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28 The PCB Magazine • November 2015 the process to repeatedly and reliably meet the product specifications (Figure 1). In this situa- tion, you would need to take steps to improve or redesign the process. If this cannot be done, the only remaining solution would be to present your capability study (more on capability stud- ies later) to the customer, quantitatively dem- onstrating that their specifications are outside of the current process capabilities and request a specification adjustment based on your study. It is important to note here that creatively expanding the specification limits to artificially improve the capability numbers will not only fool no one, but will also not provide any true capability improvement. For a process to be both in control and capable, the process must meet the following criteria: • Must remain stable over time • Must operate in a stable and consistent manner (in control) • Must be set at the proper level (centered) • The natural process variation must not exceed the product's specified tolerance (capable) Data Collection, Sample Size & Pre-Control Once the processes, and aspects for each that need to be controlled, have been estab- lished, the next step is to begin your data col- lection phase. One very successful data collec- tion strategy is to take five measurements per day, which will be then be averaged. A standard SPC control chart can be modified to use during the data collection phase, as long as it is clearly labeled as "Data Collection" and not "SPC." A better choice may be to use a tool that is called a pre-control chart. A pre-control chart is a "visual process stoplight" and an excellent precursor to a full-blown SPC control chart. Pre- control is effective because it uses (stoplight) color-coded zones (green, yellow, and red) to make control chart interpretation very easy for operators. Pre-control limits are established based on the following and again, by collabo- rating with your key suppliers: 1. The middle 50% of the chart is the green zone, which makes up 50% of the specification tolerance for the feature being measured. FeATure DATA AnALyTICS THroUGH STATISTICAL TeCHnIQUeS Figure 1: control versus capability.