Errors in cables can occur from the most innocent actions, even binding a group of wires too tightly. This type of error is called a compression failure and is caused when insulation narrows from its normal thickness. This happens when physical features or components either in the assembly or in the in the environment of the installed applications contort the insulation to the point that it cannot function correctly.
What causes insulation compression failures?
Insulation on a wire has "plastic characteristics" which under pressure will cold flow (deform due to constant pressure) and reduce the insulation wall thickness. If there is another wire, a metal connector shell, or other metal components close by, the insulation between wires or between a wire and another metal component will likely breakdown with high voltage (hipot) testing. High voltage testing is a good method to predict shorts that may appear later (a latent defect). The "softer" the insulation, the higher the susceptibility to this type of failure.
Compression errors can also occur if heat is applied to components under compression, such as adding heat shrink coverings or soldering to shields. PVC insulations and even very "hard" insulations such as TFE can also cause compression failures.
What conditions are necessary to cause this failure?
Insulation pressure increases in the following conditions:
- Cable Ties
- Routing of wires through holes in sheet metal where the holes do not have soft grommets
- Tight fitting strain reliefs
- Wrapping bare wire around exposed shielding to make a soldered connection
- Insulated wires on the factory floor that get stepped on with hard soled shoes, run over with wheeled carts, etc. during the assembly process
- Heat from soldering shields
- Unusually soft wire insulation such as is found in highly flexible cables or coax cable with "foam" insulation
- Sharp edges on components brought in contact with wire insulation
- Thin-wall wire insulation
How does the user recognize this failure?
Failures indicated while testing the cable:
- Shorts both intermittent and constant
- Failed Hipot - dielectric withstand and "Over Current"
One or more of the following might be used to verify the defect:
- A solid short usually can be isolated with a VOM or low voltage continuity tester. Sometimes this requires cutting the failed wire at a point before and after the compression to prove that the failure occurs at the compression point.
- If the wire insulation cannot be easily inspected for problems or none are visible you can use the following methods to identify the error.
- Divide and Conquer method: Perform a destructive test by cutting a cable in two and separating the cut wires. Perform a shorts and/or a hipot test on each separate side of the cable. Continue this process discovering which end has the problem and at what point the problem is detected or eliminated. On Cirris testers you can perform a "no connects" shorts/HV test. Be sure to use, "apply HV to all pins" with "Single Net Failure" set to "Fails Hipot," or "High Cap Shield" set to "NO."
- Confirmation method: Heat the cable to increase the rate of cold flow. In other words, bake the cable without exceeding the acceptable storage temperature of the cable in order to more quickly distort its shape. A heat gun is less controlled, but could also be used. After the application of heat, the problem should be more consistent.
Another way to surface this problem is to apply salt water in the area of a compression (if it is not near any un-insulated areas such as connector contacts). The wire insulation should be impermeable to salt water. Wires that are insulated, including the shield which may not be insulated, should still read off-scale for resistance with a VOM. If the resistance drops, then begin the divide and conquer approach with resistance measurements.
After confirming the point of failure you may want to inspect the damage under a microscope by removing the wire(s) from the bundle in the compression area.
How is this failure resolved?
If you are using wire with a soft insulation, do not rely on wire insulation in any compression zone. If the compression cannot be eliminated or appropriately reduced, insulated tubing over the wire might add to the normal insulation.
If you are using heat near wire insulation where wires are bundled together, carefully evaluate the application to avoid insulation damage.
The following examples are based on actual events from Cirris customers.
A cable assembly house experienced a jump in dielectric withstand failures in their cables. The cable contained a number of TFE insulated and shielded twisted pair wires. The shields were terminated as a group by exposing about .3" of the shield and wrapping a bare wire around the exposed shields and then soldering them. Previously these cables had been built with a separate shield termination for each wire.
Using the “confirmation” methods, they discovered that the wrapped bare wire was creating compression and the soldering was allowing the TFE insulation of the twisted pairs to short to the shields. Assemblers returned to using shield terminations that were designed to heat shrink and solder terminate.
Another cable assembly house was building cables used in GPS systems for motor homes. A large percentage of the cables failed hipot testing, but since no visually apparent problem was found and the customer didn't require hipot testing, it was dropped. In time, assemblies were returned as defective. Testing with low voltage, most of these defectives still passed as good.
When testing on a high voltage tester, the returned cables failed consistently with dielectric withstand failures. Using the "divide and conquer" method, the problem was traced to pieces of braided shielding that were penetrating extremely soft insulation on wires that were tightly fastened with a cable tie.
Extra insulation was added to the areas where the insulation breach compression failures were occurring.