Page 218 - Six Sigma for electronics design and manufacturing

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Six Sigma Quality and Manufacturing Costs of Electronics Products
these specifications and the defects occurring during the PCB manu-
facturing steps is not readily discernible. This has sometimes led to a
manufacturing process having a high-quality Cpk for the process
meeting its individual specifications, yet having a very poor effective
PCB assembly yield. This could result in a loss of credibility in the
Cpk values in manufacturing.
An example of such a problem is in the SMT assembly operation in
PCBs. The assembly consists of applying solder paste onto PCB com-
ponent pads through a thin metal stencil in a screening machine, then
placing the components onto the pads using an automatic placement
machine. The components remain on the PCBs because of the tacki-
ness of the solder paste. The final operation consists of passing the
PCB through a conveyer oven to reflow the solder. The solder paste
suppliers recommend a particular paste volume and height of the sol-
der deposited on the pads and a particular temperature profile for the
reflow oven. A Cpk of the solder paste and reflow operations can easi-
ly be obtained from control chart or process capability data.
High Cpk levels in solder deposition, oven profiles, and other indi-
rect measurements of quality do not necessarily lead to high yields in
PCB assembly. This has resulted in the need to develop composite
Cpk analysis based on direct defect analysis for each step of the PCB
assembly operations. These will be discussed in Chapter 8.
6.3.2 Printed circuit board (PCB)
fabrication technologies
Conventional PCB fabrication (raw PCBS) utilizes subtractive copper
etching to produce circuitry. This process is generally carried out in a
number of steps, as shown in Figure 6.6, where different metals are
Multi-
layer

One or two
layers

Figure 6.6 PCB fabrication steps.