Njoku, Jude Ebem (2016) A study of the thermomechanical reliability of solder joints in surface mount electronics technology. PhD thesis, University of Greenwich.

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Abstract

Solder joints have been an integral part of any electronic assembly. They serve as both the electrical and mechanical connections between surface mount component and the substrate. This function is crucial in Surface Mount Technology (SMT) owing to its capability in supporting the realisation of high density, functionality and performance of electronic devices. With the increase in miniaturisation of electronic components, enabling the manufacture of high-density products, the mechanical reliability of small component solder joints has become critical. The criticality increases with operations at elevated temperature and harsher ambient conditions. Severe conditions include vibration and shock which under-the-bonnet automotive electronics experience during vehicle drive. The transactions occurring in this ambient accelerate the damage of solder joints, which causes early crack initiation that later, propagates across the joint leading to system's failure over continued operations.

This PhD research work studies and evaluates the thermo-mechanical reliability of lead-free solder joints in surface mount electronic components assembled on substrate Printed Circuit Boards (PCBs). In carrying out the research, activities and factors, which influence solder-joint thermo-mechanical reliability, have been investigated. The events and factors are soldering processes, ambient temperature, joint's architecture, solder material composition, solder-joint common defects and duration of device operation.

Two type of components used for the investigation were Ball Grid Array (BGA) and a chip resistor. The designed research studies used the techniques of the Design of Experiment (DoE) and Taguchi methods. After conducting the trial tests that served as control experiments, next was the formation of test vehicles with components assembled on PCBs using lead-free solder paste, and later subjected to different thermal loading conditions. Numerous mechanical tests were carried out using the assembled test vehicles to determine, quantify and evaluate the effects of the activities and factors on the degradation of the solder joints. Fractured solder joint surfaces, which resulted after the shear test, were inspected and analysed for brittle and ductile mode of failure. The examination and analysis of the microstructure were done using the Scanning Electron Microscopy (SEM). The Coffin-Mansion equation helped in calculating the accelerated factor of solder joint degradation at field service conditions. The shear strengths of the joints were evaluated to determine the thermo-mechanical reliability of the solder joints.

The findings of this investigation are significant, and from the observations, the shear strengths of solder joints depend on the stages and values of the reflow parameter settings. A combination of high preheat, and low peak temperature produces joints with high shear strengths. Further studies on reflow parameters show that these two factors have a significant main effect on the integrity of solder joints. Also, results found show that elevated temperature operations changed the microstructure and morphology of the solder joints. Change from fine to coarse microstructure resulted in a decrease in shear strength of the joints. The joints are found to fail predominantly by brittle fracture occurring mostly at the boundary between Intermetallic Compound (IMC) layer and the solder bulk. From the results, the standoff height of a solder joint is adjustable as desired by a controlled variation of the bond pad diameter on the PCB. The standoff height of a solder joint is found to significantly impact on the bond structural integrity such that the lower the standoff height, the greater the shear strength of the joint. More results demonstrate that paste type, activation temperature used in reflow soldering process and the pad surface finish on the substrate PCB play a substantial role in determining the percentage of voids in solder joints. Besides, the results show that for minimum voiding in lead-free solder joints of Ball Grid Array, the paste type 97 may be used instead of type 96; an activation temperature of 200 °C should be utilised instead of 190 °C, and a Ni surface finish would be better than Cu surface finish. Other results establish that the magnitude of degradation of solder joints in electronic assemblies is linearly dependent on the duration of the device operations in the field. The cause of the degradation is found to be a change in the solder microstructure and the formation of CSH as well as the growth of brittle IMC layer in the joint.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Surface mount technology (SMT); microelectronics; solder joints technology; thermomechanical reliability; electronic manufacturing;
Subjects:	T Technology > TJ Mechanical engineering and machinery T Technology > TK Electrical engineering. Electronics Nuclear engineering
Faculty / School / Research Centre / Research Group:	Faculty of Engineering & Science Faculty of Engineering & Science > School of Engineering (ENG)
Last Modified:	24 Apr 2019 16:32
URI:	http://gala.gre.ac.uk/id/eprint/23703

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