Hongtao Ma

Isothermal Aging Effects on the Mechanical Shock Performance of Lead-Free Solder Joints

Isothermal aging effects on lead-free solders have been extensively investigated in recent studies. Researches show that aging effects significantly degrade the mechanical properties of lead-free solders. However, limited work has been done on the effects of aging on the board-level dynamic performance of lead-free solder joints, especially for large flip-chip ball grid array (FCBGA) packages. Due to the sensitivity of aging effects on lead-free solders, it is crucial to investigate the aging effects on the board-level dynamic performance of lead-free solders. In this paper, dynamic performance of lead-free solder joints has been characterized and isothermal aging effects have been investigated. It was found that the dynamic shock performance of lead-free FCBGA packages significantly degrades up to 20-35% after aging at elevated temperatures. It was also observed that aging at 100°C shows more severe degradation than aging at 75°C and 150°C. This is an important finding because many electronic products operate in the 100°C range for long periods. The unique finding of aging effects is related to the growth of Cu3Sn and Cu6Sn5 intermetallics after aging. Even though failure analysis shows mixed failure of pad cratering and intermetallic compound (IMC) failures, the IMC growth is the most sensitive factor contributing to the failure observed.

Acceleration factor study of lead-free solder joints under wide range thermal cycling conditions

In this study, a comprehensive set of tests were performed on test vehicles with different package types, sizes, pitches, and solder joints metallurgies. Accelerated Thermal Cycling (ATC) testing was performed using four different thermal cycling profiles: 0–100 °C, −40–125 °C, −55–125 °C, and −60–150 °C. Data from the tests were analyzed for failure mode and failure rate using Weibull statistics, and the characterized life for each test condition was determined and analyzed. The impact of solder alloy metallurgies, package types, sizes, and pitches on acceleration factors was also analyzed. The data will help quantify discrepancies due to test condition variations, and will also provide valuable guidance on effects of package types, size, pitches, and solder joints metallurgies. In addition, failure analysis was performed at different stages of the tests for each thermal cycling condition. The failure modes and failure mechanism were compared and discussed across all the thermal cycling profiles.

Isothermal aging effects on the dynamic performance of lead-free solder joints

Isothermal aging effects on lead-free solders have been extensively investigated in recent studies. There is researches show that aging effects will significantly degrade the mechanical properties of lead-free solders. However, very little work been done on the study of aging effects on the board level dynamic performance of lead-free solder joints, especially for large flip-chip BGA packages for ASICs. Due to the extreme sensitivity of aging effects on lead-free solders, it is crucial to investigate the aging effects on the board level dynamic performance of lead-free solders. In this study, dynamic performance of lead-free solder joints has been characterized and the isothermal aging effects have been investigated. It was found that dynamic shock performance of the lead-free FCBGA packages significantly degraded up to 20–35% after aging at varies elevated temperatures. It has also been discovered that aging at 100 °C shows more dramatic degradation than that of aging at 75 °C and 150 °C. It is a very critical finding due to the fact of that many electronic products operate at the 100 °C range for long period of time. The results will provide crucial guidelines in implementation of lead-free solders based on the products end-use conditions. The unique finding of aging effects is related the growth of Cu3Sn and Cu6Sn5 intermetallics after aging. Even though the failure analysis shows mixed failure of pad cratering and intermetallic compounds (IMC) failures, the IMC growth is the most sensitive factor to contribute to the failure observed in this study.

Reliability of Lead-Free Solder Joints Under a Wide Range of Thermal Cycling Conditions

In this paper, we report a comprehensive set of accelerated thermal cycling (ATC) tests that were performed on test vehicles with different package types, sizes, pitches, and solder joint alloy metallurgies using four different thermal cycling profiles: 0 to 100, -40 to 125, -55 to 125, and -60 to 150°C. Samples from the tests were analyzed for their failure modes, and failure rates were calculated by using Weibull statistics. The characterized life for each test condition was determined and analyzed. The impact of solder alloy metallurgies, package types, sizes, and pitches on acceleration factors of the ATC tests to fatigue life was also analyzed and discussed. The quantified discrepancies among several acceleration factors from different studies compared to the experimental data presented in this paper are illustrated. The results provide valuable guidance on the effects of package types, size, pitches, and solder joints alloy metallurgies on various ATC test conditions. In addition, failure analysis was performed at different stages of the tests for each thermal cycling condition. Dramatic failure mode shifts at extreme ATC conditions were observed. The significance and the long-term impact of the failure modes and failure mechanism shift between various ATC test conditions to the life prediction of lead-free solders are extensively discussed.

Aging impact on the accelerated thermal cycling performance of lead-free BGA solder joints in various stress conditions

Isothermal aging effects on lead-free solders have been extensively investigated in recent studies for both bulk solders and package solder joints. Researches show that aging significantly degrades the mechanical properties of bulk lead-free solders and dynamic performances of lead-free solder joints. There are studies exploring the impact of aging on accelerated thermal cycling (ATC) performance of lead-free solder joints, however, the results are discrepant, some research shows minimal impact of isothermal aging on long term ATC performances since most of the failure mode are not related to intermetallic (IMC) growth which has been impacted more significantly during aging. Some others show significant degradation of the of ATC life due to evidence of weakening of solder joints after aging. This study is intended to explore the factors that may affect the aging impact on the lead-free solder joint fatigue life. The test vehicle is designed with different package types, pitch sizes, and solder alloy metallurgies to capture the impact of affecting factors. The test vehicles have been aged at 100°C and 150°C for different aging durations, ATC test were subsequently performed on the aged samples and with the non aged samples as control. The effects of aging on the fatigue life of lead-free solder joints are extensively explored in this study.

Effects of PCB design variations on bend and ATC performance of lead-free solder joints

Sn-Ag-Cu (SAC) solder alloys, such as Sn-3.0Ag-0.5 Cu (SAC305) are the popular choices of lead-free solders replacing SnPb solders. However, SAC solders are more brittle in nature due to high stiffness and excessive intermetallic compounds growth at the solder joint to pad interface. This leads to higher risks in solder joints failures. Memory module type lead-free BGA packages are constantly under dynamic stresses during handling and thermal stresses during operations. It is important to understand the dynamic performance and long term reliability of memory module lead-free BGAs. It is believed that the PCB design variations cause dynamic and long term failure discrepancies in the fields. In this study, different pad and trace designs were introduced to evaluate the effects of PCB design variations on the bend and Accelerate Thermal Cycling (ATC) performance of lead-free solder joints. Pad designs with NSMD (Non-solder mask defined), SMD (Solder mask defined), and a unique web design were assembled and tested. Different solder alloys including SAC305, SAC105, and SnPb solders, have been evaluated in this study. Different PCB materials have also been evaluated in the test. Four point monotonic bend tests were performed to characterize the bending performance variations with different PCB designs and compared with conventional Sn-Pb solder. SMD pad is shown to have the best bend performance among all other types of designs in this study. In addition, this design also shows improvement in mitigation of PCB pad cratering with lead-free solders. Wide trace width seems to degrade the strength and is not preferred. Same as its superior shock resistance comparing with SAC305, SAC105 solder alloy shows better bend performance. There is no significant improvement of bend performance with Web Design. After aging treatment, bend performance of both SAC305 and SAC105 degraded by up to 34% and 29% respectively. However, the bend performance of eutectic SnPb was actually improved after aging. ATC tests were performed to investigate the effects of design variations on the long term reliability of lead-free solder joints; SMD design shows less reliability life than others. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.

Effects of Board Design Variations on the Reliability of Lead-Free Solder Joints

Sn-Ag-Cu (SAC) solder alloys, such as Sn-3.0Ag-0.5 Cu (SAC305) are the popular choices of lead-free solders replacing SnPb solders. However, SAC solders are more brittle in nature due to stiffness and excessive intermetallic compounds growth at the solder joint to pad interface. This leads to higher risk of solder joints failures. Memory module-type smaller lead-free ball grid array (BGA) packages are constantly under dynamic stresses during handling and thermal stresses during operations. It is important to understand the dynamic performance and long-term reliability of memory module lead-free BGAs. It is believed that the printed circuit board (PCB) design variations cause dynamic and long-term failure discrepancies in the fields. In this paper, different pad and trace designs are introduced to evaluate the effects of PCB design variations on the bend and accelerated thermal cycling (ATC) performance of lead-free solder joints. Pad designs with nonsolder mask defined, solder mask defined (SMD), and a unique web design are assembled and tested. Different solder alloys, including SAC305, Sn-1.0Ag-0.5Cu (SAC105) SAC105, and SnPb solders, have been evaluated in this paper. Different PCB materials have also been evaluated in the test. Four-point monotonic bend tests are performed to characterize the bending performance variations with different PCB designs and compared with conventional Sn-Pb solder. The SMD pad is shown to have the best bend performance among all other types of designs in this paper. In addition, this design also shows improvement in mitigation of PCB pad cratering with lead-free solders. Wide trace width seems to degrade the strength and is not preferred. Just as it shows superior shock resistance when compared with SAC305, the SAC105 solder alloy also shows better bend performance. There is no significant improvement in bend performance with web design. After aging treatment, bend performance of both SAC305 and SAC105 degraded by up to 34% and 29%, respectively. However, the bend performance of eutectic SnPb is actually improved after aging. ATC tests are performed to investigate the effects of design variations on the long-term reliability of lead-free solder joints; SMD design shows less reliability life than others. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.

Effects of Multiple Reworks on the Accelerated Thermal Cycling and Shock Performance of Lead-Free BGA Assemblies

In this paper, the effect of multiple rework cycles (up to 5×) on reliability of lead-free assemblies is investigated. The test vehicle is designed to capture the reliability impact of rework processes on interconnects of ball grid array (BGA) devices, the solder joints of its adjacent components, and BGA devices of clamshelled devices. The effects on high aspect ratio plated through hole (PTH) vias and microvias are also considered. A variety of BGA packages, such as flip-chip BGA, plastic BGA, and chip array BGA are selected to represent different package technologies and stress levels. The effect of multiple reworks on the behavior of lead-free solder joints during accelerated thermal cycling (ATC) and mechanical shock is extensively explored in this paper. The experimental and analytical findings showed that multiple reworks significantly degraded the ATC performance of the reworked assemblies by up to 50% in terms of characteristic life. It is also found that multiple reworks affect the PTH vias significantly. The combination of increased lead-free rework temperature excursions and high-end printed circuit board (PCB) aspect ratios may incur higher stresses in the PTH copper barrel. Early failures are observed post multiple reworks and the subsequent ATC test due to PTH barrel cracking. However, the effects of multiple reworks are less severe for devices with microvias compared to those with PTH vias. Multiple reworks also cause severe degradation of long-term performance of devices with clamshell designs. With proper control in rework process, multiple reworks have minimum effects on the adjacent BGA solder joints. In addition, failure analysis is performed in order to study the impact of multiple rework on the microstructure of SnAgCu solder joints. It is found that the intermetallic compounds (IMC) layer grows thicker as the number of reworks increases; however, the IMC growth rate and morphology are dependent on the surface finish of the package substrate. The failure mode and failure mechanism after ATC are also compared for different number of rework cycles. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.

Microstructure Development: Solidification and Isothermal Aging

With the thermodynamic driving forces in mind, the observed microstructures and microstructural evolution that follows nonequilibrium solidification are discussed, with consideration of defect formation, intermetallic phases, and the influence of surface finish and microalloying on microstructure evolution.

Challenges in Future-Generation Interconnects: Microstructure Again

Understanding multifaceted microstructural evolution mechanisms is a key enabling foundation that will enable computational modeling and prediction of electronic system lifetimes before anything is built.