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

Properties of Mixing SAC Solder Alloys with Bismuth-containing Solder Alloys for a Low Reflow Temperature Process

The subject of extensive research has been the establishing of lower temperature soldering of electronic assemblies that are similar to the once common yet still preferred eutectic Tin-Lead (SnPb) soldering manufacturing processes that are below 217˚C. This research opportunity will contribute data on mixed solder alloy assemblies that can be formed at lower process temperatures. There are many environmental and economic benefits of avoiding the current reliability concerns of assembling electronics at the standard high temperatures which peak at 230˚C 260˚C. To reduce this temperature the use of Bismuth containing solder pastes are mixing with the standard high temperature SAC solders for electronic assemblies. The materials evaluated are the (in weight percentages) 96.5Tin/3Silver/.5Copper (Sn/Ag/Cu) solder ball mixed with each solder paste, the eutectic 58Bismuth/42Tin (58Bi/42Sn), 57Bi/42Sn /1Ag and a propriety alloy that has a lower Bismuth content along with various micro alloys, 40-58Bi/Sn/X (X representing proprietary micro alloys or doping). In the assembly portion of this research the solder alloys were exposed to three different peak temperatures 180˚C, 195˚C, 205˚C. Another reflow profile attribute of focus was times above 138˚C the melting point of the eutectic Sn58Bi alloy. The ball and paste assembly portion of this research used the times above melting of 120sec and 240sec to represent process extremes and verify their significance on improving mixing level results. These times above melting did not consistently improve the mixing levels and therefore are not recommended or required during mixed low temperature solder assemblies. The results in this study suggest the recommended and optimum reflow profile to have a time above the melting point to be less than or equal to 90 seconds for mixed solder alloy assemblies in “low” (20mm a side) component is a SAC405 solder balled BGA with the dimensions of 42x28x0.8mm. With any large component the temperature gradient across the component is a risk factor and the results show that there are significantly differences of mixing from the center of the component to the edge due to an average 2.3 ˚C temperature difference during convection reflow. The average mixing % levels recorded for Tpeak= 180˚C for the solder pastes with a 58Bi = 47%, 57Bi = 47% and 40-58Bi = 44%. The average mixing % levels recorded for Tpeak= 195˚C for the solder pastes with a 58Bi = 69%, 57Bi = 77% and 40-58Bi = 57%. The conclusions found also match previous work identifying the reflow peak temperatures remain a significant factor on the mixing %. This work’s goal was to add to the knowledge of the electronics industry to better understanding the microstructure and mixing mechanisms of Bi/Sn/X-SAC solder joints for low temperature reflow assembly processes

Thermal Cycling Life Prediction of Sn-3.0Ag-0.5Cu Solder Joint Using Type-I Censored Data

Because solder joint interconnections are the weaknesses of microelectronic packaging, their reliability has great influence on the reliability of the entire packaging structure. Based on an accelerated life test the reliability assessment and life prediction of lead-free solder joints using Weibull distribution are investigated. The type-I interval censored lifetime data were collected from a thermal cycling test, which was implemented on microelectronic packaging with lead-free ball grid array (BGA) and fine-pitch ball grid array (FBGA) interconnection structures. The number of cycles to failure of lead-free solder joints is predicted by using a modified Engelmaier fatigue life model and a type-I censored data processing method. Then, the Pan model is employed to calculate the acceleration factor of this test. A comparison of life predictions between the proposed method and the ones calculated directly by Matlab and Minitab is conducted to demonstrate the practicability and effectiveness of the proposed method. At last, failure analysis and microstructure evolution of lead-free solders are carried out to provide useful guidance for the regular maintenance, replacement of substructure, and subsequent processing of electronic products

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

HARMONIC AND RANDOM VIBRATION DURABILITY INVESTIGATION FOR SAC305 (Sn3.0Ag0.5Cu) SOLDER JOINT

ABSTRACT Title of Dissertation: HARMONIC AND RANDOM VIBRATION DURABILITY INVESTIGATION FOR SAC305 (Sn3.0Ag0.5Cu) SOLDER INTERCONNECTS Yuxun Zhou, Doctor of Philosophy, 2008 Dissertation directed by: Professor Abhijit Dasgupta Department of Mechanical Engineering Vibration loading is commonly encountered during the service life of electronic products. However, compared to thermal cycling durability, vibration durability is more complex and has been less investigated. In surface mount technology, solder joints are the primary mechanical, thermal and electrical interconnects between the component and the PWB. So the reliability of solder joints is very crucial for most electronic assemblies. The vibration durability of Pb-free solder joints is the focus of this dissertation. The characteristics of the stress from vibration loading are low amplitude and high frequency, while those from cyclic thermal loading are high amplitude and low frequency. In this study, several exploratory vibration tests were conducted, using both narrow band and broad-band, step-stress excitation at several different isothermal and thermal cycling conditions. The effect of thermal pre-aging on solder joint vibration failures was also investigated. Some of the vibration durability results were analyzed in detail, to obtain quantitative insights into the vibration fatigue behavior of the SAC305 solder material. A time-domain approach was adopted to investigate the durability of solder interconnects under different kinds of vibration and quasi-static mechanical loading. First, the solder interconnects were subjected to narrow-band (harmonic) vibration loading. The test were conducted at the first natural frequency of the test board using constant-amplitude excitation and solder fatigue properties were extracted with the help of a time-domain analysis that is based on quasi-static finite element simulation. Compared to broad-band step-stress vibration durability tests, the advantage of the harmonic constant-amplitude test is less complexity in the model extraction process, hence, less uncertainty in the desired fatigue constants. Generalized strain-based S-N curves have been obtained for both SAC305 and Sn37Pb solder materials. The strain-life model constants show that SAC305 solder material has superior fatigue properties compared to Sn37Pb solder material under low-cycle fatigue loading, while the reverse is true for high-cycle fatigue loading. These results are consistent with test results from other researchers. In actual application, SAC305 assemblies almost always fail before Sn37Pb assemblies under comparable vibration excitation because of (i) higher solder strain at a given excitation level; and (ii) multiple failure modes such as copper trace cracking. Next, durability was investigated under step-stress, broad-band (random) excitation. These test results show that SAC305 interconnects are less durable than Sn37Pb interconnects under the random excitation used in this study, which agrees with the harmonic durability results. The random and harmonic durability results were quantitatively compared with each other in this study. Finite element simulation was used to investigate the stress-strain response in the interconnects. The output of this simulation is the strain transfer function due to the first flexural mode of the PWB. This transfer function is used to obtain the solder strain from the measured board strain. This fatigue assessment method demonstrated that the model constants obtained from the harmonic test overestimate the fatigue life under random excitation by an order of magnitude. The causes for this discrepancy were systematically explored in this study. The effects of cyclic loading and mean stress on the vibration durability were addressed and found to be minimal in this study. The stress-strain curves assumed for the solder material were found to have a very large effect on the durability constants, thus affecting the agreement between harmonic and random durability results. The transient response of the components on the test board under both harmonic and random excitation was also included in the strain transfer function with the help of dynamic implicit simulation, and found to have a much stronger effect on the vibration durability at the high frequencies used in broad-band excitation compared to the low frequency used in narrow-band test. Furthermore, the higher PWB vibration modes may play a strong role and may need to be included in the strain transfer-function. This study clearly reveals that the solder strain analysis for broad-band random excitation cannot be limited to the quasi-static strain transfer-function based on the first PWB flexural mode, that has been used in some earlier studies in the literature. The time-domain approach used in this study provided fundamental and comprehensive insights into the key factors that affect vibration durability under different types of excitation, thus leading to a generalized S-N modeling approach that works for both harmonic and random vibration loading