Ball Grid Array Assemblies Research Articles

Assessment of board level solder joint reliability for PBGA assemblies with lead‐free solders

Evaluates the board level reliability of plastic ball grid array (PBGA) assemblies under thermal and mechanical loading, with the objective of characterizing the reliability of lead‐free solder joints for various assembly conditions. A five‐leg experiment was designed which included various combinations of solder materials and peak reflow temperatures. It was found that the lead‐free solder joints have a much longer thermal fatigue life than the 63Sn–37Pb solder. The 63Sn–37Pb solder joints seem to perform slightly better than the lead‐free solder under mechanical loading.

Finite-element analysis of a PBGA assembly under isothermal/mechanical twisting loading

In order to reduce the reliability testing time for new electronic products, a new highly accelerated mechanical-thermal (HAMT) test system has been developed. In this system, a cyclic out-of-plane deformation is imposed on a printed circuit board (PCB) assembly by twisting the PCB mechanically inside a thermal chamber. The combined effects of the mechanical deformation and the coefficient of thermal expansion (CTE) mismatch of different materials accelerate the failure of solder joints, so as to reduce the reliability testing time of the PCB assembly. To verify the test method, HAMT tests of plastic ball grid array (PBGA) assemblies were carried out in an isothermal environment. Finite-element (FE) simulations were conducted to predict the reliability of the PBGA assemblies under the isothermal/mechanical twisting (ITMT) loading condition. In this paper, the simulation results are presented. The failure locations in the PBGA solder joints under the ITMT loading condition are investigated, and the fatigue life of the interconnections is predicted using Darveaux's crack initiation and growth model. The simulation results exhibit a good correlation with the test results.

Quick assessment methodology for reliability of solder joints in ball grid array (BGA) assembly—Part II: Reliability experiment and numerical simulation

In the present study, a facility, i.e., a mechanical deflection system (MDS), was established and applied to assess the long-term reliability of the solder joints in plastic ball grid array (BGA) assembly. It was found that the MDS not only quickly assesses the long-term reliability of solder joints within days, but can also mimic similar failure mechanisms in accelerated thermal cycling (ATC) tests. Based on the MDS and ATC reliability experiments, the acceleration factors (AF) were obtained for different reliability testing conditions. Furthermore, by using the creep constitutive relation and fatigue life model developed in part I, a numerical approach was established for the purpose of virtual life prediction of solder joints. The simulation results were found to be in good agreement with the test results from the MDS. As a result, a new reliability assessment methodology was established as an alternative to ATC for the evaluation of long-term reliability of plastic BGA assembly.

Reliability Modeling for Ball Grid Array Assembly With a Large Number of Warpage Affected Solder Joints

A reliability model is established to study thermal fatigue behavior of solder joints in plastic ball grid array (PBGA) assemblies. The model is able to simulate a configuration with a large number of warpage affected solder joints. For efficient computation, regression models are used to calculate the force acting on each solder joint to determine its height under different warpage conditions. With the height and specified solder parameters, the shapes of selected solder joints are calculated using the Surface Evolver. In addition, the displacements of these solder joints can be determined by a macro model using equivalent beams to represent hundreds of solder joints. With the shapes and displacements, three-dimensional micro models for the selected joints are established to compute strain energy densities during temperature cycling. The energy densities can be used to estimate fatigue lives through an empirical correlation. Two PBGA assemblies with 72-I/O cavity-up and 540-I/O cavity-down packages are studied using the reliability model. Silicon chip size and substrate thickness are critical to solder fatigue in the cavity-up assembly. Their effects are reduced substantially for the cavity-down assembly, which is more reliable due to small global thermal mismatch. However, its reliability is strongly affected by the warpage. The warpage changes the shapes of solder joints and can reduce the corner joint’s fatigue life from 20,000 to 7800 temperature cycles for an arch-type warpage of 0.28 mm across a 42.5 mm×42.5 mm region.

Effective Local Flexural Stiffness of Ball Grid Array Assemblies

Parts on the circuit board serve as reinforcements and produce local stiffness, which influences the deflection of the assembly under vibration. The curvature of the circuit board, combined with material properties, produces stresses that lead to the high-cycle fatigue failure of interconnecting soldering joints. Use of a conventional finite element method (FEM)—referred to as “h-method”—for circuit board analysis is cost prohibitive, as numerous parts, each containing many soldering joints, would need to be analyzed for a typical board. Instead, a direct-stress analysis method—referred to as the multi-domain method (MDM)—can be used to calculate effective local stiffness of ball grid array assemblies. The fast and accurate MDM is based on nested multi-field displacement superposition and is similar in concept to p-type FEM. It is similar to conventional FEM only in its use of the Rayleigh-Ritz methodology. The computational advantages of MDM over conventional FEM for computing thermal stresses caused by thermal coefficient mismatch have been documented previously. In present work, the use of MDM as a direct-stress analysis method to extract the effective local stiffness of ball grid-array assemblies for determining high-cycle fatigue life has been extended. This method simulates a three-point bend test for flexural stiffness calculation. It demonstrates that the force-deflection relationship at the center of the system can be accurately achieved with proper constraints at the ends. The flexural stiffness is then calculated on the basis of beam theory. This calculation produces numerical results for various part-board connections, both with and without underfill. The accuracy of the formulation is examined for layered assembly. The results for long-layered beam theory agree with those based on layered beam theory.

Quick assessment methodology for reliability of solder joints in ball grid array (BGA) assembly—Part I: Creep constitutive relation and fatigue model

In this study, a new unified creep constitutive relation and a modified energy-based fatigue model have been established respectively to describe the creep flow and predict the fatigue life of Sn−Pb solders. It is found that the relation successfully elucidates the creep mechanism related to current constitutive relations. The model can be used to describe the temperature and frequency dependent low cycle fatigue behavior of the solder. The relation and the model are further employed in part II to develop the numerical simulation approach for the long-term reliability assessment of the plastic ball grid array (BGA) assembly.

Reliability of PBGA assemblies under out-of-plane vibration excitations

Vibration testing of plastic ball grid array (PBGA) assemblies to assess the reliability of the solder joints under external vibration excitations was reported in this paper. The test vehicle was an assembly with four 256 I/Os PBGA modules assembled on a printed circuit board (PCB). During the test, the assembly was clamped at two opposite sides to a vibration shaker. It was found that the dynamic displacement of the assembly under external out-of-plane sinusoidal vibration excitations was highly nonlinear. In order to determine the reliability of the solder joints, the resistance of each module was continuously monitored during the test. Under an out-of-plane sweep sinusoidal excitation with a constant acceleration amplitude of 2.5 G within a narrow frequency band around the fundamental resonant frequency of the assembly, the first-time-to-failure and mean-time-to-failure of the solder joints were found to be 0.46 and 6.19 million vibration cycles respectively. The failed solder joints were at the corners of the PBGA module. Most failures were due to cracks near the copper pad on the PCB side.

Three-dimensional x-ray laminography as a tool for detection and characterization of BGA package defects

Non-destructive examination of the layers of a built-up substrate was achieved using three-dimensional (3-D) x-ray microlaminography, with successful separation of layers as thin as 8 /spl mu/m. The same technology was used to create reconstructed images of both surface and internal details of inner solder balls in a ball grid array package soldered to a printed circuit board. Microlaminography was also used to identify bond-wire shorts in the plane of the solder resist of a ball grid array assembly, and these were subsequently verified by destructive physical analysis. This plane is 20 /spl mu/m thick and immediately adjoining a plane of copper traces; the success demonstrates the capability of microlaminography to resolve and separate very fine detail internally within IC structures. The limits of capability of this machine were also determined; it was found that a crack of approximately 5 /spl mu/m wide in a copper trace of a BGA was not detected by the machine. As an introduction, the technology and methodology of 3-D x-ray microlaminography are explained. The results from a microlaminography system adapted for failure analysis in integrated circuit packaging are presented. It is shown that such results could not be extracted by two-dimensional (2-D) x-ray or other nondestructive methods.

The effects of in-plane orthotropic properties in a multi-chip ball grid array assembly

It has long been recognized that the properties of printed circuit boards and BGA laminates in the out-of-substrate direction differ greatly from the properties in the plane of the substrate. In this paper the variation of in-substrate-plane properties is examined, both in mechanical testing and in finite element analysis. It is shown that the in-substrate-plane coefficient of thermal expansion and modulus of elasticity are influenced significantly by the direction of the traces in the copper layers of the substrate. The impact of these on warpage is explored for a BGA assembly. It is concluded that for multichip products it is necessary to consider the effects that differences in in-substrate-plane mechanical properties may have on the finished product.

Effect of thermal cycling on the solder joints' reliability of ceramic ball grid array

Solder joints are known as one of the weakest elements of microelectronics assemblies. The integrity of solder joints in most microelectronics assemblies is of concern in systems where reliability is of great importance since the stresses experienced by the joints during service can lead to premature failure. This is mainly because the solder joints often constraint materials of different coefficient of thermal expansion (CTE) that imposes cyclic strain (fatigue) through the joints when thermal fluctuations are encountered. In addition, the high homologous temperature at which solder operates at constant stress, makes creep predominant in the solder joints. Due to viscoplastic nature of plastic, large creep and plastic deformations accumulate in a solder joint during thermal cycling, which eventually lead to thermal fatigue failures [1, 2]. Therefore, such fatiguecreep interaction effect in microelectronic packages must be considered when evaluating the reliability of solder. The reliability assessment of solder joints under creep-fatigue effect can be carried out by thermal cycling (TC) test. TC imposes cyclic strains over the solder joints of electronic components. Made by surface mount technology (SMT), which may cause the joints to fail by thermo-mechanical fatigue [3]. Pb-Sn solder used in microelectronics is expected to function at relatively high homologous temperatures over a wide range of temperatures. This is especially true for space and avionics applications where the solder joints may be required to endure extreme thermal fluctuations of −62 ◦C and 140 ◦C which correspond to 0.46TM and 0.91TM respectively for eutectic Pb-Sn alloys. The focus of this project is to study the failure characteristics of solder joints in ceramic ball grid array (CBGA) assembly that have been subjected to thermal cycling. Metallographic sections and fracture surfaces of the failed solder joints were investigated using both optical and scanning electron microscopy (SEM). The most likely location to fail first was identified by calculation of the strains imposed upon each solder joint by the following formula:

361 FEM による溶融はんだの形状及び安定性の解析

In development of surface mount technology for Ball Grid Array (BGA) or flip-chip assemblies, it is important to reduce stress concentration in solder joints as it is immediately effective in improving the fatigue life of the assembly. Thus, the ability to predict and control the joint geometries is critical to obtaining robust and reliable designs of interconnects. Other than the issue of the bump shape, there are many problems concerning soldering such as bridging or self-alignment in which surface tension is definitely involved. This paper attempts to apply the FEM approach in solving these problems. Rigid-plastic FEM which is based on iteration for the velocity field in an incompressible viscous fluid is an approach to large deformation analysis. The flow stress is described by the viscosity of the fluid and the strain rate. By introducing an automatic mesh updating procedure, transient problems with free boundary can be treated. We applied this concept to the prediction of solder joint shapes. In this kind of problem, effects of surface tension dominate. Since surface tension is a distributed load that depends on surface curvature, we employed 2D and 3D methods in which the load is updated based on instantaneous state of surface. To verify the accuracy of this method, we analyzed some shape and stability problems of liquid drops for which theoretical solutions were given. Practical applications of the method were also performed for the 2D and 3D solder joint problems, and the results showed a good agreement with experimental ones.Copyright © 2002 by ASME

A study of factors affecting solder joint fatigue life of thermally enhanced ball grid array assemblies

In this study, a finite element study of design factors, including material constants and geometry parameters, affecting fatigue life of solder joints of a typical cavity‐down Thermally Enhanced Ball Grid Array (TEBGA) assembly subjected to an accelerated temperature cycling load is performed. In order to precisely characterize the fatigue life of solder joints, the geometry profile of solder joints within the package is substantially predicted using an energy‐based method‐the Surface Evolver (see, e.g., Brakke, 1994; Chiang et al., 2000). In addition, a finite‐volume‐weighted averaging technique is proposed to describe the strain/stress response of solder joints at material/geometry discontinuities. The analysis employs a plane strain FE model with the purpose of reducing the computational cost. To validate the FE modeling so as to corroborate the nonlinear and complex mechanical behaviors of materials in the assembly, a typical interferometric displacement measurement method— Moire interferometry (see, Han and Guo, 1995) and 3‐D solid modeling are used. Through the parametric design of the fatigue life of solder joints, the reliability characteristics of the TEBGA assembly are then effectively identified.

Aging studies of PBGA solder joints reflowed at different conveyor speeds

In this paper, the shear cycle fatigue properties of plastic ball grid array (PBGA) assemblies' solder joints reflowed with three different profiles, and aged at 125/spl deg/C for four, nine, 16, 25, and 36 days are studied. The profiles were devised to have the same heating factor, which was defined as the integral of the measured temperature above the liquidus (183/spl deg/C) with respect to dwell time in the reflow profile, but to have different conveyor speeds. The effects of conveyor speed on the solder joint (nonaged and aged) fatigue lifetimes were investigated. It was found hat with increasing the conveyor speed the solder joint shear fatigue lifetime could be improved substantially. Also, the shear fatigue lifetimes of aged solder joints decreased with increasing aging time and variation in fatigue lifetimes increased for faster conveyor speed. SEM and optical micrographs show that faster cooling rate caused a rougher interface of solder/IMC and less crystallization microstructure in solder joints. Rougher interface solder joints have a longer nonaged fatigue life. The thickness of IMC increases with increasing aging time and the growth rate for solder with faster cooling rate was larger. SEM cross section views reveal that cracks initiated at the acute position near the solder pad, then propagated along the interface of the bulk solder/IMC layer. Thicker IMC layers deteriorated fatigue life, so the fatigue lifetime variation of aged solder joints with fast cooling rate was larger.

Vibration reliability characterization of PBGA assemblies

Generally, the low-cycle fatigue induced by thermal cycling is the major concern in the reliability of surface mount technology for electronic packaging, but the high-cycle fatigue induced by vibration can also contribute significant effect, especially for applications in automobile, military, and avionic industries. To assess vibration induced fatigue failures, the dynamic properties of printed circuit board (PCB) assemblies play a very important role. In this paper, the dynamic properties of a plastic ball grid array (PBGA) assembly were characterized by using experimental modal testing and finite element analysis. The bare PCB and PCB assembly with PBGA modules mounted were tested and analyzed separately, so that the influence of PBGA modules on the PCB’s dynamic properties could be identified. It was found that mounting PBGA modules to PCB increased the stiffness of the PCB. Results of constant-amplitude vibration reliability testing of the PBGA assembly are also reported. It was found that the PBGA assembly was vulnerable to vibration, and fatigue failure always occurred at the corner solder balls of the PBGA module.

Accelerated Thermal Cycling and Failure Mechanisms for BGA and CSP Assemblies

This paper reviews the accelerated thermal cycling test methods that are currently used by industry to characterize the interconnect reliability of commercial-off-the-shelf (COTS) ball grid array (BGA) and chip scale package (CSP) assemblies. Acceleration induced failure mechanisms varied from conventional surface mount (SM) failures for CSPs. Examples of unrealistic life projections for other CSPs are also presented. The cumulative cycles-to-failure for ceramic BGA assemblies performed under different conditions, including plots of their two Weibull parameters, are presented. The results are for cycles in the range of −30°C to 100°C, −55°C to 100°C, and −55°C to 125°C. Failure mechanisms, as well as cycles to failure for thermal shock and thermal cycling conditions in the range of −55°C to 125°C, were compared. Projection to other temperature cycling ranges using a modified Coffin-Manson relationship is also presented. [S1043-7398(00)00104-3]

Impact properties of PBGA assemblies reflowed in nitrogen ambient and compressed air

In this paper, the solder joint brittleness of plastic ball grid array (PBGA) assemblies has been studied by means of impact testing. The results show that the maximum impact force and the impact energy for PBGA assemblies reflowed in nitrogen ambient is higher than for those reflowed in compressed air. Reduction of the oxygen content from 1000 ppm to 50 ppm, reduces the initiation energy of impact. The impact fractured surfaces occur between the printed circuit board (PCB) pads and solder joints, i.e., in the brittle intermetallics region. The pores and impurities are mainly distributed within a ring at perimeter of the interface, with greater density at the edges than at the center. Reflow in a nitrogen ambient reduces the defect density, with reduction in oxygen content further reducing the density. Impact cracks begin from the defects of the Cu/solder interface, and propagate along the intermetallics.

Reliability and failure analyses of thermally cycled ball grid array assemblies

The reliability of ball grid arrays (BGAs) was assessed as part of a consortium effort led by the Jet Propulsion Laboratory. Nearly 200 test vehicles, each with four packages, were assembled and tested using an experiment design. The most critical variables incorporated in for the experiment were package types, ceramic and plastic; board materials, FR-4 and polyimide; surface finishes, OSP, HASL, and Ni/Au; solder volumes, low, standard, and high; and environmental conditions. Thermal cycling test results to 3000 cycles for ceramic packages with 625 inputs/outputs (I/Os) and depopulated full array plastic packages with 313 I/Os were presented in a previous paper by the authors (see Proc 30th Int. Symp. Microelectron., Philadelphia, PA, Oct. 1997, p. 396-400). This paper presents the most current thermal cycling test results (>6000 cycles) for plastic OMPAC and SuperBGA packages with 352 I/Os and 560 I/Os. Failure mechanisms detected by electrical continuity interruptions as well as optical and scanning electron microscopy results are also compared for ceramic and plastic package assemblies.

CSP compatibility in the SMT assembly process

To achieve integration of chip scale package (CSP) devices into main stream surface mount technology (SMT) assembly, various experiments have been required. In process development, experiences learned from flip chip attach and ball grid array (BGA) assembly were utilized. Key process parameters for CSP assembly were defined and some of those key factors were optimized. They will be presented in this paper. Some observations during prototype build have been documented for correlation with reliability results in the future. The requirements for further CSP assembly studies will also be addressed in this paper.

The Effect of Solder Paste Volume and Reflow Ambient Atmosphere on Reliability of CBGA Assemblies

To investigate the effect of stencil thickness and reflow ambient atmosphere on the reliability of ceramic ball grid array (CBGA) assemblies, three levels of stencil thickness, 0.10, 0.15, and 0.20 mm, were used to print solder paste on printed circuit board (PCB). After the CBGA modules were placed on PCBs, the specimens were divided into two groups, and reflowed in nitrogen and compressed air separately. Properties of the six groups of assemblies, such as shear strength, bending fatigue life, thermal shock cycles, and vibration fatigue life, were tested to find out the optimum assembling process. The results show that assemblies prepared with a stencil 0.15 mm thick yield maximized performance. And the nitrogen ambient atmosphere demonstrates a remarkable effect on improving the fatigue life. Theoretical models are given to qualitatively explain the relationship between the solder joint volume and performance. This work provides a guideline on how to determine the soldering process parameters of CBGA assemblies.

Process development for ball grid array assembly using a design of experiments approach

The focus of this work was to develop a robust assembly process for low and high input/output (I/O) ball grid arrays (BGAs). The test vehicles considered had a wide variety of components including full and perimeter array BGAs, and ultrafine pitch peripheral leaded devices. Experimental design techniques were applied in order to identify, screen, and control the parameters that affect the stencil printing process. The factors that affect the process were screened using Taguchi-based methodology. These significant factors and their interactions were then further analysed using full factorial designs. The volume and the quality of the solder paste deposits were considered as the response variables. Response surface plots were developed to provide process windows for paste deposition. Process capability studies were performed to evaluate the capability of the solder paste deposition process. Boards were assembled using optimal settings obtained from the experiments. No process-induced defects were found, and 100% yield was obtained.