Graduation Semester and Year

2015

Language

English

Document Type

Thesis

Degree Name

Master of Science in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Agonafer Dereje

Abstract

The emphasis of this study is to find out how the solder joints of the BGA package are affected with change in the stack up PCB layer copper thickness and the prepeg layer stiffness. In general the effect of the PCB layer thickness is to increase the stress and strain range and to decrease the thermal fatigue life. Basically a PCB consists of different layers with different material properties (Eg: Cu, FR4 etc.).In this paper we deal with the PCB with 1-6-1 configuration (6 core layers and 1 prepeg layer on the top and bottom). A Prepeg is a fiber glass impregnated with resin. The resin is pre-dried, but not hardened, so that when it is heated, it flows, sticks, and is completely immersed. The different layers with different material properties inside the PCB make the PCB highly orthotropic. Additionally, prepreg materials are visco-elastic and they provide some sort of stress relaxation and creep characteristics. Now when the PCB/package assembly is subjected to accelerated thermal cycling tests to assess the reliability, induced stresses are relaxed at a rate which is equivalent to the creep/stress relaxation rate. The PCB is characterized experimentally. The CTE is calculated using the Digital Image Co relation (DIC) Technique and the young's modulus of elasticity of the PCB is determined from tension test using INSTRON micro Tester. A 3D model of the whole package is formulated in ANSYS 15.0 and static structural analysis is carried out to determine the accumulated volume averaging plastic work. The fatigue co relation parameter is determined by using energy based and strain based models. The thickness of the Cu layers is increased and decreased by 50% and Prepreg layer stiffness in increased and decreased by 50% and the comparative study is done to predict the fatigue life of the package. The main purpose of the study is to understand the root cause for the solder joint failure and to obtain better methodology for a reliable design.

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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