Effective temperature profiling requires that you thoroughly define the process, take steps to ensure an accurate profile and then improve the process based on the feedback provided.

Figure 1. Thermal profiling charts temperature at multiple points as it passes through a process.
When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is unsatisfactory" -- Lord Kelvin

Thermal profiling is the charting of temperature at multiple points on a given product as it passes through a thermal process (figure 1). When manufacturing requires a heat processing step, there must be a method to ensure that the product is heated to a specified temperature for a specified period of time. That method is thermal profiling. State-of-the-art thermal process setup and monitoring equipment can allow you to achieve significant improvements in process performance.

Define the Process. The first, basic question to ask when attempting to improve a thermal process is "How hot for how long?" For most processes, the required temperature is defined as a range called the process window. Al-though it seems obvious, it is critical that this process window be defined. Unfortunate-ly, in many facilities, it is not. The oven or furnace has been set up by a technician, product has been run successfully, and the oven continues run without any form of process control until a problem develops. When the inevitable problems occur, an expensive technician or consultant is called in to fix it.

In order to gain control of a process, it must be defined. Start with materials suppliers. Adhesives, coatings, solder and brazing pastes have recommended process limits. Some software packages include databases of process specifications for various applications.

Figure 2. Profilers range from simple data loggers to sophisticated, real-time systems. Most only record product profile and process data.
Measure. Once the process has been defined and process limits set, the next step is to measure the process. For thermal processes, the most common measurement tool is a profiler; in some applications, a strip chart recorder still is used. Many types of profilers are available, ranging from simple data loggers to modern real-time systems (figure 2). Most profilers only record product profile and process data and do not allow the user to set process limits, relying instead on target temperatures. However, target temperatures do not indicate how well the profile fits the process window.

One method for simplifying process measurement is to use profiling software that ranks profile performance with the process window index. The process window index is a statistical method for instantly determining whether a thermal process is in or out of spec and ranking the profile based on its fit to a specified process window.

Generally, when a thermal profile is run, it is judged subjectively as being either acceptable or unacceptable. The process window index allows users to rank process profiles objectively on the basis of how well they fit the critical process statistics. The center of the process window is defined as zero, and the extreme edge of the process window is defined as 99%. A process window index of 100% or more indicates the profile will not process product in spec. A process window index less than 99% indicates that the profile is in spec and tells users what percentage of the process window is being used.

Figure 3. Process window index shows users the single best profile a process is capable of achieving and how robust the current profile is.
The process window index tells users exactly how much of the process window a given profile uses, and thus how robust that profile is (figure 3). It allows users to find the single best profile that a given process is capable of achieving and allows users to implement a true SPC program for the reflow process.

Ensure an Accurate Profile. Several factors can affect profile accuracy. The most common are improper thermocouple attachment and thermocouple wear.

Improper thermocouple attachment can cause noise (fuzzy lines) on the profile graph, higher than normal temperature readings, lower than normal temperature readings and erratic temperature readings. A good thermocouple attachment will yield a smooth profile. Good thermocouple attachment is defined as solid contact with the product surface, and there are numerous ways to achieve this. For applications below 482oF (250oC), aluminum tape provides a simple, effective and nondestructive method of thermocouple attachment.

Be aware that thermocouple wire deteriorates over time. Thermocouples should be checked with a thermocouple simulator on a regular basis to verify correct readings. Also, keep in mind that the higher the temperature, the shorter the thermocouples' life. At process temperatures below 572oF (300oC), thermocouples can last upwards of 30 profiles; process temperatures of 1,832oF (1,000oC) can destroy a thermocouple in less than 10 profile runs.

When profiling, it is valuable to simulate production as closely as possible. Have the product enter the oven in the same manner as it would during production. If possible, profile the product with a fully loaded oven, as the oven's characteristics can vary significantly between loaded and unloaded. It also is necessary to allow the product to cool to room temperature between profiling runs. If this is not done, it will introduce an exponential error.

Improve the Process. Once the process has been defined and measured, the final step is to improve it. Process improvements can yield multiple benefits, including:

  • Reduced defect and scrap rates.
  • Increased efficiency and cost reductions.
  • Improved labor and equipment utilization.

Technology is available to automate thermal process improvement. Automated oven setup tools utilize an oven recipe search engine to find the best profile for a given product in a given oven in less than a minute.

The installation of an automated thermal manager assures users that their process is always in spec. The technology includes automatic SPC charting, will alarm on any process drift, and can be set to automatically shut off the feed conveyor in case of a process failure. With advanced thermal process technology, users can be certain they have set up the best possible process for their product and that every product will be processed at that spec.

References
1. "A Comparison of Methods for Attaching TCs to PCBs for Thermal Profiling", Cameron Sinohui, KIC. Presented at Nepcon West 99.

For more information on thermal profiling:
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