Better thermal profiling and process optimization tools reduce energy use.

Due to the higher melting point of SnAgCu alloys, higher reflow soldering temperatures are required for Pb-free PCB assembly. Consequently, reflow oven energy consumption grows too. However, modern thermal profiling and process optimization software has the potential to reduce energy requirements.

To determine energy consumption during a Pb-free reflow process, a convection oven was equipped with a multifunctional energy meter. Measurements were taken over several days, during which leaded and Pb-free versions of the same product were processed.

Experiments were conducted using a Heller 1912 EXL reflow oven manufactured in July 2005 and a KIC SlimKIC 2000 profiler equipped with Auto-Focus optimization software.

For the tests, a representative telecom product was chosen from a family of products to be converted to Pb-free in the near future. A comparison was performed on the same product manufactured using both technologies. The experiment consisted of four sets of measurements:

• Leaded product with non-optimized reflow profile.

• Leaded product with optimized profile.

• Pb-free product with non-optimized reflow profile.

• Pb-free product with optimized reflow profile.

None of the optimized oven recipes used a conveyor speed slower than the slowest cycle time in the production line. In other words, the reflow oven was not a 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 peak temperature of the process window. Based on the solder paste, substrate and components, the process window for peak temperature was 205° to 225°C.

Oven settings for Case A were 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/min. (37"/min.).

The reflow parameters for Case A:

Peak temperature (°C): 223.5
TAL² (s): 82.0
DT (°C): 11.06

Once the profile had been set, the hourly energy consumption of the oven was measured. As is typical, the data fluctuated somewhat, but the average energy consumption for Case A was 10.4 kWh.

Case B - Optimized leaded Focus. The optimized profile was created with the help of profiling software with an Auto-focus option. Before engaging the software, the Case A profile was chosen as starting point.

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/min. (36"/min.).

The reflow process parameters for Case B:

Peak temperature (°C): 216
TAL (s): 81.3
DT (°C): 9.18

At a glance, both the peak temperature and the DT across the board are significantly lower.

Once the profile was set, the hourly energy consumption at the oven was measured. Energy consumption for Case B was 8.8 kWh, 15% lower than Case A.

Case C - Non-optimized Pb-free profile. The non-optimized Pb-free profile was chosen in the same manner as the non-optimized leaded profile (Case A). One difference was peak temperature; the process window in this case was 235° to 260°C.

Oven settings for Case C were:

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/min. (35"/min.).

The reflow parameters for Case C:

Peak temperature (°C): 252.7
TAL (s): 88.8
DT (°C): 9.71

Energy consumption for Case C was 11.5 kWh, a 10.6% increase in energy use compared to the equivalent leaded application and a 30.7% increase over the optimized leaded process.

Case D - Optimized Pb-free profile. As for Case B, the optimization software was used to find the best oven recipe. The resultant settings:

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/min. (32"/min.).

Reflow paramaters:

Peak temperature (°C): 241.4
TAL (s): 85.0
DT (°C): 4.67

Significant DT reduction can be observed.

The average energy consumption for the optimized Pb-free reflow profile is 10.6 kWh. This is very similar (1.9 higher) to non-optimized leaded profile consumption.

Compared to the non-optimized Pb-free profile, the optimized Pb-free profile shows an improvement of 7.8%.

Conclusion

The data below show average energy consumption for leaded and Pb-free profiles. The lower use of energy when using optimized profiles can be seen.

	Leaded	Pb-Free
Non-optimized	10.4 kWh	11.5 kWh
Optimized	8.8 kWh	10.6 kWh

The data can be related to the financial impact using the following formula:

Total cost of energy/year = Hourly consumption * Cost per kWh * 24 hrs. * 7 days * 52 weeks

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 Pb-free process.

Thus, using a modern thermal process optimization tool can result in production cost reduction. As a side note, the optimized oven recipes not only save energy, but also operate in the "sweet spot" of the process window, hence improving quality and productivity as well.

References

• M. Apell, J. Dautenhahn, T. Formella and J. Morris, "Power Consumption and Nitrogen Control in Lead-Free Reflow," OnBoard Technology, April 2004.

Piotr Kaênica is NPI engineer at Flextronics (flextronics.com); piotr.kaznica@pl.flextronics.com.