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Process Optimization
The process window index (PWI) can be used to quickly determine the impact of process changes and compare different process parameters.
Improving product quality in electronic assembly processes is important to profitability and is especially significant in the highly competitive electronics manufacturing services (EMS) industry. In practical terms, the foundation step is choosing a process setup that minimizes defects. A process setup consists of material selections, equipment configurations and assembly sequences. By adjusting the process setup, product quality can be improved or degraded.
Quality levels must be maintained throughout extended or multiple production runs. Maintaining quality requires robustness, which is defined as "the insensitivity of the product's functional characteristics to variations caused by noise factors:'l Accordingly, the process setup chosen must be sufficiently robust so that defects do not result from any normal variation of setup parameters. Ideally, the process setup should also provide some protection from abnormal variations.
Specified Process Window
An excellent philosophy for reducing defects is the integrated sequence of Define - Measure Improve (DMI). By following these three steps, deficiencies can be systematically identified and improvements achieved. The first step in the DMI approach is to define the process, ultimately in terms of quantitative specifications that are known to determine product quality.
Specifications are not single, discrete values. Instead, they are ranges consisting of a minimum value, a maximum value or, most commonly, both. Some examples are:
* each chip component must have a minimum contact of 95 percent on the pad
* the height of a solder paste deposit must be no less than 4 mils and no more than 8 mils
* the preheat slope of all solder joints must be less than 4degC/second during reflow.
These ranges are the process windows within which zero or minimum defects will be achieved. For some parameters, the minimum and maximum are true, binary statistical specification limits where any value inside the range is good and any value outside the range is a defect. The classic example of this type of specification limit is the pin in hole: Any pin less than the specified diameter fits in the hole, any pin larger does not.
Other specification ranges are more analog, exhibiting good quality for values within the...