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Crimp Height
The missing link in conventional CQM systems
By Werner Lothmann,
Tyco Electronics
Today, controlling the accuracy and quality of wire terminations is a big issue.
In fact, there is an increasing emphasis on 'Zero Defect' performance programmes
requiring fully assessed terminations, guaranteed performance levels and reliably
documented evidence.
But, while CQM (Crimp Quality Monitoring) systems have existed for a while,
recent developments have highlighted the fact that until now, they have had
distinct limitations.
Crimp Quality Monitor
Why? The answer to this is that most conventional systems are designed
around a false assumption: that crimp force conformance is sufficient to
ensure quality compliance. But this, quite simply, is wrong. Although it is
an approach which can be adequate, it is not designed, for example, to
validate the initial design intent of a terminal. Crimp force analysis is,
in essence, merely a method to control certain aspects of the crimping process.
So how can crimp quality be assured? The answer is by monitoring not just
crimp force, but crimp height. Which, in turn, begs the question: why?
The answer to this lies with yet another question: what makes a good termination?
Well, a good termination will conform to specific mechanical and electrical
performance characteristics. For example, it will have high tensile strength
and low contact resistance. The crimping process must deliver these characteristics.
When mapped against crimp height, a typical well-designed terminal will show
mechanical behaviour such as in Figure 1. From this, it can be seen that there is
a particular crimp height which provides optimal mechanical strength, and a
range of heights over which this value varies only slightly from the optimum.
Figure1
Although a typical graph of electrical behaviour (See Figure 2) is less clear cut,
there is, however, a range of crimp heights which give the best performance.
Ideally, of course, a terminal would yield optimum electrical performance at the
same crimp height as optimum electrical performance, but nature is rarely so kind. Nonetheless, because there is usually some overlap of tolerance bands (See Figure 3),
a crimp height usually exists which delivers close-to-optimum levels of both. This
relates to the design intent element of crimp quality, where only analysis of crimp
height ensures compliance.
Figure2
Figure3
The fact that such an index exists is useful, because it means that it can be
combined with the conventional measurement of force to derive a force
vs displacement curve. This 2-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 for any specific
wire – terminal combination 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, or even tooling wear.
Despite this, and put simply, traditional CQM approaches allow the measurement
and analysis of force with, specifically, ramp force and peak force considered most
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. In addition, various manufacturers also employ
alternative evaluation techniques of the crimp curve, sometimes analyzing
specific parts of the curve, referred to a zones.
The Tyco Electronics' CQM system, though, goes much further. 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. If crimp height fluctuates
excessively, drifts in time, or falls outside tolerance limits, the CQM detects
the problem, and an optional auto-adjust facility triggers automatic crimp
height adjustment of the applicator. And the system is very accurate. In fact,
with a measurement accuracy of 5 µm (0.0002"), 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, i.e. typically 8.3 µm (0.00033")).
Achieving this accuracy is no easy matter. To do so, Tyco 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 spring back of the
crimp height (due to the elastic-plastic terminal and wire deformation process).
A straightforward calibration procedure then relates crimp height measurement
data to real numbers for each wire size setting.
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 semi-automatic 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 the optional 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.
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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