Sn-Ag-Cu (SAC) alloy is currently recognized as the standard lead-free solder alloy for packaging of interconnects in the electronics industry, and high- Ag-content SAC alloys are the most popular choice. However, this choice has been encumbered by the fragility of the solder joints that has been observed in drop testing as well as the high cost of the Ag itself. Therefore, low-Ag-content SAC alloy was considered as a solution for both issues. However, this approach may compromise the thermal-cycling performance of the solders. Therefore, to enhance the thermal-cycling reliability of low-Ag-content SAC alloys without sacrificing their drop-impact performance, alloying elements such as Mn, Ce, Ti, Bi, In, Sb, Ni, Zn, Al, Fe, and Co were selected as additions to these alloys. However, research reports related to these modified SAC alloys are limited. To address this paucity, the present study reviews the effect of these minor alloying elements on the solder joint reliability of low-Ag-content SAC alloys in terms of thermal cycling and drop impact. Addition of Mn, Ce, Bi, and Ni to low-Ag-content SAC solder effectively improves the thermal-cycling reliability of joints without sacrificing the drop-impact performance. Taking into consideration the improvement in the bulk alloy microstructure and mechanical properties, wetting properties, and growth suppression of the interface intermetallic compound (IMC) layers, addition of Ti, In, Sb, Zn, Al, Fe, and Co to low-Ag-content SAC solder has the potential to improve the thermal-cycling reliability of joints without sacrificing the drop-impact performance. Consequently, further investigations of both thermal-cycling and drop reliability of these modified solder joints must be carried out in future work.

High-Reliability low-Ag-ontent Sn-Ag-Cu solder joints for electronics applications

Potting encapsulation inside plastic housing of electronic components has been investigated to study the encapsulation failure. Crack is one indicator of encapsulation failure. This type of defect causing the rejection of the product; thus, the crack occurrence must be assessed with respect to the potting encapsulation. Potting material was established inside the plastic housing of the surface-mount device (SMD) transformer. Cross-sectional observation of the soldered component was studied through optical and electron microscopy. Results reveal that the presence of cracks at the corner of the SMD transformer was detected. The further cross-sectional analysis suggested the presence of voids in the region around the cracks. Further crack formation and propagation was found from potting encapsulation to the plastic housing region. This behavior is observed in the corner of the SMD transformer.

Effect of Potting Encapsulation on Crack Formation and Propagation in Electronic Package

After approximately 50 years, NASA is restarting efforts to develop nuclear thermal propulsion (NTP) for interplanetary missions. Building upon nuclear engine tests performed from the late 1950s to the early 1970s, the present research and testing focuses on advanced materials and fabrication methods. A number of transient tests have been performed to evaluate materials performance under high-temperature, high-flux conditions, with several more experiments in the pipeline for future testing. The measured data obtained from those tests are being used to validate the Griffin reactor multiphysics code for this particular type of application. Griffin was developed at Idaho National Laboratory (INL) using the MOOSE framework. This article describes the simulation results of the SIRIUS-CAL calibration experiment in the Transient Reactor Test Facility (TREAT). SIRIUS-CAL was the first transient test conducted on NASA fuels, and although the test was performed with a relatively low core peak power, the test specimen survived a temperature exceeding 900 K. Griffin simulations of the experiment successfully matched the reactor’s power transient after calibrating the initial control rod position to match the initial reactor period. The thermal-hydraulics model largely matches the time-dependent response of a thermocouple located within the experiment specimen to within the uncertainty estimate. However, the uncertainty range is significant and must be reduced in the future.

/pdf/multiphysics-simulation-of-the-nasa-sirius-cal-fuel-2jigptho.pdf

Multiphysics Simulation of the NASA SIRIUS-CAL Fuel Experiment in the Transient Test Reactor Using Griffin

Parallel to the development of new lead-free solders, electronic packaging has gone through a considerable evolution. A redistribution of layers allows the increase of functionality by increasing the number of inputs/outputsin the packagewhile reducing the size. The reliability of the package is strongly influenced by the reliability of the interconnects. During production and service life, there are thermal processes involved that may lead to thermal fatigue. In this work, a two-dimensional finite elementmodel of a Fan-Out Wafer Level Packaging (FO-WLP) was built, and simulations of thermal test cycles were carried out varying the solder interconnect material: SAC305, SACQ, SACR, orInnoLot. A thermal oscillating load from –40°C to 125°C was applied to the packaging for three hours.State of the art concerning solder joint reliability models based on creep behavior reveals the benefit of using energy-based parameters, as cycles to failure are inversely proportional to the average creep strain energy density.Based on theaverage creep strain energy density simulation results, the reliability of the package withdifferent solderswas compared.The qualitative results suggestthat SACQ has a significant advantage in the operational lifetime compared toSACR, InnoLot, and SAC305.

/pdf/solder-joint-reliability-based-on-creep-strain-energy-1i7lva3n6a.pdf

Solder joint reliability based on creep strain energy density for SAC305 and doped SAC solders

Thermal characteristics of refrigerant flow boiling in two mini-channel heat sinks of different aspect ratios for battery thermal management

Thermal modeling, analysis, and design

This chapter focuses on stress and reliability analysis of interconnection such as lead-free solder joint. Reliability tests of electronic assemblies are crucial to ensure long-term operation of electronic products. It is often neither practical nor economical to conduct tests at service condition due to unaffordable test time and cost. Accelerated life tests (ALT) under elevated stress levels and/or higher stress-cycle frequency are employed to accelerate the test over much reduced time. Typical accelerating stresses test include thermal cycling, cyclic bend, drop impact and vibration tests. Such tests can facilitate formation of failure for reliability tests cost-effectively, shorten the product time and improve long-term reliability. The accelerated test methods and reliability analysis for advanced electronic packaging are examined by finite element analysis (FEA). FEA modeling and simulation are conducted for the stress analysis for lead-free solder joints subjected to thermal cycling, cyclic bend, vibration and drop impact loads. Different types of material response are used for different loading conditions, such as temperature-dependent, strain rate-dependent, visco-plastics properties. Life prediction is also presented for low cycle and high cycle fatigue behaviors. Failure analysis for advanced interconnection is conducted and correlated with FEA analysis results. Finally, a case study is presented to improve package design and assess reliability for 3D stacked package with through silicon via (TSV) chips by FEA.

Stress and reliability analysis for interconnects

Reliability and failure analysis of encapsulated packages

In the package development stage, various tests are involved to qualify the package. The thermal and mechanical related tests include tests at both the materials and package levels. In the thermal testing in Section 6.1, the thermal test methods involve not only the standard tests for thermal metrics such as Theta JA and Theta JC but also the engineering tests during package manufacturing and qualification processes such as burn-in and system level tests. Material tests are introduced in Section 6.2 to obtain mechanical properties such as stress–strain relationship, Young’s modulus, yield stress, creep properties, fatigue behavior, viscoplastic and viscoelastic properties, and so on, which is the cornerstone for the finite element modeling of packaging mechanical behavior. Solder joint material and epoxy molding compound are selected as two typical advanced electronic packaging materials to demonstrate material test method and result analysis.

Thermal and mechanical tests for packages and materials

Electrical analysis is extremely important for advanced packaging as it should provide chips with signal and power distributions. In this chapter, we focus on the electrical modeling and characterization of TSV packages. First, the basic knowledge of signal and power integrity is briefly depicted. Second, the modeling, characterization, and design of TSV packages are presented in detail. The circuit models are developed for TSV pair and TSV array, and the effects of metal-oxide-semiconductor capacitance and floating silicon substrate are considered and treated appropriately. Third, the TSV-based 3-D power distribution network is characterized based on the equivalent circuit model. At last, dummy TSVs are utilized for the design of integrated passive devices, which are believed to be beneficial for saving valuable chip areas.

Hengyun Zhang

Papers

Thermal modeling, analysis, and design

Stress and reliability analysis for interconnects

Reliability and failure analysis of encapsulated packages

Thermal and mechanical tests for packages and materials

Electrical modeling and design