The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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Page 35 of 115

36 Defining the Mechanical Constraints It is important to consider the complete definition of the PCB outline. There is more to a PCB outline than simply four lines to define a rectangle. In fact, PCB outlines are rarely defined as simple, rectangular shapes as PCB design continues to shrink electronics into the tightest useable spaces. Modern PCB outlines originating from advanced 3D packaging can contain complex curved edge requirements. Beyond mechanical consideration for the PCB outline is the defined thickness for the PCB. PCB thickness can range from very thick .200 [5.08] or more, down to .018 [0.46] or less. There are often mounting holes and slots, fixed component locations, and all the keep-out and keep-in definitions for the design. Sometimes, component height restrictions are defined in certain areas. Again, this type of constraint can either be imported or be defined manually after communicating with the mechanical engineer. One or more specific layers are reserved for the mechanical constraints. Additionally, intelligent auditing features can be added to the constraints definition. Most layout tools will allow the designer to define "rooms" or set up cross-hatched, keep-out areas which will alert the designer during design rules checking if there is a part within a keep-out zone. Once all the mechanical constraints are defined, the layout is ready for the next step. "One of the things I see that is a big, big problem is that today's ECAD/MCAD output files are not realistic enough. Sometimes components are only represented as blocks and reflective only of a couple of overall dimensions an EE might have given some thought to as far as height, length, and width. That's it. As a mechanical designer, when you design something like an RF housing, you really need to know what those components look like and exactly where they are placed." Parts Placement Once the mechanical constraints of the PCB are set, it is time to bring in the data from the schematic. When the component footprints are import ed, they are automatically connected by fine lines to show their pin's connectivity with other component pins. All these connections are based upon the previously established connectivity defined in the schematic. The imported part footprints usually appear to be piled up randomly off to the side of the defined PCB outline after being imported into database. A design with hundreds of components imported in this way will have so many connections crossed over one another, it will make the pile of parts and connections look like a rat's nest (which is what it is often called). To a new designer, this state of confusion can appear rather hopeless, but this is where the fun begins. This is the stage in which the designer can begin adding organization to what appears to be chaos. Most designers consider the placement stage of layout their reason for being—they know this first layout step will drive the success of the entire project because parts placement effects every manufacturing process and therefore every manufacturing stakeholder. Pro-Tip from Joe Bouza To a new designer, this state of con- fusion can appear rather hopeless, but this is where the fun begins.

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