MANUFACTURING Winbond Verifies Low-Temperature Soldering


Compliance for CO2 Savings and Enhanced Product Performance


Low-temperature soldering (LTS) offers environmental benefits and reduces manufacturing stresses on boards and components, explains William Chen, Deputy Director of Flash Product Marketing, Winbond. LTS Introduced


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 would significantly reduce the running costs and


CO2 emissions. 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. 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, therefore 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 enhance the reliability of electronic assemblies by soldering at lower temperatures, Winbond began its own research into LTS. The aim being to provide customers with necessary information when assembling Winbond memory products using this new, less energy-intensive and less thermally stressful process. “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,” explains Winbond. “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


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LTS consumes significantly less energy than conventional reflow soldering.


and should help them introduce LTS with ease and efficiency in their factories.”


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. This has proved critical to enable the soldering process to be performed at a significantly lower temperature than needed with conventional lead-free formulas (SAC: tin-silver-copper), however some additional techniques still must be applied to enhance the mechanical properties of solder joints. For example, increasing the ductility is important to ensure the solder joints can withstand normal mechanical stresses experienced during day-to-day life in the field. In addition to optimising 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 optimised for greater strength. Besides saving energy, the reduction in process temperature 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


DECEMBER/JANUARY 2024 | ELECTRONICS FOR ENGINEERS


joint lifetime in the field. While delivering this improvement, the adhesion strength is similar. In fact, data shows an 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, 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 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.


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