Issue link: https://iconnect007.uberflip.com/i/1256432
52 DESIGN007 MAGAZINE I JUNE 2020 critical dimensions, pin function and assign- ment, polarity, and component footprint pat- tern for assembly. But all too often, datasheets from reputable suppliers are missing critical information. I have seen datasheets for com- ponents improperly mirrored or missing criti- cal alignment dimensions. Recommended foot- print patterns for smaller parts often do not specify or take in to account the type of solder that will be used or reference the stencil thick- ness for optimum solder deposition. This has led to misdirected finger-pointing between de- sign and manufacturing stakeholders. One re- lies on the datasheet for recommended geom- etry, and another must account for those rec- ommendations and then select an appropriate solder paste, stencil thickness, and oven pro- file to get each component to process success- fully with all others. When I build a component footprint to fit onto a circuit board, I'm faced with the options of relying on the component supplier's specifi- cally recommended footprint or ignoring it and creating the footprint geometry based upon the IPC-7351 land pattern calculator. The way I see it, there are holistic design and manufacturing benefits to be gained by put- ting aside the supplier's recommended foot- print specs and opting for the IPC-7351 solu- tion. Unique supplier parts are not consistent with manufacturing criteria required by oth- er supplier's parts. Why not use a consistent component land calculation for all component parts? This philosophy usually rests well with our manufacturing assembly stakeholders. I encourage all new designers to check in with their assembly manufacturing stakeholders while in the process of creating footprint land pattern geometries as it affects the outcome of their target solder fillet conditions. Matties: We've heard from a few veteran de- signers that an app note is often nothing more than the sales pitch for the device. Is that how you see it? Dack: I can relate to that. An app note might be like the operating criteria for a four-wheel- drive vehicle. While the salesperson on the commercial shows the vehicle going up a steep canyon at a 45-degree angle, the app note says, "Don't do this or you could die." That's the app note. But the physical specification data- sheet is in another book that lists the physical properties—its engine size, horsepower, and turning radius. Data specs are legal and factual in context but do not infer performance. Matties: The datasheet is sort of the mechani- cal or physical specification, and the app note is where you start crossing over into the func- tional spec. Dack: Yes, from a component standpoint. There are a lot of different contexts here. Even PCB laminate materials have app notes. Teflon ma- terials and polyamide materials are going to have performance specifications and app notes or application engineers who will show their best utilization for performance. I wouldn't doubt the reliability of those app notes or specifications. But I would definitely seek wise council from an applications engineer before incorporating them into a design. Matties: Right. I understand that the app notes for a component will say that it performs at this level on this certain material. However, it may also perform with a lower-grade material as well but not give you quite the same perfor- mance. What triggers in your mind, as a de- signer, the need to question this or not? Be- cause you can't question every single app note that comes through, can you? You don't have the time. Dack: Correct. I am not a scientist. I'm a design- er with lots of layout experience who works with a customer who may have a need to de- sign a complex part into a legacy product de- sign. Sometimes, the customer is a scientist or an engineer who will specify what they want. Other times, they're trying to spec a compo- nent into a design realm that is totally outside the configuration it was intended for. In this case, an app note can be a helpful base start- ing point to at least put some context into the required design intent.