Many multi-board PCBs do not rely on board-to-board connectors or simple wired cables for interconnections between PCBs. When only two PCBs need to connect, a single cable or board-to-board connector may suffice. However, in products with multiple PCBs that need to interconnect, the number of cables can quickly become unmanageable within an enclosure. A straightforward solution to organize all those wires and cables is to build a wiring harness.
Electronic wiring harnesses come in various shapes and sizes, and they can become quite complex both mechanically and electrically. While designing and assembling a wire harness requires some effort, it's well worth it, especially during product assembly, where proper wiring can prevent mistakes. If you're considering what type of wiring harness to use in your product, explore these available options.
Point-to-Point Cable Bundles
The simplest type of wiring harness consists of point-to-point cable bundles, which are packaged together into a single harness assembly. These bundles can be secured using adhesives, tapes, sheathing, or mesh jackets. The cables in these bundles may include off-the-shelf cables, custom cables, or a mix of both. The interconnect topology that typically results from this setup is point-to-point, due to the direct connections facilitated by these cables.
Bifurcated Cables
Bifurcated cables involve cabling from a single connector that splits into two different destinations. This configuration allows a single receptacle on one PCB to connect to at least two other PCBs. It represents a more advanced type of wiring harness, which involves combining different connectors and receptacles across multiple boards.
Flex Printed Circuit (FPC) Cable
Flex ribbons can also serve as wiring harnesses, allowing a combination of edge connectors, surface mount connectors, and even through-hole connectors to interface with a group of PCBs. Flex connectors have the advantage of being extremely flat, making them ideal for low-profile products. In some designs, a traditional wire bundle in a standard wiring harness may not fit, making a flex cable an attractive alternative.
When used as a wiring harness, the flex cable follows the same design rules as a flex PCB. The key difference lies in the selection of connectors. For flex cables that plug into PCB receptacles, it's generally recommended to avoid surface-mounted connectors if the flex ribbon will be repeatedly plugged and unplugged. The reason is that the flexible polyimide material could easily separate from the SMD pads. Instead, choose SMD connectors with through-hole mounting assists or through-hole welding tabs to prevent solder fractures near the connector plug and to ensure the cable can be connected and disconnected repeatedly.
Parts for Wiring Harnesses: Connectors and Crimps
When building a wiring harness, such as with bifurcated cables, you'll need to select appropriate receptacles and mating connectors to make the necessary connections. Connector vendors typically do not specify their plug connectors specifically for wiring harnesses. Instead, crimp connectors are commonly used, allowing an assembler to attach a wire and then create a cable assembly or wiring harness. For example, consider the Molex connector.
A wiring harness can easily be built using this connector by employing crimp contacts, which slide into the connector. The crimp contact attaches to a wire, allowing it to secure itself inside the connector body. Once the connector is mated to its receptacle on a PCB, the electrical connection is complete.
For a wiring harness assembler, the PCB and product documentation must include the necessary parts for the wiring and harness components in the BOM. You will need to provide:
- The part number for the mating connector in the wiring harness.
- A part number for the required crimp contacts.
- A part number for the wiring, including the wire gauge.
These materials should be supplied to the wiring harness assembler to ensure that a complete wiring harness can be procured. The wiring harness itself should also have its own bill of materials and a detailed drawing indicating pin connections on each connector plug, along with part numbers for compatible crimps. Documenting wiring harnesses can be challenging and may require some manual drawings due to a lack of standardization. Be sure to contact your cable assembly or wiring harness manufacturer to ensure they receive the necessary documentation for proper assembly.
Leverage MCAD Collaboration for Sizing Cabling
When building a custom wiring harness or cable assembly, designers have traditionally relied on running cabling through a prototype enclosure using spooled wire or even string. This method requires hand-assembling the harness to check its fit within the enclosure. If a flex cable is used, a prototype must be constructed, which can be quite costly when working with thin polyimide flex ribbons.
This approach is both expensive and time-consuming, especially when the goal is simply to estimate wiring length within the enclosure. Today, design teams should leverage MCAD applications and 3D models of the enclosure to estimate and size cabling more efficiently. These tools can easily incorporate cabling of various shapes and sizes, as well as insert bends to visualize cable paths within a wiring harness.
A side benefit of this approach is that it also enables experimentation with connector placement within a 3D mechanical model of the PCB inside the enclosure. Many connector manufacturers provide 3D models, such as STEP files, which can be easily integrated into both ECAD and MCAD applications.
