Surface Mount Technology(SMT) is a relatively new technology which has been used by manufacturers to attach electronic components to a printed circuit board. Previous research has shown that the reliability of the SMT product is influenced by the integrity and the geometry of the solder joint. The problem of predicting the shapes of solder joints as a function of the manufacturing process parameters has received less attention in the literature. The reported research has been devoted mainly to problems involving the formation of joints with simple boundary condition geometry for which analytical models and solution methods are available.
This thesis discusses the development and application of a numerical method for predicting the equilibrium shapes of solder joints, including the leadless, gullwing and J lead, which are formed during a reflow process. A rigorous formulation based on the variational methodologies is derived in 2-D integral form so that the standard numerical techniques for plane problems can be applied. In order to transfer the volume integral to area integral leadless model is adopted by height function. Geometric complexity stemming from the inclined plane of the gullwing lead is resolved by employing three element types. These element types are used to describe the joint profile formed on the vertical, inclined and interfacial planes. The curved shape of J lead is modeled by employing a transformed coordinate and corresponding element types. The unknown nodal coordinates can be determined by deriving and then solving the corresponding finite element equations. These equations, which contain an unknown Lagrangian multiplier arising from the volume constraints, are nonlinear and require iterative numerical solution.
During the soldering process, the molten solder has been observed to move upward and solidify along the gullwing lead, which is called the wicking phenomenon. In this thesis, possible causes of the wicking are investigated, and its effects on the solder joint profile are quantitatively estimated by introducing the wicking constant. The free energy reduction by intermetallics formation between the copper and tin seems to be the major source of wicking action. Numerical examples representing the leadless, gullwing and J leads are tested for validation of the developed simulator and illustration of its overall capability. The calculated results show reasonably good agreements with the experimental data when the wicking effects are considered. Effects of the pad length, inclined lead angle and solder volume on joint profiles are also investigated.
Finally, the CAD tools were developed for the determination of the pad and metal mask dimensions in design phase.
