Surely many people can relate to a phone charger that only works when the wire is placed in a certain position. Being a bit of a packrat, I have several frayed chargers within the same nightstand that I save for extra parts and replacements. For all cables, but especially high gauge wire subject to many connection/disconnection events, constant or extended periods of strain eventually results in poor performance or intermittent connectivity.
Fortunately, PCB cable strain relief exists for a myriad of installation environments. Creating the best solution for a particular cable or build requires knowledge of how stress is induced within a system and different approaches to mitigate it.
The Stress-Strain Relationship and Testing
Strain is closely related and sometimes mischaracterized as stress. To be precise, strain is the deformation process caused by stressors. This commonly refers to the pairing of normal stress and strain arising from an axial tension. However, developed strain can be multidimensional and originate from various stress applications, such as torsion, rotation, and compression.
The resulting deformation from stress can manifest in two general categories for solid bodies:
- Elastic: The material can recover its original shape and characteristics after the stressors are relaxed. This deformation is, therefore, temporary and reversible.
- Plastic: The shape and property of the material are permanently affected after applying enough stress to bypass the elastic region of deformation. Additional stress leads to the maximum tensile strength of the material and then failure.
It’s important to understand that although elastic deformation can be undone, its occurrence can still create issues depending on how a cable is attached to a system. The elastic deformation only describes the behavior of the material of the cable and does not account for issues created at connectors caused by a cable increasing in length. As the underlying material properties of the cable are changing, simply entering into the elastic region of strain can result in mechanical failure.
Some standardized tests will measure their response and determine how materials and cable dimensions will react to different loads. While some tests are reserved for specific cable constructions, some basic universal tests are routinely applied:
- Retention: A load is applied to a cable using a predefined maximum weight to establish that the cable pull strength is below the loading. Successful retention at greater weights indicates the cable will not slip from the connector.
- Anchorage: Unlike in retention, where a max loading is applied, anchorage looks for fatigue in a cable by applying a pull every second for slightly under one minute; cable displacement post-test may not exceed 2 mm. Following this, a cable undergoes a secondary torque test that looks to keep rotation less than or equal to 45°.
- Impact: A cable sample is chilled to the desired temperature and placed in an apparatus with a suspended load. When a mechanical switch is thrown, the load drops and carries its momentum to the point where the machine holds the wire in place. A cable must show no signs of wear or crack after the test.
PCB Cable Strain Relief Considerations
Bend radius is important in determining cable strain calculus. Too much length and the bend is flimsy and collapses without additional support, but too little length in the cable kinks and damages the internal conductor. Wires of a finer gauge can undergo further flexibility than wider gauge wires, and additional objects like jackets (and the rigidity of their materials) further affect the total bend allowance of the cable. Even without severing the conductor, an extreme bend radius can collapse the cable to the point where connectivity becomes intermittent, and current flow suffers.
Strain relief for a cable undergoing some bend radius has multiple approaches for surmounting the various fixture and installation challenges. First, the proper threading, which is usually tied to the originating region for local manufacturing or the targeted region for international markets, must be considered.
Though there are some similarities between the major regions, notable differences include:
- PG: A German standard with smaller thread depth but a long straight line pitch.
- NPT: The National Pipe Thread (based in the US) utilizes a 1/16th taper rate to help form a compression seal when torqued.
- Metric: A common thread that sees global usage; possesses the parallel threading of the PG with the pitch of the NPT.
| Metric | NPT | PG | |
| Thread angle (°) | 60 | 60 | 80 |
| Threads | Parallel | Tapered | Parallel |
The point where the wire meets the connection point also has two branches of strain relief design. Dome fittings protect from abrasion and shear caused by the cable rubbing against the enclosure. In contrast, flex fittings inhibit kinking and eventual breakage at the connection point by introducing a bend into the cable.
Your Contract Manufacturer Can Handle the Intricacies of Your PCB Installation
PCB cable strain relief has the possibility of being overlooked in fixture design and installation of the board. Still, it’s a necessary element for determining how rugged and reliable the mechanical parameters of the system are. It’s a good reminder of the granularity of detail that must be considered for a successful PCB installation, whether the board or associated equipment. Let VSE’s partners and engineers assist you if you need help capturing the enormity of your next fixture design. We aim to build quality electronics for our customers in various industries and applications.
