The Relationship between Reflow Soldering Processes and Power Consumption
December 31, 1969 |Estimated reading time: 7 minutes
Differences in the melting point of lead-free alloys compared to that of eutectic tin-lead typically result in soldering processes characterized by changes in the operating temperature of the forced convection oven's heating zones.
By Denis Barbini and Jack Geibig
This simple observation has far-reaching ramifications, including an impact on product peak temperature, product temperature range or ΔT, time above liquidus (TAL), profile shape, oven configuration, and the power required to operate the oven. While optimizing the reflow process for product and material specifications has been a priority, less effort has been focused on the impact these parameters have on the power needed to achieve an optimized process.
In support of an industry-funded effort to evaluate the potential environmental impacts associated with the use of lead and lead-free solders, Vitronics Soltec teamed with the University of Tennessee Center for Clean Products to assess the affect of solder selection on power consumption during the reflow assembly process. This study was initiated with three specific goals in mind:
- Compare and contrast reflow soldering process characteristics for three alloys with melting points of 139°, 183° and 217°C.
- Characterize power consumption of the three different reflow soldering processes under both idle and loaded oven conditions.
- Correlate the board-level reflow characteristics to the power required to achieve them.
Figure 1. A ramp-soak-spike reflow profile for SnAgCu.
The first step in the methodology employed to study the power requirements for each alloy was standardization of specific parameters for a fair comparison. Profiles were designed for the following alloys: Sn1.0Ag57Bi (139°C), eutectic SnPb (183°C), and Sn3.9Ag0.6Cu (217°C). A heated process time of four minutes was selected, based upon typical manufacturing considerations for a reflow soldering profile. As a result, conveyor speed was kept constant in an eight-heating zone, two-cooling zone forced convection oven. The product investigated was a motherboard measuring 9.6 × 9.6". The reflow profile type selected for this investigation was a ramp-soak-spike profile as illustrated for a SnAgCu reflow process. (Figure 1)
The reflow process attributes for the three alloys were as closely matched as possible to facilitate the comparison of the power requirements for the achieved processes. This procedure was necessary for comparing the power required to achieve the profiles. Table 1 lists the peak temperature range, ΔT and TAL for the three profiles. Changes in the process attributes illustrate the realities of implementing a reflow process as the melting point of the alloy increases, including a loss in process window and flexibility for adjustments.
Product Peak Temperature Requirements
The product's minimum and maximum peak temperatures were established for each process in relation to the respective alloy's melting point. The minimum peak temperature is influenced by two factors:
- Minimum temperature necessary to properly melt and reliably form a solder interconnect. For SnAgCu, this temperature is typically between 230° and 235°C.
- Complexity of the board and resulting ΔT so that the maximum peak temperature does not exceed 250°C.
For the maximum peak temperature, efforts are focused on maintaining a temperature less than 250°C. This depends on product complexity and thermal capacity of the components/board. In this investigation, the minimum peak temperature achieved was 23°, 26° and 18°C above the melting point of the three alloys, respectively. For the Sn1.0Ag57Bi and SnPb processes, the minimum peak temperatures achieved are 14 percent and 16 percent above the respective alloy melting point, while the minimum peak temperature ach-ieved for the Sn3.9Ag0.6Cu process was only 8 percent above the 217°C melting point. This result was dictated by the restrictions on the maximum allowable peak temperature of 245°C.
TAL
TAL can impact solder joint quality, as well as that of the reflow process. From the process attribute perspective, extending the TAL allows more time for the board to attain increased, uniform thermal equilibrium, thereby minimizing ΔT. In this investigation of three alloys and processes, TAL decreases from 74 sec for the Sn1.0Ag57Bi to 62 sec for Sn3.9Ag0.6Cu. This 16 percent change is due to flexibility in the process window. With lower required temperatures for processing, developing a profile within the length of the forced convection oven is simpler. Raising required peak temperature from 160° to 235°C (45 percent) decreases end-user flexibility in developing a specific reflow profile. However, the three TALs achieved are within a reasonable range that allows for direct comparison of ΔT, as well as power consumption.
This investigation showed that as the alloy melting point increases, so does ΔT. Best efforts were made to keep many parameters constant so that direct comparisons were valid. As discussed, the TAL varied by an amount that would preclude direct comparisons of ΔTs. The ΔT increases, as does operating reflow process temperatures, by approximately 50 percent (6.3° to 9.7°C) from Sn1.0Ag57Bi to Sn3.9Ag0.6Cu, and by approximately 33 percent (7.3° to 9.7°C) from SnPb to Sn3.9Ag0.6Cu processes. This trend is at the heart of lead-free implementation. The convergence of material specification limits with enlarging ΔTs results in a process on the edge of control. Exceeding the "window of opportunity" opens up the possibility of material damage and resulting defects.
In summary, as the melting point of the alloy increases, the operating temperature range, TAL and ΔT become more critical and less flexible to achieve both a reliable solder interconnect and a functional component. At this juncture, characterizing the power requirements of the reflow oven itself is equally important.
Costs Associated with a Reflow Process
In the drive to become environmentally friendly by eliminating lead-based alloys or through the transition to water-based fluxes, the power required to process these materials increases. However, the degree of the increase is subject to variables such as set point temperatures, design of soldering equipment, loading conditions and process time. The design of reflow equipment and its ability to handle manufacturing conditions is critical for product quality and also in calculating cost of ownership. In this investigation, power consumption was monitored for the three alloys, both with the oven under idle operation (without any boards) and under loaded operation, where in this case the board spacing is defined by two board lengths or 19.2" spacing from the leading edge to leading edge of two consecutive boards. Figure 2 illustrates the power requirements for the three reflow processes in Table 1.
Figure 2. Power consumption for the three processes in Table 1 under idle and loaded conditions.
The results indicate two power requirement trends. The first is that as the melting point and associated temperature requirements of the solder increase, power consumption of the reflow equipment does also. While appearing obvious, this statement does not define consumption statistically. In the transition from SnPb to Sn3.9Ag0.6Cu, power consumption increases approximately 6 percent under idle conditions and more importantly, by 9.3 percent under loaded operation. The second trend observed is the stability of the power consumption in transitioning from idle to loaded conditions, 0.7 to 1 kWh. This is due to the stability and design of the forced convection oven. Previous research has shown that with poorly designed forced convection reflow ovens, both the absolute power consumption and difference between idle to loaded conditions are subject to increased power consumption and deviation.1
Even a small increase in power consumed is environmentally significant, given that the resulting impacts from reflow energy consumption dominated 13 of 16 different environmental impact categories (e.g., ozone depletion) and were found to account for as much as 96 percent of the total global warming impacts for the lifecycle of solder.1,2
Conclusion
Implementing high melting point lead-free alloys in the manufacture of electronic products presents the assembler with many challenges. This study focused on optimization of the reflow process to meet material specifications and still produce a quality solder interconnect. This study also characterized the power required to achieve the optimized process. Understanding this relationship is as critical as any other challenge to be overcome in achieving successful lead-free process implementation. It has a direct relationship and effect on cost of ownership and oven capability and stability. Tradeoffs are unavoidable in implementing an overall optimized lead-free process.
A previous study focused on tradeoffs involved in changing profile type and characterizing its impact on ΔT, TAL and power consumption.3 Conclusions from that study also found that profile type exerts a significant impact on process attributes and power consumption.
Given the recent and ongoing focus on the optimization and refinement of the lead-free reflow process, this investigation illustrates that equipment operating costs over the lifetime of the reflow oven are critical to the end user's ability to economically produce products soldered with SnAgCu. A matrix of parameters, including peak temperature range, ΔT, TAL, profile type, process time, conveyor speed, cooling rates and power consumption is necessary for a comprehensive study of lead-free reflow process implementation, as well as in forced convection reflow equipment selection. In this manner, the real cost of implementing a lead-free reflow process may be qualified and quantified for its long-term application.
References
- Geibig, J. et al. "Lifecycle Comparison of Energy Use During the Application of Lead-free Solders," APEX 2003 Proceedings.
- Geibig, J. and Socolof, M. "Lifecycle Impacts of Lead and Lead-Free Solder use in Reflow Soldering of Electronics," 2004 Electronics Goes Green Proceedings.
- Barbini, D. and Bourgelais, P. "Profiling Tradeoffs for a Lead-free Process," 2004 Five Steps to Lead-free CD.
Denis Barbini, Ph.D., advanced technologies manager, Vitronics Soltec, may be contacted at (603) 772-7778; E-mail dbarbini@us.vitronics-soltec.com. Jack Geibig, acting director, Center for Clean Products and Clean Technologies at the University of Tennessee, may be contacted at (865) 974-6513.