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Requirements for Implementation of Lead-free Soldering
December 31, 1969 |Estimated reading time: 2 minutes
Implementing a lead-free process for electronic assembly will only be successful when certain challenges are addressed and overcome.
By Denis Barbini
These challenges, including supply chain logistics, cost, material selection, plus equipment choices and upgrades, require planning and organization. However, the heart of lead-free implementation and assembly is the soldering process.
Process development is determined by two factors:
- Impact of various material specifications. Alloy, flux, board laminate and finish, and component packaging and metallurgy exert individual restrictions on the soldering process that result in an overall window of opportunity. This window defines the process whereby a proper joint can be successfully formed while maintaining component and board integrity.
- Flexibility and compatibility of soldering equipment. Reflow, wave and selective soldering equipment must deliver a precisely controlled process. This process must not only address tight material restrictions but also must meet throughput requirements. In addition, the equipment must be compatible with exposure to more extreme operating conditions.
Regardless of the particular soldering process, the technologies developed and employed are designed to deliver soldering processes that are robust and reproducible, as well as enable the end user to tailor the individual processes to match material requirements.
Figure. A lead-free profile for a BGA and PLCC on a medium-sized board.
The process engineer must acquire a thorough understanding of the necessary changes from the standard eutectic tin-lead reflow process, as well as how the reflow oven can achieve a compatible process. A typical lead-free soldering process for a medium-sized board is illustrated in the figure. Emphasis is placed on four critical aspects of the reflow process:
- Preheating. Soak vs. straight ramp preheating types exert influence on flux behavior and optimizing the relationship between ΔT and time above liquidus (TAL).
- Peak temperature and TAL. Achieving proper minimum and maximum peak temperature, along with an appropriate TAL is necessary to maintain component and board integrity and facilitate proper joint formation.
- Cooling rate. With SnAgCu-bearing alloys, the cooling rate exerts influence on joint microstructure at t0 as well as bulk solder intermetallics. Faster cooling rates result in a finer, homogeneous microstructure that creates stronger joints. Exit temperature also is determined by cooling rate.
- Time. Resulting throughput is determined by the relationship of board size to conveyor speed. Ultimately, throughput is a function of the oven's heated length.
Flexibility, heat transfer capability, accuracy and precision all are critical to developing and fine-tuning a reflow process that meets all of the specifications necessary for high quality. From the end user's perspective, a lead-free-compatible oven features the ability to optimize the preheat, ΔT, TAL and cooling rate to fit process window restrictions while maintaining cycle time when compared to a tin-lead process.
Selection of materials and soldering equipment are critical to final product quality. While a lead-free reflow soldering process was used as an example in this article, wave and selective soldering processes have equally demanding restrictions that can only be met with flexible, compatible equipment.
References
For a complete list of references, contact the author.
Denis Barbini, Ph.D., advanced technologies manager, may be contacted at Vitronics Soltec Inc., 2 Marin Way, Stratham, NH 03885; (603) 772-7778; E-mail: dbarbini@us.vitronics-soltec.com; Web site: http://www.vitronics-soltec.com