This design criteria was not met with SAC305, which is much closer to the maximum processing temperature for some common board materials and components. Since the SnPb eutectic alloy melts at 183☌, Pb-free replacements were highly desired in the same temperature range. Table 1 shows the composition of the alloys as well as the liquidus and solidus temperatures of each.Īlloys A and B would be considered mid-range melting point alloys. Five profiles were developed for use on six different lead-free alloys-SAC305, and five other next generation alloys. A eutectic solder may be ideal for reducing voiding, however, it can introduce tombstoning defects on passive components.Īn experiment was conducted to investigate how a selection of Pb-free solder alloys perform under different reflow profiles. If an alloy is eutectic (where the liquidus and solidus temperatures are equal) the wetting and solidification rates will be much quicker. If an alloy has a wide plastic range (large difference between the liquidus and solidus temperatures), the wetting rate and also the solidification rate will be slow. The plastic range exists for any non-eutectic alloy and is the temperature range between the solidus and the liquidus of the alloy where the alloy is neither fully solid nor fully liquid but instead has a pasty-type consistency. It is important to consider the plastic or pasty range of these alloys and how it affects the wetting characteristics of the solder. A balance of ductility and rigidity in a solder alloy needs to be achieved in order to have a reliable solder joint for high reliability under a wide service temperature environment 1. When added, antimony (Sb) increases tensile strength, while bismuth (Bi) can make the alloy more brittle. The next generation lead-free alloys have been developed as solutions to finding the perfect balance between being strong and flexible. However, developing the ideal combination to meet all of the desired properties has proven to have limitations within the Pb-free space. Different compositions of these elements added into a tin-silver-copper matrix can improve thermal fatigue and strength. SAC305 has proven itself a worthy replacement for Sn63, although requirements in the automotive, aerospace, and military industries have driven further research for alloys with even higher reliability expectations.Įlemental additions of indium, bismuth, antimony, and nickel have been candidates for the next generation Pb-free alloys. SAC305 (96.5% Sn, 3.0% Ag, and 0.5% Cu) has emerged as the go-to lead-free alloy by popular choice. The introduction of RoHS regulations in the electronics industry has prompted extensive research into the development of Pb-free soldering alloys with the reliability the industry came to expect from SnPb alloys.
SOLDER REFLOW TEMPERATURES HOW TO
This experiment focused on developing the best profile for next generation SAC alloys and how to best modify the reflow process when a new material is chosen.
SOLDER REFLOW TEMPERATURES SOFTWARE
To keep up with these industry needs, reflow profiling software and best practices have continued to develop. New materials and process constraints have also limited process windows, making assemblies that much more sensitive to profile. Companies with a high mix of assembly designs and materials are even more challenged with using the best reflow process for each one.
The reflow process is at the top of the list for optimization. Process development and continuous improvement are critical to success in PCB assembly. KEY WORDS: high-reliability, SAC alloys, reflow profiling, reflow process window In addition, as high-temperature applications that traditionally used high-Pb alloys for high operating temperatures come close to losing RoHS exemptions, new alloys are emerging to achieve the high performance expectations associated with applications in this range. There will also be a discussion on the critical points of successful reflow profiling for the wider range of Pb-free alloys now on the market. This paper will review common profile shapes and recipes, showing data for how they differ in the resulting solder joints. These choices really expand the playbook of possible reflow profiles. More than a decade later, a large segment of the industry is moving to adopt modified SAC alloys, as well as new Pb-free alloys for higher reliability, or even low melting point alloys. MB Allen, KIC San Diego, CA, Sloan, Kim Flanagan, Brook Sandy-Smith, Indium Corporation Clinton, NY, the Pb-free transition, most of the industry settled on using predominantly SAC305 solders. Reflow Profiling for Next-Generation Solder Alloys
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