Solder Assembly Research Articles

FEA Assisted Optimization of Bump Shear Test Method for Assessment of Back-End-of-Line Mechanical Integrity

An enhanced Cu pillar bump shear test (BST) method has been developed to quantify wafer level back-end-of-line (BEOL) mechanical integrity and assess the risk of white bump failure. First, three different types of failure mode in the BST and the one that corresponds to the white bump failure in which delamination occurs at an ultra-low-k (ULK) dielectric layer, were identified by studying the load-displacement data together with focused ion beam (FIB) and scanning electron microscopy (SEM) analysis. To find the reason for different failure modes in BST and the effects of BST conditions, first, the difference in the stress modes between the bump shear test and the assembly solder reflow process was compared using finite element analysis (FEA) stress simulations. Then, parametric studies on the bump shear angle and the bump shear height were performed to find the optimal conditions at which the BST stress modes best mimic those of the solder reflow. Based on these studies, special high Cu pillar bump test samples were built in order to apply the optimal shear height and angle condition. The new optimal BST method has shown to significantly increase the probability of ULK delamination failure mode and was shown to be in good agreement with reflow hammer test results. Also, the method was used to verify the effect of metal density as well as process and design changes. Finally, a machine learning based correlation for quantifying the pure BEOL integrity regardless of the bump and far-BEOL dimensions was developed for application of the method to various products.

Solder Joint Defect Inspection Method Based on ConvNeXt-YOLOX

ConvNeXt-YOLOX model, an improved deep learning algorithm based on the YOLOX neural network, is proposed for surface mount technology (SMT) production line printed circuit board assembly (PCBA) solder joint defect detection with high accuracy and speed. The improved model employs the classification neural network ConvNeXt to enhance the feature extraction capability of the YOLOX backbone feature extraction network, thus enhancing the target detection accuracy and speed as well as the small target detection capability while maintaining the same number of parameters. A solder joint defect detection dataset containing 759 PCBA solder joint defect images is constructed, on which the improved model ConvNeXt-YOLOX, the original model YOLOX, and the lightweight model YOLOX-s are trained. Subsequently, the training results of the three models are compared. The comparison shows that the mean average precision (mAP) of the improved model ConvNeXt-YOLOX is 97.21%, which is 0.82% and 3.02% higher than that of YOLOX and YOLOX-s, respectively, while the mAP at (0.5:0.95) is increased from 76.3 to 77.5. Moreover, the detection speed is increased from the original 27.06 to 27.88 frames/s. In summary, the improved model ConvNeXt-YOLOX has strong small target feature extraction and detection capabilities, which are consistent with the actual requirements of solder joint defect detection.

A New Low-Temperature Solder Assembly Technique to Replace Eutectic Sn-Bi Solder Assembly.

We successfully achieved low-temperature assembly by reflowing the 13.5Sn-37.5Bi-45In-4Pb quaternary eutectic solder paste and the SAC 305 solder ball together at 140 °C for 5 min. The wetting angle of the mixed solder joint is 17.55°. The overall atomic percent of Pb in the mixed solder joint is less than 1%, which can be further reduced or eliminated. Moreover, after aging at 80 °C for 25 days, we observed no obvious decrease in shear strength of the fully mixed solder joint, which is the most advantage of this assembly technique over Sn58Bi solder assembly. The Bi phase segregation at the interface is slowed down compared with Sn-Bi solder joint. This low-temperature assembly is promising to be applied in advanced packaging technology to replace the eutectic Sn-Bi solder.

Reflow Soldering Capability Improvement by Utilizing TaN Interfacial Layer in 1Mbit RRAM Chip.

We investigated the thermal stability of a 1Mbit OxRRAM array embedded in 28 nm COMS technology. A back-end-of-line (BEOL) solution with a TaN–Ta interfacial layer was proposed to eliminate the failure rate after reflow soldering assembly at 260 °C. By utilizing a TaN–Ta interfacial layer (IL), the oxygen defects in conductive filament were redistributed, and electromigration lifetimes of Cu-based damascene interconnects were improved, which contributed to optimization. This work provides a potential solution for the practical application of embedded RRAM beyond the 28 nm technology node.

A general design strategy for thermoelectric interface materials in n-type Mg3Sb1.5Bi0.5 single leg used in TEGs

The reliability of thermoelectric power generators (TEGs) depends heavily on the contact interface between thermoelectric (TE) materials and electrodes. We propose a general alloying approach for generating TE interface materials (TEiMs) for n-type Mg3Sb1.5Bi0.5 systems. The TEiM serves as a metallisation layer or barrier that precedes the soldering assembly. We first selected Fe from 15 elements as the matrix element considering the criteria of high bonding strength and low interfacial resistivity. Following the principles of high bonding propensity, coefficient of thermal expansion (CTE) matching, diffusion passivation, and dopant inactivation, two types of ternary alloys (Fe7Mg2Cr and Fe7Mg2Ti) with shear strengths (σs) of > 40 MPa and the specific contact resistivities (ρc) of < 5 µΩ cm2 were obtained. Furthermore, the thermal stability of the TEiM/Mg3Sb1.5Bi0.5 contact interface was investigated employing aging treatment. The contact interface exhibited high shear strength (σs>30MPa), low specific contact resistivity (ρc<10μΩcm2), and excellent thermal stability after aging treatment at 400 °C for 15 days. The general TEiM design strategy presented herein will contribute to further optimization of contact interfaces in TEG devices.

Bond shear strength of Al2O3 nanoparticles reinforced 2220-capacitor/SAC305 solder interconnects reflowed on bare and Ni-coated copper substrate

The influence of Al2O3 nanoparticles addition in trace amounts and electroless Ni–P substrate coating on the microstructure development and bond shear strength of Sn-3.0Ag–0.5Cu (SAC305) solder joint were investigated. The performance and reliability of the 2220-capacitor joints with Al2O3 nanoparticle reinforced nanocomposites reflowed on Cu and Ni–P coated substrate were analyzed under varying high-temperature environments. The addition of nanoparticles enhanced the wettability and microhardness of the solder and considerably refined the joint microstructure. The dispersion and adsorption of Al2O3 nanoparticles resulted in the suppression of intermetallic (IMC) growth at the interface and refinement of the β-Sn grains as well as IMC precipitates into the matrix. The Ni–P coating on the substrate significantly retarded the IMC growth kinetics resulting in the formation of a thin and uniform IMC layer at the joint interface. The thermal stability and performance of the joint under high-temperature environments were enhanced due to the Ni–P coating on the substrate. Compared to the unreinforced SAC305 solder joint with bare Cu substrate, joints with SAC305 + 0.05Al2O3 composite showed about 17% higher shear strength with bare Cu substrate and about 27% higher strength with Ni–P coated substrate. The Weibull distribution analysis indicates a significant improvement in joint reliability of the 2220-capacitor/SAC305 solder assembly using SAC305 + 0.05Al2O3 nanocomposite and Ni-coated substrate. The ANOVA study suggests that the solder joint performance majorly depends on the operating environment, solder composition, and the substrate finish.

Impact of inter-metallic compound thickness on thermo-mechanical reliability of solder joints in solar cell assembly

This study evaluates the impact of intermetallic compound (IMC) thickness on thermo-mechanical reliability of lead-free SnAgCu solder joints in crystalline silicon solar cell assembly with regard to fatigue life. Finite element modelling is used to simulate the non-linear thermo-mechanical deformation of the joints. Five geometric models of solar cell assemblies with different IMC thickness layers in the range of 1 to 4 μm are utilized. The models were subjected to accelerated thermal cycling from 40 °C to 85 °C employing IEC 61215 standard for photovoltaic panels. Creep response of each of the assembly's solder joints to the induced thermal load were simulated using Garofalo-Arrhenius creep model. Simulation results indicate that when IMC thickness grows incrementally to 1, 2, 2.5, 3 and 4 μm, thermo-mechanical fatigue life of solder joints diminishes to 13,800, 11,800, 10,600, 9400 and 7800 cycles to failure respectively. Thus, solder joint fatigue life decreases as the IMC thickness increases during service lifetime. Therefore, proper design of solder joint in crystalline silicon solar cell assembly must include consideration of IMC layer thickness to prevent premature failure and to ensure fulfilment of desired functional lifetime of 13,688 cycles to failure (25 years).

Securing Printed Board Performance and Assembly Reliability in Automotive Applications through IPC Standards

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Industry consensus developed IPC standards for the automotive electronics manufacturer have been available since 2016. We will look at the current IPC standards available in development and in revision, the standards development process, and the automotive application addendums for printed board fabrication and assembly soldering that provide unique criteria for the reliability of electronic interconnects that must survive the harsh environments within the automotive industry. Attention will be given to a new effort in development to provide links between existing automotive-centric IPC standards and other industry standards applicable to automotive applications for printed board material selection, design, and solder joint reliability testing. We will also discuss challenges facing the industry, including process changes and cleanliness requirements, e.g. ionic contaminants, that affect every manufacturer and customer.&lt;/div&gt;&lt;/div&gt;

Thermoelectric cooling module with damping conductor on cold spays

Subject of Research.The paper presents results of study on a thermoelectric cooling module with damping heat and electrically conductive material for cold junctions. Telluride obtained by synthesis in a melting furnace is used as a thermoelectric element material. The size of a thermoelectric element is 2.0×2.0×1.5 mm. The modules were mounted on ceramic plates made of 96% Al2O3 with dimensions of 30×30×0.89 mm. Copper coated with a layer of nickel is used as the switching bus material. Modules were attached to the board by soldering. Tin and bismuth solder paste (melting point is equal to 139°С) was used as an assembly solder alloy. An electrically conductive EX-A302L silicone with specific resistance of 0.02 Ohm/cm, densityof 3.7 g/cm3 and thermal conductivity of 2.1 W/(m·K) was used for the damping compound. Methods. For efficiency increase of a thermal contact of a thermoelectric module, associated with large temperature gradient between hot and cold energy, especially for electrical n-elements, it is recommended to use an elastic electrically conductive adhesive. Elastic conductive adhesive based on silicone is proposed. Conductive silicone EX-A302L adhesive sealant consisted of single-component cold-vulcanized silicone with splashes of conductive microgranules. Working temperature from -50 to +120°C allows for the application of this adhesive both for conventional refrigeration modules, and for the cold junction of medium-temperature modules. A phased module assembly technology is developed. Main Results. Comparative tests are carried out for the experimental thermoelectric module and the standard industrial module of TB-31-2.0-1.5 thermoelectric module at measuring installations manufactured by Kryotherm company. The integrity of the modules was tested on Testo thermal imager. It is established that the difference between the working characteristics of experimental and industrial modules does not exceed 10% and lies within the permissible limits. It is found out that destruction dynamics of the modules after temperature cycling is virtually the same. Practical Relevance. The results obtained demonstrate the application possibility of electrically conductive adhesive as a contact layer for a thermoelement. The proposed technology will allow for replacing the materials of thermoelectric module branches with more efficient ones, but having different coefficients of temperature expansion, thereby increasing the efficiency of the thermoelectric device.

Novel Solder Alloy: Wide Service Temperature Capability for Automotive Applications

With automotive electronics booming, more sensors and power moderators are increasingly required for electrical vehicles and self-driving cars. Lead-free tin-silver-copper (SnAgCu), also known as SAC , has been a popular choice for surface-mount technology (SMT) assembly solder alloy in the electronics industry.

Experimental and Theoretical Assessment of Thin Glass Substrate for Low Warpage

This paper compares the warpage after lead-free solder assembly of ultrathin glass and organic substrates for microelectronic packages. Smart mobile devices demand packages to be thinner, which exacerbates warpage, which in turn affects interconnect pitch scaling and reliability. Although low-coefficient-of-thermal-expansion (CTE) organic laminates have been developed to reduce the warpage during assembly, glass substrates offer added benefits of much higher modulus and thermo-mechanical stability, with the potential to reduce warpage beyond low-CTE organic materials. The focus of this paper is on modeling and measurements to quantify the warpage of glass and organic laminates at 100- $\mu \text{m}$ core thickness after thermo-compression bonding at 260 °C peak temperature. In the experiments, four-metal-layer glass substrates were fabricated at the panel level, diced into $18.4 \times 18.4$ -mm2 coupons, 10-mm $\times $ 10- $\text {mm}\,\,\times 630-\mu \text{m}$ silicon dies were thermo-compression bonded, and the warpage of the assembled packages was measured using shadow moire interferometry. In parallel to the experiments, numerical models that account for the viscoplastic behavior of the solder as well as the sequential build-up materials and processes have been developed. The predicted warpage from the models was compared to the experimental measurements. It was seen that the low-CTE glass had lower warpage than the organic core of equivalent CTE, while increasing the CTE of the glass increased the warpage. Also, the underfill fillet size was found to influence the warpage for thin substrate packages, and an optimal fillet size has been identified for minimizing warpage. B-staged and capillary underfill materials were compared and found to have similar warpage values.

Lead-Free Solder for Assembly of Thick-Film Hybrid Modules for Use in High-Temperature Applications

High lead solders have long been used for the assembly of underhood automotive thick-film hybrid modules. With the move to lead-free solders in the electronics industry, there is an increasing interest in alternatives to high lead solders in these applications. A AgBiX solder paste has been evaluated for SiC die and resistor attach for 200 °C applications in vehicles. Die attached to thick-film Ag metallized substrates showed no degradation in shear strength after 2000-h aging at 20 °C, while die attached to PdAg thick-film metallization exhibited a decrease in shear strength of $\sim 30$ %. After 2000-h aging at 200 °C, chip resistors showed a decrease in shear force at fracture of $\sim 44$ % and 60% for assembly on thick-film Ag and thick-film PdAg, respectively.

Analysis of the natural activities of radionuclides, heavy metals, and other poisonous elements in lead-free Sn–6.5Zn solders with different alloying elements

Problem statementSn–Zn solders are candidate alternatives to Sn–Pb-based solder alloys. With the increasing requirement for lead-free solders, the reliability of successor solders in microelectronics assemblies is in high demand. The elimination of radionuclides, heavy metals, and other poisonous elements in lead-free Sn–6.5Zn solders in the microelectronics industry is a worldwide goal. As a result, it is useful to identify the natural concentrations of radioactive nuclides, heavy metals and other poisonous trace elements (both macro- and micro-element contents), such as Au-196, Th-227, Ag-110M, Fe-59, Zn-65, Rb-89, Rh-106M, Bi-207, Cs-137, Eu-154, Sb-126, Eu-152, Co-56, Co-58, Co-60 and K-40. If this lead-free solder contains high concentrations of natural radioactive nuclides, then workers handling it might be exposed to significant levels of radiation. Therefore, it is important to determine the levels of radioactive nuclides in this solder to protect workers; these levels provide background for the safety rules and precautions that should be applied for those working in this field. ApproachThe levels of natural radionuclides (238U, 232Th, and 40K) and their daughter products contained in Sn–6.5Zn solders have been estimated via gamma-ray spectrometry using a 100% Hyper-Pure Germanium (HPGe) detector. ResultsThe mean activities due to radionuclides, heavy metals, and other poisonous elements were measured, and the results are summarised in tables. ConclusionThe results of this assessment obtained by gamma-ray spectroscopic analysis indicated the presence of high levels of natural radioactivity, heavy metals and other poisonous elements. This result implies that a comprehensive study should be conducted to protect workers from high doses of hazardous substances.

Investigation of Thermoelectric Parameters of Bi2Te3: TEGs Assembled using Pressure-Assisted Silver Powder Sintering-Based Joining Technology

Operation of thermoelectric generator (TEG) modules based on bismuth telluride alloys at temperatures higher than 250°C is mostly limited by the melting point of the assembly solder. Although the thermoelectric parameters of bismuth telluride materials degrade for temperatures >130°C, the power output of the module can be enhanced with an increase in the temperature difference. For this, a temperature-stable joining technique, especially for the hot side of the modules, is required. Fabrication and process parameters of TEG modules consisting of bismuth telluride legs, alumina ceramics and copper interconnects using a joining technique based on pressure-assisted silver powder sintering are described. Measurements of the thermal force, electrical resistance, and output power are presented that were performed for hot side module temperatures up to 350°C and temperature differences higher than 300°C. Temperature cycling and results measured during extended high-temperature operation are addressed.

Interfacial reaction and mechanical evaluation in multi-level assembly joints with ENEPIG under bump metallization via drop and high speed impact test

The criteria of mechanical reliability in solder joints can be identified and described by comparative evaluation via drop test and high speed pendulum impact test. Systematic samples of assembly and attachment joints with various Pd additions were employed and investigated in this study. The statistical values of mechanical performances were calculated and compared. Better high speed impact performance of SAC305/ENEPIG attachment joints with 0.06μm Pd layers was confirmed owing to the single Cu6Sn5 phase growth. However, the comparative measurement of the better performance on drop testing exhibited in ENEPIG/SAC305/immersion Sn assembly joints with 0.1μm Pd layers deposit resulted from the thinner and layer-type IMC growth. The correlation between the cracks propagation and Pd addition was established on the basis of the elemental X-ray color mapping via Field-Emission Electron Probe Microanalyzer (FE-EPMA). It is expected that through comparison between impact and drop test in mechanical reliability, a criterion of joints reliability can be established. Besides, the optimal Pd layer deposit for the ENEPIG surface finish in the attachment and assembly solder joints was demonstrated and confirmed.

Large Form Factor Hybrid LGA Interconnects; Recent Applications and Technical Learning

Recent high-end server design trends have continued to challenge electronic packaging engineers to design and integrate larger form factor land grid array (LGA) attached modules within their assemblies. These trends have included the application of larger, denser, organic packaged modules whose electrical performance and postencapsulation physical characteristics have been enabled by both the continued development of hybrid LGA connectors as well as new module actuation hardware designs. In this paper, we'll discuss these recent trends, including the specific technical attributes and challenges that need to be addressed to ensure a repeatable and reliable assembly design is developed. Initially, overviews of the latest connector and module design trends, including styles and physical sizes and their implications to the module's bottom surface metallurgy (BSM) flatness requirements, etc. are provided. Pursuant to this, recent system integration trending is reviewed; including both the module quantity per system assembly as well as module to module physical placements and how each of these impact printed wiring board (PWB) design (i.e., layer count, LGA site flatness, etc.) as well as the PWB assembly's solder processing characteristics (i.e., LGA pre- and post-solder attach contact co-planarity, etc.). Completing the application portion, is a description of some recent LGA actuation hardware and module external cooling apparatus designs (e.g., air-cooled heats sinks and water-cooling cold-plates and thermal interface materials (TIMs), etc.). The remaining portion of the paper is dedicated to the description of the mechanical analysis efforts completed to provide a fundamental understanding of the design's “as-assembled” attributes and a review of the associated evaluation completed to verify the integrated assembly's reliability characteristics. From the analysis methodology perspective, both the means of including each of the integrated assembly's key attributes (e.g., module mechanical construction and as encapsulated flatness, LGA contact compliance and stiffness as well as soldered contact coplanarity, TIM stiffness, actuation hardware, heat sink and cold-plate mechanical construction, etc.) and the resulting estimation of the predicted module internal TIM and hybrid-LGA's Pb-free soldered interface strains, actuation hardware stresses and LGA contact load variation are provided. Completing the discussion is a review of the variety of testing executed to validate the design's intended reliability. Included in this is a description of the test vehicle's design, the environmental stress testing conducted (i.e., mechanical pre-conditioning, accelerated thermal cycling (ATC), mixed flowing gas (MFG) and heat aging (HA), etc.) and the resulting data.

Quantitative Bump Height Analysis in ENIG Using Design of Experiment

Flip chip technology has grown by leaps and bounds and is getting even smaller in size. Optimization of process parameters in manufacturing is eminent due to reliability issues. This paper reports the parameters that affect the quality of the bump height in electroless nickel immersion gold (ENIG) and their relationships between each other. A total of four different combinations of parameters have been carried out for this investigation using the design of experiment (DOE) approach. It can be concluded that higher temperature of electroless nickel permits an increase of bump height where as the increment in immersion gold temperature does not nessasarily affect the value of bump height. All four samples recorded a higher value of bump height than the controlled bump height value. This implies reliability of the solder joint and assembly process robustness can be improved with an increase of bump height by increasing the time.

Failure mechanism of solder bubbles in PCB vias during high‐temperature assembly

PurposePb‐free soldering challenged printed circuit board (PCB) assembly with high temperature. The purpose of this paper is to explain the failure mechanism of printed circuit board (PCB) assembly with solder bubbles of vias to avoid the problems of via‐drilling defects and solder joint failure.Design/methodology/approachThe failure of PCB vias with solder bubbles was investigated through cross sections and SEM microstructure inspection, TMA measurement, moisture absorption analysis and DSC measurement. The moisture absorption and CTE of FR4 laminate matched with manufacturing requirement to avoid the effects of solder bubbles. The effects of via drilling with a dull drill bit were compared to that with a new drill bit.FindingsThe moisture absorbed inside holes of via plating layers could be exposed to induce solder bubbles during Pb‐free soldering assembly and dull drill bits should be prevented during the drilling process to avoid the no‐bearing drilling effects.Originality/valueThe failure of PCB vias is not only involved in the voiding in solder joints but manufacturing processes of PCB. The paper designs an approach to analyse the properties of PCB materials and the drilling effects of vias to find out the mechanism resulting in solder bubbles of vias. The problem of drill bits should be considered to prevent the moisture absorbed in drilling vias with defects.

Development of the bus joint for the ITER Central Solenoid

The terminations of the Central Solenoid (CS) modules are connected to the bus extensions by joints located outside the CS in the gap between the CS and torodial field (TF) assemblies. These joints have very strict space limitations. Low resistance is a common requirement for all ITER joints. In addition, the CS bus joints will experience and must be designed to withstand significant variation in the magnetic field of several tenths of a Tesla per second during initiation of plasma. The joint resistance is specified to be less than 4nΩ.The joints also have to be soldered in the field and designed with the possibility to be installed and dismantled in order to allow cold testing in the cold test facility. We have developed coaxial joints that meet these requirements and have demonstrated the feasibility to fabricate and assemble them in the vertical configuration. We introduced a coupling cylinder with superconducting strands soldered to the surface of the cable that can be installed in the ITER assembly hall and at the cold test facility. This cylinder serves as a transition area between the CS module and the bus extension.We made two racetrack samples and tested four bus joints in our Joint Test Apparatus. Resistance of the bus joints was measured by a decay method and by a microvoltmeter; the value of the current was measured by the Hall probes. This measurement method was verified in the previous tests. The resistance of the joints varied insignificantly from 1.5 to 2nΩ.One of the challenges associated with a soldered joint is the inability to use corrosive chemicals that are difficult to clean. This paper describes our development work on cable preparation, chrome removal, compaction, soldering, and final assembly and presents the test results.

Review on microstructure evolution in Sn–Ag–Cu solders and its effect on mechanical integrity of solder joints

The use of Pb-bearing solders in electronic assemblies is avoided in many countries due to the inherent toxicity and environmental risks associated with lead. Although a number of “Pb-free” alloys have been invented, none of them meet all the standards generally satisfied by a conventional Pb–Sn alloy. A large number of reliability problems still exist with lead free solder joints. Solder joint reliability depends on mechanical strength, fatigue resistance, hardness, coefficient of thermal expansion which are influenced by the microstructure, type and morphology of inter metallic compounds (IMC). In recent years, Sn rich solders have been considered as suitable replacement for Pb bearing solders. The objective of this review is to study the evolution of microstructural phases in commonly used lead free xSn–yAg–zCu solders and the various factors such as substrate, minor alloying, mechanical and thermo-mechanical strains which affect the microstructure. A complete understanding of the mechanisms that determine the formation and growth of interfacial IMCs is essential for developing solder joints with high reliability. The data available in the open literature have been reviewed and discussed.