To Fail or Not to Fail, That is the Question – Act II
Dr. Bob reviewed the issue of failure criteria in the previous issue of ConnectorSupplier.com. He said that two different types of failures have to be addressed: catastrophic and systematic. He attached definition to both, noting that catastrophic failure results from quality issues in assembly or manufacturing, and systematic failures result from design issues (material selection, low normal force, etc.) Quite frankly, I never thought about that in the perspective as presented. It’s a good way, however, to prioritize failure mechanisms and to define them properly.
Bob also implied that the two critical criteria are to establish resistance stability and address the concerns about intermittent behavior of contacts. This is true for signal, data processing, power, etc.
So let’s discuss these two topics in more detail. The classic technique used to establish criteria for resistance is to specify an allowable magnitude of resistance. OK, let’s go with 20 milliohms maximum at its initial usage, or never to exceed that value for its total life. Many documents will use the option of specifying the initial and final allowable magnitudes contingent on anticipated environmental considerations. Other specs will specify both a maximum average and a maximum for an individual contact. The values established are all over the spectrum. Each company’s qualifying agency seems to have its own ground rules and they are different from each other.
When I was a young engineer and asked how this was determined, the answer I got turned out to be less than satisfactory. The common technique was to measure the resistance over a sample of contacts, take the maximum resistance observed, and add an additional amount (based on judgment), and that became the requirement. In many cases, that value was arbitrarily changed to enable additional manufacturers to comply with the requirement. This appeared to be a rather inadequate—and, quite frankly, weird—way to specify one of the most important connector variables.
Further discussions with other engineers indicated that no one understood what the definition of a failure was, which would cause a fault. The initial thinking was, “what is the electrical resistance composed of.” There is a simple basic equation to explain this:
R = RT x 2 + RB + RF + RI where…
R = Contact Resistance
RT = Termination Resistance (x2 = 2 Terminations)
RB = Bulk Resistance of the Material
RF = Surface Film Resistance
Now, termination resistance does not change unless there’s a major quality issue, and it has a magnitude in the mid-MicroOhm level. Bulk resistance at room ambient also does not change (except with temperature fluctuations). The film resistance is also very low on “pristine parts” and is easily displaced during the mating operation. This is the natural film that’s in the air at ambient conditions. The interface resistance was generated by ‘a’ spots on the two contact elements. Thus, when ‘a’ changes occur, it is due to something at the contact interface being altered.
By this time, being a frustrated product designer, I started to realize that the actual values that were specified were not the main issue. The change in resistance was. I’m sure that many readers have had similar feelings, so, I’ve established a table defining different magnitudes of changes:
Change in Resistance
- 0.0 to +5.0 stable
- 5.1 to 10.0 stable, with minor change
- 10.1 to 15.0 stable, with significant change
- 15.1 to 50.0 marginal
- >50.0 unstable
Experience has shown that the magnitudes of “a” through “c” would not result in faults (99 percent confidence). Item “d” is a problematical one, requiring a risk assessment to be made in these magnitudes. In some instances, the resistance increases and levels off, finally becoming stable. In other instances, the opposite is true.
Observing the voltmeter will also help determine stability. A stable resistance will “lock” down a valve, with very little observed drifting of the valve. Designs which are suspect will tend to have drifting plus or minus10 milliohms, generally more. Item “e” is when there is significant drifting, and it will not settle down.
Further discussions indicated that when an initial resistance is observed, the resistance remains the same, with some increase being tolerated at the end-of-life.
It is obvious that the change in resistance is the key element that will, or should, establish pass/fail criteria and stability. Starting in the late 1980s and early 1990s, due to the rapid expansion of the application areas, the change in resistance was specified in various specifications and standards, and has generally been accepted as the proper way to establish stability.
The second major topic Bob mentioned was the intermittent behavior of contacts. In this instance, very high-resistance spikes will occur for a very short period of time, and then disappear. In essence, a voltage shift occurs, which triggers equipment to fail or behave erratically. This is commonly called the “blue screen” syndrome, or “ghost faults.” The basic problem with evaluating this mechanism is that no one can predict when it is likely to occur and what the driving factors are. But it does exist; it is not just a laboratory occurrence. Some factors that cause this situation are durability, vibration, high shock levels (well above 100 Gs) and/or a dry or wet thermal-cycling (during ramp-up and -down periods).
This phenomenon could become increasingly important, particularly as newer high-speed devices are developed, in which triggering voltages are reduced. There is work being performed to establish how to determine the susceptibility of a contact that will result in intermittent behavior. Some of these activities include:
- The development of low nano second event detection
- Near-continuous monitoring of resistance
- “Glitch” detection
Item “a” will detect an event of a given definition, and how often it occurs, but it is not qualitative in nature. Item “b” will generate millions of data points requiring special analysis techniques. Item “c” is a combination of “a” and “b,” but has a time limitation.
Since the intermittent behavior has not been specifically defined and the magnitude of the problem has not been determined, now that funding research has been significantly reduced throughout the industry, work in this area is very slow, at best.
However, as new high-speed designs are developed and other technical changes are incorporated—such as the use of thin gold systems (<10.0)—the reduction of gold as a plating system, both in usage and thickness, will make this phenomenon increasingly important.
So, as I get older, a bit more fussy in the brandy I drink and the cigars that I smoke, I do feel confident that technology will continue to evolve and connectors will not disappear. The hope is that bright, young engineers will come forth to quench their thirst by being challenged to answer that which may appear to be the unanswerable.
Max Peel is a Senior Fellow at Contech Research, an independent test and research lab located in Attleboro, Massachusetts, U.S.A.
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