The Case for Automatic Profiling: Why It Outperforms Manual Profiling

by Bjorn Dahle, President, KIC

The electronics assembly industry is constantly evolving to manufacture requested quality while driving down cost. One area of recent focus is perhaps one of the last manual tasks in the production line, namely thermal profiling. This article describes the technology for real-time automatic profiling (AP) and the inherent benefits over the old manual profiling routine.

Limitations of Manual Profiling

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 the relevant process limits.

Before describing the technology behind AP and its benefits over manual profiling, it is important to also review the strengths and limitations of the until-now accepted profiling routine.

Manual profiling essentially relates to attaching a number of thermocouples (TCs) to specific areas or components on a printed circuit board (PCB), plugging in a profile device and sending this train through the reflow oven or wave solder machine. The profiler records and displays the resulting time vs. temperature graph, along with some selected data such as peak temperature, time above liquidous and more.

This method has the benefit of utilizing a hard-wired connection to a measuring point using TCs. It is therefore based on direct contact readings of temperature and time.

However, there also are numerous weaknesses and limitations to this approach:
1. 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 as the tape itself only minimally impacts the TC measurement.
2. In order to get 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 After a few runs, the PCB already is lighter (material has burned off) and no longer represents the production PCB.
3. When reattaching TCs, either because the PCB has deteriorated or because a TC has popped off, inaccuracies are introduced. It is almost impossible to attach a TC anew and get the same reading. In other words, variances are introduced to the instrument that is trying to measure consistencies. Again, the use of aluminum tape helps reduce this source of inaccuracies.
4. Perhaps the biggest weakness of manual profiling is that when the thermal process changes, the profiler is sitting on a shelf somewhere unaware. The spot checking nature of manual profiling means that the thermal process runs blind almost all the time.
5. Manual profiling can interrupt production, is labor intensive and is based on human intervention, making it inconsistent and not conducive to process control.

Automatic Profiling Overview

There are numerous automatic systems on the market (including several systems from KIC, an industry leader in automated thermal process tools and systems. We also develop and sell manual profilers. The systems rely on sensors that are permanently embedded in the oven or wave solder machine.

Like most automatic machines, the AP system first needs to be programmed before going into the automatic mode. To program the system, the technician runs a single manual profile. The programming generates two sets of separate data streams:
1. The profile on the PCB measured with TCs attached to strategic locations on the board.
2. The environment inside the chambers along the path where the PCB runs, in addition to the pace and position of the PCB during the programming run.

How an object heats up and cools down is not a mystery, but rather it is a result of several variables that influence each other in accordance with the laws of thermodynamics. If we can measure these variables we can then accurately calculate the time versus temperature profile of the object. 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 delta T between the PCB and the adjacent air and more
• The duration to which the PCB is exposed to the dynamic environment at all times
• The mass and thermal properties of the PCB

By utilizing the 30 temperature sensors located along the path of the PCB, the automatic system continuously measures the environment that affects the PCB during its run through the thermal 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. Finally, the automatic system learns the thermal properties of the PCB by observing how the PCB heats up and cools down during the programming run. A model can therefore be built that accurately calculates the PCB profile during future production runs based on the 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 hence performed without human intervention and without affecting the production.

Benefits of AP

There are significant benefits to AP over manual profiling.

1. The continuous nature of AP means that the thermal process no longer runs blind. The profile for every single PCB is measured. The profile also is checked against the relevant process window to verify that it is in-spec
2.  Data can be stored for easy retrieval at any time for full thermal process traceability
3. AP’s real-time nature lends itself to effective 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 the engineer to adjust the process or oven setup typically before any defects have been produced
4. It lowers the cost of production by reducing production downtime due to manual profiling as well as by reducing scrap, rework and labor
5. AP can be used as a troubleshooting tool when a production line suddenly develops a yield issue. Even though most engineers believe that only 5-10 percent of all defects are attributed to reflow and wave soldering, the same engineers often start troubleshooting the yield issue 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 industry, additional capabilities are coming to the fore. One of these is the ability to complement automatic optical inspection (AOI) and X-ray. 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 specs and the AOI machine finds no defects, then a more complete inspection will result. The combined inspection provides a high level of confidence that the entire PCB is good.

An X-ray system has the benefit of seeing through component bodies onto the solder joints below, but again it cannot go inside the microstructure of the joints to determine the health of the solder joint. Defects like cold solder and head in pillow are hard 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, where only a tiny fraction of the PCBs get inspected, the AP system can help make the selection for inspection more effective by flagging those PCBs that have been processed out of spec in the thermal process.

AP Accuracy
A common question is how accurate can the automatic measurement of the PCB profiles be when there is no physical contact between a TC and the PCB. The answer is that the accuracy is not only extremely high and better than manual profiling due to the weaknesses of manual profiling listed above, but also the accuracy can easily be measured. An AP system is event-based, meaning that the profile measurement is based on each PCB exiting the oven or wave solder machine. When attaching TCs to a PCB and running a manual profile, we acquire two sets of data for the same PCB — the AP measurement and the manual profile measurement. We can do this repeatedly over a period of time and then run an accuracy and repeatability study of the two data streams to determine how well they correspond to each other.

Today, there are thousands of factories around the world that rely on AP. They 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.

Another question often raised is whether a continuous proofing system is required when modern reflow ovens and wave solder machines are so stable. There are several opportunities for ovens to suffer from significant process variations that are beyond the oven’s control. The best example of this would be changes in the factory exhaust system. Also, as can be deducted from the text above, AP is not about babysitting ovens and wave solder machines, it is about automation, traceability, quality improvements and lowering production cost.
Bjorn Dahle, President, may be contacted at KIC, 16120 Bernardo Center Dr., San Diego, CA 92127; 858-673-6050; E-mail: [email protected]; Web site: www.kicthermal.com.

Reprinted with permission from Global SMT & Packaging June 2013