Due to the higher melting point of lead-free Sn-Ag-Cu alloy, higher reflow soldering temperatures are required for lead-free PCB assembly. Consequently, reflow oven energy consumption increases as well.
This research work is focused on the potential opportunity to reduce higher energy requirements with the use of modern thermal profiling and process optimization software.
Piotr Kaênica, Flextronics, Poland
Introduction
In order to determine the energy consumption during the lead-free reflow process, a convection oven was equipped with a multifunctional energy meter. Measurements were performed over a period of several days, during which the oven processed leaded and lead-free versions of the same product respectively.
Experiment description
The experiments were conducted using a Heller 1912 EXL reflow oven manufactured in July 2005, and a SlimKIC 2000 profiler equipped with Auto-Focus optimization software. For the tests, a representative product was chosen from the telecom family group of products that was scheduled to be converted to lead-free in the near future. A comparison was performed on the same product manufactured in both technologies. The experiment consisted of four sets of measurements:
- leaded product with non-optimized reflow profile
- leaded product with optimized profile
- lead-free product with non-optimized reflow profile
- lead-free product with optimized reflow profile
Care was taken to make sure that no optimized oven recipes used a conveyor speed slower than the slowest cycle time in the production line. In other words, the reflow oven did not become the bottleneck in the production line for any of the tests in this report.
Case A – Non-optimized leaded profile
For the non-optimized profile a recipe was chosen manually that fit the process window in terms of peak temperature. Based on the solder paste, substrate and components, the process window for peak temperature used the range from 205º to 225ºC. Once the profile had been set, the hourly energy consumption at the oven was measured. As is typical, the data were fluctuating somewhat, but the average energy consumption for Case A was 10.4 kWh.
Case B – Optimized leaded profile
The optimized profile was created with the help of a profiling software system with an Auto-Focus option. This automatic prediction optimizer has the ability to pick up the lowest available peak temperature that fits into the process window. Before engaging the software, the Case A profile above was chosen as the starting point. What can be observed at first glance is that both the peak temperature and the delta T across the board are significantly lower. Once the profile had been set, the hourly energy consumption at each oven was measured. Average energy consumption for Case B was 8.8 kWh, which is 15 percent lower than in Case A.
Case C – Non-optimized lead-free profile
The non-optimized lead-free profile was chosen in the same way as the non-optimized leaded profile (Case A). The difference is the peak temperature.
The process window (in the terms of peak temperature) was defined as the temperature range from 235º to 260ºC. Once the profile had been set, the hourly energy consumption at the oven was measured. Average energy consumption for Case C was 11.5 kWh. This represents a 10.6 percent increase in energy use compared to the equivalent leaded application and a 30.7 percent increase over the optimized leaded process.
Case D – Optimized lead-free profile
As for Case B, the optimization software was used to find the best oven recipe (See Tables 7 and 8). We can observe significant delta T reduction. The average energy consumption for the optimized lead-free reflow profile is 10.6 kWh. It is very similar to the non-optimized leaded profile consumption (1.9 percent higher). Comparing the optimized lead-free profile to the non-optimized lead-free profile, there is a 7.8 percent improvement.
Conclusion
The table shows all average energy consumption for leaded and lead-free profiles. The lower usage of energy with optimized profiles can be seen. The above numbers can simply be translated into the financial impact using Formula 1. Assuming an average energy cost of $0.076/kWh, the annual energy savings per oven as a result of optimizing the process is $1,062.30 for the leaded process and $597.54 for the lead-free process. It has been proven that using a modern thermal process optimization tool can result in production cost reduction. An added benefit is that the optimized oven recipes do not only save energy, but they also operate in the ‘sweet spot’ of the process window, hence improving quality and productivity.
Figure 1. The experiments were conducted using a Heller 1912 EXL reflow oven and a SlimKIC 2000 profiler with Auto-Focus optimization software.
References
M. Apell, J. Dautenhahn, T. Formella, J. Morris, “Power Consumption And Nitrogen Control In Lead-Free Reflow,”
OnBoard Technology, April 2004, pp. 24-26.
Piotr Kaênica is NPI engineer at Flextronics (flextronics.com); [email protected]
Reprinted with permission from Global SMT & Packaging magazine, May 2006