Issue link: https://iconnect007.uberflip.com/i/1337116
32 DESIGN007 MAGAZINE I FEBRUARY 2021 ger than tin-lead solders, their land patterns were much, much smaller than ours. Matties: So, Tom, when you are perhaps instructing a class or educating your custom- ers, what would you tell your students regard- ing footprints and how to avoid a failure? Hausherr: If I were going to do a class, I would say that accurate 3D models would have to be mandatory for quality control. I went to work for a company in Orange County, California, five years ago and I worked side by side with a gentleman who knew SolidWorks inside out. He did everything backward: He took the package dimensions and meticulously drew the 3D component model in SolidWorks, then took that model and designed a pad pattern underneath it. He created his own toe, heel, and side solder joints from the 3D model. It was kind of like reverse engineering. You create the 3D model first and then you cre- ate the solder pattern aer the 3D model. Matties: If you follow that strategy, whether you do it f ir st or second, the modeling shouldn't have any errors related to footprint mismatch. Hausherr: Yes, the 3D modeling must have the highest integrity. e model must be created using the exact package and terminal lead dimensions. When you have the 3D STEP modeling, it's really easy to detect an error in a through-hole footprint. But for surface mount, you put the 3D model on a surface mount pat- tern, you look at it and you think, "Well, I don't know if it's right or not. Is the toe correct? Is the heel correct?" A basic understanding of terminal lead style rules and their respective solder joint goals would help you make that determination. If I were to do a class on land patterns the first topic would be that 80% of the parts on a circuit board are chip resistors, capacitors, inductors, and filters. I would say, "Let's design a pattern for each package size because there's not one magic formula." An 01005 chip pack- age requires a unique land pattern, then then you have a 0201 and it requires its own unique toe, heel, and side. You have a 0402, a 0603, a 0805, and they all require unique patterns. e IPC-J-STD-001 standard is my leading document. When I took the IPC-J-STD-001 class, I learned how to hand solder and learned all about solder joints. I learned a lot of infor- mation about solder joint acceptability for assembly. e very first sentence in IPC-7351 land pattern guideline reads, "is document should adhere to the IPC-J-STD-001." Actu- ally, it doesn't, as far as solder joint goals for various chip sizes and gull wing pin pitches. However, the IPC-7351 mathematical model for calculating an accurate land pattern has stood the test of time for the past 34 years, but the solder joint goal values for toe, heel, and side need to be updated for microminiature components and fine pitch packages. Matties: How are the datasheets? When you come in and you're dealing with a new com- ponent, do you validate the datasheets on your own? Do you trust them? What's your strategy there? Hausherr: e datasheets provide the package dimensions. And most datasheets today pro- vide the recommended pattern. Some of the patterns are getting so complex that it's like designing a mini circuit board. It takes five hours to design one pattern. Today's manufac- If I were going to do a class, I would say that accurate 3D models would have to be mandatory for quality control.