SOLDER PASTE FOR LOW-TEMPERATURE PB-FREE APPLICATIONS

Posted by Alphaassembly on November 27th, 2017

Imagining the modern electronics industry without the existence of solder is pretty much like trying to picture the human body without its nerves. Solder plays the critical role of holding all the components in a PCB assembly together while ensuring that nothing obstructs the flow of electricity from one point to another. Traditionally, Tin lead solder pastes have been the go-to choice for manufacturers looking to fabricate PCB units, but the recent push towards miniaturization of gadgets and the advent of RoHS and WEEE directives are bringing about new changes. The industry is shifting towards the use of low-temperature lead-free solder paste to help reduce the amount of lead reaching landfills with electronic wastes, thereby preserving the environment. Low-temperature solder pastes like the Sn/Bi/Ag alloy are perfect for use in a variety of SMT assembly applications and offer various features for the benefit of manufacturers. Let’s explore further:

Low-temperature Pb-Free solder pastes – the need
With the movement towards the use of Pb-free solder in electronics assembly, a lot of research and focus has been placed in the search for viable alternatives. The most popular option to emerge over the years in the lead-free solder paste category is Sn/Ag/Cu or the SAC alloy. However, with all their environmentally friendly hype, this alloy combination presents obvious challenge for users - it has a higher melting temperature. With a staggering 35°C (approx.) melting point difference, SAC alloy becomes a tough choice for applications where the SMT components and assembly design are ill-equipped to handle the temperature needed to make the alloy eutectic. Low-temperature solder pastes were developed to counter this specific problem. Adding Bismuth to the alloy combination can bring the solder paste’s melting temperatures down considerably, making them more conducive for use in specific PCB manufacturing applications.

Low-temperature Pb-Free solder pastes – the uses
Solder pastes like the Sn/Bi/Ag alloy are used today in a variety of temperature sensitive applications across the electronics manufacturing industry, including in:
•    Low-melting or low-Tg flex circuitry that is popularly used in smartphones, IoT devices and modern gadgets.
•    Temperature sensitive components like MEMS devices, some semiconductors and crystal oscillators.
•    Large area arrays to avoid head-in-pillow (HIP) and non-wet-open (NWO) type issues.
•    Substrates that cannot withstand high temperatures needed to make other Pb-free solder pastes eutectic.
•    Processes where step soldering is involved which necessitates a secondary lower temperature reflow requirement.

Low-temperature Pb-Free solder pastes – the benefits
•    These solder pastes require less energy and fuel consumption as compared to standard Pb-free alloys that have to be heated to much higher temperatures to obtain optimum eutectic consistency.
•    They can reduce the reflow process cycle time by a considerable amount.
•    They exhibit excellent resistance to solder balling and enhanced electrical reliability.
•    These pastes are compatible with most lead-free surface finishes that are commonly used in the industry.
•    They do not need much rework, thereby increasing your first time yield levels.
•    Their low-temperature melting feature allows users to adopt less expensive PCB substrates and components.

Solder pastes such as Sn/Bi/Ag alloy are designed to allow effective and economical assembly of PCB units without dependence on lead-based materials to manage melting temperatures.

About the Author:
Alpha Assembly Solutions, Inc. is the global leader in the development, manufacturing and sales of innovative specialty materials used in a wide range of industry segments, including electronics assembly, power electronics, die attach, LED lighting, photovoltaics, semiconductor packaging, automotive and others. Since its founding in 1872, Alpha has been committed to developing and manufacturing the highest quality specialty materials.