Isothermal aging causes detrimental changes in the microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies. These material changes also degrade the reliability of solder joints in assemblies subjected to aging prior to field use. In the current work, we have extended our previous research on the effects of aging on lead free solder material behavior to explore the effects of prior aging on solder joint (board level) reliability in actual assemblies. Our overall objective was to develop new reliability prediction procedures that incorporate aging effects, and then to validate the new approaches through correlation with thermal cycling accelerated life testing experimental data for pre-aged assemblies.

Thermal cycling reliability of aged PBGA assemblies - comparison of Weibull failure data and finite element model predictions

Solder joints in electronic assemblies are often subjected to cyclic (positive/negative) mechanical strains and stresses. Such exposures can occur in variable temperature application environments or during accelerated life thermal cycling tests used for qualification. Cyclic loading leads to damage accumulation, crack initiation, crack propagation, and eventually to fatigue failure. In this investigation, we have examined the effects of prior aging conditions, strain range, strain rate, and testing temperature on the cyclic stress-strain behavior of SAC305 lead free solder. Newly designed micro-cylinder shaped uniaxial lead free solder test specimens have been prepared in glass tubes using a vacuum suction process. Prior to testing, the samples were aged for various durations (0 to 360 days) at 125 °C. After aging, the fabricated samples were then subjected to cyclic stress-strain loading under several different conditions. From the recorded cyclic stress-strain curves, we have been able to characterize and empirically model the evolution of the solder hysteresis loops with aging duration. It has been observed that aging leads to the microstructural coarsening and degrades the mechanical fatigue properties, and those degradations are much more significant at the first few days of aging. The effects of aging on the cyclic stress-strain behavior have also been quantified for the first time for different testing temperatures, strain rates, and plastic strain ranges. At elevated test temperatures or lower test strain rates, an increase in the plastic strain range and a drop of the peak stress and loop area were found. Additionally, the plastic strain range, loop area, and peak load increased as expected with greater applied total strain range.

Cyclic Stress-Strain Behavior of SAC305 Lead Free Solder: Effects of Aging, Temperature, Strain Rate, and Plastic Strain Range

Fatigue failure of solder joints is one of the major causes of failure in electronic devices. Fatigue life prediction models of solder joints were first put forward in the early 1960s, and since then, numbers of methods were used to model the fatigue mechanism of solder joints. In this article, the majority fatigue life models are summarized, with emphasis on the latest developments in the fatigue life prediction methods. All the models reviewed are grouped into four categories based on the factors affecting the fatigue life of solder joints, which are: plastic strain-based fatigue models, creep damage-based fatigue models, energy-based fatigue models, and damage accumulation-based fatigue models. The models that do not fit any of the above categories are grouped into “other models.” Applications and potential limitations for those models are also discussed.

A State-of-the-Art Review of Fatigue Life Prediction Models for Solder Joint

Lead free solder materials are susceptible to significant creep deformations in harsh high temperature environments including automotive, avionics, military, and oil exploration applications. In addition, dramatic degradations will occur in the creep responses of lead free solder alloys when they are exposed to long term isothermal aging during product applications at high temperatures. Such degradations in the creep compliance of the solder material are universally detrimental to reliability of solder joints in electronic assemblies. In this work, we have characterized the high temperature creep behavior of SAC405 (95.5Sn4.0Ag0.5Cu) lead free solder, which is the most creep resistant of the standard SACN05 alloys. In addition, we have studied the creep behaviors of two doped SAC solders, SAC_Q and Innolot, which have been previously shown to out-perform SAC405 in simple mechanical stress-strain tests at room temperature. Tensile specimens were formed in rectangular cross-section glass tubes using a vacuum suction process, and a water quenched (WQ) solidification profile was utilized to yield fine microstructures and the upper limits of the mechanical properties for each alloy. The samples were then aged for 10 days at room temperature to stabilize their microstructures. After aging, creep testing was performed at two different stress levels (10, 15 MPa) and several different extreme/high testing temperatures (T = 100, 125, 150, 175, and 200 °C). For each set of conditions, the creep performances of the three alloys were compared. The results showed that the doped SAC alloys were more resistant to creep at high temperatures. The creep rates of SAC_Q are roughly 50% of those for SAC405, while the creep rates of Innolot are roughly 33% of those for SAC405. It is likely that the dopants can significantly block the movement of dislocations and thus increase the creep resistance of these solders.

High temperature creep response of lead free solders

Reliable lead free solders are needed for products exposed to extreme environments such as those used in the automotive, avionics, and oil-exploration industries, as well as in military applications. In this study, stress-strain curves have been measured for several doped Sn-Ag-Cu (SAC) solder materials at high temperatures up to 200 °C, and their performances have been compared to those for standard SAC alloys. The doped lead free solder materials are referred to as SAC_R (Ecolloy), SAC_Q (CYCLOMAX), and Innolot by their vendors. SAC_R and SAC_Q are formulated with Sn, Ag, Cu, and Bi (SAC+Bi), while Innolot includes an engineered combination of six elements. Tensile specimens were formed in rectangular cross-section glass tubes using a vacuum suction process, and a water quenched (WQ) solidification profile was utilized. This profile resulted in extremely fine microstructures, and mechanical properties near the upper limit possible for each alloy. Uniaxial tensile testing was performed on the three doped alloys at temperatures of 25, 50, 75, 100, 125, 150, 175, and 200 °C, and a strain rate of 0.001 sec-1. For the SAC_Q alloy, testing was also performed at strain rates of 0.0001 and 0.00001 sec−1, and the Anand constitutive parameters were calculated. The results for the doped solders were compared to standard SAC105 and SAC405 lead free alloys. The mechanical properties of SAC_R were found to exceed those for SAC105 at all temperatures, even though SAC_R does not contain any silver. In addition, the mechanical properties of SAC_Q and Innolot were found to match or exceed those of SAC405 at all temperatures. The new alloys show great promise for use in extremely harsh environments.

Mechanical characterization of doped SAC solder materials at high temperature

Pb-free solder joints exposed to elevated isothermal temperatures for prolonged periods of time undergo microstructural and mechanical evolution, which degrades the joint electrical performance. We report the effect of isothermal aging on the reliability of Sn–Ag–Cu (SAC) assemblies on three different surface finishes [immersion Ag (ImAg), electroless Ni/immersion Au (ENIG), and electroless Ni/electroless Pd/immersion Au (ENEPIG)]. The characteristic life for SAC alloys in 10- and 15-mm ball grid array packages on ImAg degraded over 40% after an 85 °C/12 months aging and over 50% during a 125 °C/12 months aging. ENIG and ENEPIG outperformed ImAg for all aging treatments. For passive components (2512 resistors) on ImAg, the reliability performance degraded 16.7%/28.1% after a 1-year aging at 85 °C/125 °C. Failure analysis showed dramatic intermetallic binary Cu–Sn and ternary Ni–Cu–Sn film growth at the bottom of the solder joint interfaces for ImAg and ENIG/ENEPIG. For 125 °C-aged samples, the cracks appeared at the corners of both package and board sides of the solder ball and propagated along (near) the intermetallic compound location. For the case of aged fine-pitch packages, the failures tended to start at the component side solder ball corner and then propagate along an angled path downward into the bulk solder.

Reliability Comparison of Aged SAC Fine-Pitch Ball Grid Array Packages Versus Surface Finishes

This study illustrates test results and comparative literature data on the influence of isothermal aging and thermal cycling associated with Sn-1.0Ag-0.5Cu (SAC105) and Sn-3.0Ag-0.5Cu (SAC305) ball grid array (BGA) solder joints finished with ENIG and ENEPIG on the board side and ENIG on the package side compared with ImAg plating on both sides. The resulting degradation data suggests that the main concern for 0.4 mm pitch 10 mm package size BGA is package side surface finish, not board side. That is, ENIG performs better than immersion Ag for applications involving long-term isothermal aging. SAC305, with a higher relative fraction of Ag3Sn IMC within the solder, performs better than SAC105. SEM and polarized light microscope analysis show cracks propagated from the corners to the center or even to solder bulk, which eventually causes fatigue failure. Three factors are discussed: IMC, grain structure, and Ag3Sn particle. The continuous growth of Cu-Sn intermetallic compounds (IMC) and grains increase the risk of failure, while Ag3Sn particles seem helpful in blocking the crack propagation.

Packaging Reliability Effect of ENIG and ENEPIG Surface Finishes in Board Level Thermal Test under Long-Term Aging and Cycling.

This paper investigates the effect of long term isothermal aging and thermal cycling on the reliability of lead-free solder mixes with different solder compositions, PCB surface finishes, and isothermal aging conditions. A variety of surface mount components are considered, including ball grid arrays (BGAs), quad flat no-lead packages (QFNs) and 2512 Surface Mount Resistors (SMRs). 12 lead-free solder pastes are tested; for BGA packages these are reflowed with lead-free solder spheres of SAC105, SAC305 and matched doped solder spheres (“matched” solder paste and sphere composition). Three surface finishes are tested: Organic Solderability Preservative (OSP), Immersion Silver (ImAg), and Electroless Nickel Immersion Gold (ENIG). All test components are subjected to isothermal aging at 125°C for 0 or 12 months, followed by accelerated thermal cycle testing from −40°C to 125°C. Data from the first 1500 cycles is presented here, with a focus on the effect of surface finish on package reliability. Cur...

Reliability Analysis of Lead-Free Solder Joints with Solder Doping on Harsh Environment

This study illustrates test results and comparative literature data on the influence of isothermal aging and thermal cycling associated with Sn-1.0Ag-0.5Cu (SAC105) and Sn-3.0Ag-0.5Cu (SAC305) ball...

Sn-Ag-Cu Solder Joints Interconnection Reliability of BGA Package during Thermal Aging and Cycling

Pb-free solder joints undergo microstructural and mechanical evolution due to alloy coarsening and growing intermetallic compounds which degrade the joint electrical performance. Electronics assemblies containing solder joints are frequently exposed to elevated temperatures for prolonged periods of time. The purpose of the study is to discover the effect of isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies. After studied different surface finishes, we employed Immersion Ag (ImAg), Immersion Sn (ImSn), Electroless Ni/Immersion Au (ENIG) and Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) which have potential for higher reliability and better performance and received increased attention for both packaging and subtracted applications. A full experiment matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch μBGAs and in additional, 0.65mm MLF and 2512 resistors be particularly tested. S...

Chaobo Shen

Papers

Reliability Comparison of Aged SAC Fine-Pitch Ball Grid Array Packages Versus Surface Finishes

Packaging Reliability Effect of ENIG and ENEPIG Surface Finishes in Board Level Thermal Test under Long-Term Aging and Cycling.

Reliability Analysis of Lead-Free Solder Joints with Solder Doping on Harsh Environment

Sn-Ag-Cu Solder Joints Interconnection Reliability of BGA Package during Thermal Aging and Cycling

Effects on the Reliability of Lead-Free Solder Joints under Harsh Environment