Issue link: https://iconnect007.uberflip.com/i/1466505
60 SMT007 MAGAZINE I MAY 2022 "Let's see if we can figure out the questions first without looking them up," Sue suggested. "Good idea," Andy said. "We already know that SAC means S = tin (from Sn), A = silver (from Ag), and C = copper (from Cu). I also remember hearing Chuck say that tin is the fundamental metal for solders, and almost all solders start with tin." "So what about SAC305? Isn't silver expen- sive?" Sue mused. "If so, there probably isn't that much silver in it or the solder paste would be under lock and key." Andy responded, "You know Pete Smith, right? He's really smart. He once mentioned that there are solder pastes with only silver and no copper, so I'm thinking the amount of cop- per is likely very small." ey hemmed and hawed as they spiritedly debated on the answer. First, it was 30% sil- ver and 5% copper, but those seemed too high. Aer a few minutes, they settled on 3.0% silver and 0.5% copper since they felt the numbers would be in order with the letters SAC. "Okay, let's use our phones to look up the answers," Sue suggested. She entered the search and found a website 1 with the answer. "We were right!" she said loudly. "is is kind of fun." "I agree," Andy said, laughing. "Let's look at the next question." 2. The formula for aspect ratio (AR) for a stencil aperture is: d/t, where t is the stencil thickness and d is the line width. Assuming the stencil is 5 mils thick, what is the finest line that can be reliably printed? A rule of thumb is the AR should be 1.5. a. 5.5 mils b. 3.5 mils c. 7.5 mils d. 10 mils e. 4.5 mils "at's obvious, the answer is C. 7.5 mils," Sue exclaimed. "Whoa, wait a minute, how could you get the answer so fast?" Andy asked, feeling a bit dejected. "Well, look at the formula," she responded brightly. "If t = 5 mils and AR is 1.5 = d/t, d must be 1.5 times t or 7.5." "Yikes, I see it now, but not nearly as fast as you did," Andy said, letting out an audible groan. "Even though I didn't see its value at the time, I was a good student in Mrs. McGillicud- dy's ninth grade algebra class," Sue shared. Upon hearing this from Sue, Andy started thinking he needed to "up his game," and then suggested they look at the next question. 3. Assume that component placement is the "gate" in an SMT assembly process line, which has one chip shooter and one flexible placement machine. The chip shooter takes 60 seconds to place its components while the flexible placer takes 45 seconds. Some chips are being placed by the flexible placer. To improve productivity, what should be done? a. Nothing, everything is fine. b. Chips should be taken off the flexible placer and put on the chip shooter as chip shooters are best to place chips. c. Chips should be removed from the chip shooter and placed on the flexible placer to time balance the line. d. The chip shooter is so slow, so move all the components to the flexible placer. "at one is obvious, right? e answer is C," Andy exclaimed. "I agree," said Sue. "Chuck had talked about this from the first day Maggie and John took over the company. 'e line should be time balanced,' he would say." "It's sort of hard to understand how some- one would not see this," Andy said. "You will assemble more boards if the line cycle time is at minimum, and it is minimized by time bal- ancing the component placement machines."