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66 SMT007 MAGAZINE I JANUARY 2023 with circular apertures for BGA components— is that correct, Carlos?" "Si, señorita," Carlos responded. Sue continued, "Printing square apertures provides about 25% more surface area, hence 25% more paste volume. However, experi- ments 1 have shown that square apertures also enable improved transfer efficiency over circu- lar apertures. It is believed that this is due to the curvature of the circular aperture, which makes the solder paste adhere more to the sten- cil than the pad. If we change the stencil aper- tures to square, the problem might go away." To illustrate her point, Sue pulled out a cou- ple photos showing the differences between reflowed solder deposits from both circular and square apertures (Figure 3). Miguel seemed impressed. "Wow, that is a dramatic difference," he said. "I see your PWB pads are solder mask defined, and that helps keep the solder paste flux from flowing away, which is a good thing," Sue said. "In addition, the reflow profile is a ramp-to- peak, which also minimizes graping. So, my hope would be that changing from circular to square apertures might solve the problem 1 ." Meanwhile, Andy was meeting with José and the four process engineer candidates: Juan, Santiago, Jesús, and María. José explained to the group that Andy wanted to train the candi- dates to become process engineers. e train- ing would be voluntary and would be per- formed on overtime, so the candidates would be adequately compensated for their time. All four looked nervous. José had selected them because they had worked at the company for more than five years, were all good employ- ees, had good attitudes, and most importantly, were eager to learn. "What if we're not smart enough?" Santiago asked anxiously. "Andy and I discussed what you would need to learn and I'm confident all of you can do it. You are all high school graduates, know how to run almost all the equipment, are good at math, and most importantly, you're all curious and interested in learning new things," José answered. "How do you know we are good at math?" María asked. "Because, at one time or another, each of you has asked me either how line balancing or reflow throughput is calculated. Aer showing you how to do those calculations, I've asked you to perform them on numerous occasions, and you've always gotten them right," José responded. Will square apertures fix the graping prob- lem? How will Andy prepare these new pro- cess engineers for success? Stay tuned to find out. SMT007 References 1. "Square vs. Circular Apertures and the Five Ball Rule Revisited," by Ron Lasky, hotwires.net. 2. For more details on minimizing graping, see The Printed Circuits Assemblers Guide to Solder Defects, by Christopher Nash and Dr. Ronald C. Lasky, 2021. Ronald C. Lasky is an instructional professor of engineering for the Thayer School of Engineering at Dartmouth College, and senior technologist at Indium Corporation. To read past columns, click here. Download The Printed Circuit Assembler's Guide to… Solder Defects by Christopher Nash and Dr. Ronald C. Lasky. You can also view other titles in our full I-007eBooks library. Figure 3: Solder paste reflowed after being printed from a circular aperture (left) and a square aperture (right).