Issue link: https://iconnect007.uberflip.com/i/1369942
MAY 2021 I DESIGN007 MAGAZINE 43 the available space where all the components are to be placed, and then looking at the extent of the board itself, can be daunting on some very small boards; remember that most boards (without having to go to truly embedded com- ponents to minimize de-coupling caps, for instance) only have their external layers avail- able to be populated with components. In many cases the customer may not want any components on one side or the other, further limiting the available space. Perhaps the back side must lay flat against another board. Components cannot be simply placed wherever they fit on a given design—for example, bypass caps that need to be as close to the power pin they are associated with. In a low frequency/DC context, a bypass cap opposes changes in the voltage line by charg- ing or discharging. e capacitor functions like a low impedance battery that can sup- ply small amounts of transient current. In a high frequency context, the capacitor is a low impedance path to ground that protects the IC from high-frequency noise on the power line. Consider also trace and space limitations based on power functions: e greater the power the greater the voltage; this requires wider tracks that also eat up board area. Assembly also eats up board space. Auto insertion devices can place components extremely accurately, but they still require enough room to operate in. In addition, parts that cannot be placed by automated placement equipment must be hand placed, requiring additional space for the technician and access to get his or her fingers (or tools) within. An example would be devices such as switches or connectors. Additional space is also required for de-bugging or reworking by the technician. IPC has some great information on guide- lines for space such as IPC 2221B—the spec that deals with design—and it has solid infor- mation on voltage spacing and other electrical considerations. • IPC-A-610: is is the generic accept- ability/rejection spec and covers how hardware is to be assembled onto PCBs. • IPC 7351B: is is the land pattern spec for surface mount parts with details on pad size and spacing pertaining to PCB design. Finally, test—namely test point access— must also be considered. ere needs to be enough room to probe the test points. #3: Information Not Initially Covered by the Customer and Changes on the Fly is one is inevitable and generally happens aer the board has gone back to the customer for part placement and/or final route review. It might be information either not shared by the customer in the kick-off meeting or informa- tion you as the designer did not ask about. Fre- quently, previously unforeseen things pop up and need to be dealt with. Additionally, there may be some feedback from different engi- neers on the project; mechanical and power engineers may have different ideas on what they would like to see or imagine they would like to see from the design. More on this topic later. In conjunction, there may be physical changes that the customer may require, such as new components due to part availability or part obsolescence. is one is quite common as components can be hard to find and a sub- stitute must be used. It is the board designers' responsibility to incorporate these changes with a minimal effect and loss of time. #2: Requests for Things that 'Don't Play Well Together' What I mean by this is things like small/tight pitch BGAs with high copper for high current. Sometimes the customer wants things that are just not possible from either a design or fab- rication standpoint; for example, three- or four-ounce finish with 0.003"/0.003" trace and space, or very tight pitch components with