Overview: What is a Process Window Index (PWI)?

To truly grasp a process window index, you must first understand the process window. A process window is a range within which a thermal process must remain, or the quality of the part produced will suffer.

The process window index measures how well the process remains within specified limits. For example, a PWI of 70% suggests that the process “consumes” 70% of the defined window. Conversely, a PWI over 100% means the process will not make the product within specifications.

If the PWI is 99%, the profile will build the product within the defined limits but operates at the edge of its process window.

5 Benefits of a Process Window Index

Before the process window index became commonplace, comparing a thermal process to its window was a subjective judgment with little uniformity. Consequently, results could vary between engineers or operators. It was also difficult to compare different products.

1. Simplifies Profiling

The PWI reduces all profiling statistics to a single number which is easy to understand. As a result, even inexperienced operators can set up an oven with an optimal profile for a thermal process. Before, it could take even experienced engineers hours to accomplish this task.

2. Confidence in the Thermal Process

The PWI ensures managers use the best profile possible to make quality products. The PWI takes the uncertainty out of the “baking” process and enables precision.

3. Makes It Easy to Compare Processes

The PWI enables engineers and managers to compare the performance of different lines within one factory. They can also quickly see how processes at different plants or using various equipment measure up. Instead of a subjective judgement, engineers can now use an objective metric to make these comparisons.

4. Improved Quality Control

The PWI lets engineers know if a specific setup will produce good products.

5. Reduces Production Waste

The PWI helps reduce staff training, limit defects caused by operator error, improve troubleshooting and enables managers to identify the best equipment to perform a process.

Why is It Important to Understand the Process Window Index?

The process window index reveals a lot of information about a process.

If the PWI is close to 100%, minor measurement errors could mean the process actually exceeds the process window. Also, a process with a PWI of 99% is vulnerable to changing conditions which move the process outside the process window (which causes poor product quality and waste).

Also, the PWI fits the DMI (Define, Measure, and Improve) model that most lean six sigma management systems depend upon for continuous improvement.

An Industry Example of a Process Window Index

A typical industry example where PWI applies is soldering parts together while producing a computer motherboard. To avoid thermal shock, engineers will measure how quickly heat increases in the solder (slope), the peak temperature it reaches, the thermal soak (how heat spreads through the solder), and reflow. These parameters must sit within specified limits to ensure reliable connections between parts.

3 Best Practices When Thinking About Process Window Index

The following tips will help you evaluate processes using the PWI percentage metric:

1. You Want Low PWI For Your Processes

While this seems common sense, you want processes with a large margin for error. If the processes only “occupy” a narrow range well within the specified limits, it is less prone to error. Managers must also consider the costs of a process, but low PWI measures are generally preferred.

2. A Reliable Process Sits In the Middle of the PWI Range

Ideally, your process doesn’t come close to the high or low range limit. Processes that come close to the specification limits can actually exceed these limits due to measurement errors and are vulnerable to variations in real-world conditions.

3. Use PWI to Compare Processes Recorded with Different Measurement Standards

Suppose you have factories in the United States and Germany in a new company that merged from firms based in each country. The U.S. factories kept records in inches and pounds, while the German facilities used the metric system. Instead of converting one firm’s data to a common measurement system, you can use the PWI percentage measurement to compare these processes.

Frequently Asked Questions About the Process Window Index

Managers typically ask the following questions about the PWI:

What is the difference between the process window index and the process window?

The process window defines the range within which a process must exist to be reliable. Meanwhile, the process window index (PWI) is the percentage of the process window range that a process occupies in practice.

Are processes with the same PWI entirely identical?

No. The PWI doesn’t tell managers exactly where a process sits within the specified range. For example, two processes can both have a 70% PWI, but one of them comes within 1% of the upper or lower limit. The process close to the range limit will be less reliable.

Why is PWI useful if there can be differences between processes with the same PWI?

PWI shows its benefits when there are multiple parameters that a process must remain within. Trying to compare how reliably a process remains within each range can be difficult. However, a single percentage that cuts across these different ranges can give managers a quick evaluation of how they compare.

Simplicity and Objectivity Are the Main Benefits of the Process Window Index

PWI offers a simple way to compare operators, different lines on the same shop floor, manufacturing equipment, and processes at separate facilities. While processes with the same PWI can have hidden differences, it provides a single objective number to compare manufacturing methods that formerly relied upon subjective judgments.

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