Issue link: https://iconnect007.uberflip.com/i/881969
22 SMT Magazine • October 2017 uct to choose for a customer. For most lead-free assemblies, there are few basic materials that re- ally work well. The other big question to ask is whether you're going with a no-clean process. Where you process the paste and the amount of thermal energy put into it is going to render the flux benign. If you're planning to wash off the residue, you should still use a no-clean paste and wash it. There are also water-soluble pastes that that can be washed off with just water." Customers often follow the components manufacturers' recommendations at the start- ing point in their board designs—for instance, how a land pattern should be put on the board for a particular component. This results in a manufacturing challenge for the assemblers. "We have to be innovative on our manufactur- ing capability or ability to make solder joints for them because they want the smallest pad possi- ble or they want the least voids possible in the bottom terminated components because that affects the performance of those components," says Nunenkamp. "So, what can we do from a manufact uring perspective to ensure that hap- pens? Do we innovate in the pad design, and adjust our process and really narrow our pro- cess window so that we're successful every sin- gle time? Those are the types of things that they expect from us. Just getting that quality solder joint to fill its size and consistency is our tar- get. So, if you can give them feedback upfront, we're going to have a little bit better possibility. We utilize software rules to give that feedback and identify a rule set that would provide con- sistency for us or highlight the nuances of those small solder joints that Brook was mentioning. The other day, I found a board that was going to give us 7.8 mil solder pads, which is going to provide challenges in our stenciling process. They don't want to change the pad design, so I have to alert them that I have to change my stencil thickness to support that release prop - erties and adequate soldering requirements. Those are the types of things that we can give design feedback on to ensure success and qual- ity products in really a timely manner for our customers." "I would agree with that," says Sandy-Smith. "Many times, if you look at the root cause from a design choice that was made, the issue stems from the board designer and it's something that they can't change. So that's what pushed us to develop materials that go above and beyond to really fix QFN voiding or BGA warpage-induced defects. For instance, in the case of a bottom terminated component like a QFN, if there are plated vias in the pad that are very large and they're not plugged, the right amount of solder paste can be put down, but the paste will flow into the vias and end up with very high void- ing in that part. Aside from adding more sol- der paste in this case, there isn't really a way to get rid of the voiding. If you could go back to choose a different via design, you could fix the voiding issue entirely. But if you can't change that design, then you have to find material so- lutions that go above and beyond in order to reach the goals that you want." Vias, and Other DFM Challenges Speaking of vias, designers most of the time put the via in the pad and don't bother to even think or ask questions about what happens to the solder paste during reflow. "There are times where someone came to me complaining of high voiding just on this one component, so we did a lot of research or we got them set up with a low-voiding materi- al. Then, when we looked at the actual X-rays and found that there were huge vias that pulled the solder away from the solder joint," says San- dy-Smith. "Often, people don't realize that they need to do a cross-section because it's hard to measure stand-off height for these components without doing destructive analysis. Because ev- erything is underneath the component with ACHIEVING THE PERFECT SOLDER JOINT: THE MANY PERSPECTIVES ON SOLDERING " The other day, I found a board that was going to give us 7.8 mil solder pads, which is going to provide challenges in our stenciling process. "