Nonlinear analysis of plastic ball grid array solder joints

A nonlinear finite element analysis was carried out to investigate the viscoplastic deformation of solder joints in a ball grid array (BGA) package under temperature cycling. The effects of constraint on printed circuit board (PCB) and stiffness of substrate on the deformation behavior of the solder joints were also studied. A relative damage stress was adopted to analyze the potential failure sites in the solder joints. The results indicated that high inelastic strain and strain energy density were developed in the joints close to the package center. On the other hand, high constraint and high relative damage stress were associated with the joint closest to the edge of the silicon chip which was regarded as the most susceptible failure site if cavitation instability is the dominant failure mechanism. Increasing the external constraint on PCB causes a slight increase in stress triaxiality and relative damage stress in the joint closest to the edge of the silicon die. The relative damage stress is not sensitive to the Young's modulus of the substrate.     

Thermomigration and electromigration in Sn58Bi ball grid array solder joints

In the present study, individual effect of thermomigration (TM) and combined effects of TM and electromigration (EM) in Sn58Bi ball grid array (BGA) solder joints were investigated using a particular designed daisy chain supplied with 2.5 A direct current (DC) at 110 °C. Driven by the electric current, Bi atoms migrated towards the anode side and formed a Bi-rich layer therein. With a thermal gradient, Bi atoms tended to accumulated at the low temperature side. When the effects of TM and EM were in same direction, TM assisted EM in the migration of Bi, otherwise it counteracted the effect of EM. The effect of electron charge swirling were detected when the electric current passed by the Cu trace on the top of the solder bump instead of entering into it. For the joint without current passing by or passing through, only TM induced the migration of the Bi atoms.     

Wavelet-based approach for ball grid array (BGA) substrate conduct paths inspection

The aim was to detect boundary defects such as open, short, mousebite and spur on ball grid array (BGA) substrate conduct paths using machine vision. The 2-D boundaries of BGA substrate conduct paths are initially represented by the 1-D tangent curve. The tangent angles were evaluated from the eigenvector of a covariance matrix constructed by the boundary coordinates over a small boundary segment. Since defective regions of boundaries result in irregular tangent variations, the wavelet transform was used to decompose the 1-D tangent curve and capture the irregular angle variations. A boundary defect can then be easily located by evaluating the wavelet coefficients of the 1-D tangent curve in its high-pass decomposition. The proposed method is invariant with respect to the rotation of the BGA substrates and does not require prestored templates for matching. Real BGA substrates with various boundary defects were used as test samples to evaluate the performance of the proposed method. Experimental results show that the proposed method achieves 100% correct identification for BGA substrate boundary defects by selecting appropriate wavelet basis and decomposition level.     

激光加热条件下 SnPb共晶钎料与Au/Ni/Cu焊盘界面反应动力学

激光重熔在电子封装领域中SnPb共晶钎料凸点制作方面存在极大的优势。采用扫描电子显微镜（SEM）分析了激光加热条件下SnPb共晶钎料与Au/Ni/Cu焊盘之间的界面反应，探讨了钎料中的溶解与扩散动力学。结果表明：CnPb共晶钎料在激光加热瞬间与Au/Ni/Cu焊盘中的Au发生反应，生成Au-Sn金属间化合物，其形貌和分布与激光输入能量密切相关；随着激光输入能量的增加，Au-Su化合物由边境连续层状转变为针状，最后以细小颗粒弥散分布在钎料内部。     

Plastic ball grid array (PBGA) overview

As integrated circuit functionality and clock speed continue to rise, innovative packaging approaches are in great demand. Recently, the plastic ball grid array (PBGA) technology has been gaining industry-wide interest and commitment as the potentially lowest-cost package for high-I/O devices and even for some lower-pincount applications. Drivers include the density advantages of an area array, quickly achieving six-sigma assembly yields with existing assembly equipment, the potential for excellent electrical and thermal performance, along with the traditional low cost of plastic packages. Because some perceived weaknesses are being eradicated, worldwide evaluation of the PBGA has accelerated. Although various aspects of this technology are discussed frequently, an overall assessment is still under development. In this paper, a systematic and comprehensive evaluation of PBGA technology will be described to identify (1) its technical advantages and limitations, (2) unique application areas where PBGA is the package of choice, and (3) current major hurdles for acceptance of PBGA and possible approaches to overcome these problems. The PBGA will be compared with PQFP, CQFP and CBGA in terms of package characteristics and their impact on system assembly. The characteristics include package attributes (i.e., package size, I/O counts and lead pitch), performance (i.e., electrical, thermal) and reliability (moisture). At the system level, solder joint fatigue, board routing, solder assembly yield, solder reparability and board delay are key metrics. The cost implication of various package families will be discussed. By analogy with SMT, the infrastructure for PBGA will take time to develop. The key elements and the current status of this infrastructure will be discussed.     

Interfacial reactions of Sn-3.5% Ag and Sn-3.5% Ag-0.5% Cu solder with electroless Ni/Au metallization during multiple reflow cycles

The increasing industry awareness of lead-free activities has prompted original equipment manufacturers and suppliers to investigate lead-free solder systems in detail. The reliability of lead-free solders has been studied a lot recently, but the knowledge of it is still incomplete and many issues related to them are under heavy debate. In this study, the interfacial reactions of Sn-3.5Ag and Sn-3.5Ag-0.5Cu (wt.%) solders with Cu/Ni(P)/Au ball grid array (BGA) pad metallization were systematically investigated after multiple reflows. The peak reflow temperature was fixed at 260°C. It was found that relatively high consumption of Ni(P) was observed in the case of Sn-3.5%Ag solder alloys during multiple reflow cycles. A white layer of P rich Ni-Sn compound was observed above the dark Ni₃P layer for Sn-3.5%Ag solder after several reflows. It was noticed that the mean thickness of the intermetallics and the dark P-rich Ni layer at the interface was decreased just by adding 0.5% Cu in Sn-3.5%Ag solder alloy with less overall interfacial reaction at the solder joint.     

Stability of AuSn eutectic solder cap on Au socket during reflow

The stability of Au20Sn eutectic cap formed during reflow of Sn/Au bump was studied under multiple reflow cycles. For a 5 μm thick Sn layer, the eutectic caps formed for the bumps of 60 and 40 μm in diameters were quite stable up to 10 reflows and the ζ compound layer at the interface was a good barrier layer to prevent the exhaustion of the eutectic solders. However for 20 μm bumps, the relative larger eutectic volume resulted in side wall wetting. Therefore only small part of eutectic alloy was left on the top of the Au socket after one reflow. After three reflows Au socket was almost covered by a single ζ compound layer. The results identified that the maximum cap height formed during reflow should be smaller than half of the bump diameter to maintain it on the Au socket.     

Ball grid array (BGA) substrate conduct paths inspection using two-dimensional wavelet transform

The objective of this paper is to implement two-dimensional wavelet transform (2-D WT) in the detection of mousebite, spur, open, and short defect candidates on ball grid array (BGA) substrate conduct paths. Once the defect candidates are located, traditional BGA substrate inspection algorithms can further detect true defects among these suspicious defects. Therefore, the scope and effort during the inspection stage can be significantly reduced. The binary BGA substrate image is processed that shows only conduct path boundaries, which are decomposed directly by 2-D WT. Then, the inter-scale ratio from the wavelet transform modulus sum (WTMS) across adjacent decomposition levels for the edge pixels on BGA substrate conduct path boundaries is calculated. Since irregular edges in a small domain can preserve much more wavelet energy, an edge pixel is considered as an abnormal one or a defect candidate if its inter-scale ratio is less than a predefined threshold. The proposed approach is template-free and easy to implement, so it is suitable for small batch production. Real BGA substrates with synthetic boundary defects are used as testing samples to evaluate the performance of the proposed approach. Experimental results show that the proposed method is able to capture all the true mousebite, spur, open, and short defects without any missing errors by appropriate wavelet basis, decomposition level, and image resolution.     

Effects of nano-Al2O3 particles on microstructure and mechanical properties of Sn3.5Ag0.5Cu composite solder ball grid array joints on Sn/Cu pads

A Sn3.5Ag0.5Cu (SAC)–XAl₂O₃ nano-composite solder was prepared by adding 100 nm Al₂O₃ to SAC (wt.%) solder. The interfacial microstructures and mechanical properties of SAC–XAl₂O₃ nano-composite solder balls on immersion Sn surface finished BGA joints after multiple reflows was investigated. As a whole, adding Al₂O₃ nanoparticles to SAC solders significantly changed in the interfacial microstructure, and both scallop-type and prism-type modes were observed in the plain SAC solder and SAC–XAl₂O₃ nano-composite solder after reflowing, respectively. The nanoparticles suppressed the growth of the Cu₆Sn₅ layer, significantly improving the shear strength. The fracture surfaces of the plain SAC solder showed a semi-brittle fracture mode, but those of the SAC–XAl₂O₃ nano-composite solder exhibited typical ductile failures.     

Three-dimensional (3D) visualization of reflow porosity and modeling of deformation in Pb-free solder joints

The volume, size, and dispersion of porosity in solder joints are known to affect mechanical performance and reliability. Most of the techniques used to characterize the three-dimensional (3D) nature of these defects are destructive. With the enhancements in high resolution computed tomography (CT), the detection limits of intrinsic microstructures have been significantly improved. Furthermore, the 3D microstructure of the material can be used in finite element models to understand their effect on microscopic deformation. In this paper we describe a technique utilizing high resolution (< 1 µm) X-ray tomography for the three-dimensional (3D) visualization of pores in Sn-3.9Ag-0.7Cu/Cu joints. The characteristics of reflow porosity, including volume fraction and distribution, were investigated for two reflow profiles. The size and distribution of porosity size were visualized in 3D for four different solder joints. In addition, the 3D virtual microstructure was incorporated into a finite element model to quantify the effect of voids on the lap shear behavior of a solder joint. The presence, size, and location of voids significantly increased the severity of strain localization at the solder/copper interface.     

Interfacial reaction and elemental redistribution in Sn3.0Ag0.5Cu-xPd/immersion Au/electroless Ni solder joints after aging

Sn3.0Ag0.5Cu solder doped with 0, 100, and 500 ppm Pd was reflowed with electroless Ni/immersion Au substrate. As Pd concentration increased in the solder, formation and growth of (Cu,Ni)₆Sn₅ were suppressed. After thermal aging, Cu₄Ni₂Sn₅ and Cu₅NiSn₅ were observed at interface of Sn3.0Ag0.5Cu-xPd/Au/Ni systems. As compared to Cu₄Ni₂Sn₅, more Pd dissolved in Cu₅NiSn₅. In addition, Pd doping enhanced the growth of Cu₄Ni₂Sn₅ and slowed the formation of Cu₅NiSn₅, which would stabilize the intermetallic compound. Based on quantitative analysis by field emission electron probe microanalyzer, the correlation between Pd doping and elemental redistribution in solder joints was probed and discussed. This study described a possible mechanism of the formation of different intermetallic compounds in Pd-doped lead-free solder.     

Tin whiskers growth of SnAgIn solder on Kovar substrate with Au/Ni plating

In this research, the interactions of SnInAg solder on Kovar leadframes (Fe29Ni17Co) with Au/Ni UMB was studied, to reveal the whiskers growth mechanism. Samples were prepared by reflow process and aging, with SnInAg solder paste on the substrate. Scanning electron microscope was used to observe the morphology and the cross-sectioned structure of the whiskers. Focused ion-beam was used for the preparation of the transmission electron microscope (TEM) samples, and energy dispersive X-ray and TEM were then used to distinguish the different intermetallic compound (IMC) phases. It was found that, a special solder thickness range would be needed for the growth of whiskers. The root cause of this strip whiskers growth region was studied. It is worthy to mention that, the Sn–Cu reactions, which was reported by many former researches, was not the main source of the compressive stress here. It was found that the Au–Sn reactions, the IMCs transformation during the aging process, and the oxidation layer growth were dominating the whiskers growth. Effects of cracks on the whiskers growth were carefully observed and discussed, and it was found that cracks under a certain width could exempt the breaking of the oxidation layer at the solder surface, thus help the whiskers growth. On the other hand, wide cracks might accommodate the deformation of the solder layer and prohibit the whiskers from being erupted out.     

Studies of the Ti-W/Au metallization on aluminum

The temperature stability of a composite film structure of 1500 Å of Ti_0.3W_0.7 and 3000 Å of gold r.f. sputtered sequentially onto an evaporated aluminum film 9000 Å thick on silicon was investigated by Auger depth profile analysis. Annealing of this structure at 300 °C for 6 min is shown to result in extensive intermixing. This phenomenon appears to proceed via hillock growth in the aluminum and a grain boundary diffusion mechanism involving the Ti-W. Thorough characterization of the Ti-W by transmission electron microscopy, helium ion backscattering spectrometry and deuterium activation analysis suggests that the Ti-W is of reasonable purity. These data indicate that Ti_0.3W_0.7 exhibits marginal diffusion barrier properties when deposited in a relatively pure state. In addition, it is shown that the barrier properties improve by orders of magnitude when Ti-W is sputtered in N₂-Ar. Thus it appears that optimum diffusion barrier properties are present only when Ti-W is deposited in a relatively impure state.     

The effect of 0.5 wt.% Ce additions on the electromigration of Sn9Zn BGA solder packages with Au/Ni(P)/Cu and Ag/Cu pads

It has previously been established that Sn-9Zn-0.5Ce alloy possesses mechanical properties superior to those of undoped Sn-9Zn alloy, and is free of the problem of rapid whisker growth. However, no detailed studies have been conducted on the electromigration behavior of Sn-9Zn-0.5Ce alloy. In this research, Sn-9Zn and Sn-9Zn-0.5Ce solder joints with Au/Ni(P)/Cu and Ag/Cu pads were stressed under a current density of 3.1 × 10⁴ A/cm² at room temperature for various periods of time. Due to finer grain sizes, the electromigration effects were more severe in Sn-9Zn-0.5Ce solder joints than in Sn-9Zn solder joints when joint temperature was around 80 °C. In addition, both solder joints (Sn-9Zn and Sn-9Zn-0.5Ce) with Au/Ni(P)/Cu pads possess longer current-stressing lifetimes than those with Ag/Cu pads because Ni is more resistant than Cu to migration driven by electron flow.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

再流条件对Ni颗粒增强复合钎料组织形态的影响

主要针对不同的再流次数带来的不同热输入对Ni颗粒增强复合钎料IMC形态的影响进行了深入研究。由前一阶段研究表明，决定Ni颗粒增强复合无铅钎料组织变化的关键因素是钎料的钎焊温度与钎料熔点的温度差△T以及在熔点以上保温时间t。其本质即外界对钎料的热输入量的大小。随着热输入的增加，Ni颗粒周围的IMC以及钎料/基板界面处的IMC都相应变化发展。由于Ni颗粒的加入。基板＼钎料界面层的结构形态均与Sn-Ag共晶钎料有较大不同，Ni与cu6Sn5的相互作用起到了关键影响。界面层厚度的变化随再流次数增加呈现线性增长。