Page 185 - Six Sigma for electronics design and manufacturing
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Six Sigma for Electronics Design and Manufacturing
                     154
                           Table 5.5 Example of process capability studies for PCB assembly line
                                              Cpk
                                                                Check
                                      Cpk
                                                       Status
                                                                          limit
                     Process
                                                                method
                                                                         ± 0.005
                                              1.61
                                                                n = 100
                                      1.42
                     Lead form
                                      1.41
                     Screen print
                                                                P chart
                                                     Check OK
                                              1.41
                                                                 n = 30
                                              1.99
                                                     Check OK
                     Adhesive apply
                                      1.99
                                                                         ± 0.005
                                                                         ± 0.002
                                              2.66
                                                     Recalculate
                                      1.70
                     Place components
                                                     Check OK
                                      1.06
                     Solder reflow
                                                                 n = 30
                                                                         ± 0.005
                                              1.06
                                                                 n = 30
                                              1.18
                                                                         ± 0.005
                     Manual solder
                                                     Check OK
                                      1.18
                                      1.06
                     Connector install
                                                                         ± 0.005
                                                                 X  , R
                                                     Check OK
                                              1.06
                                                                         ± 0.010
                                              1.72
                                                     Check OK
                                                                MR = 20
                                      1.72
                     Hardware assembly  baseline  this QTR  Recalculate  MR = 10  Specification
                     Conformal coat   1.70    1.70   Check OK    X  , R  ± 0.005
                     tions using Cpk. It shows the baseline and the present quarter per-
                     formance. The data could also be plotted versus time, with the man-
                     agement goals shown prominently on the graph plots.
                       Table  5.5  shows  a  process  capability,  measured  in  Cpk,  for  each
                     step of the process. The process capability is checked each quarter,
                     and the source of the check is shown. Some checks are performed by
                     using  existing  control  charts,  including  moving  range  (MR)  charts,
                     whereas others are checked using sampling methods. Note that two
                     process capabilities had to be changed, since the quality performance
                     has changed dramatically.
                     5.3  Determining Gauge Capability
                     An  important  part  of  capability  studies  when  measuring  the  total
                     variability in manufacturing is to account for gauge or test process
                     variability.  Variability  is  not  limited  only  to  the  manufacturing
                     process; the variability of the measurement system needed to test the
                     manufactured parts should also be considered. Figure 5.6 shows typi-
                     cal sources of variation and error in a process and its measurement
                     system. The majority of measurement errors, including those due to
                     the operator (appraiser) or the equipment (gauge), can be measured
                     and quantified through gauge reliability and reproducibility (GR&R)
                     methodology.  The  use  of  GR&R  to  evaluate  measurement  systems
                     quality is mandatory in achieving six sigma quality.
                       The following is an explanation of the terms used in Figure 5.6.
                       Short and long variations in the manufacturing process are due to
                        time-dependent  parameters,  such  as  incoming  part  quality
                        changes,  age  of  equipment,  and  methods  for  maintaining  equip-
                        ment. They will be discussed in the next section.
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