Process window index

Process Window Index (PWI) is a statistical measure that quantifies the robustness of a thermal process. PWI measures how well a process fits into a user-defined process limit known as the specification limit. PWI is used in the electronics manufacturing industry, particularly the soldering industry, where the scale and rankings were developed for various thermal processes. For maximum efficiency, separate PWI values are computed for peak, slope, reflow, and soak processes of a thermal profile.

Using PWI values, thermal processes can be accurately measured, analyzed, compared and tracked at the same level of statistical process control and quality control that are available to other manufacturing processes.

Statistical process control

Process capability is the ability of a process to produce output within specified limits. To help determine whether a manufacturing or business process is in a state of statistical control, process engineers use control charts, which help to predict the future performance of the process based on the current process.

Process Window Index for a thermal process. Green blurbs denote that the PWIs are within specification, red depicts out of specification.

To help determine the capability of a process, statistically determined upper and lower limits are drawn on either side of a process mean on the control chart. The control limits are set at three standard deviations on either side of the process mean, and are known as the upper control limit (UCL) and lower control limit (LCL) respectively. If the process data plotted on the control chart remains within the control limits over an extended period, then the process is said to be stable.

Within these control limits are the tolerance values specified by the end-user, known as specification limits – the upper specification limit (USL) and lower specification limit (LSL) respectively. If the process data plotted on a control chart remains within these specification limits, then the process is considered a capable process, denoted by ${\hat {C}}_{pk}$.

The electronics manufacturing industry has developed a specialized specification limit known as the Process Window Index (PWI), which provides a quantifiable and reproducible measure of how well a thermal profile performs relative to critical process limits. By using PWI values, thermal processes can be accurately measured, analyzed, compared and tracked at the same level of statistical process control and quality control available to other manufacturing processes.

PWI in electronics manufacturing

Main article: Reflow soldering

A graphical representation of the Process Window Index for a thermal profile

In the electronics manufacturing industry, particularly those involving soldering, a thermal profile is a complex set of time-temperature values for a variety of processes such as slope, thermal soak, reflow, and peak. (See reflow soldering).

Each thermal profile is ranked on how it "fits" in a process window (the specification or tolerance limit). The center of the process window is defined as zero, and the extreme edge of the process window as 99%. A PWI greater than or equal to 100% indicates that the profile does not process the product within specification. A PWI of 99% indicates that the profile processes the product within specification, but runs at the edge of the process window. A PWI of 60% indicates a profile uses 60% of the process specification. By using PWI values, manufacturers can determine how much of the process window a particular thermal profile uses. A lower PWI value indicates a more robust profile. In this way, raw data is normalized in terms of a percentage.

For maximum efficiency, separate PWI values are computed for peak, slope, reflow, and soak processes of a thermal profile. To avoid the possibility of thermal shock affecting the output, the steepest slope in the thermal profile must be determined and leveled. Manufacturers use custom-built software to accurately determine and decrease the steepness of the slope. In addition, the software also automatically recalibrates the PWI values for the peak, slope, reflow, and soak processes. By setting PWI values, engineers can ensure that the reflow soldering work does not overheat or cool too quickly.

Formula

Example of a Process Window Index for peak, soak, and slope values

The Process Window Index for a complete set of thermal profile statistics is calculated as the worst case (ie highest number) in the set of statistics. For example, a thermal profile with three thermocouples, with four profile statistics logged for each thermocouple, would have a set of twelve statistics for that thermal profile. In this case, the PWI would be the highest value among the twelve values, expressed as a percentage. The formula for PWI is calculated as follows:

${\text{PWI}}=100\times \max _{i=1\dots N \atop j=1\dots M}\left\{\left|{\frac {{\text{measured value}}_{[i,j]}-{\text{average limits}}_{[i,j]}}{{\text{range}}_{[i,j]}/2}}\right|\right\}$

where:

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 high and low limits of the statistic

range [i, j] = the [i, j]^th high limit minus the low limit of the statistic

Thus, the PWI is the worst case profile statistic that is the maximum, or highest percentage of the process window used.

See also

• Reflow soldering
• Wave soldering

External links

• Process Window Index on YouTube
• What is your PWI? – KIC Thermal