Process Window Index

The Process Window Index is a quantifiable, reproducible, statistical measure of how well a thermal profile performs relative to critical process limits. Every thermal profile is ranked on the basis of how it "fits" within the process window. The center of the process window is defined as zero, and the extreme edge of the process window as 99%. A PWI of 100% or more indicates that the profile will not process product within specification. A PWI of 99% indicates that the profile will process product within spec, but it is running at the very edge of the process window. A PWI of 70% indicates a profile is using 70% of the process spec.The PWI tells you exactly how much of your process window a given profile uses, and thus how robust that profile is. The lower the PWI, the better the profile. The thermal process can now be reliably measured, analyzed, compared and tracked with the same level of SPC and Quality Control available to other manufacturing processes.

Calculating the PWI

The Process Window Index for a complete set of profile statistics is calculated as the worst case (highest number) in the set of statistics. For example: if you run a profile with three thermocouples, and four profile statistics are logged for each thermocouple, then there will be a set of twelve statistics for that profile. The PWI will be the worst case (highest number expressed as a percentage) in that set of profile statistics.

The Process Window Index is calculated using a formula that includes all statistics for all thermocouples. The formula for the PWI is calculated as follows:

$PWI\; =\; 100\; x\; MAX\; ((measured\; value\; (i,j)\; -\; average\; limits\; (i,j)\; /\; range\; (i,\; j)+\; 2))$

# 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 statistic's high and low limits
# range [i,j] =the [i,j] th statistic's high limit minus the low limit

Thus, the PWI is the worst case profile statistic (Maximum, or highest percentage of the process window used), and all other values are less.

Links

Process Window Index on YouTube [http://www.youtube.com/watch?v=YWkHU8ft-Cg]

ee also

* Reflow soldering
* Wave soldering