Proper PCB Mechanical Constraints Drawing

Before beginning layout, we first need to know how large the PCB needs to be. This is frequently a bit of back and forth between the EE and the ME with a little give and take for each. Once the size is known, the ME will need to create a Mechanical Constraints drawing. This is the “contract” between the ME and the EE: as long as the EE obeys the limits in this drawing, the board should fit in the enclosure. A good mechanical constraints drawing includes all the information that the EE will need. This includes the board shape, location of key components, keep out areas, and height limits.

The board shape should include the dimensions of each side of the board including the radius for any arcs. If the board has any inner cutouts, bear in mind that inner cutouts are routed and a smaller radius will increase the cost. For most applications an inner radius of 0.62” is sufficient, going down to 0.31” where space is really critical. Most PCB layout programs use the lower left corner of the board as the origin of the coordinate system. So when defining the board shape, use the lower left corner of the board as the reference point and measure from there. This will make dimensioning the board shape easier to match the mechanical design. 

For more complex board outlines it can be handy to also include a DXF of the board outline but please be sure to include the dimensioned PDF so we can use it as verification. The biggest gotcha is putting too much stuff into the same layer.  When creating the DXF, put only the outline on one layer and all the other stuff on other layers. Otherwise the layout program will have a difficult time with the import.

The mechanical constraints drawing should also include the locations of key components, like buttons, LEDs, pretty much anything that may affect or be affected by the enclosure. When placing dimensions to critical components, please provide dimensions from origin to a component hole, if it’s a through-hole component, or to the component center, if it’s a surface mount component.

We also need to know the height constraints for the various parts of the board. Again, this is part of the “contract” between the ME and the EE. The height constraints may be different for different sections of the board but these should be called out on the mechanical constraints drawing. 

Once the EE has received this drawing, the next step is to place the components in the layout program. After the key components have been placed, I like to generate a STEP file of the board and components and send it to the ME to ensure that there are no collisions yet. This can be very helpful in preventing issues later down the road when routing is complete and changes are more time-consuming.

Derek Smith