A review : On the development of low melting temperature Pb-free solders

IMC growth of Sn-3.5Ag/Cu system: Combined chemical reaction and diffusion mechanisms

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 behavior of lead-free solder joints in electronic components

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

The thermal fatigue performances of Sn98.5Ag1.0Cu0.5 (SAC105), Sn97.5Ag2.0Cu0.5 (SAC205), Sn96.5Ag3.0Cu0.5 (SAC305) and Sn95.5Ag4.0Cu0.5 (SAC405) solder alloys with Pb terminations were investigated by accelerated temperature cycling with and without thermal preconditioning. The performance of the SAC alloys was compared to eutectic SnPb and aged SAC alloys. The test vehicle consists of commercial 2512 ceramic chip resistors soldered to printed wiring boards using the different solder alloy compositions. The solder joints were monitored continuously during a thermal cycle of 0°C -100°C with a ramp rate of 9°C/min and a 30 min dwell between temperature extremes. Failures were defined in accordance with the IPC-9701 A industry test guidelines and failure data are reported as characteristic life η (number of cycles to 63.2% failure) from a two-parameter Weibull distribution. The microstructural evolution was characterized using metallographic techniques and back-scattered scanning electron microscopy. The findings show that the lifetime of the alloys can be ranked as follows: SAC 305 ~ SAC 405 >; SAC 205 >; SAC 105 >; SAC305 aged >; SAC 105 aged >; SnPb and to determine mechanisms of failure, electron microscopy analysis and fractography were performed on post-cycled components.

Thermal Fatigue and Failure Analysis of SnAgCu Solder Alloys With Minor Pb Additions

Temperature profile effects in accelerated thermal cycling of SnPb and Pb-free solder joints

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

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

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.

Yan Qi

Papers

Temperature profile effects in accelerated thermal cycling of SnPb and Pb-free solder joints

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

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