The Use of Implied Cpk in Product and Assembly Tine Manufacturing and Planning Activities

2017-08-19 16:56:59 viya 76

As discussed earlier,some industries have adopted a form of six sig- ma that is based on target values of Cpk. Examples are the auto industries with the QS 9000 (Cpk 1.67 for new and 1.33 for old products), and the defense industry with various Cpk values for weapon systems (Cpk = 1.33 for the P22 jet fighter). In these cases, an implied Cpk value is used to characterize the quality of the process or the product being evaluated.

Cpk

When using implied Cpk, it is assumed that defects are occurring because of violation of a particular or a composite set of specifications. The composite specification can be one-sided or two-sided, depending on the interpretation of the defects. For example, a wire bond could be treated as one-sided, since it is assumed that in testing the bond, only a minirnum specification value is given. For solder defects, a composite specification can be assumed to be two-sided, since solder defects can be caused by too much solder (solder shorts),or too little solder(insufficient solder defects). The difference between implied one- or two-sided specifications is that the number of defects representing the f{z) value under the normal curve should be halved for two-sided specifications or used directly for one-sided specifications, resulting in different implied Cpk interpretations. The decision for one- or two-sided specifications for implied Cpk should be left to the appropriate design and manufacturing engineers. A description of the use of such an implied Cpk process is given in Chapter 2.


The use of an implied Cpk process in assembly line activities is similar to the DPMO process. Individual manufacturing processes are analyzed for quality,with a DPU (PPM) and an implied Cpk calculated for each. For each assembly, such as a PCB running through the line, the parts counts and process steps are calculated, then multiplied by the DPU rate to obtain the defects for each step in the assembly line. Finally the defects are added and then reflected in a total yield using Equation 4.8 and an implied Cpk. Alternately, the yields could be calculated for each step, then multiplied together to form a total yield. A decision has to be made for each process as to the type of quality data to be collected. In the PCB assembly line case, the choices of the quality data for each process can be as follows:

• The use of a particular defect parameter for each process step. For component types, defect data can be collected on the following: axial insertion for through-hole components, pick and place operations for SMT components,odd-shape components for automatic as well as manual placement, and mechanical parts assembly such as with screws and special connectors. For terminations,defect data can be collected on manual as well as automatic soldering.

This division of defect data according to the process used can help in identifying lower-quality process steps and in targeting these processes for quality improvements.

Data collection can be based on a selected attribute. For example, placement quality data can be based on components, leads, or a combination of both.

Guidelines have to be established in order to handle defects from prior operations that might influence defects in subsequent operations; for example, a placement defect that can cause multiple terminations defects. This can skew the termination data. Decisions have to be made and training programs offered to operators in order to follow guidelines on apportioning and analyzing defects ac- cording to source.



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