Verifying Low-Temperature Soldering Compliance For CO2 Savings And Enhanced Product Performance

In a conventional lead-free soldering process reflow ovens operate continuously at about 260°C. An effective alternative capable of operating at a lower temperature could significantly reduce the running costs and CO₂ emissions associated with electronic manufacturing. There are also advantages to be gained from the reduction in the thermal stresses imposed on circuit boards and components by reflowing at a lower temperature.

In 2017, Intel and Lenovo announced low-temperature soldering (LTS), a new process suitable for surface-mount assembly that permits reflowing populated boards at 190°C (figure 1). This is significantly below the temperature needed for lead-free soldering.

Fig. 1: LTS consumes significantly less energy than conventional reflow soldering.

LTS launch presentations quantified the potential environmental benefits that can be achieved through reducing the soldering temperature by up to 70°C. Each SMT line can save 57 tons of CO₂ emissions annually, which suggests that the industry overall could prevent 35-50,000 tons of CO₂ entering the atmosphere each year.

Winbond commits

Seeing the opportunity to reduce the environmental footprint of manufacturing and also enhance the reliability of electronic assemblies by soldering at lower temperature, Winbond began its own research into LTS. The aim: to provide customers with necessary information when assembling Winbond memory products using this new, less energy-intensive and less thermally stressful process.

The move resonates with Winbond’s ongoing commitment to reducing its environmental footprint. Between 2017 and 2021, the company saved more than 1,167,000 GJ of energy, equivalent to the consumption of more than 92,000 households, and saved 230 tons of CO₂ emissions. Other improvements include increasing water recycling to almost 11 million cubic meters across all factories, reducing general waste by more than 7 tons through recycling, and removing up to 98% of volatile organic compounds (VOCs) from processes. In addition, Winbond has joined the SEMI ESG initiative to support sustainable development for the semiconductor manufacturing industry.

Our industry can greatly reduce consumption of the world’s resources by adopting modern processes like LTS. However, manufacturers must typically do a great deal of research and qualification before they can introduce such a radically different process for high-volume production. In particular, reliability is critical and manufacturers need to be sure the end-of-line yield will be adequate. Our initiative to verify the compatibility of our products answers critical questions for our customers and should help them introduce LTS more easily and quickly in their factories.

To complete the project, Winbond engineers tested and inspected prototype assemblies to verify reliability and quality. Reliability testing was performed according to JEDEC procedures, including drop, vibration, and temperature cycling. Compliance with the JEDEC standard was officially acknowledged in November 2022.

LTS in practice

LTS is performed using a tin-bismuth (SnBi) solder metallurgy. While this has proved critical to enabling the soldering process to be performed at a significantly lower temperature than is needed with conventional lead-free (SAC: tin-silver-copper) formulas, some additional techniques are applied to enhance the mechanical properties of solder joints; in particular, increasing the ductility is important to ensure the solder joints can withstand the normal mechanical stresses experienced during day-to-day life in the field. In addition to optimizing the ratio of bismuth to tin, so-called precipitation strengthening is performed, which includes doping with copper and nickel, and the metal grain size is also optimized for greater strength.

Besides saving energy, the reduction in process temperature made possible with the LTS SnBi solder also alleviates the thermal stresses that cause boards to warp during reflow soldering. Warpage can be reduced by more than 50%, resulting in improved coplanarity of soldered assemblies from reduced intrinsic stresses when cooled after reflow soldering. This helps to extend solder joint lifetime in the field. While delivering this improvement, the adhesion strength is similar. In fact, data shows a small increase from typically 2.28 kgf to 2.40 kgf.

In addition, production of some types of assemblies can be simplified. Where sensitive components are included, which cannot be exposed to high reflow temperatures, these are usually added after reflowing thereby necessitating additional assembly processes and equipment. Adopting LTS can ease production by allowing products like these to be fully assembled before reflowing.

LTS also enables faster technology scaling for chip makers by saving any need to ensure thermal robustness in packaging, allowing thinner, lightweight package construction, and easing the transition to finer interconnect pitch. This is possible because the reduction in board warpage prevents bridging defects due to compression as well as open-circuit faults caused by expansion.

Conclusion

Through its in-house research, Winbond has ensured that its memory products are compatible with LTS assembly and enable manufacturers to achieve high standards for yield, quality, and reliability in the field. It is one of the first companies in the world to take such an initiative and demonstrate compliance with JEDEC standards. By 2027, more than 20% of electronics manufacturers are expected to have introduced LTS in their factories.

In addition to saving electricity costs and CO₂ emissions, implementing LTS saves exposing chips and assemblies to the high reflow temperatures associated with conventional SAC soldering thereby helping to increase reliability and accelerate technology scaling. We hope that other global leaders will now join us in taking action for a greener and more sustainable future.