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... each solder paste material listed in Table 1 the three components identified in Figure 2 were cross-sectioned to measure key attributes of their solder joints. The number and location of the cross-sectional cuts for each component is shown in Figure 3. As seen in the figure, 8 solder joints were inspected for each of the cross-section cuts made. ...
Citations
... Other studies examined the collapse of the BGA solder joints after reflow. The collapse of BGA solder balls was analyzed by measuring the length of a diagonal line in a solder ball (edge-center-edge) [7] or by measuring the stand-off height after reflow [9]. Several works have also predicted the shape (geometry) of a BGA ball after one or several reflows using the finite element method [10][11][12][13][14][15] or computational fluid dynamic simulation [16]. ...
This study presents a new approach to investigating the impact of repeated reflow on the failure of ball grid array (BGA) packages. The issue with the BGA package collapse is that the repeated reflow can lead to short circuits, particularly for BGAs with a very fine pitch between leads. A novel approach was developed to measure the collapse of BGA solder balls during the melting and solidification process, enabling in situ measurements. The study focused on two types of solders: Sn63Pb37 as a reference, and the commonly used SAC305, with measurements taken at various temperatures. The BGA samples were subjected to three different heating/cooling cycles in a thermomechanical analyzer (TMA) at temperatures of 250 °C, 280 °C, and 300 °C, with a subsequent cooling down to 100 °C. The results obtained from the TMA indicated differences in the collapse behavior of both BGA solder alloys at various temperatures. Short circuits between neighboring leads (later confirmed by an X-ray analysis) were also recognizable on the TMA. The novel approach was successfully developed and applied, yielding clear insights into the behavior of solder balls during repeated reflow.
... 4,5 Soldering at lower temperatures also reduces the energy consumption and energy costs. 6,7 With a eutectic temperature of 139°C, Sn-Bi alloys are one of the most promising alloy systems on which to base LTS. [1][2][3][5][6][7] Bi is much cheaper than Ag, a major alloying addition in the current default Pb-free solder. ...
... 6,7 With a eutectic temperature of 139°C, Sn-Bi alloys are one of the most promising alloy systems on which to base LTS. [1][2][3][5][6][7] Bi is much cheaper than Ag, a major alloying addition in the current default Pb-free solder. That, as well as being nontoxic, provides additional incentives for the use of solders based on Sn-Bi in consumer electronics. ...
With low liquidus temperatures, low raw material costs, and non-toxicity, Sn-Bi low-temperature solders are promising candidates for the replacement of the currently widely-used lead-free solders in situations in which process temperatures have to be reduced. Electrical resistivity is one of the most important properties of solder alloys, as one of their primary functions is to conduct electrons between the connected components. The electrical resistivity of an alloy of a given composition at a specific temperature and pressure is affected by the microstructure and the crystal structure of the phases present. For Sn-Bi solders, the solubility of Bi in Sn is highly temperature-sensitive and increases from 3 wt.% at room temperature to 21 wt.% at 139°C, the eutectic temperature of the Sn-Bi system. As the temperature increases within that interval, Bi will dissolve in Sn, while it will precipitate as the temperature decreases. The resulting significant changes in the overall microstructure and the lattice parameters of the Sn phase can be expected to affect the electrical resistivity. In this study, the electrical resistivity of hypo-eutectic Sn-37wt.%Bi and near-eutectic Sn-57wt.%Bi alloys was measured as a function of temperature and the temperature coefficient of resistance (TCR) calculated. It was found that the electrical resistivity increases linearly with increasing temperature up to 70°C, while above 80°C, a deviation from the linear relationship was observed. This deviation is attributed to the rapid dissolution of Bi in Sn at 80°C and above.
... Low-temperature soldering can reduce warpage caused by thermal stress, improve product reliability, and reduce nearly 40% of the CO 2 emissions and production costs [11,12]. In recent years, low-temperature lead-free solders have attracted much attention, in which the SnBi eutectic solder alloy is the most widely used in low-temperature soldering connections for device-level packaging and system-level packaging [13]. ...
The Sn–Bi solder paste is commonly used in electronic assembly and packaging, but its brittleness causes poor reliability in shock environments. In this study, the mechanical reliability of Sn–Bi solder paste and Sn–Bi composite solder paste with thermosetting epoxy (TSEP Sn–Bi) was investigated with the board level drop test. The crack characterizations of solder joints were evaluated using a stereomicroscope after the dye and pull test. The microstructure characterization and failure types of solder joints were analyzed by a scanning electron microscope (SEM), and an energy dispersive spectrometer (EDS) was employed to investigate the initial phase identification and elemental analysis. Compared with Sn–Bi solder paste, the results show that the TSEP Sn–Bi solder pastes reduced the proportion of the complete failure and partial failure of the solder joints during the drop test. The microstructure observation of the crack path showed that the Sn–Bi/TSEP Sn–Bi solder joints were reinforced through the cured epoxy resin. The number of drops of the Sn–Bi/TSEP Sn–Bi-x (x = 3, 5, 7) solder joints had 1.55, 2.57, and over 3.00 times that of Sn–Bi/Sn–Bi solder joints after the board level drop test.
... However, the recent technology associated with these complex devices cause temperature variations and thermally induced stresses due to the larger degree of distribution caused by the expanded size of the package, which could cause challenges in the process of assembly due to warpage, a higher degree of interconnect reliability degradation, and eventually an impact to the long-term component lifetime [3]. As a potential solution to the large package warpage induced challenges, Low Temperature Solder (LTS) interconnects are of considerable interest and development, which can lower the total thermal stress with reducing dynamic warpage in large body size systems [4,5]. Among LTS alloys, eutectic Sn-Bi binary alloy systems are good options due to their lower melting temperature around 138-142 o C [6][7][8][9][10]. ...
... Four types of low-temperature Sn-Bi-X solder pastes of 180 lm in thickness were stencil printed on the PCB, as listed in Table 1. The hybrid solder joints were soldered under various reflow profiles (185-195°C) to achieve a Bi mixing percentage of 40-50%-the area of Bi distribution occupied the area of the whole hybrid solder joint [17]. ...
Low-temperature soldering technology (185–195 °C) is applied for assembling consumer electronics with various Bi contents, and board-level drop reliability of hybrid Sn–Ag–Cu/Sn–Bi–X ball grid array (BGA) solder joints interconnected chip components and PCB boards was evaluated. A hybrid solder joint structure of a partial melting Sn–Ag–Cu BGA solder ball wrapped by a continuous Bi-containing region was fabricated, and the effects of low Bi content and minor additives of Sn–Bi–X solders on microstructure and board-level drop reliabilities were revealed. The thicker interfacial Cu6Sn5/(Cu,Ni)6Sn5 intermetallic compounds (IMCs) accompanied with larger Bi-rich phases formed at the solder/Cu interface with decreasing Bi content of Sn–Bi–X solder. Drop impact-induced cracks initiated and propagated within the larger Bi-rich phases in a wear-out failure mode, compared with cracks initiated and propagated along the solder/IMC interface in an early failure mode if fine Bi-rich phases segregated at the solder/IMC interface. The Sn–49Bi–1Ag solder was the optimum low-temperature solder for assembling Sn–Ag–Cu/Sn–Bi–X hybrid solder joints, which failed in a wear-out failure mode with the highest drop characteristic lifetime of 420 drops, significantly higher than those failed in an early failure mode.
... The latest researches by Intel and the INEMI (International Electronics Manufacturing Initiative) [1,2] found that the key to solving these problems is to employ low-temperature leadfree solder assemblies. The Sn-58Bi eutectic solder with a low melting point of 138°C has shown advantages in the assembly process [3,4]. However, in the aging process SnBi/Cu solder joints exhibited a series of problems, such as the formation of the brittle Bi-rich phase, serious coarsening of IMC, and poor stability of the interface layer [5][6][7][8]. ...
Development of lead-free solders with a melting point lower than that of the most widely used Sn-3.0Ag-0.5Cu (SAC305) is crucial for coping with the welding defects of ultra-thin microchips. The Sn-58Bi solder with a low melting point of 138 °C is a promising one. However, some severe issues of this solder, such as the brittle failure of soldering interface and the serious coarsening of the intermetallic compound (IMC) in the solder/Cu matrix interface, have hindered its applications in microelectronic assembly industries. In this work, the morphologies and layer thicknesses of interfacial intermetallic compounds between the Sn-51Bi-0.9Sb-1.0Ag solder and Cu substrate during the isothermal aging at 125 °C have been investigated. The results show that the scallop-like intermetallic compound layers of different curvature radii are formed at the solder joint interfaces. The main phase composition of the IMC layer is Cu6Sn5. After the isothermal aging treatment, the IMC layer gradually becomes a flat layer. The IMC thickness of the Sn-51Bi-0.9Sb-1.0Ag/Cu soldering interface at the stable state is thinner by 33% compared to that of the Sn-58Bi/Cu interface. By fitting the experimental data, the growth rate constant of the IMC layer in the Sn-51Bi-0.9Sb-1.0Ag/Cu solder joint is 2.060 × 10–18 m²·s⁻¹, while it is 7.302 × 10–18 m²·s⁻¹ in the Sn-58Bi/Cu solder joint. The present study indicates that the addition of Sb and Ag elements can substantially suppress the IMC layer growth.
... There is an ongoing need for assembly of electronic components that cannot safely withstand high reflow temperatures of typical near eutectic SnAgCu (SAC), Pbfree solders [1][2][3][4]. Furthermore, lowering reflow temperatures can reduce warping in printed circuit boards (PCB) [5]. ...
The use of eutectic SnBi-based solder alloys allows lower temperature assembly solutions. But, the majority of solder bumped components available in the electronics supply chain come with SAC solder balls attached. Thus, fabrication can result in mixed assembly SAC/BiSn solder joints. But, the addition of two to twelve percent of Bi to SAC results in dramatic variation of its mechanical properties, including a drastic decrease in elongation, and a doubling of shear strength. The formation of brittle regions in the SAC region of the SAC/Bi-Sn solder joint could threaten the reliability of the mixed assembly. We examined the effect of heat treatment and current stressing on Bi transport in SAC/BiSnAg mixed solder joints. After reflow, a large gradient in the concentration of Bi in the SnBiAg region of the assemblies was observed. The diffusion of Bi into the SnAgCu (SAC) matrix in SAC/SnBiAg mixed solder caps structures was studied at elevated temperature between 85°C and 125°C for up to 500 h. The effects of current stressing (electromigration) on the distribution of Bi atoms were also examined. Solder joints were current stressed at temperatures up to 125°C, current densities up to 8 x 10 3 amps/cm 2 , and times up to 820 h. Samples were cooled to room temperature, cross-sectioned, polished, and characterized using scanning electron microscopy (SEM). The migration of the Bi atoms was characterized with a simple SAC/SnBiAg interface penetration depth model.
... There is an ongoing need for assembly of electronic components that cannot safely withstand high reflow temperatures of typical near eutectic SnAgCu (SAC), Pbfree solders [1][2][3][4]. Furthermore, lowering reflow temperatures can reduce warping in printed circuit boards (PCB) [5]. ...
The use of eutectic SnBi (139°) based solder alloys allows lower temperature assembly solutions. But, the majority of solder bumped components available in the electronics supply chain come with SAC solder balls attached. Then, fabrication results in mixed assembly SAC/Bi-Sn solder joints. But, the addition of two to twelve percent of Bi to SAC results in dramatic variation of its mechanical properties, including a drastic decrease in elongation, and a doubling of shear strength. The formation of brittle regions in the SAC region of the SAC/Bi-Sn solder joint could threaten the reliability of the mixed assembly. The effects of reflow parameter values on the microstructure of SnAgCu/SnBiAg mixed assemblies were examined. The variation of the volume fraction of the hypoeutectic SnBiAg phase with respect to the peak temperature during reflow, and the initial volumes of the SnAgCu and SnBiAg phases, was characterized. An equation was developed to predict the volume of the SnAgCu and SnBiAg phases after reflow, as a function of the peak temperature during reflow, and the initial volume of the SnBiAg phase. This theory was based upon a one-dimensional Sn/eutectic SnBi mixed assembly.
... Comparing conventional Sn37Pb eutectic solder (183 • C), the higher melting temperature of SAC305 (217-221 • C) limits its application when facing miniaturization challenges associated with emerging ultra-mobile computing, wearable devices, and the Internet of Things (IoT) markets [11]. Therefore, further studies have been carried out globally by industrial as well as academic consortiums on new Pb-free alternatives [12][13][14][15]. ...
Microstructural and mechanical properties of the eutectic Sn58Bi and micro-alloyed Sn57.6Bi0.4Ag solder alloys were compared. With the addition of Ag micro-alloy, the tensile strength was improved and this is attributed to a combination of microstructure refinement and an Ag3Sn precipitation hardening mechanism. However, ductility is slightly deteriorated due to the brittle nature of the Ag3Sn intermetallic compounds (IMCs). Additionally, a board level reliability study of Ag micro-alloyed Sn58Bi solder joints produced utilising a surface-mount technology (SMT) process, were assessed under accelerated temperature cycling (ATC) conditions. Results reveal that micro-alloyed Sn57.6Bi0.4Ag has a higher characteristic lifetime with a narrower failure distribution. This enhanced reliability corresponds with improved bulk mechanical properties. It is postulated that Ag3Sn IMCs are located at the Sn-Bi phase boundaries and suppress the solder microstructure from coarsening during the temperature cycling, hereby extending the time to failure.
... Comparing conventional Sn37Pb eutectic solder (183 • C), the higher melting temperature of SAC305 (217-221 • C) limits its application when facing miniaturization challenges associated with emerging ultra-mobile computing, wearable devices, and the Internet of Things (IoT) markets [11]. Therefore, further studies have been carried out globally by industrial as well as academic consortiums on new Pb-free alternatives [12][13][14][15]. ...
Ag microalloyed Sn58Bi has been investigated in this study as a Pb-free solder candidate to be used in modern electronics industry in order to cope with the increasing demands for low temperature soldering. Microstructural and mechanical properties of the eutectic Sn58Bi and microalloyed Sn57.6Bi0.4Ag solder alloys were compared. With the addition of Ag microalloy, the tensile strength was improved, and this was attributed to a combination of microstructure refinement and an Ag3Sn precipitation hardening mechanism. However, ductility was slightly deteriorated due to the brittle nature of the Ag3Sn intermetallic compounds (IMCs). Additionally, a board level reliability study of Ag microalloyed Sn58Bi solder joints produced utilizing a surface-mount technology (SMT) process, were assessed under accelerated temperature cycling (ATC) conditions. Results revealed that microalloyed Sn57.6Bi0.4Ag had a higher characteristic lifetime with a narrower failure distribution. This enhanced reliability corresponds with improved bulk mechanical properties. It is postulated that Ag3Sn IMCs are located at the Sn–Bi phase boundaries and suppress the solder microstructure from coarsening during the temperature cycling, hereby extending the time to failure.
