Process Window Index

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.

Example of a Process Window Index for peak, soak, and slope values

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:

pwiformula

where:

i = 1 to N (number of thermocouples)

j = 1 to M (number of statistics per thermocouple)

measured value [ij] = the [ij]th statistic’s value

average limits [ij] = the average of the [ij’]th high and low limits of the statistic

range [ij] = the [ij]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

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Is Go-No-Go profiling good enough?

What is go-no-go profiling? Well basically you get a pass/fail, green light/red light indication at the end of your profileGo no Go letting you know whether you are in spec or not.

How often are most of us, when we get the “go” or green light at the end of the profile indicating that we finally got the profile in spec, ready to button it up and turn the thermal process over to production?

If I have and in-spec profile does this mean I’m ready for production?

Well, if you think about it, if you are just relying on a go-no-go indication for your profile results you are never sure if your process is at the very limits or somewhere deep within spec.   In the case of the former,  depending on the demands of your production on your thermal process you could be operating at an out of spec condition during production, even though your profile gave you a green light!

If you are like most production facilities being in spec during production not only matters but having an indication of how far within spec is your process is just as important since depth of being in spec is the only way to ensure continuous quality production.  A common tool used for not only determining whether you are in or out of spec as well as how far in or how far out (depth) is using PWI (process window index).

See the blog entry PWI.

https://profilingguru.com/process-window-index-pwi/

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