Hamid R. Ghorbani

Bio: Hamid R. Ghorbani is an academic researcher from University of Toronto. The author has contributed to research in topics: Finite element method & Creep. The author has an hindex of 5, co-authored 5 publications receiving 137 citations.

Thermal Fatigue of SnPb and SAC Resistor Joints: Analysis of Stress-Strain as a Function of Cycle Parameters

Abstract: Accelerated thermal cycling (ATC) has been widely used in the microelectronics industry for reliability assessment. The relative effects of thermal cycling parameters (temperature range, dwell time, and ramp rate) and the failure mechanisms they induce have been the subject of many studies; however, uncertainty remains, particularly regarding the role of a very high ramp rate such as encountered in a thermal shock chamber. In the present research, thermomechanical finite-element (FE) models were used to analyze the stress/strain response of a resistor test vehicle during ATC testing under three accelerated test conditions: two ramp rates (14 degC/min and 95 degC/min) and two temperature ranges (DeltaT=0degC-100degC and -40degC-125degC), denoted 14-100 (ramp rate-temperature range), 95-100, and 95-165. The temperature gradients through the thickness of the assemblies were measured during the ATC test with the high ramp rate and were used to create an FE model that included transient stresses and strains. The effect of the transient temperature gradients during thermal shock was found to be negligible in these resistor joints. The FE models were then used to simulate the above three ATC test conditions with either SnPb or Pb-free (SAC) solders and were compared with previously published thermal fatigue lives for this resistor test vehicle. For both SnPb and SAC resistors, the maximum total solder strains (sum of elastic, plastic, and creep) and strain energy dissipation per cycle predicted by the FE models in the 95-165 test condition were much greater than those in either the 14-100 or 95-100 test conditions, which produced almost identical strains and energy dissipation. In all cases, the strain energy density dissipation per cycle due to creep was much larger than that due to plastic deformation. The trend of these results was in accordance with the ATC tests, which showed that the thermal cycling lives decreased in the same order for both the SAC and SnPb solders; i.e., the fatigue life decreased as the predicted total strain increased

Accelerated thermal fatigue of lead-free solder joints as a function of reflow cooling rate

Abstract: Leadless chip resistor (LCR) assemblies were manufactured using both traditional tin-lead (Sn37Pb) and lead-free (Sn3.8Ag0.7Cu) solders. The leadfree test vehicles were assembled using three different cooling rates: 1.6°C/sec, 3.8°C/sec, and 6.8°C/sec. They were then exposed to accelerated thermalcycling (ATC) tests between 0°C and 100°C with a 10–14°C/min ramp rate and a 5-min dwell time. The test results indicated that these lead-free solder joints had better creep-fatigue performance than the tin-lead solder joints. The LCR built with the medium cooling rate showed the longest fatigue life compared with the resistors built with the normal cooling rate of 1.6°C/sec and the higher cooling rate 6.8°C/sec. The number of cycles to failure was significantly correlated to the void defect rate. Failure analyses were done using cross-sectioning methods and scanning electron microscopy (SEM). Finite-element models were built to analyze the inelastic, equivalent strain range in solder joints subjected to thermal-cycling conditions with different degrees of solder wetting. The results indicated that poor wetting increases strains throughout the joint significantly, which is in accordance with the ATC results.

Reliability behavior of lead-free solder joints in electronic components

Abstract: With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.

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

Abstract: 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.