Min Pei

Constitutive modeling of lead-free solders

Solders are used extensively as electrical interconnects in microelectronics packaging. Because of environmental concerns, lead-based solders are being replaced by Sn/Ag and Sn/Ag/Cu based solder materials. Since the thermomechanical reliability of modern electronic devices depends on, to a large extent, the fatigue and creep behavior of the solder joints, it is imperative to understand the deformation behavior of these new lead-free solders. This study conducted extensive thermomechanical testing on several commercial lead-free solder alloys. Anand viscoplastic model was used to describe the behavior of these materials with new curve fitting techniques. A modified Anand models was proposed that can yield a more accurate description of lead-free solders.

Effect of finite element modeling techniques on solder joint fatigue life prediction of flip-chip BGA packages

Solder joint fatigue life in thermal cycling has been studied for decades using the finite element method. A great variety of modeling methodologies such as global/local modeling (sub-modeling) and sub-structure modeling (superelement) has been developed. Many different types of constitutive equations for solder alloys, various loading assumptions, and several definitions of damage parameters have been used. However, the accuracy of these different modeling approaches has not been completely evaluated in literature. There has been some long-standing confusion regarding the modeling assumptions and their effect on the accuracy of models, such as the initial stress-free temperature setting, selection of damage parameters, and choice of element type. This paper presents a comprehensive study of finite element modeling techniques for solder joint fatigue life prediction. Several guidelines are recommended to obtain consistent and accurate finite element results

Reliability analysis of SnPb and SnAgCu solder joints in FC-BGA packages with thermal enabling preload

Modern semiconductor devices in many applications require a thermal solution to remove the heat away from the device and maintain a certain operating temperature. These thermal solutions typically use a heat sink and a thermal interface material (e.g. thermal grease) between the device and the heat sink. A compressive load is applied to reduce the thermal resistance of the interface and facilitate better heat transfer from the device to heat sink. Depending on the magnitude, this compressive preload may affect the fatigue behavior of second level solder joints connecting the device to PCB in a thermal cycling environment. This paper describes the experimental setup and test results to evaluate the reliability of solder joints in the presence of a preload. 3-D nonlinear finite element analysis is performed to simulate the effect of compressive load in thermal cycling. Both SnPb and SnAgCu solder alloys are studied with various levels of preload

Creep and Fatigue Behavior of SnAg Solders With Lanthanum Doping

In this paper, extensive testing was conducted to study the effects of Lanthanum (La) doping on the creep and fatigue behavior of SnAg lead free solder alloys. Variables considered in this paper include doping amount, aging temperature, and aging time. The experimental data show that rare earth element (RE) doping increases SnAg solders creep resistance by about 15%. Meanwhile, RE doping does not affect thermal aging behavior of the solder alloy. A microstructure dependent Anand viscoplastic model is proposed to capture the RE doping effect on the creep behavior. Good agreement between the model predictions and experimental data are obtained. In addition, fatigue tests were performed with bulk specimen. It is found that La doping increases the fatigue life by about five times. The optimal doping level for better fatigue performance is around 0.1%.

Field condition reliability assessment for SnPb and SnAgCu solder joints in power cycling including mini cycles

Actual field conditions that computer components such as microprocessors experience are different from the accelerated thermal cycling tests typically used to perform reliability assessment. Field conditions can include longer dwell times, different temperature ramp rates driven by power ON/OFF events, and temperature fluctuations during the power ON state due to workload patterns. Series of numerical simulations have been performed to study the effect of the field conditions on the solder joint fatigue life of electronic packaging. The simulation results show that SnPb and SnAgCu solders respond differently with respect to dwell time and mini cycles. Without considering the effect of mini-cycles, it is possible that SnAgCu may fail earlier than SnPb with very long dwell time. However, it is important to note if mini-cycles contribute to damage as seen in this analysis, the SnPb solder will probably still fail before SnAgCu solder

Constitutive Modeling of Lead-Free Solders

Solders are used extensively as electrical interconnects in microelectronics packaging. Because of environmental concerns, lead-based solders are being replaced by Sn/Ag and Sn/Ag/Cu based solder materials. Since the thermomechanical reliability of modern electronic devices depends on, to a large extent, the fatigue and creep behavior of the solder joints, it is imperative to understand the deformation behavior of these new lead-free solders. This study conducted extensive thermomechanical testing on several commercial lead-free solder alloys. Anand viscoplastic model was used to describe the behavior of these materials with new curve fitting techniques. A modified Anand models was proposed that can yield a more accurate description of lead-free solders.Copyright

Define electrical packing temperature cycling requirement with field measured user behavior data

In this paper, a user behavior based solder joint reliability modeling approach has been proposed to estimate the design and test requirements for the second level interconnect (SLI) reliability prediction. This approach uses a numerical tool to integrate solder joint creep damage during the actual use condition that was collected from a large user sample size. The resultant damage per time period was then input to the solder joint fatigue model to estimate equivalent damage to testing duration. The is a physics based approach and is expected to provide more accurate product life prediction and reliability performance demand for BGA package designs.

A preliminary solder joint life prediction model by experiment and simulation for translation of use condition to temperature cycling test condition

This paper introduces a new preliminary solder joint fatigue model based on thermo-mechanical finite element analysis (FEA) simulation results and the use of extensive solder joint reliability (SJR) experimental data for ball grid array (BGA) packages. A comprehensive FEA modeling method for temperature cycling (TC) loading was defined based on thorough and detailed convergence studies on modeling approaches, mesh sensitivities, analysis parameters, material parameters, boundary conditions and thermal loading conditions. Extensive reliability data was collected for various package designs, form factors, board thicknesses and testing conditions to demonstrate feasibility. The result is a solder joint fatigue model derived from FEA thermal mechanical modeling results and empirical reliability data regression fitting. Next, this FEA modeling method was coupled with a transient heat transfer method to integrate thermal gradients that exist in actual product use condition (UC) duty cycles. A new UC method is demonstrated based on a common physical damage metric calculated from numerical simulations for UC (with real user behavior data and temperature gradient) and TC (uniform temperature) conditions. The derived SJR fatigue model was combined with the newly developed UC method to establish new TC test requirements based on the actual use condition duty cycling.

Effect of Rare Earth Elements on Lead-Free Solder Microstructure Evolution

In this paper, quantitative microstructure studies were performed on multiple length scales to investigate effect of rare earth element (RE) doping on SnAg lead free solder materials. Variables considered in this paper include doping amount, aging temperature and aging time. It was found that RE doping refines the microstructure and reduces microstructure coarsening rate, but the inter-particle spacing remains unaffected. Therefore, higher RE doping level leads to higher volume fraction of the eutectic phase due to the increased total number of Ag3Sn particles.

Accelerated stress testing methodology to risk assess silicon-package thermomechanical failure modes resulting from moisture exposure under use condition

IC components are exposed to moisture and thermal cycles during chip-package-board assembly and in their end use conditions. Moisture exposure influences the mechanical integrity of silicon backend dielectrics, assembly/packaging materials and packages. Reliability performance under accelerated stresses that simulate use conditions are often a critical factor in choice of materials, processing options and design rules. A complete assessment of the cumulative environmental exposure from chip-package assembly, shipment/storage, board system assembly, through end-customer use is required to guarantee product performance and reliability. This paper will detail these end user environments and use failure mode/mechanism specific acceleration models to develop accurate accelerated life testing plans and requirements. These requirements will then be compared to JEDEC standards based requirements and a need for re-calibration of these standards to more appropriate temperatures and stress durations will be highlighted.