The Effect on Energy Use from an Optimized Reflow Oven Recipe in Lead-Free Applications
Abstract
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 is growing 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.
Introduction
In order to determine the energy consumption during the lead-free reflow process, a convection oven was equipped with a multifunctional energy meter. Then measurements were performed for several days, when 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 the Auto-Focus optimization software.
For the tests, a representative product was chosen from the telecom family group of products that were 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 was used as the range from 205° to 225°C.
Case A Settings
Oven settings for Case A were set as follows:
Zone 1 2 3 4 5 6 Setpoint 101 117 131 155 161 161
Zone 7 8 9 10 11 12 Setpoint 171 180 199 239 239 220 Conveyor speed: 95 cm/minute (37”/minute)
The reflow process parameters for Case A:
Peak temperature [°C]: 223.5 TAL [s]: 82.0 ∆T [°C]: 11.06
Case A Results
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.
Case B Settings
The optimized oven settings for Case B were:
Zone 1 2 3 4 5 6 Setpoint 101 117 131 155 162 162
Zone 7 8 9 10 11 12 Setpoint 172 183 193 229 230 217 Conveyor speed: 92.3 cm/minute (36”/minute)
The reflow process parameters for Case B:
Peak temperature [°C]: 216 TAL [s]: 81.3 ∆T [°C]: 9.18
Case B Results
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 the 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 235° to 260°C.
Case C Settings
Oven settings for Case C were set as follows:
Zone 1 2 3 4 5 6 Setpoint 120 130 150 179 190 222
Zone 7 8 9 10 11 12 Setpoint 235 235 243 263 262 222 Conveyor speed: 90 cm/minute (35”/minute)
The reflow process parameters for Case C:
Peak temperature [°C]: 252.7 TAL [s]: 88.8 ∆T [°C]: 9.71
Case C Results
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 with Case B, the optimized profile was created with the help of a profiling software system with an Auto-Focus option.
Case D Settings
The optimized oven settings for Case D were:
Zone 1 2 3 4 5 6 Setpoint 116 104 140 166 193 211
Zone 7 8 9 10 11 12 Setpoint 219 231 257 247 245 208 Conveyor speed: 80.6 cm/minute (32”/minute)
The reflow process parameters for Case D:
Peak temperature [°C]: 241.4 TAL [s]: 85.0 ∆T [°C]: 4.67
Case D Results
We can observe significant delta T reduction. The average energy consumption for optimized lead-free reflow profile is 10.6 kWh. It is very similar to 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.
Leaded Lead-Free Non-optimized 10.4 kWh 11.5 kWh Optimized 8.8 kWh 10.6 kWh
The above numbers can easily be translated into the financial impact using the following formula:
Total annual cost of energy = Average kWh consumption • Cost/kWh • 24 hours • 7 days • 52 weeks
Assuming an average energy cost of $0.076/kWh, the following table illustrates the total annual cost of energy per oven and the total annual savings per oven as a result of optimizing the process.
Leaded Lead-Free Non-optimized $6,904.93 $7,635.26 Optimized $5,842.64 $7,037.72 Cost Savings $1,062.29 $597.54
This proves that using a modern thermal process optimization tool can result in production cost reduction. Further, the optimized oven recipes operate in the “sweet spot” of the process window. This improves quality and productivity in addition to reducing operating expenses.
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