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74 SMT007 MAGAZINE I AUGUST 2022 them were constantly asking questions of their co-workers to understand all the aspects of electronic assembly," Maggie elaborated. "is trait became contagious, as the other workers wanted to learn more, too." "One of the chaps, who was a little rough around the edges and a sloppy dresser, came into work one day sporting a complete make- over. He was well groomed and dressed in smart business casual attire," John said. "I complimented him, and he told me that seeing Andy so 'with it and enthusiastic' encouraged him to make a change." Meanwhile, at their favorite pizza shop, Sue and Andy were working on their homework and preparing to give their second class at Ivy-Benson. "How is 'Miss Can't Speak in Front of a Group' doing?" Andy teased. "anks for helping me in the first class," Sue said. "Toward the end, I actually felt comfort- able speaking to the group. It helped that you lectured, and I just answered questions." "Well, at the end you were speaking more than I was," he responded. "I think I could explain the derivation of the area ratio formu- las, but you did it much better than I could." "is has been a big breakthrough for me," Sue said, with a little shakiness in her voice. "Anyway, are you ready for tonight's math class?" Sue asked. "Yikes, trigonometry," he said, jokingly. "ough, I have gotten sine, cosine, and tan- gent memorized with the mnemonic: Oscar had a heap of apples 1 ." Off they went to their math class and, aer class, ended up in their favorite ice cream par- lor to discuss the next SMT lesson they were teaching. "In three days we teach the very basics of how solder paste is made and how it functions," Sue began, with Andy responding, "I found it fascinating." "I think it's neat that the solder powder can be made by taking liquid solder and splashing it on a fan blade," Sue commented. "Yeah, then they put the powder through screens to get the desired particle size distribu- tion," he added (Figure 1). "What I never appreciated was that solder paste is a multifaceted material. What every- one refers to as flux is really a complex mix- ture of materials that not only enables effective soldering but must have the right viscosity for printing and tack to hold the components," she said. "Another interesting property that a solder paste must have is good response-to-pause 2 ," Andy said. "at's one thing I haven't heard of," she replied. "Well, when you stop the line and the sten- cil printer is not printing, some solder pastes stiffen and the first print aer starting up is not useable. So, that board has to be wiped clean and reprinted. Chuck Tower said that Ivy- Benson had used a paste that had this prob- lem, and it caused a lot of downtime. e paste was very cheap but ended up causing the com- Figure 1: Solder powder is sieved by using screens of differing fineness. The numbers are the number of openings per inch. A Type 4 solder powder uses a coarse screen of 400 and a fine screen of 635. The powder on the 635 screen is the Type 4 powder.