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Control and Stability in Lead-free Reflow
December 31, 1969 |Estimated reading time: 8 minutes
As the reflow process window closes, a number of enabling technologies are opening doors in a brave new lead-free world. This article focuses on several factors of a successful lead-free reflow soldering process.
By Richard Burke
It seems as though it was more than 15 years ago that the SMT industry migrated from organic acid-based solder pastes and rosin to the less-forgiving realm of low-solid, no-clean formulations. Then the evolution from air to a nitrogen process expanded the reflow process window - allowing production that generated little to no flux residue. For many manufacturers, this meant retiring the cleaning process altogether, eliminating the need for these machines, and reducing production hours significantly.
Around the same time, our industry again demonstrated its flexibility and embraced convection heating over the previous standard of IR heat transfer. PCB manufacturers recognized the growing importance of the thermal profile, and saw immediate gains in stability, overall throughput, and process quality. As a result of these industry-wide changes, other advances such as flux-management systems and variable-speed convection were born, and continue to play important roles in current reflow systems.
Recently, the industry was faced with the challenge of transitioning to lead-free manufacturing to meet the European Union’s (EU’s) RoHS Directive. This change in reflow standards is similar to previous examples in that it further reduces the process window. In this environment, the importance of flexibility, process control, and stability cannot be overstated. Key enabling technologies are being developed to pry the process window open - turning lead-free conversion from “want to do” to “need to do” for more manufacturers.
Turning up the Heat
One important factor in any successful reflow soldering process has been the thermal profile. It has the most direct effect on the quality of solder joints and overall product reliability. New thermal profiles are required for lead-free alloys, representing a significant reduction in the reflow process window - because the liquidus temperature has increased from 183°C (eutectic tin/lead) to 217°-221°C (lead-free SAC alloy). This increase of 34°-38°C in liquidus temperature means that the required peak temperature range must also increase. Current lead-free solder pastes require peak temperatures ranging from 230°-250°C. As shown in Table 1 these higher peak temperatures approach the uppermost limits of many SMT components. The time of the exposure is a determining factor in whether these temperatures will prove to be detrimental.
Table 1Temperature LimitsPeak temperatureMax. duration260°C5 seconds250°C15 seconds240°C25 seconds230°C40 secondsThese reflow soldering profile and temperature limits apply to ceramic chip capacitors, thick film resistors, resistor arrays, tantalum chip capacitors, chip thermistors, 0402/0603 chip inductors, ferrite chip beads, and silicon and Schottky barrier rectifier diodes.*
This makes thermal control and stability of reflow ovens more critical. How much does it close your reflow process window? Compared to tin/lead alloys, which maintain a 30°-40°C margin within this component danger zone, peak temperatures for lead-free alloys will shave that margin to within 10°C.
The heat-transfer efficiency of modern reflow ovens has more effect on operating costs than ever before. Given the higher operating temperatures of lead-free alloys, heat-transfer efficiency also requires strict temperature-variation reduction across the entire assembly. Consider the upper temperature limit of components against the peak temperatures of lead-free alloys, and the importance of thermal differential becomes clear.
Reflow soldering systems must transfer thermal energy across the assembly’s varying thermal mass with the smallest variation between low and high thermal mass components. Otherwise, small thermal mass components are at risk of overheating, and large thermal mass components are at risk of not reflowing.
Control Issues
There is a world of possibility for lead-free alloy thermal efficiency to be found in enabling technologies such as variable-speed convection, controlled convection rates, heat-on-intake designs, and diffusers that provide a nozzle-like flow capability to modern reflow soldering systems. While these technologies provide improved heat-transfer efficiency, resulting in reduced temperature gradients across assemblies, they do not necessarily improve process control - essential to a successful, repeatable, and reliable lead-free reflow process.
One technology worth studying is recipe-driven, closed-loop, blower-speed control. These systems work by monitoring and controlling the RPM of an individual blower through a feedback loop. As the vast majority of in-market reflow ovens are convection-dominant, this is a point of concentration for most reflow soldering operations. Without this control, any variation in blower speed (RPM) will almost certainly affect the thermal profile, compromising process control and operational stability.
For example, imagine that a lead-free profile is generated under blower speeds operating at high convection to reduce ∆T across the assembly. For some reason, the upper-spike-zone blower-speed changes and begins operating at the equivalent of a low convection speed. Depending on the product, such a change in RPMs could result in a 20°C drop in peak temperature, and as much as a 1°-5°C difference in cross-assembly thermal variation. If so, products processed under these conditions likely would be outside the process window.
Conversely, consider a profile generated at low convection, so as to reduce costly power and nitrogen consumption. In this case, consider the potential impact of the upper-spike-zone blower operating at high convection. This process change could result in component damage - as peak temperatures are increased. The impact of closed-loop, blower speed control contributes significantly to oven stability and production repeatability.
There are measurable and significant effects of a convection-rate reduction in just one blower on the sustainable peak temperature of an assembly. Figure 1 shows a ramp profile that minimizes this differential; however, a ramp-soak-peak profile would look quite different. In any case, precise control over convection speed, and careful adherence to operational specifications throughout the profile, will provide unparalleled process control.
Figure 1. Effects of cooling slopes in lead-free reflow.
Slippery (Cooling) Slopes
This more complete thermal control also provides numerous benefits during the cooling portion of the reflow process. Solder paste suppliers typically recommend aggressive cooling slopes (2°C/sec. to 4°C/sec.) for lead-free alloys. Some studies have shown an aggressive cooling slope can provide optimal diffusion of material and fine eutectic grain structures. Figure 2 shows the effect of an aggressive cooling slope. It is important to note that the desired fine dendrite size is a result of the tin-rich forms being more random in size and shape, and therefore, more diffused within the solder joint. Figure 3 depicts tin-rich forms that are longer and more layered, the results of which can be a discernible grainy solder joint structure.
Figure 2. Effects of an aggressive cooling slope of −6.31°C/sec.
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Figure 3. Tin-rich forms are longer and more layered with a cooling slope of -1.27°C/sec.
In past cooling zones it sufficed to provide some level of variable convection; however, current ovens integrate features and functions that focus on more complex control issues. While closed-loop convection powers repeatability, cooling-zone convection variability ensures flexible cooling-slope development. Add to that closed-loop temperature control, and the power of this combination becomes apparent in cooling-zone profile flexibility, repeatability, and overall process control.
Opening the Window
As mentioned, one of the major process changes in our industry was the introduction of nitrogen process systems to a world using air. While the nitrogen process has not had the impact in reflow soldering that it has had in wave soldering, reflow has benefited from this inert environment. It pushed open the reflow process window by improving paste spreading, preventing re-oxidation, and reducing wetting times (with peak temperature reductions up to 10°C). It has also led to more aesthetically pleasing solder joints.
While few would argue the value of nitrogen systems, many consider the total cost when calculating overall value. Major cost considerations take the form of necessary capital equipment investment and recurring consumable costs. A majority of today’s reflow soldering systems processing lead-free assemblies use air rather than nitrogen. The adoption rate may be further hampered by the fact that most lead-free solder pastes are formulated to run in either air or nitrogen, and many nitrogen-configured ovens are also being operated in an air environment.
For these and other reasons, manufacturers remain focused on process control - and many see quality improvement opportunities in technologies such as process verification and product traceability. Advancements in both are providing detailed data logging and redundant monitoring of critical process parameters. Product-level traceability - taking the shape of barcode scanning devices - is experiencing wide adoption within operations and industries requiring high reliability, such as the automotive, medical, aerospace, and military markets. If past experience is any indication, market acceptance may accelerate, particularly as more emphasis is placed on process control, and as quality demands continue to rise.
Conclusion
While the mandate for lead-free alloys has complicated reflow soldering systems, it also has yielded advanced technologies that enable higher reliability, greater yields, and lower total costs of ownership. As the lead-free future unfolds, new challenges will likely present themselves, and the industry will pull together to meet them. On the horizon, we can see the fast-approaching implementation of 01005s, and their requisite assembly issues. As the prevalence of more challenging 0201 and 01005 components grows, it will result in a decline of traditional 0603 and 0402 components. This trend will place more importance on tight process control, operational stability, and repeatability (Figure 4). Future studies and operational experience will offer a better understanding of the impact of profile and atmosphere (air vs. nitrogen) on smaller components.
Figure 4. LCR passive component size trends. Courtesy of Prismark.
The reflow soldering process continues to show solid growth and provides incremental process improvements - even in the face of the well-publicized challenges of lead-free conversion. Manufacturers continue to commit to the next level of reflow, and prove that reflow process optimization is alive and opening windows worldwide.
* NIC Components Corp., Melville, N.Y.
Richard Burke, product manager, Speedline Technologies, may be contacted via e-mail: rburke@speedlinetech.com.