needed. I think jetting is a great ap- plication right now, but I think it's going to take some time for the miniaturization of printed circuit boards to really push us to using it on a wide scale." "Another consideration from a solder joint standpoint is that with more oxidation and finer powders, reflow has a more pitfalls or it's more challenging," says Sandy-Smith. "So, with finer and finer deposit sizes and smaller and smaller components, there might also be limits that we can reach as far as solder joints becom- ing so small. You might not be able to use air re- flow or it might change other processes, so the issue is not just how you deposit the solder paste." Customer Collaboration is Key Close cooperation or collab- oration between customers and suppliers always yield the best re- sults in the SMT industry. San- dy-Smith agrees. "That's most of what I do. I test our solder paste fairly rigorously when we have a new solder paste so that I can make sure we get the product and process recommendations out to our customer base," she says. "From a very basic standpoint, when the goal is to form a good solder joint, it's critical to have clean surfaces that are not oxi- dized. It's critical to have a flux that will stand up to the thermal profile that you're expecting to put it through, and it's also critical to keep all the surfaces oxide-free so the best metallur- gical connection can be formed. It is important to choose the right alloy to meet your process and the temperature limits that you want to conform with. These three things are really im- portant when I'm thinking about which prod- ACHIEVING THE PERFECT SOLDER JOINT: THE MANY PERSPECTIVES ON SOLDERING Robert Roush