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STEP 7: Soldering
December 31, 1969 |Estimated reading time: 6 minutes
Lead-free compliance is now a reality in the electronics manufacturing industry. Progress has been considerable, and the industry is learning about reliable lead-free alternatives. High-performance boards are being produced, and new standards for lead-free processing have been established. This article focuses on reflow process considerations with lead-free solder processing
The lead-free initiative continues to affect all areas of manufacturing - none more so than the reflow process. Higher melting temperatures and soak duration of lead-free solder formulas require a change in the way people manage the reflow process. Soak times, peak temperatures, time above liquidus (TAL), and ramp rates call for tight control within a shrinking process window. There also are additional considerations for cooling needs, exit board temperatures, and flux management.
Until all components and boards are lead-free we have the additional burden of working with mixed materials. For example, lead-free solders combined with components and/or boards that have lead-based finishes affect the reflow profile in contradictory ways. Because every board configuration is different, each assembly has the potential to be unique. Thus, no standard thermal profile fits all assemblies or lead-free solder formulas. However, many problems related to lead-free reflow are induced by a reflow process that is out of specification. Some common problems include poor solder joint reliability due to intermetallic layer thickening, a direct result of longer TAL; board warpage; and fillet lifting, or separation of the solder fillet from the copper pad during cool down. Voids in solder joints, resulting in interconnection failures, also are an issue. When dealing with lead-free solders containing large amounts of tin, tin whiskers can occur - a direct result of thermal stresses introduced in the soldering ramp profile. Aside from these problems, basic eutectic reflow challenges must be dealt with, including skips, solder balls, bridges, and tombstones. Tight control of the reflow process is the only way to prevent such defects.
Standard tin/lead (SnPb) eutectic solder has a melting point of 183ºC, with a generous reflow window. Because of this, many manufacturers have been able to use one or two thermal profiles to process a wide range of board assemblies. When dealing with lead-free solders, the process window for reflow shrinks considerably (Figure 1). Longer soak times and an increased melting point for lead-free solder (between 217º and 221ºC) approaches the component maximum temperature of 250ºC. In addition to tighter thermal control, more profiles are required to ensure good quality solder joints without damaging components.
Much of the focus to-date on lead-free solder has been on tin/silver/copper alloys (SnAgCu - SAC). Industry groups such as iNEMI and IPC have officially named slightly different variations of SAC as the standard for lead-free solder. Both systems contain about 4.0% Ag and 0.5% Cu. From an equipment standpoint, implementation differences are negligible, but challenges do exist.
Reflow Ovens and Lead-free
Some reflow ovens are not adequate for the demands of lead-free solder processing. Other ovens can be upgraded with retrofit kits, or used to process light boards. More sophisticated ovens - those with ample power, temperature control, and the ability to work at higher temperatures - can process lead-free solder without modification. The major differences between eutectic tin/lead and SAC profiles are peak temperatures required for reflow. Tin/lead has a liquidus temperature of 183ºC, while SAC is 217ºC. This 34ºC difference translates directly to a profile requiring a higher peak temperature and slightly longer processing times. While tin/lead is normally processed between 210º and 220ºC, SAC should see a peak with a minimum of 235º to 245ºC. This temperature, however, is close to the danger zone of 250ºC for many components. The previous safety factor of 40ºC has now dropped to 10ºC. Thus, the process window has shrunk, and tighter control is necessary to avoid overheating components. Likewise, higher temperatures require a longer processing time if current heating and cooling ramps are maintained.
Forced Convection and Closed-loop Systems
With smaller process windows, a reflow oven equipped with control of the convection process is needed to maximize thermal uniformity and repeatability. Controlled convection rates using static-pressure generation make this possible. A higher static pressure increases peak temperature, dwell times, and temperature uniformity. While temperature differences across a board can be found, particularly on large boards with various sized components, controlled convection rates ensure constant, uniform heat transfer. With higher convection rates, all points on a board can fit into the tight process window; reflow can occur without component damage.
Some reflow ovens have advanced closed-loop convection systems and control algorithms that limit variations. These self-controlling systems provide feedback so the oven can adjust itself and remain within the process window. Sophisticated mathematical algorithms determine when adjustments are needed and how much change is needed to keep the parameters within target. This control ensures repeatability and consistency, a critical factor for a lead-free reflow oven.
Figure 1. Lead-free process impact.
Cooling control has also become important with SAC. Studies show that the shear strength of SAC is slightly lower than eutectic solder when cooled at 1.5º to 2.5°C/sec. Smaller grains with increased strength can be obtained by faster cooling, but IPC/JEDEC limits the cooling rate to 6°C/sec., because of concerns for board integrity. Some ask for cooling rates in the range of 4º to 5°C/sec. On the other hand, manufacturers that reflow large BGAs are advocating slower cooling rates (as low as 0.5°C/sec.) to minimize ball shearing. Controlling the convection in the cooler can assist in increasing or decreasing the cooling rate. As in heating, closed-loop control in the cooling section ensures repeatability and consistency.
Oven Atmospheres
Over the years, many manufacturers have learned to process tin/lead solder without nitrogen, and it is highly unlikely they will return to it, because of the added cost. When lead-free processing originally began, most manufacturers bought new reflow ovens with nitrogen options in case it was needed, because upgrading an air-only oven to nitrogen can be costly or impossible. Currently, there is a mix between air-only and air/nitrogen ovens being purchased. While air is being used with consumer products, nitrogen processing occurs on high-quality boards where cosmetics and high reliability are needed. The nitrogen atmosphere protects metal surfaces from oxidation during heating and ensures proper flux activation. Comparing the wetting force of different fluxes in air and nitrogen shows that nitrogen coverage can improve the process. Nitrogen also reduces the residue of some fluxes, minimizing in-circuit test failures. Some lead-free studies have found that nitrogen enhances solder quality. The actual oxygen residue allowed in the nitrogen also should be evaluated prior to implementation to minimize overall cost of ownership. Nitrogen cost vs. benefits also must be evaluated prior to implementation.
Flux is another important lead-free issue. Higher reflow temperatures mean that fluxes need to be less susceptible to burning and must stay on parts longer. Many solder suppliers have developed fluxes that work well at higher lead-free processing temperatures. From the oven perspective, the flux-collection system must be able to handle increased temperatures and larger quantities of flux.
Conclusion
Beyond lead-free solder standards, successful lead-free solder processing involves important consideration of reflow oven capabilities, especially in the areas of temperature control, oven power, handling of flux, and profile development. As usual, the issues of oven temperature capability, consistency, repeatability, and flexibility remain important, but are even more critical when processing lead-free solders.
REFERENCES
For a complete list of references, contact the authors.
Rob DiMatteo, product manager for SMT, BTU International, may be contacted at (978) 667-4111, ext. 143; rdimatteo@btu.com. Fred Dimock, senior process engineer, BTU International, may be contacted at (978) 667-4111, ext 207; fdimock@btu.com.