What is your PWI?

Process Window Index (PWI): the thermal profile fit within the process window

A thermal profile provides the temperature vs. time data collected via a data logger with thermocouples attached to a PCBA or other assembly, as it passes through a conveyorized oven or other thermal process (for soldering or curing).

The job of any manufacturer is to make sure this profile accommodates the relevant process specifications and control limits – the process window. The Process Window Index (PWI) measures how well the thermal profile fits within this window. It characterizes this relationship with a single number!

Due to the complexity of both the thermal profile and the process window, it 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 of 100 and above means your profile is out-of-spec
  • A PWI of less than 100 means your profile is in-spec
  • 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

Let us take a look at the last benefit. As 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 fast and accurate recipe search engine and profile prediction AI you arrive at the ability to select the best oven recipe to run production 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:

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 measured 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.

PWI data sheet

PWI size A4 data sheet