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KICstart Version 1.1.0.x
KICstart User Manual 45
APPENDIX A:
The Process Window Index
A Method for Quantifying Thermal Profile Performance
The Problem
While there are currently statistically valid methods for quantifying pick and place and screen printer
performance, there is no widely accepted method for comparing performance of thermal profiles, and thus,
no quantifiable system of ranking thermal process performance. Once a thermal profile has been run, it is
judged as being either in or out of spec, and perhaps subjectively judged as being OK, good, or really good.
Efforts to track process performance for SPC or QC generally focus on a single, or a small group, of profile
statistics; for example, peak temperature of one or two thermocouples on a golden board. The Process
Window Index is a statistical method for ranking thermal profile and thermal process performance.
Defining the Process Window Index
The Process Window Index is a measure of how
well a profile fits within user defined process
limits (see Figure 1).
This is done by ranking process profiles on the
basis of how well a given profile “fits” the
critical process statistics. A profile that will
process product without exceeding any of the
critical process statistics is said to be inside the
Process Window. The center of the Process
window is defined as zero, and the extreme
edge of the process window as 99%.
A “Process Window Index” of 100% or more
indicates that the profile will not process
product in spec. A “Process Window Index” of
99% indicates that the profile will process
product within spec, but it is running at the very
edge of the Process Window. A “Process
Window Index” of less than 99% indicates that
the profile is in spec and tells users what
percentage of the process window they are
using: for example, a PWI of 70% indicates a
profile that is using 70 percent of the process
spec.
The PWI tells users exactly how much of their
process window a given profile uses, and thus
how robust that profile is. The lower the PWI, the better the profile. A PWI of 99% is risky because it
indicates that the process could easily drift out of control. Most users seek a PWI of below 80%, and
profiles with a Process Window Index between 50% and 60% are commonly achieved (if the oven is
sufficiently flexible and efficient).
Figure 2 shows the Process Window Index for the Peak Temperature of a single thermocouple. The
Process Window Index for a complete set of profile statistics is calculated as the worst case (highest
number) in the set of statistics.
Figure 1: Process Window and PWI
Figure 2: The Process Window Index
(Single Statistic—Peak Temperature of one
Thermocouple)