Experimental characterization of the mechanical behavior of two solder alloys for high temperature power electronics applications

An experimental investigation of two potential candidate materials for the diamond die attachment is presented in this framework. These efforts are motivated by the need of developing a power electronic packaging for the diamond chip. The performance of the designed packaging relies particularly on the specific choice of the solder alloys for the die/substrate junction. To implement a high temperature junction, AuGe and AlSi eutectic alloys were chosen as die attachment and characterized experimentally. The choice of the AlSi alloy is motivated by its high melting temperature Tm (577°C), its practical elaboration process and the restrictions of hazardous substances (RoHS) inter alia. The AuGe eutectic solder alloy has a melting temperature (356°C) and it is investigated here for comparison purposes with AlSi. The paper presents experimental results such as SEM observations of failure facies which are obtained from mechanical shear as well as cyclic nano-indentation results for the mechanical hardening/softening evaluation under cyclic loading paths

Viscoplastic behavior of diamond die attach subjected to high temperature conditions

In power electronic applications, diamond based semi-conductors appears to be a new way to widely increase the capabilities of power electronic converters. The main prospective expected is an increasing in system integration and power capabilities. The Diamonix project concerns the elaboration of a single-crystal diamond substrate with electronic quality and its associated packaging. The designed structure has to resist to temperatures varying between -50°C and +300°C. This paper deals with an experimental and numerical study of the diamond die attach solution. The development of a packaging for diamond component relies in particular on a specific choice of solder’s alloys for the junction die/substrate. To carry out this junction, AuGe and AlSi eutectic alloys were chosen and characterized; the choice of these two kinds of solders i.e. AuGe and AlSi is motivated by the practical elaboration process and the restrictions of hazardous substances (RoHS). The first solder has a melting temperature of 356°C; the second has a higher melting point of 577°C. In this paper, we present some numerical results obtained from FE simulations of two 2D configurations of simplified electronic packaging. The power electronic packaging is composed of a diamond die and a copper metallized Si3N4 ceramic substrate which are brazed together with either AuGe or AlSi solder alloy. To predict the thermomechanical behavior of the solders, a particular constitutive behavior law was implemented as a User MATerial subroutine which is based on a viscoplastic unified McDowell formulation, coupled with porous damage equations. The mechanical law can describe precisely the viscoplastic damage phenomenon of solder subjected to high thermal cycling and to optimize the thermo-mechanical modeling for advanced package developmen

The durability of solder joints under thermo-mechanical loading; application to Sn-37Pb and Sn-3.8Ag-0.7Cu lead-free replacement alloy

Solder joints in electronic packages provide mechanical, electrical and thermal connections. Hence, their reliability is also a major concern to the electronic packaging industry. Ball Grid Arrays (BGAs) are a very common type of surface mount technology for electronic packaging. This work primarily addresses the thermo-mechanical durability of BGAs and is applied to the exemplar alloys; traditional leaded solder and a popular lead-free solder. Isothermal mechanical fatigue tests were carried out on 4-ball test specimens of the lead-free (Sn-3.8Ag-0.7Cu) and leaded (Sn-37Pb) solder under load control at room temperature, 35°C and 75°C. As well as this, a set of combined thermal and mechanical cycling tests were carried out, again under load control with the thermal cycles either at a different frequency from the mechanical cycles (not-in-phase) or at the same frequency (both in phase and out-of-phase). The microstructural evaluation of both alloys was investigated by carrying out a series of simulated ageing tests, coupled with detailed metallurgical analysis and hardness testing. The results were treated to produce stress-life, cyclic behaviour and creep curves for each of the test conditions. Careful calibration allowed the effects of substrate and grips to be accounted for and so a set of strain-life curves to be produced. These results were compared with other results from the literature taking into account the observations on microstructure made in the ageing tests. It is generally concluded that the TMF performance is better for the Sn-Ag-Cu alloy than for the Sn-Pb alloy, when expressed as stress-life curves. There is also a significant effect on temperature and phase for each of the alloys, the Sn-Ag-Cu being less susceptible to these effects. When expressed as strain life, the effects of temperature, phase and alloy type are much diminished. Many of these conclusions coincided with only parts of the literature and reasons for the remaining differences are advanced

Design of the Contact Metallizations for Gold-Tin Eutectic Solder-A Thermodynamic-Kinetic Analysis

This dissertation focuses on the design of reliable interconnections using Au-20wt.%Sn solder with the assistance of thermodynamic calculations. In this work, three commonly encountered contact metallizations, namely Ni, Cu, and Pt, have been selected. In order to assess the reliability of the Au-20wt.%Sn|X (X=Ni, Cu and Pt) interconnections from the metallurgical viewpoint, firstly, the phase diagrams of the Au-Sn-X ternary systems have been thermodynamically established with the Calphad method. Secondly, the diffusion couple method was employed to study the interfacial reactions experimentally. The microstructures of the as-soldered and subsequently aged Au-20wt.%Sn|X interconnections were characterized by means of scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDX) and scanning transmission electron microscopy with EDX. The observed interfacial reaction phenomena have been rationalized by combining the experimental results with the thermodynamic considerations. Emphasis has also been placed on collecting the mechanical properties of the IMCs formed at the solder/Cu and Ni interfaces since these values are essential for evaluating the reliability of the interconnections.  The results in this dissertation show that Au-20wt.%Sn|Pt was more thermally stable than Au-20wt.%Sn|Ni and Au-20wt.%Sn|Cu when these as-soldered reaction couples were subsequently aged at 150 °C for a long-term. When a short bonding time is employed, Pt contact metallization is superior to the Ni and Cu contact metallizations for the Au-20wt.%Sn solder

MICROSTRUCTURAL CHARACTERIZATION AND THERMAL CYCLING RELIABILITY OF SOLDERS UNDER ISOTHERMAL AGING AND ELECTRICAL CURRENT

Solder joints on printed circuit boards provide electrical and mechanical connections between electronic devices and metallized patterns on boards. These solder joints are often the cause of failure in electronic packages. Solders age under storage and operational life conditions, which can include temperature, mechanical loads, and electrical current. Aging occurring at a constant temperature is called isothermal aging. Isothermal aging leads to coarsening of the bulk microstructure and increased interfacial intermetallic compounds at the solder-pad interface. The coarsening of the solder bulk degrades the creep properties of solders, whereas the voiding and brittleness of interfacial intermetallic compounds leads to mechanical weakness of the solder joint. Industry guidelines on solder interconnect reliability test methods recommend preconditioning the solder assemblies by isothermal aging before conducting reliability tests. The guidelines assume that isothermal aging simulates a "reasonable use period," but do not relate the isothermal aging levels with specific use conditions. Studies on the effect of isothermal aging on the thermal cycling reliability of tin-lead and tin-silver-copper solders are limited in scope, and results have been contradictory. The effect of electrical current on solder joints has been has mostly focused on current densities above 104A/cm2 with high ambient temperature (≥100oC), where electromigration, thermomigration, and Joule heating are the dominant failure mechanisms. The effect of current density below 104A/cm2 on temperature cycling fatigue of solders has not been established. This research provides the relation between isothermal aging and the thermal cycling reliability of select Sn-based solders. The Sn-based solders with 3%, 1%, and 0% silver content that have replaced tin-lead are studied and compared against tin-lead solder. The activation energy and growth exponents of the Arrhenius model for the intermetallic growth in the solders are provided. An aging metric to quantify the aging of solder joints, in terms of phase size in the solder bulk and interfacial intermetallic compound thickness at the solder-pad interface, is established. Based on the findings of thermal cycling tests on aged solder assemblies, recommendations are made for isothermal aging of solders before thermal cycling tests. Additionally, the effect of active electrical current at 103 A/cm2 on thermal cycling reliability is reported

MODELING RATE DEPENDENT DURABILITY OF LOW-Ag SAC INTERCONNECTS FOR AREA ARRAY PACKAGES UNDER TORSION LOADS

The thesis discusses modeling rate-dependent durability of solder interconnects under mechanical torsion loading for surface mount area array components. The study discusses an approach to incorporate strain-rate dependency in durability estimation for solder interconnects. The components under study are two configurations of BGAs (ball grid array) assembled with select lead-free solders. A torsion test setup is used to apply displacement controlled loads on the test board. Accelerated test load profile is experimentally determined. Torsion test is carried out for all the components under investigation to failure. Strain-rate dependent (Johnson-Cook model) and strain-rate independent, elastic-plastic properties are used to model the solders in finite element simulation. Damage model from literature is used to estimate the durability for SAC305 solder to validate the approach. Test data is used to extract damage model constants for SAC105 solder and extract mechanical fatigue durability curve

Characterization of Thermo-Mechanical Damage in Tin and Sintered Nano-Silver Solders

abstract: Increasing density of microelectronic packages, results in an increase in thermal and mechanical stresses within the various layers of the package. To accommodate the high-performance demands, the materials used in the electronic package would also require improvement. Specifically, the damage that often occurs in solders that function as die-attachment and thermal interfaces need to be addressed. This work evaluates and characterizes thermo-mechanical damage in two material systems – Electroplated Tin and Sintered Nano-Silver solder. Tin plated electrical contacts are prone to formation of single crystalline tin whiskers which can cause short circuiting. A mechanistic model of their formation, evolution and microstructural influence is still not fully understood. In this work, growth of mechanically induced tin whiskers/hillocks is studied using in situ Nano-indentation and Electron Backscatter Diffraction (EBSD). Electroplated tin was indented and monitored in vacuum to study growth of hillocks without the influence of atmosphere. Thermal aging was done to study the effect of intermetallic compounds. Grain orientation of the hillocks and the plastically deformed region surrounding the indent was studied using Focused Ion Beam (FIB) lift-out technique. In addition, micropillars were milled on the surface of electroplated Sn using FIB to evaluate the yield strength and its relation to Sn grain size. High operating temperature power electronics use wide band-gap semiconductor devices (Silicon Carbide/Gallium Nitride). The operating temperature of these devices can exceed 250oC, preventing use of traditional Sn-solders as Thermal Interface materials (TIM). At high temperature, the thermomechanical stresses can severely degrade the reliability and life of the device. In this light, new non-destructive approach is needed to understand the damage mechanism when subjected to reliability tests such as thermal cycling. In this work, sintered nano-Silver was identified as a promising high temperature TIM. Sintered nano-Silver samples were fabricated and their shear strength was evaluated. Thermal cycling tests were conducted and damage evolution was characterized using a lab scale 3D X-ray system to periodically assess changes in the microstructure such as cracks, voids, and porosity in the TIM layer. The evolution of microstructure and the effect of cycling temperature during thermal cycling are discussed.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

Deposition and application of electroless Ni–W–P under bump metallisation for high temperature lead-free solder interconnects

A reliable and robust diffusion barrier, commonly known as under bump metallisation (UBM), is indispensable in solder interconnects in order to retard the interfacial reaction rate, hence the growth of intermetallic compounds (IMCs). However, electroless Ni-P coatings are not adequate to inhibit interfacial reactions effectively since the formation of columnar structure and voids in the crystalline Ni3P layer in hybrid automotive devices (operating temperature above 300ºC) can significantly deteriorate the mechanical integrity of solder joints. In this thesis, electroless Ni-W-P coatings, as an effective UBM capable to serving under high temperature (up to 450ºC), are developed, characterised and subsequently applied onto the high temperature lead-free solder interconnects. [Continues.