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

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

In this investigation, Scanning Electron Microscopy (SEM) and Scanning Probe Microscopy (SPM) have been utilized to examine aging induced microstructural changes occurring within lead free solders. Unlike many prior studies, fixed regions in the solder joint cross-sections were monitored throughout the aging process, rather than examining different samples and/or different regions after the various aging exposures. Sn-3.0Ag-0.5Cu (SAC305) lead free solder joint samples were formed with reflowed (RF) and water quenched (WQ) cooling profiles and resulting initial microstructures, and then polished microscopy cross-sections were prepared. Nanoindentation marks were added to the cross-sections at certain locations to facilitate locating the fixed regions of interest in subsequent microscopy observations. After preparation, the joints were then aged at T = 125 oC, and the microstructures were observed and recorded in the selected regions after various aging exposures using SEM and SPM. With this approach, time-lapse imagery of the microstructure evolution in a particular region of a solder joint has been recorded as a function of the aging time. When placed together sequentially, these images can be used as frames to create experimentally recorded movies of the microstructural evolution in SAC solder joints exposed to aging. This approach has allowed us to visualize phenomena such as the coalescence and migration of intermetallic compounds (IMCs) and the coarsening of beta-Sn dendrites. The results presented in this paper detail our findings for small aging times up to 10 hours, where the most rapid and dramatic changes in microstructure occur. In particular, this work has concentrated on observing the coalescence, change in shape, and migration of Ag3Sn intermetallic particles. The aging induced changes in number of IMCs, total area of all IMCs, average particle area, and average particle diameter have been quantified for several fixed regions in the joints.

Visualization of Microstructural Evolution in Lead Free Solders during Isothermal Aging Using Time-Lapse Imagery

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.

Jiawei Zhang

Papers

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