The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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65 Additionally, it is always a good idea to check in with the appropriate manufacturing process stakeholder when initially setting these values. For best DFM, never set values to the stakeholder's minimum or maximum capability. Design rules in a PCB layout tool can be a blessing if understood and set properly. However, they can also be a curse if not carefully considered and reset from the default settings. As an alternative to readjusting the preset design values in a new PCB layout database each time a new design is started, PCB design engineers often create their own PCB layout templates. A personal PCB design template can be easily established by making all the desired settings such as color preferences, DRC settings, via size, layer stack-up, formats, and title blocks to a generic design database and filing it as a design template. Saving PCB design templates for various types of designs can save a lot of setup time at the start of a design. Design templates may be created for layer count for 2, 4, 6 (& more) layer rigid designs or for special types of designs, such as flexible circuits or boards with standard card edge connectors. Defining Vias Via size requirements need to be considered at the beginning of a layout. Vias are small holes in the PCB which are plated to make interstitial connections to multiple circuit layers. In other words, making connections from one side of the board to the other or to inner layers requires a via. There are three general types of mechanically-drilled vias: through-hole, buried, and blind. Vias require a pad etched in copper on each layer which will be electrically connected. The pad diameter needs to be small enough to not eat up valuable routing area; however, the pad diameter must be large enough to accommodate a hole-forming operation such as drilling or laser-forming. Drilled Vias Mechanically-drilled via holes are kept small for space concerns and have a special relationship with PCB thickness due to the process by which the holes are plated. For the walls of the via to form a plated through-hole (PTH), a very fine solution of copper (referred to as CuPosit) must thoroughly coat the surface of the holes. CuPosit forms a micro-layer of copper on the bare wall of the drilled hole. The process requires the CuPosit to wash freely through the holes to coat the entire surface. However, this process is impeded when the hole diameter is too small in relationship to the thickness of the PCB material it goes through. In the PCB manufacturing world, this relationship is known as the aspect ratio of the PCB. Saving PCB design templates for various types of designs can save a lot of setup time at the start of a design.

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