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 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!
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 profile is out of spec
- A PWI of less than 100 signifies an in-spec 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:
- Transcends language barriers
- Is equipment and personnel independent
- Instant analysis
- Forms the foundation for process optimization
Let us take a look at the last 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 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 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 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 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 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 12 statistics for that profile. The PWI will be the worst case (highest number expressed as a percentage) in that set of 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:
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.