Automated reflow process control: a new system provides electronics assemblers with automated real-time control of their reflow process.

Process data is readily available for the screen printer and pick-and-place phases of surface-mount production. Thus, determining whether these steps are in control is a relatively simple task. However, until recently, this task has not been true for the reflow process.

Systems have been available for many years that deliver real-time reflow process data, but these systems have been configured as engineering tools. An engineering tool captures all available data on a given process, but then a process engineer must analyze that data to determine whether the process is in control.

However, in today's economy, where electronic assemblers must realize every possible efficiency, this allocation of scarce engineering resources is not acceptable. Technology that gives assemblers complete control of their reflow processes while minimizing the need for human resources is required.

Status Quo Process Control

The status quo method of controlling the reflow process is to first set the oven to previously determined temperature setpoints and conveyor speed. Then, when the oven reports that the control points have reached setpoint value, production can begin.

The user must assume that the temperature setpoints and conveyor speed that gave an in-spec profile yesterday will give an in-spec profile today. However, the more time that has elapsed since the oven was profiled, the less likely that the process will still be in-spec. Most users are aware that they are essentially "flying blind" so they verify their reflow process on a regular basis: once a month, once a week, once a day or even once a shift.

One problem with this method is that if the profile has drifted out-of-spec, every product manufactured since the last in-spec profile is suspect. Another problem is that, if a catastrophic change occurs in the process, periodic profiling will not likely catch this change. Instead, catastrophes will be discovered at final inspection, which of course is too late, and inevitably some product must be reworked or scrapped. But the most serious problem with this method is that it is expensive in terms of both line downtime and human resources.

The status quo method of reflow process control relies on two basic technologies: pass-through profilers and real-time thermal monitoring systems. Pass-through profilers have evolved into sophisticated process setup tools, but they still only offer a snapshot of the process.

One work-around that has been tried with limited success is the use of a pass-through profiler with a fixture that characterizes the oven rather than the actual process. This method provides some data for tracking oven performance, but, because the sampling is intermittent, the method does not yield adequate process data or true control.

Real-time thermal monitors, with sensors embedded in the oven, do offer continuous data (in essence, a video of the process) and the opportunity for true process control. However, because currently available real-time thermal monitors are configured as engineering tools, true process control is resource intensive.

Automated Reflow Management

Automated reflow management systems that combine continuous statistical process control (SPC) charting, line balancing, documentation and production traceability into an integrated software package have recently been introduced. They are designed to automatically feed real-time process data to engineers and managers, allowing them to make critical decisions affecting production costs and quality.

These systems can record real-time thermal process data for every product, instead of the conventional practice of only periodically checking oven performance. These systems automatically catch potential defects before they happen, rather than discovering actual defects during the inspection stage.

Automated reflow management systems use thermocouples permanently embedded inside the conveyorized oven at the process level (Figure 1). The thermocouples are configured in probes, which are installed along the conveyor at product level. The probes are connected to a device that sends the probe data to a personal computer.

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The basic function of an automated reflow management system is to accurately and automatically collect data on product passing through the reflow oven. This system provides several benefits:

* eliminates the need for process verification profiles

* provides real-time feedback and alarms for zero-defect production

* completely automates reflow process data collection

* provides automated SPC charting of the reflow process and alarming of variances in process capability (Cpk).

One significant difference between automated reflow management systems and previous real-time thermal monitors is that the new systems are a production solution rather than an engineering tool. System software has been designed to be completely intuitive for maximum ease of use, which means that the process can be monitored with minimal human resources.

An automated reflow management system's ability to interface directly with the oven controller offers additional process efficiencies. When setpoints are changed in the software, such as changing to a previously profiled product, the data can be downloaded automatically to the oven, eliminating the need for separate data entry.

Product Virtual Profiles

The means for verifying the profile of every board produced is a virtual profile (Figure 2). A virtual profile is established by running a baseline profile of the product with a real-time profiler while simultaneously collecting real-time data from the thermocouple probes in the oven. The mathematical correlation between the temperatures at product level and the temperatures on the product itself allows the software to accurately simulate changes in the product profile.

[FIGURE 2 OMITTED]

Once a virtual profile has been established, the system goes to monitoring mode with real-time simulation of how the product profile changes based on probe readings. Process temperature or airflow cannot change without affecting the product temperature, and the software's algorithms accurately extrapolate changes in process temperature to changes in the product profile.

Once a profile has been established within a user-defined process window, the automated reflow management system monitors production for that particular product. In the real-time monitoring mode, the system produces a real-time profile chart and a table of data that has been selected based on the process window. The system can also provide SPC charts for each statistic, as well as a control chart for the overall process window index (PWI) of the product itself. Data is updated and saved for each board as it exits the oven.

The PWI is a statistical method for ranking process performance. The PWI measures how well a process fits within user-defined process limits by ranking process profiles on the basis of how well a given profile fits the critical process statistics (Figure 3).

[FIGURE 3 OMITTED]

The PWI reflects the performance of the whole process, which is a much better indicator of process capability than tracking a single statistic. Thus, the PWI provides excellent data for SPC and other quality control monitoring programs. Automated reflow management systems use the PWI to calculate the overall process capability (Cpk) for every board that goes through the oven.

The Heat Transfer Issue

One recent assertion is that measuring the heat transfer rate in the reflow process is critical. Tools are available that can measure the heat transfer rate in an oven. However, such measurements can only be performed intermittently and only on a fixture, not on the actual product. Research has established what is intuitively obvious: That temperature at the product level will vary significantly if any change occurs in process temperature or airflow; for example, the loss of a fan.

Automated reflow management systems do not directly measure the heat transfer rate inside reflow ovens. Instead, these systems take advantage of the fact that modern ovens are finely tuned, highly repeatable, reliable machines. In a properly functioning oven, the temperature change ([DELTA]T) between the oven setpoints and the conveyor in the controlled zones is extremely stable in normal operation. Thus, temperatures measured at the conveyor are very consistent if the heat transfer rate is consistent.

Automated reflow management systems characterize the oven and determine its stability. The temperature as measured at the product level is a function of the oven's heat transfer rate, which is determined by the zone setpoint temperature and the velocity of the air in the zone. Thus, any change in zone setpoint temperature or air velocity will cause a change in temperatures at the product level. So the automated reflow management system does not need to monitor the heat transfer rate; it is already continuously monitoring temperatures at the belt, and the heat transfer rate cannot change without those temperatures also changing.

Automated Real-Time SPC

Once the virtual profile has been established, the system can automatically generate SPC data (Figure 4). Every time a board exits the oven, the data set is plotted on frequency histograms. Data can be charted for all critical process specifications including peak temperature, soak time and time above liquidus.

[FIGURE 4 OMITTED]

The data is plotted on real-time control charts, and the process capability (Cpk) can be calculated for each specification. The overall PWI can be charted, providing a real-time Cpk for the entire process. Any process drift outside of control limits will bring an immediate alarm. Real-time Cpk tracking enables the system to flag an out-of-control process before the oven has produced a single defect.

Data collection is automated and Windows-based. The system will allow users to examine the profile for every board produced, thus providing valuable process documentation. The file system is product name-based, making such data as profiles, production data and alarm events readily accessible. All events and profiles can also be time and date stamped.

The alarm record shows when alarms occurred and when they were acknowledged, allowing supervisors to monitor their operators' performances. Alarms can be set to trigger whenever the virtual profile statistics or the PWI exceed user-defined limits. The system can also alarm variations in process Cpk, notifying the user that the process is varying and allowing the situation to be corrected before the process goes out of spec. Another alarm function can act as a fail-safe to alert operators of any significant change in process temperatures.

When an alarm occurs, operators can access the troubleshooting function in the software to isolate the source of the alarm. The positive and negative deviation for each probe thermocouple can be tracked, and the amount of deviation can be displayed. The software will correlate the probe thermocouples to the oven zones, so the exact location of the temperature deviation will be readily apparent.

Conclusion

An automated reflow management system offers many benefits including:

* automatic SPC charting that provides Cpk and process data for every product

* real-time process information for improved decision-making

* ease of use for improved staff allocation and reduced training costs

* oven controller interface

* traceability for every product

* no verification profiles

* zero-defect thermal process.

With an automated reflow management system, electronics assemblers can be assured that their reflow process will be inspec 24 hours per day and seven days per week.

Greg Jones is special projects manager with KIC, San Diego, CA; e-mail: [e-mail].