Research on Thermal Fatigue Life Prediction Technology of PoP Laminated BGA Products

In this paper, a fast prediction method for thermal fatigue life of PoP laminated BGA Product was proposed. Firstly, the stress and strain of the solder joints of each layer of the laminated device in thermal fatigue test was determined by finite element simulation method. According to the research idea of relative stress and strain, the solder joints were divided into sensitive solder joints and reliable solder joints. Secondly, sensitive solder joint were connected with PCB traces through the internal pads, bonding wires, TSVs and reliable solder joints to form a daisy chain. Through real-time dynamic monitoring of the resistance change of the daisy chain in thermal fatigue test to judge whether the solder joints fail, and record the occurrence time of the first failure solder joint. Finally, the thermal fatigue life of the product was estimated by the Norris-Landzberg formula.

Design and analysis of a spatially shaped laser beam used for temperature field simulation

Laser has controllable space-time characteristics and can be loaded on materials as a heat source in laser manufacturing, laser medical and other fields. In the thermal fatigue test of engine parts, thermal load can also be considered by laser beam. In this paper, a large aperture spatially shaped laser beam with complex and uneven intensity distribution is selected as the heat source for temperature field simulation. Using the method of key nodes selection, the temperature distribution of a few key nodes can reflect the desired temperature field of the whole nodes. Based on finite element model (FEM), through the numerical simulation of the temperature field, optimized intensity distribution of the shaped laser beam is calculated, and a diffractive optics element (DOE) is designed to achieve the shaping effect. The results show that a spatially shaped laser beam can be used for temperature field simulation.

Thermal Fatigue Test on Dissimilar Welded Joint Between Gr.91 and 304SS

This paper describes a thermal fatigue test on a structural model with a dissimilar welded joint. In the present design of Japan sodium cooled fast reactor (JSFR), there may be dissimilar welded joints between ferritic and austenitic steels especially in intermediate heat exchangers (IHX) and steam generators (SG). Creep-fatigue is one of the most important failure modes in JSFR components. However, the creep-fatigue damage evaluation method has not been established for dissimilar welded joint. To investigate the evaluation method, structural test will be needed for verification. Therefore, a thermal fatigue test on a thick-wall cylinder with a circumferential dissimilar welded joint between Mod.9Cr-1Mo steel and type 304 austenitic stainless steel (304SS) was performed. Since the coefficients of thermal expansion of these steels were significantly different, buttering layer of Ni base alloy was installed between them. After the completion of the test, deep cracks were observed at the heat affected zone (HAZ) in 304SS, as well as at HAZ in Mod.9Cr-1Mo steel. There were many tiny surface cracks in base metal (BM) of 304SS. According to the fatigue damage evaluation based on the finite element analysis results, the largest fatigue damage was calculated at HAZ in 304SS. Large fatigue damage was also estimated at BM of 304SS. Fatigue cracks were observed at HAZ and BM of 304SS in the test, so that analytical results are in a good agreement with the observations. However, though relatively small fatigue damage was estimated at HAZ in Mod.9Cr-1Mo steel, deep fatigue cracks were observed in the test. To identify the cause of such a discrepancy between the test and calculations, we performed a series of finite element analyses. Some metallurgical investigations were also performed.

Effect of Maximum Temperature on the Thermal Fatigue Behavior of Superalloy GH536

Thermal fatigue tests of superalloy GH536 were carried out at different maximum temperature. Three-dimensional numerical finite element computations were performed to simulate thermal fatigue test process. The crack initiation, propagation and thermal fatigue failure mechanism of GH536 plate at different maximum temperatures were obtained by experiments and numerical methods. Result shows that the crack initiation life is shortened and the crack growth rate is accelerated with the increase of the maximum temperature of thermal fatigue test. The numbers of appearing 1 mm length cracks are 180, 74 and 37, respectively, when the maximum temperature is 800°C, 850°C and 900°C respectively. So the thermal fatigue performance decreases with the increase of the maximum temperature. But in the thermal fatigue tests of different maximum temperature, the thermal fatigue crack initiation is all caused by a single crack initiation source, and the thermal fatigue cracks initiate transgranularly, develop and propagate intergranularly.