In teaching process optimization and failure analysis, one of the most helpful concepts is understanding he difference between common cause and special cause defects. A special cause defect, in a well tuned process, occurs when something unpredictable changes. As an example, let's say you get a batch of printed wiring boards (PWBs) that have oxidation on the pads. This is a defect and the boards shouldn't used, however we will assume that somehow they made it through the company's receiving inspection process. It should not be too surprising that when the boards are assembled that they have a poor first pass yield, say 35%. Typical first pass yield in this optimized process is 95%. It is obvious that the poor yield was due to this "special cause," the oxidized pads.
Common cause failures are a little more difficult to explain and comprehend. In a process, there are multiple entities that can vary, within the specifications, such as the solder paste viscosity, the temperature and humidity of the room, the reflow profile, the wettability of the component leads and PWB pads etc. Statistically, within the specifications, the variation can be such as to result in a small number of fails......say the 5% we get with this process when everything is as it should be. These types of fails are called common cause fails.
It is fundamentally crucial to understand the differences between special and common cause fails to successfully monitor and improve processes. One of the tragedies that I often see when the failure rate increases, due to special cause fails, is the process engineers changing the process parameters (e.g. raising the reflow temperatures when the pads in the special cause example above did not wet). In a well optimized process the process parameters are determined by designed experiments, any collapse of process yields is the result of a special cause. You can only fix special causes by identifying and rectifying them, not by changing the process!