Each thermal profile is ranked on how it fits in a process window (the specification or tolerance limit). Raw temperature values are normalized in terms of a percentage, relative to both the process mean and the window limits. The center of the process window is defined as zero and the extreme edges of the process window are ±99%. A PWI greater than, or equal to, 100% indicates that the profile does not process the product within specification. A PWI of 99% indicates that the profile processes the product within specification, but that it runs at the edge of the process window. For example, if the process mean is set at 200 °C, with the process window calibrated at 180 °C and 220 °C, respectively; then a measured value of 188 °C translates to a process window index of -60%.
By using PWI values, manufacturers can determine how much of the process window a particular thermal profile uses. A lower PWI value indicates a more robust profile. For maximum efficiency, separate PWI values are computed for peak, slope, reflow, and soak processes of a thermal profile.
To avoid the possibility of thermal shock affecting production, the steepest slope in the thermal profile is determined and leveled. Manufacturers use custom-built software to accurately determine and decrease the steepness of the slope. In addition, the software also automatically recalibrates the PWI values for the peak, slope, reflow, and soak processes. By setting PWI values, engineers can ensure that the reflow soldering work does not overheat or cool too quickly.
The Process Window Index is calculated as the worst case (i.e. highest number) in the set of thermal profile data. For example, a thermal profile with three thermocouples, with four profile statistics logged for each thermocouple, would have a set of twelve statistics for that thermal profile. In this case, the PWI would be the highest value among the twelve values, expressed as a percentage. The formula to calculate PWI is:
i = 1 to N (number of thermocouples)
j = 1 to M (number of statistics per thermocouple)
measured value [i, j] = the [i, j]th statistic’s value
average limits [i, j] = the average of the [i, j']th high and low limits of the statistic
range [i, j] = the [i, j]th high limit minus the low limit of the statistic
Thus, the PWI is the worst case profile statistic that is the maximum, or highest percentage of the process window used.
Source: Wikipedia: http://en.wikipedia.org/wiki/Process_Window_Index