The Influence of Design Parameters on Solder Joint Reliability in Electronic Packages PDF Download

Author: Aylin Yenilmez
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Languages : en
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Book Description
A typical electronic package generally consists of a die (Integrated Circuit chip), die attach, substrate and moulding compound. The major functions of an electronic package are: to provide a path for the electrical current that powers the circuits on the Integrated Circuit (IC) chip, to distribute the signals onto and off of the IC chip, to remove the heat generated by the circuits and to support and protect the IC chip from environmental hazards. Power distribution involves the distribution and conditioning of the electrical current necessary for the ICs to function. Signal distribution involves creating electrical connections between various components in a module and providing interfaces to the next level of assembly. Thermal management is necessary to remove heat generated by the electronic components so that they stay within an allowable temperature range. Circuit protection involves mechanical support and protection from physical damage as well as protection from environmental hazards such as moisture, contaminants or ionising radiation.There are many electronic packaging technologies that have facilitated Printed Circuit Board (PCB) assembly choices that have advanced packaging developments, e.g. solder-bumped flip-chip technology, solder Ball Grid Array (BGA)technology and solder Chip-Scale Packaging (CSP) technology. These are allSurface Mount Technology (SMT) assemblies. There are also many kinds of BGAsdepending on their substrates. These are ceramic BGA (CBGA), tape-automatedbonding BGA (TBGA), plastic BGA (PBGA), metal BGA (MBGA), and dimple BGA(DBGA), etc.For these electronic packaging the solder joint is the only mechanical and electrical way of attaching them to the PCB. Because of this, solder joint reliability is one of the most important issues in electronic packaging and interconnect systems.Solder alloys are used to bond dissimilar materials that have different thermal expansion coefficients. Once the structure is bonded together, the components are subjected to cyclic thermal stresses due temperature changes during operation. These stresses arise from mismatch in thermal expansion coefficients. Because the solder is above half of its melting point at room temperature, it presents a non-linear creep (viscoplastic) response.The actual mechanism by which a solder joint fails is due to crack initiation and propagation through a joint. The location and nature of the cracks depend on the joint configuration, intermetallic structure, strain, strain rate and thermal loading. Based on extensive testing in electronics industry, the number of cycles to solder joint is usually predicted based on the volume weighted average plastic work density in conjunction with empirical constants as part of a life prediction model.This study concerns the determination of design parameters with the largest impact on the solder joint life. The design parameters consist of the amount of the solder volume, die thickness, die size, pad thickness, pad size, mould compound, mould size and substrate thickness. Functional relationships between the average plastic work and these design parameters are established.This is achieved by considering three different package types provided by the companies in the electronics industry .The material properties, methodology andboundary conditions are consistent in each package analysis. The analysis isconducted by constructing three dimensional non-linear finite element models of the package assemblies. The solder material is modelled as a viscoplastic solid, the printed circuit board as orthotropic linear elastic solid and the rest of the materials as linear elastic solids. In each calculation, thermal cycles are simulated in order to establish a stable stress-strain hysteresis loop. These packages are subjected to a specified temperature cycle. In the finite element analysis of each package, a non-linear global model with a relatively coarse mesh for the substrate, printed circuit board and the solder balls provides the critical joint for the subsequent non-linear sub modelling of the critical solder joint. The critical joint for sub modelling is identified based on the amount of inelastic (plastic) work density at the end of the last cycle. The sub modelling permits refinement of the mesh. The displacement boundary conditions are determined from the solution of the global analysis through the use of cut boundary interpolation method. The number of cycles to crack initiation and the crack growth rate per cycle are both correlated with plastic work density. Using the crack initiation, growth constants and characteristic crack length, the number of cycles to solder joint failure is calculated. The empirical constants used in the life prediction model are well accepted in industry.