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Why Automatic Thermal Profiling Outperforms Manual Approaches
Manual profiling: attach a number of thermocouples to specific areas on a PCB, plug into a profile device, and send this train through a reflow oven or wave solder machine.
By Bjorn Dahle, President, KIC, San Diego, CA
Electronics manufacturers are constantly seeking improved performance for reduced cost. As part of electronic manufacturing, one of the last manual tasks to make the transition to automation is thermal profiling. Yet, the technology for real-time thermal automatic profiling (AP) certainly exists and offers inherent benefits over the older manual profiling routines.
For decades, the electronics assembly industry has accepted manual profiling as the best way to set up a reflow oven and wave solder machine, and to verify and document that the thermal process adheres to relevant process limits. Manual profiling essentially involves attaching a number of thermocouples (TCs) to specific areas or components on a printed circuit board (PCB), plugging into a profile device, and sending this train through a reflow oven or wave solder machine. The profiler records and displays the resulting time-versus-temperature graph, along with some selected data such as peak temperature, time above liquid state, and more. This approach, with the benefit of a hard-wired connection to a measuring point using TCs, is based on direct contact readings of temperature over time.
There are a number of weaknesses and limitations to this method:
TC attachment is critical and often represents a source of inaccuracies. When using high-temperature solder or epoxy for attachment, the TC bead (where the actual reading takes place) does not measure the surface of the PCB or component but rather the blob of material covering the bead. The use of aluminum tape for attachment greatly reduces this inaccuracy since the tape itself only minimally impacts the TC measurement.
For readings on a specific product (as opposed to an unrelated fixture), one of the production PCBs must be used for profiling. Repeated runs through the oven/wave solder machine deteriorate the PCB. After a few runs, the PCB already is lighter (material has burned off) and no longer represents the production PCB.
When reattaching TCs, either because the PCB has deteriorated or because a TC has fallen off, inaccuracies are introduced. It is almost impossible to attach a TC anew and achieve the same reading. Variances are introduced to the instrument that is trying to measure consistencies. Again, the use of aluminum tape helps reduce this source of inaccuracies.
One of the biggest weaknesses of manual thermal profiling occurs when the thermal process changes, and the profiler is sitting on a shelf somewhere unaware. This spot-checking nature of manual profiling means that the thermal process runs blind almost all the time.
Manual profiling can interrupt production, is labor intensive, and is based on human intervention, making it inconsistent and not conducive to process control.
A number of automatic thermal profiling systems are currently available from various suppliers, including KIC. The systems rely on sensors that are permanently embedded in the oven or wave solder machine. Like most automatic machines, these AP systems must first be programmed before being capable of performing automatically. To program the system, a technician runs a single manual profile. The programming procedure generates two sets of separate data streams: a profile on the PCB to be measured with TCs attached to strategic locations on the board, and the environment inside the test chambers along the path on which the PCB runs, in addition to the pace and position of the PCB during the programming run.
Automatic profiling (AP) provides increased automation, traceability, quality improvements and lower production costs, relying on sensors that are permanently embedded in an oven or wave solder machine.
The manner in which an object heats up and cools down is the result of numerous variables that influence each other according to the laws of thermodynamics. By measuring these variables, it is possible to accurately calculate the time-versus-temperature profile for an object under test. In the case of the PCB traveling through the thermal process, the PCB profile is a function of the following variables: the environment to which it is exposed, i.e., the heat, the difference in temperature (ΔT) between the PCB and the adjacent air, and more; the duration to which the PCB is exposed to the dynamic environment at all times; and the mass and thermal properties of the PCB.
By using 30 temperature sensors located along the path of a PCB, an automatic system continuously measures the environment that affects the PCB during its run through the thermal measurement process. At the same time, the position of the PCB, and the duration for which it is exposed to the varying environmental forces, are measured by the board sensor and speed encoder. The automatic system learns the thermal properties of the PCB by recording how the PCB heats up and cools down during the programming run. Therefore, a model can be built that accurately calculates the PCB profile during future production runs based on real-time variables that are being measured continuously. The end result is an accurate real-time profile measurement of each PCB being produced. The measurements are performed without human intervention and without affecting the production.
Benefits of AP
There are significant benefits to AP over manual profiling:
The continuous nature of AP means that the thermal process no longer runs blind. The profile for every PCB is measured and checked against a relevant process window to verify that the PCB meets required specifications.
Data can be stored for easy retrieval at any time for full thermal process traceability.
The AP's real-time nature lends itself to effective statistical-process-control (SPC) charting. SPC acts like a proactive information system that informs the responsible engineers of negative trends or an out-of-control thermal process. This allows an engineer to adjust the process or oven setup before any defects have been produced.
AP can help lower the cost of production by reducing production downtime due to manual profiling as well as by reducing scrap, rework, and labor.
AP can be used as a troubleshooting tool when a production line suddenly develops a yield issue. Even though most engineers believe that only 5 to 10 percent of all defects are attributed to reflow and wave soldering, the same engineers often start troubleshooting for yield problems by running a profile. This is because the oven or wave solder machine is essentially a "black box" as opposed to having relatively good information from all the other machines and processes in the production line. Executing an unscheduled profile may take 30 minutes and sometimes much longer. If the profile indicates that the problem is not in the thermal process, then expensive production downtime has been used looking for the problem where it did not exist. An AP system, on the other hand, immediately informs the engineer whether the process was acceptable or not.
As AP becomes more popular in the electronics manufacturing industry, additional capabilities are being developed for these systems. One of these is providing complementing automatic optical inspection (AOI) and x-ray capabilities. AOI machines cannot inspect solder joints hidden from view underneath the component body, for instance in the case of ball grid array (BGA) and package-on-package (PoP) components. Even where the AOI machine can view the solder joints, it cannot look inside the solder's microstructure to determine the health of the joint. If the AP system verifies that the BGA components and solder joints were processed in accordance with the component tolerances and solder paste specifications and the AOI machine finds no defects, then a more complete inspection will result. The combined inspection provides a high level of confidence in the quality of a PCB being produced.
An x-ray system has the benefit of seeing through component bodies onto the solder joints below, but it cannot go inside the microstructure of the joints to determine the health of the solder joint. Defects such as cold solder joints and head in pillow are difficult to detect. Knowing, however, whether these solder joints were processed correctly makes for a more complete inspection. Because most factories are running batch x-ray inspections, where only a small fraction of PCBs are inspected, an AP system with this dual inspection capability can help make the selection for inspection more effective by flagging those PCBs that have been processed out of specification in the thermal process.
Although there is no physical contact between a TC and a PCB under test, the automatic measurements are quite accurate and better than manual profiling due to the weaknesses of manual profiling listed above, but also the accuracy can easily be measured. The event-based measurements of an AP system are performed on every PCB exiting an oven or wave solder machine, in contrast to manual temperature profiling, where TCs must be attached to a PCB under test. Over time, it is possible to acquire two sets of data for the same PCB: the AP measurement and the manual profile measurement. By doing this over time and running an accuracy and repeatability study of the two data streams, the two approaches can be studied for how well they correspond with each other.
Given the quality and stability of today's reflow ovens and wave solder machines, some may question whether a continuous proofing system such as an AP system is really necessary. But there are several opportunities for ovens to suffer from significant process variations that are beyond an oven's control. The best example would be changes in a factory exhaust system. AP is not as much about babysitting ovens and wave solder machines, as much as it is about automation, traceability, quality improvements, and lowering production costs.
At present, thousands of electronic manufacturing factories around the world rely on AP. These facilities span the full spectrum of EMS and OEM as well as small, medium and large companies. Many of these facilities have world-class production and process control, providing further credibility to the accuracy of AP.
Contact: KIC, 16120 Bernardo Center Dr., San Diego, CA 92127
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