Connectors, Tooling Holes, and Fiducial Markers for Component Placement and Orientation
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When you’re a new systems designer, manufacturability of your designs is probably the last thing you think about. When I was younger and worked on a new electro-optics system, I was dismayed to learn that a system I designed needed a complete redesign. In its current state, the thing was unlikely to ever make it off the manufacturing line.
PCBs should also be designed such that manufacturing is streamlined. Orientation, landing type, and holes for through-hole components all need to be considered during design. Come assembly time, you can aid your PCB manufacturer by placing fiducial markers on your board. This helps expedite assembly of your board and reduce fabrication/assembly errors.
When you think about it, there are multiple ways some components can be attached to a PCB. For some simple components like resistors, orientation doesn’t really matter; the device is not sensitive to signal polarity. Other components, like square and rectangular ICs, can be placed on a board with multiple orientations, and only one of these arrangements will be correct.
Providing your manufacturer/assembler with some information about the correct orientation of your components and your board is critical to ensure your finished board is fabricated properly and will operate as designed. Automated pick-and-place machines need some reference point to identify the correct location and orientation of a board in space. These reference points appear on a PCB in the form of fiducial markers.
What may seem like random pieces of copper on your PCB are actually important features that improve profitability during manufacturing. Fiducial markers play the double role of providing a reference point for pick-and-place machines, as well as an indicator of orientation for nearby components. Square and rectangular ICs do not have interchangeable orientation, and your assembler can determine the correct orientation based on the location of a nearby fiducial marker.
Square ICs have a mark in one corner of the chip that can be used to indicate the proper orientation of the component on the board. Likewise, rectangular ICs have a notch at one end of the chip indicating their orientation. A fiducial marker can be placed on the PCB at one corner or end of the area where the IC is to be mounted. The orientation notch on the IC will then sit adjacent to the fiducial marker when properly oriented.
Integrated circuits with orientation markers
Aside from component placement, a fiducial marker can be placed on panelized PCBs to indicate the correct orientation of a panel. If you are manufacturing a rectangular board, fiducial markers should be placed at opposite corners of the board, as well as at opposite corners of the panel. This allows a pick-and-place machine or other assembly machinery to determine if the panel was loaded in the correct orientation.
The IPC standards require that you place three fiducials on your panel, with one at each corner of the panel. This allows a pick and place machine to distinguish the entire panel from an individual board. This also allows a pick-and-place machine to identify whether a panel has been mistakenly rotated.
Fiducials can also be used to check the length of a PCB during manufacturing. The dimensions of a PCB will vary slightly, and these variations accumulate in larger boards. The diagonal or lateral distance between two fiducials can be measured with a vision system and checked against allowed tolerances. This allows a pick-and-place machine or other assembly machinery to reject a board that does not fit within the allowed tolerances.
If you are working with a board that has surface-mount components on both sides of the board, then you will need to place fiducials on both sides of the board. This allows assembly machinery to locate the proper reference on both sides of the board. The same applies to a panelized board. The size of the fiducial markers you should place on a board depends on the capabilities of the assembly machinery. Again, check with your manufacturer for their requirements.
You may opt to include various tooling holes if your PCB needs to be mounted to its packaging or some other surface. Since tooling holes pass through the entire PCB, there should be sufficient clearance between the hole, nearby traces, and any power/ground planes on the inner layers. Tooling holes should be left unplated so that they can remain isolated from electrical connections and reduce the chances of shorts.
Connectors on your PCB usually have an obvious orientation. A connector for an external cable or device will generally appear at one edge of the board and will point outwards from the edge. When an outward-facing connector appears at the corner of a board, it could have two different orientations. A fiducial marker can be useful to indicate the correct orientation of the connector.
Vertical connectors, whether surface mount or through-hole connectors, also have a specific orientation that should be indicated directly on the board. These connectors are usually keyed, which forces the cable to insert with a specific orientation. Fiducial markers can be used to indicate the location of the key with respect to the connector holes or landing pads.
USB connectors on a black PCB
All the PCB elements discussed here can be placed in your next device when your PCB design software includes the best CAD tools and integrated component libraries. Altium Designer 18.1 gives you access to industry-standard CAD tools, component libraries with a multitude of connectors, and tools for generating deliverables for your manufacturer.