Since the dawn of the electronics era, people have been actively pursuing the miniaturization of electronic products. Manufacturers too have striven to reduce the size of semiconductor chips and electronic components to be fixed on substrates and circuit boards via soldering and plugging-in. However, the rise of environmental protection awareness worldwide in the 1990s had a negative impact on traditional leaded solder, which has excellent electrical and mechanical properties. In 2003, the European Union became the first to announce the “Restriction of Hazardous Substances (RoHS)“ directive, which went into effect in 2006. This directive prohibited the use of materials containing heavy metals such as lead, mercury and cadmium in electronic products sold to the EU. This regulation kicked off the movement towards lead-free electronic materials and forced the industry to invest in the research and development of the mass production of lead-free solder.
Lead-free solder has a relatively high melting point, so, in order to introduce it into the manufacturing process, it is necessary to overcome the problems brought about by an increased Reflow Temperature. Components must withstand higher soldering temperatures while simultaneously ensuring the lifetime and reliability of products. Among these lead-free solder options, tin-silver-copper (SAC) alloys perform better in terms of solder joint reliability. When the process transitioned from tin-lead (SnPb) alloys to tin-silver-copper alloys, the Reflow Peak Temperature of the SMT production line rose from about 220˚C to about 250˚C. However, in the interest of meeting product reliability requirements, tin-silver-copper alloys are still being used today.
In recent years, High Performance Computing (HPC) and Artificial Intelligence (AI) have seen a sharp rise in popularity. However, the traditional Board-Level construction method has proven unable to meet the demands for high performance, high bandwidth, low power consumption, multi-chip integration and space integration required for these applications’ fast and complex calculations. The industry needs advanced packaging processes, such as the System in Package (SiP) and the 2.5D Chip-on-Wafer-on-Substrate (CoWoS) Si interposer package, to overcome these problems. However, when these packaging processes are paired with the high melting point, lead-free SAC process, it encounters substrate warpage problems caused by the reflow soldering temperature. As such, lead-free solder that can be used in Low Temperature Soldering (LTS) has become a center of attention.
The development of low temperature soldering has been going on for some time, due initially to the active implementation of environmental protection and carbon neutrality policies in various countries. One approach to reducing carbon emissions in the production process of electronic products is to adopt a lead-free low temperature solder paste process. For the sake of product yield and reliability, the process verification and time to introduction for this material have been accelerated. General purpose lead-free solder paste, SAC305 (96.6% Sn, 3% Ag, 0.5% Cu), has a melting point of 217°C. Lead-free low temperature solder pastes usually refer to solder pastes containing “bismuth (Bi)” metal. When it comes to SnBi alloys, the melting point of Sn64Bi35Ag1 is only 178°C, and the melting point of Sn42Bi58 is even lower at only 138°C. In other words, the melting point of “bismuth (Bi)” solder pastes are about 39°C～79°C lower than that of SAC305 lead-free solder pastes.
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Lead-free low temperature solder pastes can meet the needs of low temperature processes, but do they have other advantages or disadvantages? |
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Advantage 1. Reduce Costs |
Due to the low melting point of lead-free low temperature solder paste, the reflow soldering temperature of the Surface Mount Technology (SMT) can be lowered. This also reduces the cost of using high temperature resistant components and materials on the circuit board. Furthermore, because the temperature of the reflow oven does not have to be high to complete the soldering, the power consumption of the equipment can be reduced. This makes it possible to simultaneously achieve the goals of power cost reduction, energy conservation and carbon reduction. In addition, some plug-in components are made of plastic and cannot withstand high temperatures, therefore they require that the SMT process be split into two parts. However, the use of lead-free low temperature solder paste reduces the SMT process temperature, which means the process can be simplified into one, saving both time and energy.
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Advantage 2. Reduce Warpage |
Advanced packaging processes, such as SiP and CoWoS, need to seal many different types of components onto the same substrate. During the SMT process, however, substrates can become warped due to the high temperature of reflow soldering. This causes additional tensile stress at the joints between components and the substrate, which results in poor soldering yields and increases production costs. The use of lead-free low temperature solder paste, on the other hand, reduces the reflow soldering temperature of the SMT process, thus reducing the degree of substrate warpage and improving reliability.
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Disadvantages of Lead-Free Low Temperature Solder Pastes |
The comparatively poor solder joint strength is where lead-free low temperature solder paste needs the most improvement. Its performance falls short of that of mainstream tin-silver-copper alloy solder pastes in the Temperature Cycling Test, the Mechanical Shock Test and other reliability verification tests. Therefore, how to strengthen the reliability of solder joints has become a central issue for low temperature solder pastes. The solder paste material industry is committed to this research and has been trying to achieve this goal by adding other elements to the SnBi alloy. For example, it has been found that adding “silver (Ag)” can improve the strength of solder joints as well as the material’s fatigue resistance, which helps it pass the temperature cycling test. Some experts believe that this is due to the help of the Ag3Sn alloy in the material. However, the brittleness of silver compounds means that, if the silver content is too high, the solder joints will fail to pass the mechanical shock test. Therefore, current mainstream lead-free low temperature solder pastes generally have a silver content of less than 3%.
When introducing lead-free low temperature solder paste materials, it is important that they be matched with the appropriate temperature settings in the SMT process in order to obtain the best soldering effects and pass product reliability verification tests. The common practice is to reduce the peak temperature of the reflow as much as possible without affecting the soldering quality. The goal is to reduce the thermal deformation of the circuit board and substrate during the manufacturing process and to, by increasing the cooling rate after the reflow peak, speed up the curing time for low temperature solder. However, if the cooling rate after the reflow peak is increased too much, the chance that the solder will crack may increase. As such, it is best to evaluate the soldering characteristics of a lead-free low temperature solder paste through reliability verification first and then select the appropriate cooling rate.
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MA-tek provides the Total Solution to customers’ reliability verification needs, covering everything from experiment planning, PCB production, and SMT board mounting to reliability testing and failure analysis (Figure 1). We have been assisting world-class smart handheld device and server manufacturers with verifying solder joint reliability for many years. MA-tek has comprehensive analysis and testing capabilities as well as the ability to help customers find the root causes of solder joint crack failures after reliability verification (Figure 2).
Figure 1. MA-tek – Total Solution Reliability Verification Service |
Figure 2. (Top) Nondestructive Inspection of Solder Joint Cracks (3D X-ray); (Bottom) Cross Section Analysis of Solder Joint Cracks (SEM) |
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Solder Joint Lattice Analysis |
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The material composition of lead-free low temperature solder paste is markedly different from that of the current mainstream tin-silver-copper alloy solder paste. Therefore, the soldering properties of the materials after boarding are also different. The lattice distribution of solder joints in particular has a significant impact on quality and reliability. |
In lieu of this, MA-tek introduced Electron Back Scatter Diffraction (EBSD) analysis technique. EBSD can be used to conduct lattice analysis of solder joints to confirm the crystallization forms and ratio from the microstructure of the material lattice (Figure 3). It can also conduct microscopic analysis of the cracked areas of solder joints to identify the true causes of failures (Figure 4). MA-tek’s rapid and complete verification and analysis capabilities helps customers choose the most suitable lead-free low temperature solder paste, optimize the reflow process conditions, and improve the Board-Level Reliability of products more efficiently, thus accelerating customers’ time-to-market.
 Figure 3. Solder Joint Grain Structure and Crystallization Analysis (EBSD) |
Figure 4. Crystallographic Analysis of Solder Joint Cracks (EBSD) |
MA-tek strives constantly to ensure that we are the best R&D partner our customers can have. We assist customers with accelerating the mass production and launch of advanced processes and products and seizing business opportunities in the rapidly developing electronics industry.
