Issue link: https://iconnect007.uberflip.com/i/1498723
MAY 2023 I DESIGN007 MAGAZINE 27 many calories it has. I don't know what's in the bun, the burger, or the magic sauce, but I know it's really good. I don't care, because at the end of the day I put the burger in my food hole, I feel good, and everything is fine. In a simple sense, it's the same with circuit board material specifica- tion. I'm not talking about high-performance, signal integrity type of boards, but general cir- cuit boards. As I said, about 85% of what we do is an assembly-line hamburger, and it just needs to taste like a hamburger at the end of the day. Yes, the designer should specify the material, but they shouldn't over-specify it. Over-specification—over-constraint—is the problem. Most boards we see could surely work without specifying a slash sheet. When we go offshore, it's the slash numbers and brand name or trademark indicators that tend to stop the procurement process. Happy Holden: Ideally, the material selection process should be a conversation with the design engineer, designer, assembler, and the fabricator. That's ideal. I've always cautioned customers not to put this decision in the lap of the EMS provider. The EMS company is only concerned with two surfaces, and they don't know a lot about the interior or the stackup. at communication would be ideal, but the reality is, many PCB designers are not in touch with the fabricator—prototype or offshore. ey may not even know who will fabricate the prototypes. PCB designers are so far removed from the fabrication and assembly stakehold- ers who build their products that there's almost no way to execute solid DFM processes. Glass transition temperature rating (Tg) specifica- tion is a good example of this. PCB designers have no business specifying Tg if they're trying to save money. Many of our customers have standard notes, and I used to do this. I wanted the best qual- ity board, and I always specified Tg 170°C or Tg 180°C, which is a higher temp perfor- mance range for laminate material. I did that every day, all week long, not realizing that it was extremely costly in volume. We had the other stakeholders on our projects and in our corporation who wanted to make money, not spend it. Aer working for a bare board supplier and now an EMS supplier, I've found that there are other stakeholders involved who plan the assembly of the board. ey know how to effectively modify the material's Tg and aim it toward the number of thermal excursions that the board will experience—and which the designer has no idea about. For instance, a multilayer board design has surface mount on one side, no surface mount on the other side, and a couple of through-hole parts. How many thermal excursions will this board go through? It may go one pass on the top side through the oven, and then one pass down the wave solder line for the through- hole ports. at's two thermal excursions. If the volume assembly supplier were given an opportunity to change the spec to Tg 150°C, there would be an opportunity to realize a cost savings here. But if I've specified 180°C Tg as the standard, the EMS provider must get it quoted that way. Shaughnessy: Is there one document or guideline that contains these metrics? You might go to laminate suppliers' datasheets or somewhere else to get the information, but can you trust it? You must keep it simple from a designer stand- point, because the biggest culprit working against driving your board to success in vol- ume is over-specification. If we over-specify, we might be spending dollars unnecessarily. If we partner with suppliers, we build a rela- tionship of quality, delivery, and low cost. If we build trust with our suppliers, they become part of our stakeholder community and we can rely on them to do the right thing—to build