A thermal profile provides the time vs. temperature information on a
part going through the thermal process. The job of any manufacturer is
to make sure that this thermal profile accommodates the relevant thermal
process. The Process Window Index (PWI) measures how well the profile
fits the process window. It characterizes this relationship with a
single number!

This method displays the results from a reflow profiler or data logger
in a simply way, a single number showing if the reflow profile is in
spec.

Due to the complexity of both the thermal profile and the process
window, it typically is not easy to identify whether the process is in
spec, much less how deep in spec the profile is. That is until you use
the powerful and exceptionally simple PWI concept:

A PWI number of 100 and above means that your reflow profile is out of spec
A PWI of less than 100 signifies an in-spec reflow profile
The lower the number, the closer to the center of the process window

Think of it as the Richter scale of your process. A single number
provides all you need to know. It enjoys the following benefits:

Objective
Transcends language barriers
Is equipment and personnel independent
Instant analysis
Forms the foundation for process optimization

Another benefit, as modern electronics have increased in complexity and
factories strive to produce the required quality in volume, simply
setting up your thermal process in spec is no longer adequate. The
process needs to be deep in spec in order to accommodate the natural
drift that is inherent in any process over time. The PWI concept
characterizes each reflow profile in terms of how close it is to the
center – the sweet spot – of the process window. This means that it
automatically ranks alternative reflow profiles. Whether you have two or
two million alternative process setups for your application, the PWI
number will immediately alert you to the very best reflow oven setup.

When combining KIC’s extremely accurate process simulation with the
speed of modern computers, you arrive at the ability to select the best
reflow oven recipe to produce a process in the “sweet spot” of your
process window – in seconds!

Calculating the PWI

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

The PWI 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 reflow profile statistic (maximum, or
highest percentage of the process window used), and all other values are
less.