I was tasked with putting together a bookshelf a few months ago. I thought it would be simple enough until I realized the documentation provided was for a completely different piece of furniture! Thankfully, most pieces were unique enough that I could slowly work through trial and error by referencing the images on the box. This small bookshelf was not built to bear a huge load, so while I can’t be sure that everything is exactly as the manufacturer intended, it’s thus far held its shape.
However, imagine having to piece together a PCB cable assembly with lackluster instructions. The sheer complexity of most assemblies would preclude any reasonable attempt at even starting the assembly process, and any evaluation of the assembly would be uncertain. Much like PCB manufacturing, a collection of standards has been devised that provides a roadmap for general quality assurance of cable assemblies.
Reliability Classes Are Not Just for Boards
PCB manufacturing primarily focuses on the construction and assembly of the board. Still, board integration within a complicated system is sometimes highly prioritized. For builds containing many interconnections, keeping track of the cables that span the boards is necessary to ensure overall functionality. PCB cable assembly concerns not only the correctness of the external board connections but also the quality. In many ways, it can be considered a system-level analog to trace routing. In that sense, troubleshooting can be more difficult as there is no automated design rule check (DRC) for cable assemblies.
Instead, technicians must carefully build and test assemblies to validate function. Although errors related to cable and wire assemblies are more easily corrected than those occurring in manufacturing, they nonetheless represent a potential source of device malfunction and customer dissatisfaction.
Interestingly, the Institute of Printed Circuits (IPC) alone does not govern the ruleset for cable/wire assemblies. The Wiring Harness Manufacturers Association (WHMA) partners with IPC in the joint IPC/WHMA-A–620 standards to establish minimum acceptability for different grades of PCBs. Like the fabrication and assembly, cable assembly can inhabit one of three Classes denoting its reliability:
| Class 1 | Class 2 | Class 3 | |
| Quality | Lowest | Moderate | Highest |
| Cost | Lowest | Moderate | Highest |
| Yield | Highest | Moderate | Lowest |
| Industries | Common appliances | Dedicated service devices (televisions, computers, etc.) | Medical and aerospace |
| Acceptable downtime | Should be minimal | Must be minimal | Zero |
Just like for the boards themselves, certain operations require a higher level of ruggedness to maintain uptime if disruption to their function could pose a safety issue. On the reverse side, manufacturers don’t want to spend more during assembly processes and reduce yield unless necessary. Applying Class 3 techniques would improve the reliability of consumer electronics, but this would have a poor cost-benefit payoff given that most of these devices need to be competitively priced and are usually replaced by successor technology or discarded within a few years.
Reliability Classes establish the minimum acceptability for a particular device depending on its function. This feature empowers manufacturers to maximize production capabilities and limits losses while protecting end users with stringent industry standards to ensure device reliability, especially when poor or intermittent performance could be hazardous.
PCB Cable Assembly Qualifications and Testing
Explaining the entire scope of IPC/WHMA-A-620 is beyond the scope of this piece. Still, generally, the quality of the PCB cable assembly is bound to the crimping, soldering, lacing, and other similar methods of securing wires and any associated features. While high-level analysis typically focuses on defects, there are four potential ratings for any process:
- Target: The ideal process outcome; quality control should strive to meet this level wherever possible.
- Acceptable: Qualifies a process meeting minimum requirements.
- Process indicator: Acceptable with caution; a process indicator rating points toward production issues that should be analyzed further if particular instances still proliferate.
- Defect: Failure to meet acceptability criteria. A product is considered defective in the presence of any single defective process until remedied. Class 1 defects are universal, and Class 2 defects may or may not extend to Class 1.
In addition to providing a classification of processes with the above system, the joint standard also identifies electrical and mechanical tests as a final quality checkpoint of the cable assembly. Some of these tests are unique to the cable assembly, while others are extensions of earlier tests performed during board manufacturing as the cable assembly extends the functionality of the board:
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Electrical
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- Continuity: Checks the basic circuit parameters (resistance, current, voltage) of the assembly.
- Shorts: Checks for unintended connections based on the netlist using a low voltage signal (exact voltage value should be below maximum component thresholds).
- Dielectric Withstanding Voltage (DWV): A high voltage test that sends a voltage spike across components that is representative of transient signals that could be encountered during operation. A faulty component could exhibit arc-over or dielectric breakdown.
- Insulation Resistance: Determines whether the insulative material of cables (and other items) exhibit the rated resistance while subjected to a high voltage.
- Voltage Standing Wave Ratio (VSWR) – Evaluate the circuit’s power efficiency using a ratio of reflected power to input power.
- Insertion Loss: Measures signal loss across a frequency range at particular values.
- Reflection Coefficient: Similar to VSWR, compares the energy of the reflected wave to the incident wave.
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Mechanical
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- Crimp Height: Compares production crimp height against the manufacturer’s specifications; every unique terminal and conductor combination will have a different value for the height.
- Pull Force: A destructive test that ascertains the structural integrity of a crimped connection. Regardless of whether the pull force is applied until mechanical failure, the cable assembly is no longer considered fit for end users.
- Crimp Force Monitoring: Electronically measures crimp force-time combination used and referenced against a baseline.
- Coaxial Shield Pull Force: Like the general pull force test, a destructive axial force is applied, rendering the product unsuitable for end use.
- RF Connector Shield Ferrule Torsion: The cable is rotated at a fixed distance away from the connector; the cable is expected to twist but not rotate at the connector.
Your Contract Manufacturer Has Assembled a Team Dedicated to Quality
PCB cable assembly is as important to final performance as the board itself, and design teams must align themselves with an experienced manufacturer to handle all assembly needs at the board and build level. A highly-reliable assembly stems not only from cutting-edge equipment but a well-trained technician team that can accurately diagnose and correct issues encountered at all stages of production.
That’s where VSE comes in: as a team of engineers who build electronics for our customers, we pride ourselves on the quality of your product at every point throughout development. Paired with our professional manufacturing partners, we aim to deliver the best performance in NPIs and mass-volume PCB and box-build assemblies.
If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.
