Optimizing Board Design for Adequate Clamping Space

DFM_ClearanceTwo common oversights related to board design affect whether the manufacturing equipment can effectively process a board. Whether your board is manufactured using pin through hole (PTH) or surface mount technology (SMT), the manufacturing equipment requires certain space on the board for clamping, transport, and/or supporting its weight. Without sufficient clearance, boards may need to be manually assembled; a situation which can dramatically increase the cost of a product.

A good rule of thumb for any board design is to leave 0.187” (about 3/16th of an inch) of clearance around the board. That is, when the board is run on rails through the equipment, space is required at parallel edges to transport the board. Understanding, however, that in modern electronics design every fraction of an inch is precious, we are often able to minimize the clearance required.

For example, some boards (if they are sufficiently small and properly shaped) can be panelized. That is, multiple boards can be run together on a single panel on which we add an outer frame for clearance. Still, the boards must be separated at some point, so clearance is required for a v-groove score mark through which a blade will pass. Another alternative is to use biscuit routing with break-off tabs, which requires an area about 0.3″ long and 0.15″ deep along the board edges at each tab location. A combination of the two methods can also be used when called for.

Another related consideration comes into play if a board is large and/or includes heavy components (large relays, heavy power supplies, etc.). In such cases, it is likely that the board will sag once subjected to heating. This can result in unacceptable board warping or damage if solder spills on top of the board at the wave solder machine.

To adequately support large or heavy boards, fixtures can be used but this is an added cost. Consider avoiding large panels if this is possible. Another option is to add mechanical stiffeners or to employ a thicker substrate. It’s also a good idea to leave a clear strip down the centre of such boards. Some soldering equipment can use a moving support chain or wire to support the board if such a clear area exists.

At OCM Manufacturing, we work closely with our customers to ensure these types of design issues are managed to everyone’s satisfaction. To do so, we ask that the customer provide CAD or gerber files as soon as component placement has been mapped out. This gives both OCM and the customer sufficient time to make necessary adjustments without impacting timelines. The end result is a more manufacturable product: faster to produce and less expensive to assemble.

At OCM Manufacturing, we can work with designers to ensure that their plans and prototypes are manufacturable and therefore marketable. Contact one of our Program Managers for details about how we can help.