THE PROCESS WINDOW INDEX™ (PWI)—PROFILING REDUCED TO A SINGLE NUMBER
The PWI measures the profile's fit to the process window in a mathematical and objective manner by using a single
number. This aids in optimizing the process by comparing and ranking alternative profiles in terms of their fit to
the available process window. The lower the PWI, the more efficient and stable the process!
ABOUT THE PROCESS WINDOW INDEX (PWI)
There are statistically valid methods for quantifying most EMS processes, such as pick and place and screen printer
performance. However, until now there has been no widely accepted method for measuring, analyzing and comparing the
performance of thermal profiles, and no means of ranking their suitability for reflow of newly introduced boards.
The Process Window Index (PWI™), developed exclusively by KIC, is a simple way of statistically measuring and
ranking how good any given profile is relative to critical process limits.
PWI ranks every thermal profile by where it "fits" within the user defined process window. Optimum performance is
achieved at the center of this window, and is designated as 0%. The opposite or extreme edge is defined as 99%, A
PWI of 99% indicates that the profile will process product within spec, but is running at the very edge of
suitability, and of course, any boards processed with a PWI above 99% risk certain rejection. So for example, if the
highest recommended temperature is 250° C and the lowest is 235° C, the zero point or optimal PWI would be midway
between these extremes, or 242° C. The PWI reveals exactly where a given profile numerically ranks within the
process window, which is a clear designation of how viable that profile is. Although slight fluctuations or
deviations are normal in the reflow process, it's a certainty that the lower the PWI, the better the profile. It's
as simple as that!
THE BENEFITS AND ADVANTAGES OF PWI RANKING
Greatly Simplify & Speed The Profiling Process. All profile statistics are reduced to a single number
- the PWI - that anyone can understand. When using advanced KIC profiling tool, such as Navigator, the PWI aids in
speeding up the whole profiling process by making it easier to understand.
Instill Confidence in Thermal Process Capability. he PWI is not just a single statistic. It reflects
the performance of the entire profile and thus aids in generating the best profile a process can achieve, opening
the door to precise profile control.
Profiles Can Be Easily Compared .Comparisons can now be made quickly -- between individual lines on
the shop floor, between processes at multiple plants and between processes using dissimilar equipment. With PWI,
profile comparison is no longer subjective, since it ascertains that the best profile is being used.
Attain Improved Quality Control. is ideal for use with SPC and other QC monitoring platforms. It is a
significant step towards zero-defect production.
Lower Production & Equipment Costs. Immediate savings can be realized in each of these areas: staff
training; operator errors; production monitoring and data collection; process troubleshooting, and pre-purchase
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:
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.