Crimp height is key quality indicator

Werner Lothmann of Tyco Electronics argues that crimp quality monitors have traditionally focused on the wrong metric for quality.

Crimp quality monitoring is not new.

In fact, it has been around for some years, as a way of controlling the accuracy and quality of wire terminations.

But, until recently, such systems have had limited use, unable to deliver the levels of monitoring performance increasingly required in today's production environment.

Why should this be the case? Well, the answer lies essentially with the fact that, conventionally, crimp force conformance has been a standard criterion for crimp quality.

Specifically, the precise control of ramp force and peak force are particularly important.

Usually, these two parameters are calculated in relation to average value and standard deviation and, as soon as a crimp cycle occurs in which the value of either exceeds a preset multiple of the SD, the termination is evaluated for faults to determine acceptability.

Although this approach can be adequate, it is also prone to missing common quality problems, such as partial stripping, missing strands and 'short brush'.

In short, the message is clear - crimp force alone is not sufficient for 100% confidence in crimp quality.

Increasingly though, there is an emphasis on 'zero defect' performance programmes requiring fully assessed terminations, guaranteed performance levels and reliably documented evidence.

So, what's the solution? First, we need to recognise that there are many elements which have a bearing on the quality of a crimp.

There is, for example, the terminal crimp design itself, and the condition of the wire, as well as other terminal attributes and the tooling condition itself.

And any of these factors, which brings as little as a 1% deviation away from desired performance levels, can potentially lead to a significant reduction in quality.

However, although many factors can be monitored, there is one key factor which, in practice, is found to be the most effective and reliable method of measuring crimp quality.

That factor is crimp height.

By incorporating advanced, real-time monitoring to measure not only the applied crimp force, but also the displacement during the crimping cycle, a far more accurate evaluation of crimp quality can be made.

Using crimp height as a key statistical process parameter enables the derivation of a force versus displacement curve.

This two-dimensional analysis, provided by the curve, is the key to significantly increasing confidence in crimp quality.

During calibration and production, a history of good crimp curves can be built-up, against which subsequent crimp cycles are compared: this is termed the crimp signature.

Variations of the crimp signature highlight problems which may be missed by simply monitoring force.

The use of the wrong wire, for example, or improper positioning of the wire stop, or even tooling wear.

A full analysis of the force versus displacement curve can lead to significantly higher levels of confidence in crimp quality.

The measurement of crimp height is the basis of a Tyco Electronics' CQM (crimp quality monitor).

If crimp height fluctuates excessively, drifts in time, or falls outside tolerance limits, the CQM detects the problem, and an auto-adjust facility triggers automatic crimp height adjustment of the applicator.

And the system is very accurate.

In fact, with a measurement accuracy of 5um, crimp height can be monitored at a six sigma quality level.

(Six sigma processing requires that the standard deviation of representative sample of crimp heights does not exceed 1/12 of the total crimp height tolerance band, ie typically 8.3um).

Achieving this accuracy is no easy matter.

To do so, Tyco Electronics has adopted an innovative approach for crimp height determination.

This implements a solid-state displacement transducer, housed in the applicator, along with a force transducer which monitors crimp force.

The resulting signals are transmitted to the microcomputer in the CQM, where the system calculates the crimp height - corresponding with the zero force point - through a complex mathematical algorithm which takes account of the effects of friction.

A straightforward calibration procedure then relates crimp height measurement data to real numbers for each wire size setting.

With the capability to handle up to 50 different applicators, the CQM allows the calibration of data of 200 terminal-wire combinations to be stored in a single unit.

To control and inspect the crimp process performance at a glance, performance data is presented clearly on an LCD screen in the form of a chart with 120 bars, each bar representing a sample of 1 to 99 crimps.

The length of the bar is scaled between tolerance limits, indicating an average sample crimp height.

So how can you tell if a crimp is satisfactory? A satisfactory crimp, within a prescribed crimp height control limit, is confirmed by a green light, located by the applicator, and easily seen by the operator.

If crimp height tolerance limits are exceeded, through a mechanical defect, a red warning light goes on and an audio alarm sounded.

This alerts the operator of a semiautomatic terminating machine to check the crimp, or immediately shuts down the automatic leadmaker.

Of course, all tooling applications are subject to external pressures, such as thermal effects, so over a period of time crimp height can be prone to gradually drift away from the desired preset level.

To address this, an early warning system can be set within a tolerance band.

Exceeding the control limit is signalled by a single beep.

This alerts the operator to inspect the crimp, while a leadmaker can be stopped after a preset number of such events.

When the CQM is used in conjunction with its auto-adjust facility, the control limit also serves to trigger an automatic crimp height adjustment, based on the last sample's average, to bring the average crimp height back to the nominal value.

In statistical terms this improves the process capability index by defining the degree to which crimp results may fall outside the tolerance limit.

The CQM also saves time.

This is because it consists of monitor and applicator, both independent of the terminator itself, so changing the applicator for different wire/terminal combinations is quick and easy.

It is also dependable, because the monitor stores previous calibration data, for both semiautomatic and automated leadmaking machines, with or without the auto-adjust facility.

So, today, technology is available that brings a major advance in quality control, even to the most advanced tooling systems.

Providing a solution to the most vexing problems facing the industry, it enables manufacturers to meet the demand for total quality management and make enormous strides forward in the quest for 'zero defect' performance.

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