When designing a prototype PCB, it is most often desirable to break out unused I/Os on a main processor or ASIC to access points. This allows them to be probed for measurements, or connected to other devices using flying leads. The two most common ways to do this are:
- Place through-holes test points that can be used as a solder point for a wire
- Place SMD test points that can be probed with a meter
- Place pin headers that connect to the I/O bank on the main processor
The most common approach is to use pin headers. This is one reason you will development boards, evaluation products, and popular microcontroller boards full of pin headers; they are most likely being used to access I/O banks that are intended for connections to other equipment.
In a production system, this is not necessarily the case. When transitioning to production, you will generally find that all pin headers will be removed from a design, except for the functionally important headers. If you’re developing a system that is intended to transition to volume production, where should you place these headers so that final optimization is easier?
Pin Headers in Prototypes vs. Production
Suppose you’re building a prototype PCB; the typical practice is to put pin headers in any spot where they can be easily accessed or somewhere along a board edge. They might also be stacked or grouped together in one area of the board so that all I/Os can be routed to the same region. No matter where they are placed, a prototype will often have an over-abundance of pin headers out of pure necessity.
The board below is a typical example showing header placement on a small PCB.
Placement around the edges is most logical as it allows I/Os to route directly outwards from the main processor. This type of microcontroller board is most often used for development, not for production. A production version of this board would certainly not include so many pin headers unless they were absolutely necessary for the end product.
Once it’s time to transition these types of boards into production, there will be some optimization of the board, and this generally involves removing pin headers. If you were developing a prototype embedded product and you were using pin headers for I/O breakout, how can you place these to minimize your rework effort?
Place One Header, Then Make it Smaller
The first simple strategy you can use that minimizes redesigns when optimizing for production is to simply use a single large header. Once making the switch to production, the finalized production header can just be made smaller, or it can be removed entirely. Typically this would be the strategy if the production-grade assembly needed to include a pin header with minimal pins. There are high-count pin headers that can be used in prototyping, and these can later be reduced to a smaller pin header by switching out the part number.
When reducing the pin header size, there may be some required rerouting into the pinout, so the initial pinout has to be designed to be somewhat future-proofed. Doing this swap also allows for a shrouded version to be placed in the production board so that the design can interface with a custom cable, such as a ribbon cable.
Expand the Board to Fit Headers
Another option for header placement in a prototype is to place all headers along the board edge, and then simply expand the board edge from the intended form factor to accommodate the headers. The central area of the board, which contains all other components, can then be designed as would be intended for the production-grade product.
This is a better option if you are breaking out a pin header for accessing I/Os, such as for testing and measurements, but you won’t include these in the final board. This way, you could eliminate any redesign requirements in the central area of the board containing all the components. After removing the pin headers, you can collapse the board edge to the required size before production.
SWD and JTAG
Aside from the I/O headers shown above, you may find two other headers typically placed on a PCB, both for prototype and production:
- Shrouded SWD header
- JTAG header
The SWD header is normally used for programming and accessing SPI bus, while JTAG can be used for debug. If JTAG won’t be used in the field, then that header should be removed when transitioning to production, it can just be marked Do Not Populate (DNP). The SWD header might be needed for on-the-line flashing, assuming you won’t be using pre-programmed Flash memory components in assembly.
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