The previous unit looked at a portion of electronics design that is sometimes called “front-end engineering.” In this phase of design and product development, the schematics that get developed are like the foundational blueprint for a new system. Once it’s time to start creating a PCB layout, there is a specific workflow the industry follows to make sure designs fit into an enclosure and will satisfy mechanical requirements. If you decide to pursue PCB design as a career, you’ll likely need to follow the standard workflow as this will be implemented by your team members.

PCB Design Starts With the Enclosure

The PCB design and layout process starts once an initial set of schematics is created, and the schematics are ready to be transferred into a new design. Once the bare board is being created in your ECAD software, it should be designed so that the assembled board will fit and mount into its enclosure. Professional design teams start designing a PCB by considering the enclosure and how this constrains the allowed space in the physical layout.

The example enclosure shown below includes connectors at the edge and accessible through the wall of the enclosure. This is a typical configuration for connectors to interface with external devices. The connectors, the thickness of the enclosure, and the lateral dimensions of the enclosure will determine the allowed board size, the placement of some components, and the size of the components that can be included.

The desired locations of connectors and dimensions of the enclosure will place important limitations on a PCB layout.

We can look at the MiniPC project as a guide to get a sense of the mechanical requirements imposed when the board was originally designed. The connector placement, mounting, and expansion card requirements are called out in the image below. All connectors for interfacing with external devices are placed along the bottom edge, while expansion card slots are located at other edges to make room for other groups of components.

This grouping of connectors in different areas of the board is exactly what we expect to see when mechanical constraints are driving the design process. The rest of the elements in the board may have mechanical constraints that are defined early in the design process by mechanical designers, rather than being set by the PCB designer. The remaining room in the board is allocated so that the PCB layout engineer can carefully arrange components and route them within the allowed space.

Because the enclosure constrains where you can place components, and the physical size of many components, PCB design teams will implement the following design workflow when creating a new product:

1. Schematic design and mechanical design are performed before PCB layout, oftentimes in parallel
2. Any constraints on component placement, component sizes, board size, and mounting hole location are determined
3. The designer implements the required board shape and mounting style in the new layout
4. Components are arranged and checked against the size/height limits imposed by the enclosure
5. After replacing or moving any components, the design is routed and the layout and evaluated

This back-and-forth process is standard in design teams at smaller companies, and at large enterprises. PCB designers need to know a bit about mechanical design and MCAD software to ensure they can properly communicate with their team members.