00193407-02 HWWS SIPLACE Machine Accuracy EN.pdf - 第11页
© Siemens AG . . Änderungen vorbehalten SIPLACE Placement Systems Machine capability indices Cm A machine suitability for the planned appl ication is assessed with references to the indices Cm (capability machine) and Cm…
© Siemens AG . . Änderungen vorbehalten
SIPLACE
Placement Systems
Statistical Process Control
The goal of Statistical Process Control (SPC) is not to determine or to describe the quality of a process by
using 100% inspection, but to use small samples to determine the quality of the process.
Standard Distribution
The standard distribution is illustrated graphically as a bell-shaped curve .
The diagram below shows the standard deviation 12,5 um.
Expressed statistically: at the peak of the bell-shape curve, 68.26% of all placements have a max. deviation
of 12,5 um from set-point value.
It can also be explained: a component which requires a placement accuracy of 50 um can be
placed with 4 sigma, based on this statistical distribution.
The defect placement rate here is 60 dpm (defects per million).
When comparing specifications of placement machines, the statistical base must be identical.
Two values specifically determine the appearance
of the bell-shaped curve:
average value and standard distribution
graph right side:
average value (no offset)
standard distribution 12.5 um (1 sigma)
x-4
σ
-3
σ
-2
σ
σ
4
σ
3
σ
2
σ
σ
-50 -37,5 -25 -12.5
0
12,5
25
37.5 50
μ
m
1
s
i
g
m
a
Machine Capability
© Siemens AG . . Änderungen vorbehalten
SIPLACE
Placement Systems
Machine capability indices Cm
A machine suitability for the planned application is assessed with references to the indices Cm
(capability machine) and Cmk for the machine capability.
Outside disturbing influences have to be eliminated.
Cm: only dispersion in relation to the tolerance (4 sigma specification, Cm = min. 1,33)
USL = upper specification limit, e.g. + 50 um
LSL = lower specification limit, e.g. - 50 um
USL - LSL
6 x Sigma
Cm =
Machine Capability
© Siemens AG . . Änderungen vorbehalten
SIPLACE
Placement Systems
Machine capability indices Cmk
Machine capability, e.g. min. C
mk
= 1,33 (4 Sigma specification))
Cmk: position of the mean value of the series of measurements as well as the dispersion
¾ the machine capability is just a snap-shot and is performed with proper
measurement tools.
Cmk = Min (Cmk 1, Cmk 2) meaning the lowest value is only used for the Cmk
The centering index k is a measure of the deviation of the mean process value from the target value
(center of tolerance).
If the machine parameters are perfectly centered, k = equal to zero, and Cmk = equal to Cm.
Machine Capability
3 x Sigma
USG - Offset
Cmk 1 =
3 x Sigma
OSG - Offset
Cmk 2 =