Atila Mertol

Application of the Taguchi method on the robust design of molded 225 plastic ball grid array packages

A three-dimensional nonlinear finite element model of a molded plastic ball grid array (PBGA) has been developed using the ANSYS/sup TM/ finite element simulation code. The model has been used to optimize the package for robust design and to determine design rules to keep package warpage within acceptable limits. An L/sub 18/ Taguchi matrix has been developed to investigate the effect of die attach and mold compound properties along with the substrate, mold compound and die attach thicknesses on the potential defects that would possibly occur during temperature cycling. For package failures, simulations performed represent temperature cycling 165/spl deg/C to -65/spl deg/C. This condition is approximated by cooling the package mounted on a multilayer printed circuit board (PCB) from 165/spl deg/C to -65/spl deg/C. For coplanarity analysis, simulations have been performed without the PCB and the lowest temperature of the cycle is changed to 20/spl deg/C. Predicted results indicate that for an optimum design (low stress in the package and low package warpage) mold compound as well as die attach material should have low Youngs modulus and low coefficient of thermal expansion. Furthermore, it is found that the overmold should be kept as thin as possible. In addition to the optimization analysis, plastic strain distribution on each solder ball has been determined to predict the location of the probable first solder ball failure. The results indicate that higher strain levels are obtained in solder balls located one pitch inwards from the edge of the die. >

Application of the Taguchi method to chip scale package (CSP) design

A three-dimensional (3-D) nonlinear finite element model of an overmolded chip scale package (CSP) on flex-tape carrier has been developed by using ANSYS/sup TM/ finite element simulation code. The model has been used to optimize the package for robust design and to determine design rules to keep package warpage within acceptable Joint Electron Device Engineering Council (JEDEC) limits. An L/sub 18/ Taguchi matrix has been developed to investigate the effect of die thickness and die size, mold compound material and thickness, flex-tape thickness, die attach epoxy and copper trace thicknesses, and solder bail collapsed stand-off height on the reliability of the package during temperature cycling. For package failures, simulations performed represent temperature cycling 125/spl deg/C to -40/spl deg/C. This condition is approximated by cooling the package which is mounted on a multilayer printed circuit board (PCB) from 125/spl deg/C to -40/spl deg/C. For solder ball coplanarity analysis, simulations have been performed without the PCB and the lowest temperature of the cycle is changed to 25/spl deg/C. Predicted results indicate that for an optimum design, that is low stress in the package and low package warpage, the package should have smaller die with thicker overmold. In addition to the optimization analysis, plastic strain distribution on each solder ball has been determined to predict the location of solder ball with the highest strain level. The results indicate that the highest strain levels are attained in solder balls located at the edge of the die. The strain levels could then be used to predict the fatigue life of individual solder balls.

Estimation of aluminum and gold bond wire fusing current and fusing time

Wirebonding is the common method for electrically connecting the semiconductor device to the external leads or pins. Permanent damage to wirebonds would occur if the wires could not dissipate the energy, due to excessive current, delivered through the wires. This current overstress damage is recognized as fused bond wires. In this paper, the simplified model developed by Loh (1983) has been used to predict the fusing time and the fusing current of aluminum and gold wires as functions of wire length and wire diameter. In addition, the equation obtained by Loh has been put into more convenient form by eliminating the dependence on the bond wire diameter. Results from both approaches have been presented in graphical form so that package design engineers would be able to estimate the fusing current for a given fusing time. Conversely, the fusing time for a given current passing through the wires could be easily obtained from the design graphs. >

Optimization of high pin count cavity-up enhanced plastic ball grid array (EPBGA) packages for robust design

Three-dimensional (3-D) nonlinear finite element models of epoxy encapsulated enhanced plastic ball grid array (EPBGA) packages with and without an aluminum lid have been developed using ANSYS finite element simulation code. The model has been used to optimize the packages for robust design and to determine design rules to keep package warpage within acceptable limits. An L/sub 18/ Taguchi matrix has been developed to investigate the effect of die attach and encapsulant properties along with the substrate, encapsulant, die attach, and internal copper plane thicknesses on the reliability of the package during temperature cycling. For package failures, simulations performed represent temperature cycling from 165/spl deg/C to -65/spl deg/C. This condition is approximated by cooling the package mounted on a multilayer printed circuit board (PCB) from 165/spl deg/C to -65/spl deg/C. For coplanarity analysis, simulations have been performed without the PCB and the lowest temperature of the cycle is changed to 20/spl deg/C. Predicted results indicate that for an optimum design, that is low stress in the package and low package warpage, encapsulant as well as die attach material should have low Youngs modulus and low coefficient of thermal expansion. Furthermore, it is found that the substrate and the die attach epoxy thicknesses should be increased beyond the current design. In addition to the optimization analysis, plastic strain distribution on each solder ball has been determined to predict the location of the possible first solder ball failure.

Effect of heat slug and die attach material properties on plastic pin grid array (PPGA) package stress

A three-dimensional finite element model of a plastic pin grid array (PPGA) with an internal heat slug has been developed by using the ANSYS finite element simulation code. The model has been used to study the effects of heat slug as well as die attach material properties on the thermomechanical stresses developed within an electronic package during temperature cycle. Parametric studies have been performed by changing the Youngs modulus and the coefficient of thermal expansion for the die attach and heat slug materials to create a database for the predicted von Mises stresses and the package warpage. This database is then used to obtain empirical correlations for the stress and the package warpage as a function of the Youngs modulus and the coefficient of thermal expansion of the heat slug and die attach materials. These stress correlations would be useful to design engineers in selecting the die attach and heat slug materials to reduce the stresses on the critical devices. In addition to the finite element simulation, the analytical solutions for two elastic layers joined by one bonded joint are also used to predict the maximum shear stress for the joint made of die, die attach, and heat slug when the Youngs modulus and the coefficient of thermal expansion of one of the layers are independently changed. >