Solder Powder Research Articles

Optimizing Type 7 Solder Paster Formulation and Compatibility with Assembly Technology for Sub-75um System-in-Package Applications

For System-in-Package (SiP) assembly, solder paste printing remains the preferred solder application method of choice. However, as passive components (008004) and next-generation flip-chip components continue to shrink, smaller stencil openings are needed and these small stencil apertures still need to achieve consistent printing. In addition, as component standoff height continues to decrease for fine-pitch applications, the cleanability of the final SiP build becomes a concern; for many applications which require water-soluble pastes, pure DI water becomes difficult to use when the standoff height lessens. This paper will investigate important parameters from a materials supplier’s perspective for meeting the challenges of fine-feature solder paste printing and package assembly. First, innovations in solder powder technology will be discussed which allow for consistent solder paste (Type 7) printing at 60um apertures. Next, the effect of stencil technology will be discussed. Many stencil manufacturers are approaching a manufacturing limit both with respect to stencil thinness and aperture size. This paper will discuss two common stencil manufacturing designs, laser-cut and electroform, and present findings on the effect these designs have on solder paste print consistency. Lastly, solder paste cleanability will be discussed; the results of a study investigating novel Type-7 solder pastes for use with semi-aqueous cleaning technology will be presented. Important process considerations with respect to stencil and package cleaning will also be discussed.

Addressing Challenges in Dual Sided SiP Thermal Budget

System-in-Package (SiP) has been adopted as the mainstream advanced packaging technology in many consumer applications including 5G smartphones, tablets, wireless earbuds, smart wearables, etc. Heterogeneous integration, smaller form factor, better electrical performance, lower cost of assembly and faster time to market are the key differentiators of SiP as compared to conventional packaging techniques. In applications such as 5G mobile RF Front-End (RFFE) and Antenna-in-package (AiP) modules, dual sided SiP is the popular approach which brings even higher level of integration and smaller form factor. However thermal warpage control is a significant challenge during two times of solder reflow processes for the front and back sides of the package, especially if one side is with underfill and mold materials which have significant CTE mismatch to the substrate and dies, causing delamination failures and yield losses. This study examines the novel solution of reducing thermal budget with the use of a low temperature solder paste material based on SnBiAg alloy in ultra-fine pitch Type 6 solder powders. It allows a minimal peak reflow temperature of only 170 °C that effectively reduces warpage of dual sided SiP substrates. Solder alloy and paste properties and solder joint microstructures are examined in detail for tests including fine-pitch printing and component soldering, thermal aging test on different substrate finishes, thermal cycling test and mechanical drop test for reliability performance.

Study on the properties of epoxy-based Sn[sbnd]58Bi solder joints

In electronic packaging technology, Sn58Bi alloy solder has the advantages of low-temperature soldering and high shear strength of solder joint. It has a wide range of application in the electronics industry for low-temperature soldering, especially for temperature-sensitive substrates. Epoxy-based Sn58Bi solder paste is a mixture of curable flux and Sn58Bi solder powder. During the soldering process, the curable flux, in addition to the functions of a conventional flux, forms a cured epoxy shell around the periphery of the solder joint, which can effectively improve the shear strength and thermal stability of the solder joint. In this study, a self-made epoxy-based Sn58Bi solder paste was prepared and the properties of the solder joints were investigated. The results show that epoxy-based Sn58Bi solder paste has better wettability than traditional Sn58Bi solder paste without epoxy resin. And the shear strength of epoxy-based Sn58Bi solder joint is about 25 % higher than that of traditional Sn58Bi solder joint. Furthermore, the electrical conductivity of epoxy-based Sn58Bi solder joint has the same order of magnitude as that of traditional Sn58Bi solder joint, which means that non-conductive epoxy resin has no effect on the electrical conductivity of the epoxy-based Sn58Bi solder joint.

Effect of the Solder Powder Size of Sn-Bi-Ag-In Lead-Free Solder on the Wettability and Microstructure of Cu Substrate

Abstract In this study, tin-bismuth-silver-indium (Sn-Bi-Ag-In) lead-free solder powders are prepared by the consumable-electrode direct current arc (CDCA) method. Without agglomeration, the satellite-free solder powders show good sphericity. The effect of solder powder sizes on the wettability and microstructure of Sn-Bi-Ag-In solder alloy on copper substrate is systematically investigated. The solder joints prepared by solder powder in a size of 53–120 μm exhibit relatively small wetting angles. In addition, the microstructure of the solder joints is composed of bismuth-rich phases, tin-rich phases, and Ag3Sn phases. A continuous intermetallic compound (IMC) layer with a thickness in the range of 0.64–0.70 μm is formed at the interface between the solder and the copper substrate. The results show that the formation and morphology variety of the IMC layers for different solder joints are unobvious, reflecting the good wettability of the Sn-Bi-Ag-In lead-free solder powders on the copper substrate in a wide size range.

30‐5: Late‐News Paper: Development of a highly reliable Mini‐LED display module using simultaneous transfer and bonding (SITRAB) technology

In this study, we present the development of a highly reliable 64 × 64 Mini‐LED display, where the LED array is transferred/bonded onto a glass substrate using SITRAB technology and materials. The display substrate was fabricated by patterning two layers of Cu on top of heat‐resistant glass with a thickness of 500 μm. We applied SITRAB solder paste containing type 6 solder powder based on a resin matrix to the display substrate using screen printing. Subsequently, we transferred and bonded the 64 × 64 color LED array using the SITRAB process. The pixel pitch is 450 μm, corresponding to a 78" 4K display. No bad pixels were found among a total of 12288 LEDs. We analyzed the shear strength and reliability test results of LED chips bonded using SITRAB materials and processes. By applying SITRAB materials, the shear strength of the LED chips was increased by 27% before and 45% after reliability testing compared to the chips without added resin.

Innovations in Soldering Materials and Optimization of Solder Paste Printing and Inspection Parameters for System-in-Package Assembly

System-in-Package (SiP) assembly has become a popular solution for the challenges of continued miniaturization in the semiconductor industry. The continued and expected growth of the semiconductor industry in the coming years is driving more advanced assembly methods within System-in-Package and advanced packaging in general, and more advanced semiconductor assembly materials are required to meet those challenges such as solder paste specifically designed for SiP assembly, with fine solder powder and specifically formulated flux vehicles. Fine feature solder paste printing as an assembly method continues to be a challenge for SiP assembly. Small stencil aperture designs have necessitated fine-powder solder pastes, with type 5, type 6, and type 7 being common, as well as there being a need for type 8 for future, more advanced designs. Additionally, as more components are being packed into a SiP, solder paste rheology becomes an important characteristic to consider. Printing transfer efficiency, stencil life, and resistance to slump, are other important paste attributes to consider when evaluating solder paste for SiP assembly. While material properties are important to consider, proper equipment setup for a certain paste formulation is also necessary to maximize throughput and minimize assembly defects. The effects of solder paste printer parameters on fine-feature printing performance will be discussed, such as print speed, separation speed, separation distance, and stencil material. This paper will first focus on common failure modes in SiP assembly, and the technology advancements in both solder paste formulation and substrate design which help eliminate those failure modes. Then, this paper will discuss how proper stencil printer setup can reduce assembly defects, and will highlight designs-of-experiment outlining which stencil printer parameters are statistically significant in improving process yields of SiP assemblies with fine powder solder pastes, both with respect to pre-reflow inspection and post-reflow component placement. This paper will also discuss the importance of solder paste inspection with respect to assembly defects, and will highlight important parameters to optimize to accurately inspect solder paste deposits below 90 microns.

Mechanical Reliability Assessment of a Flexible Package Fabricated Using Laser-Assisted Bonding

The aim of this study was to develop a flexible package technology using laser-assisted bonding (LAB) technology and an anisotropic solder paste (ASP) material ultimately to reduce the bonding temperature and enhance the flexibility and reliability of flexible devices. The heat transfer phenomena during the LAB process, mechanical deformation, and the flexibility of a flexible package were analyzed by experimental and numerical simulation methods. The flexible package was fabricated with a silicon chip and a polyimide (PI) substrate. When the laser beam was irradiated onto the flexible package, the temperatures of the solder increased very rapidly to 220 °C, high enough to melt the ASP solder, within 2.4 s. After the completion of irradiation, the temperature of the flexible package decreased quickly. It was found that the solder powder in ASP was completely melted and formed stable interconnections between the silicon chip and the copper pads, without thermal damage to the PI substrate. After the LAB process, the flexible package showed warpage of 80 μm, which was very small compared to the size of the flexible package. The stress of each component in the flexible package generated during the LAB process was also found to be very low. The flexible device was bent up to 7 mm without failure, and the flexibility can be improved further by reducing the thickness of the silicon chip. The bonding strength and environmental reliability tests also showed the excellent mechanical endurance of the flexible package.

Effects of spreading behaviors on dynamic reflectivity in laser soldering

The random occurrence of component burn damage is a major challenge for laser soldering, but the mechanism has not been clear. In order to find the reason, a new integrating sphere platform was developed to simulate the actual process of laser soldering, and observe the real-time variation of laser reflectivity (LR) from initial fusion to fully spreading out of the solder. The results showed that the whole process is divided into 5 stages, which are initial low LR stage, rapidly rising moment, decreasing stage, re-increasing stage, and stable stage with high LR (around 64%). By analyzing the spreading behaviors and cross microstructures of solder joints, it was found that LR has no direct correlation with the spreading shape or temperature variation of solder paste, but is dependent on the interaction of laser multi-reflections within solder powders, one-time reflection of melt surface, and heat conduction from upper melted liquid to lower unmelted powders. The findings indicated that the burn damage is attributed to the combined effect of high LR and the concentrated reflection caused by crescent solder at the final stage.

Effects of Solder Powder Particle Size and Substrate Surface Finish on Degradation Properties of Solder Joints

In this study, the degradation characteristics of solder joints composed of powder-size Sn-3.0Ag-0.5Cu and the substrate surface finish are compared. Reflow is performed at a N2 ambient to apply an organic solderability preservative (OSP), electroless nickel/immersion gold (ENIG) substrates, and solder pastes of Type 6 (5-15 ㎛) and Type 7 (2-11 ㎛). Subsequently, a thermal shock test (TST) is performed for 1,500 cycles at a temperature of -40 ℃ to 125 ℃ (dwell time of 10 min) to analyze the deterioration characteristics. After N2 reflow is performed, the void content of the OSP substrate/solder joints is higher than that of the ENIG substrate/solder joints, and the shear strength of the ENIG substrate/solder joints is higher than that of the OSP substrate/solder joints. After 500 TST cycles, the shear strength at the OSP substrate/Type 7 solder joints does not decrease, whereas the shear strength at the ENIG substrate/Type 7 solder joints decreases by approximately 25%. Cross-sectional analysis shows that Cu6Sn5 and (Ni,Cu)3Sn4 intermetallic compounds (IMCs) emerged in the OSP and ENIG interfaces, respectively. The increasing rate of IMC thickness at the Type 7 solder joints is higher than that at the Type 6 solder joints. Fracture analysis shows that the OSP and ENIG joints fractured in the solder and solder/IMC interfaces, respectively. The IMC of the OSP and ENIG joints exhibit scallop and needle shapes, respectively. The ENIG joints fractured at the solder/IMC interface because of a stress concentration in the needle-type IMC. After 1500 TST cycles, the shear strength of all samples decreases by more than 50%. Cross-sectional analysis shows a crack at the solder joints of all samples after 1,000 cycles. The decrease in shear strength is caused by these cracks.

A Novel Design of High-Temperature Lead-Free Solders for Die-Attachment in Power Discrete Applications

Abstract Development of high-temperature lead-free (HTLF) solders to replace high-lead solders for die-attachment in power device applications is driven by (1) the harmful effects of lead to human health and the environment, and (2) the demand of the improved bonding materials serving under high-power density and high-junction temperatures, especially for wide-band-gap power devices. A novel design, based on a mixed solder powder paste technology—Durafuse™—has been developed to deliver a Sn-rich HTLF paste, presenting the merits of both constituent powders. The combination of the rigid, high-melting SnSbCuAgX and the ductile, low-temperature Sn-rich solder in one paste enables reflow at a relatively low temperature (barely above the liquidus temperature of the final joint composition) and maintains the joint strength above 15MPa in the temperature range between 270°C and 295°C. The sufficient high-temperature strength has demonstrated the capability of maintaining the joint integrity during subsequent multiple SMT reflows below the 270°C peak temperature, regardless of the existence of a partial melting phase. Both X-ray inspection and cross-section microstructure have not shown any damage in the Si die or any noticeable cracks in the bonding joint, even after 3000 cycles of TCT (−40 to 150°C). In summary, Durafuse™ HT, the novel design of the high-temperature lead-free pastes, has shown the feasibility as a drop-in solution to replace high-lead solders for die-attachment in power discrete applications.

Enhancing the Paste Release on 55μm pads with Water-Soluble Type 7 SAC305 Solder Paste for High Density SIP Application

Abstract The ultra-fine SAC305 solder is widely used as the lead-free solder composition for interconnection in advanced packaging based on its advantages such as good joint strength, thermo-mechanical fatigue behavior and creep resistance. Stencil printing remains the technology of choice for system-in-package (SiP) assembly because of its benefits such as economy of usage, ease of process control, flexibility of usage and fast and wide range of process window. SiP involves the high-level integration of different components, Copper (Cu) pillar flip-chip dies and chips in one package to achieve multiple functions in one system. The need to accommodate ever increasing demand for reduced footprints requires the technology to constantly invent a range of Cu pillar 55μm diameter and smaller components. Many mobile communications SiP consist of 6 or more flip chips in a single package. Conventional flip-chip attach uses flux & Cu pillar solder cap for solder joint formation as it is challenging for solder paste to be printed on fine geometries below 70μm. Additional benefits of solder paste printing help eliminate non-wet Cu pillar defect rates and improve yield over conventional flux printing. Heraeus solder paste (AP series T7) makes use of ultra-fine spherical shape of solder powder to create strong metal coalesce for bonding during reflow process of Cu pillars mounting to substrates. For example, the smallest passive component in use today is 008004 and Cu pillar diameter of 70um, which has a stencil opening of 125μm and 70μm respectively. Further reduction in Cu pillar flip-chip diameter of 55μm will require further reduction in stencil opening to 55μm, thus testing the limits of current type 7 pastes. Current solder pastes (AP series T7) can meet the consistency of printability when the stencil opening is 70μm or more. With smaller stencil opening, down to 55μm, the printing performance is less than desired. In this paper we describe the stencil printing application on the enhanced version of type 7 paste with a reinvented flux. The application tests were carried out using 55μm stencil opening. This paper will report the results of this application test.

Effect of Cu content on properties of Cu/Zn+15%SAC0307+xCu/Al joint by ultrasonic excitation at low temperature

The ultrasonic-assisted soldering joint of Cu/Al with different contents of copper particles added to solder powder at low temperature was studied. In this study, 15%SAC0307 solder powder with a diameter of 500 nm, different amounts copper particles with a diameter of 500 nm were mixed with zinc particles of a diameter of 45 μm to fill the Cu/Al joint, and the soldered was heated to 240 °C for ultrasonic-assisted under the environmental atmosphere. Successfully achieved micro-joining of Cu/Al under ultrasonic-assisted at low-temperature. The results have shown that, when the copper content was 0%-15%, two layers of complete and continuous CuxZny compounds were formed at the copper side interface. When the copper content was more than 15%, the IMC layer of the copper side interface was not obvious, and a large amount of CuxZny compounds were formed near the copper side. The base metals on the aluminum side formed a solid solution combination with zinc. When the addition amount of copper powder was 15%, the soldering seam was smooth and flat, the defect between balls was not obvious, and the IMC was uniformly distributed, so the average shear strength reached the maximum value of 47.37 MPa.

High-reliability, high-melting, lead-free, mixed solder paste System-BiAgXⓇ.

BiAgX®, a mixed solder powder paste composed of a primary high-melting solder powder and an additive low-melting solder powder, exhibited a melting temperature above 260°C and was comparable to, or even better than, the reliability of high-lead solders. The additive solder is designed to react preferentially with various surface metallizations and form a controllable intermetallic layer. Inside the joints, sub-micron AgSn particles are dispersed surrounding Bi colonies, which constrain the dislocation movement, thus enhancing strength, ductility, and associated joint reliability.

Study on the Reliability of Sn–Bi Composite Solder Pastes with Thermosetting Epoxy under Thermal Cycling and Humidity Treatment

In this study, a Sn–Bi composite solder paste with thermosetting epoxy (TSEP Sn–Bi) was prepared by mixing Sn–Bi solder powder, flux, and epoxy system. The melting characteristics of the Sn–Bi solder alloy and the curing reaction of the epoxy system were measured by differential scanning calorimeter (DSC). A reflow profile was optimized based on the Sn–Bi reflow profile, and the Organic Solderability Preservative (OSP) Cu pad mounted 0603 chip resistor was chosen to reflow soldering and to prepare samples of the corresponding joint. The high temperature and humidity reliability of the solder joints at 85 °C/85% RH (Relative Humidity) for 1000 h and the thermal cycle reliability of the solder joints from −40 °C to 125 °C for 1000 cycles were investigated. Compared to the Sn–Bi solder joint, the TSEP Sn–Bi solder joints had increased reliability. The microstructure observation shows that the epoxy resin curing process did not affect the transformation of the microstructure. The shear force of the TSEP Sn–Bi solder joints after 1000 cycles of thermal cycling test was 1.23–1.35 times higher than the Sn–Bi solder joint and after 1000 h of temperature and humidity tests was 1.14–1.27 times higher than the Sn–Bi solder joint. The fracture analysis indicated that the cured cover layer could still have a mechanical reinforcement to the TSEP Sn–Bi solder joints after these reliability tests.

Thermochemical Mechanism of the Epoxy-Glutamic Acid Reaction with Sn-3.0 Ag-0.5 Cu Solder Powder for Electrical Joining.

An epoxy-based solder paste (ESP) is a promising alternative to conventional solder pastes to improve the reliability of fine-pitch electrical joining because the epoxy encapsulates the solder joint. However, development of an appropriate epoxy formulation and investigation of its reaction mechanism with solder powder is challenging. In this study, we demonstrate a newly designed ESP consisting of diglycidyl ether of bisphenol F (DGEBF) resin, Sn-3.0 Ag-0.5 Cu (SAC305) solder powder, and L-glutamic acid (Glu), which is a proteinogenic amino acid for biosynthesis of proteins in living systems. The mechanism of the thermochemical reaction was explored and tentatively proposed, which reveals that the products of the reaction between SAC305 and Glu function as catalysts for the etherification of epoxides and alcohols produced by chemical bonding between DGEBF and Glu, consequently leading to highly crosslinked polymeric networks and an enhancement of impact resistance. Our findings provide further insight into the mechanism of the reaction between various formulations comprising an epoxy, amino acid, and solder powder, and their potential use as ESPs for electrical joining.

Effect of Zn-powder content on the property of Cu/SAC0307 powder/Cu joint under ultrasonic assisted at low temperature

PurposeLow-Ag SAC solder will lead to a series of problems, such as increased the melting range and declined the solderability and so on. These research studies do not have too much impact on the improvement of solders’ performance but were difficult to achieve satisfactory results. It is urgent to develop new soldering technology to avoid the bottleneck of lead-free solder. low-temperature-stirring soldering and ultrasonic-assisted soldering was developed in the authors’ early work, but slag inclusion and pore would gather and grow up to lead decreasing of the shear strength. In this paper, Cu/SAC0307 +Zn power/Cu joints with ultrasonic-assisted at low-temperature was successfully achieved.Design/methodology/approach45um Zn-powder and SAC0307 No.4 solder powder were mixed to fill the Cu-Cu joint, and the content of Zn-powder were 0 and 5%, 7.5% and 10%, 12.5% and 15% respectively. During the soldering process under ambient atmosphere %252C the heating platform provided a constant 220%253 F and the ultrasonic vibrator applied a constant pressure of 4 MPa to the copper substrate. The soldering process was completed after holding 70 s at 300 W.FindingsThe Zn particles made the IMC at the joint interface and in the soldering seam from scallop-type Cu6Sn5 to flat-type Cu5Zn8. The shear strength of joints without Zn was only 12.43 MPa, the shear strength of joints with 10% Zn reached a peak of 34.25 MPa, and the shear strength of joints containing 10% Zn was 63.71% higher than that of joints without zinc particles, and then the shear strength decreased. In addition, with the increase of zinc content, the fracture mode of the joint changed from the brittle fracture of the original layered tears to the mixed tough and brittle fracture.Originality/valueA new method that Zn micron-size powders and SAC0307 micron-size powders was mixed to fill the joint, and successfully achieved micro-joining of Cu/Cu under ultrasonic-assisted without flux at low-temperature.

Reliability of Cu Nanoparticles/Bi-Sn Solder Hybrid Bonding Under Cyclic Thermal Stresses

The influence of thermal cycle stress loading on hybrid bonding, which was formed by sintering a mixture of Cu nanoparticles and a eutectic Bi-Sn solder powder, has been investigated. A Si chip and a directly bonded aluminum (DBA) substrate were bonded using the hybrid bonding layer. The bonded sample was evaluated using a thermal cycle test (− 40°C and 250°C). The degradation process of the sample during the test was observed nondestructively using synchrotron radiation x-ray computed laminography. The thermal cycle stress loading had a minimal effect on the microstructure of the bonding layer, which has a high bonding strength owing to the liquid phase sintering and high decomposition melting temperature of the Cu-Sn compound formation. This property reduced the Al deformation of the DBA substrate caused by the thermal cycle loading, resulting in the suppression of the bonding layer degradation. Therefore, hybrid bonding can be instrumental in achieving the reliable operation of power modules at high temperatures.

Next-Generation Self-Assembled Anisotropic Conductive Adhesive Developed Using Solder Powder for Electrical Micro-Bonding in the Emerging IT Industry

The importance of the IT industry is becoming more and more prominent as it gradually changes to a non-face-to-face society after the COVID-19, and the next-generation smart devices of 5G mobile communication, micro LEDs, and wearable devices are emerging as the leading of the IT industry. Along with the development of these devices, limitations and problems of the Anisotropic Conductive Adhesive Film (ACF) used as a connection material for Film on Film (FOF), Chip on Glass (COG), Chip on Film (COF), and Film on Glass (FOG) are emerging. [1-5]

Solder Joint Structure of Sn-58Bi Solder Paste with Epoxy-based Flux

Non-clean soldering technology and low temperature bonding will be of great significant for applications in electronic industry. A curable solder paste containing epoxy-based flux and Sn-58Bi solder powder was developed. The solder joint covered by cured epoxy shell can be obtained by soldering process. The thermal-curing process of the curable flux was monitored by DSC to ensure the wetting and metallurgical connection between Sn-58Bi and Cu substrate. The microstructure characteristics of the solder joints were analyzed by metallurgical analysis and SEM observation. The factors influencing the structure of the solder joints were analyzed and discussed.

Microstructural evolution and micromechanical properties of SAC305/CNT/CU solder joint under blast wave condition

Purpose The purpose of this paper is to investigate the effect of carbon nanotube (CNT) addition on microstructure, interfacial intermetallic compound (IMC) layer and micromechanical properties of Sn-3.0Ag-0.5Cu (SAC305)/CNT/Cu solder joint under blast wave condition. This work is an extension from the previous study of microstructural evolution and hardness properties of Sn-Ag-Cu (SAC) solder under blast wave condition. Design/methodology/approach SAC/CNT solder pastes were manufactured by mixing of SAC solder powder, fluxes and CNT with 0.02 and 0.04 by weight percentage (Wt.%) separately. This solder paste then printed on the printed circuit board (PCB) with the copper surface finish. Printed samples underwent reflow soldering to form the solder joint. Soldered samples then exposed to the open field air blast test with different weight charges of explosives. Microstructure, interfacial IMC layer and micromechanical behavior of SAC/CNT solder joints after blast test were observed and analyzed via optical microscope, field emission scanning microscope and nanoindentation. Findings Exposure to the blast wave induced the microstructure instability of SAC305/Cu and SAC/CNT/Cu solder joint. Interfacial IMC layer thickness and hardness properties increases with increase in explosive weight. The existence of CNT in the SAC305 solder system is increasing the resistance of solder joint to the blast wave. Originality/value Response of micromechanical properties of SAC305/CNT/Cu solder joint has been identified and provided a fundamental understanding of reliability solder joint, especially in extreme conditions such as for military applications.