409-32048
Rev A
23
of 64
LIMITS
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The SPC “Min Force (Range)” and “Max Force (Range)” Limits are specification limits, which
the press force is compared against to determine if the press is good (within the limits) or bad (outside the
limits). These spec limits will affect CPK (process capability index). Of course, the wider these limits the
better the process capability appears. To keep the CPK meaningful, realistic spec limits must be entered.
These numbers correspond to the limits that you will find on the SPC chart. TE recommends that the SPC
“Min Force” limit be set equal to the “Min Force/Pin” value under the Force section; whereas the SPC
“Max Force” limit should be set slightly less than the “Max Force/Pin” value under the Force sec
tion. The
reason for the Max limit being set less is due to the purpose of the Max value in the profile. It's an error
condition; realistically, it will never be reached in a successful press cycle. Specifying a limit that is less
than the Max force value also allows SPC to alert the operator that an undesirable trend is occurring.
□
“Stop on Out of Spec Limits”
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When this box is checked, the average force within the SPC Sample
Range for each connector is compared against the specification limits entered in the SPC Limits. If the
force detected is outside the “Min Force” or “Max Force” limit, an error message is generated indicating
the problem. If the box is not checked, no error will be displayed. Whether a message is displayed or not,
the out of spec condition will always be noted within the .RAW file.
Keep in mind that the SPC Min and Max specs are not necessarily the same as Min and Max force per pin
entries, which are used as forces within the press profiles. Typically, the Min limits will be the same but
the Max SPC spec limit is less than the Max force allowed in the profile. Remember that the SPC range
should be set to capture the compliant pin section sliding down the hole. This force is generally less than
the Max force allowable on the connector because the Max force is usually applied right before
termination as the connector contacts the board’s surface.
□
“Flag 7 Point Trend”
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This check box indicates whether the points plotted on the X-bar chart should be
analyzed for a seven-point trend. A seven-point trend is seven consecutive points:
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with increasing values
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with decreasing values
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above the mean
—
below the mean
The objective is to identify trends that indicate a change in the process is occurring. If one of these trends
is found, it will be highlighted in red on the SPC chart.
□
“Flag Control Limit Violation”
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When this box is checked, the SPC button within the run screen will
display red flashing bars to indicate when a connector has an out of control limit condition. Remember that
each connector type has its own SPC Limits. An out of control limit condition on any graph will cause the
red bars to flash. To acknowledge the condition and clear the red flashing bars simply view the graph or
graphs displaying the control limit violations. The chart selection dropdown in the upper left hand corner of
the runtime SPC screen indicates which connectors have an out of control condition by showing "*****" in
front of the connector name.
We allow control limit violation flagging to be turned on and off because the pressing process is not a
typical SPC application. Remember there is no knob to turn to correct the process. All that can be done to
correct violations is to reject components. Unless the customer has a specific action plan for an out of
control condition, the flashing bars are just an annoyance and are better turned off. All SPC data will be
gathered and the SPC charts can still be viewed. This same idea applies to the "out of spec limits"
described above. Unless the operator has been trained on what to do when they see this message, it
should not be turned on.
“Run Length (Boards)”
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Run length must be set for each connector. TE recommends using a minimum
of 30, which has shown to be a good lot size for trend detection. In addition, this recommendation is due
to 30 being the maximum number of points that can be displayed on the X-bar and R charts. While points
for the last 30 boards will always be displayed, all SPC data collected since the board program was first
run is maintained in a file (C:\AEP\SPC\boardname.mdb), and is never deleted.
CPK is calculated based on the last "Run Length" boards. Typically, run length is set to 30 so the CPK
would be calculated based on the points displayed on the graph. However, run length can be set less than
or greater than 30. CPK would then be based on that number, and not on the displayed number of points.
