Board level investigation of BGA solder joint deformation strength

In this paper we present a method to determine the stress in BGA solder joints on complex, real assembled circuit boards.To be able to investigate the mechanical effects of post-reflow assembly within the solder joints of BGA components, it is necessary to undertake a mechanical investigation at board level by taking into consideration the effect of the adjacent components and the interconnection layer layouts.In our project, we have developed a method of how to investigate the board level deformation strength of BGA joints. The elastic properties of a real assembled circuit board and of a circuit bare board are measured; an FEM model is then created, both of the bare board and of the assembled printed circuit board taking into account the layout of the interconnection layers. The advantage of this PCB FEM model is that the deformation of a PCB of any size and for any load can be calculated quickly using any ordinary computer. In our project, we also have created another detailed FEM model for the BGA solder joints.Using the constructed FEM models, we are able to determine the stress in BGA solder joints on a real electronic product for a typical type of load (i.e. bending of PCB) thereby verifying our method. Since the simulated results correspond well to previous literature written on this topic, we consider that our method is appropriate for calculating stress in the solder joints of multi-lead components on complex, fully assembled circuit boards.     

Random vibration reliability of BGA lead-free solder joint

Solder joint fatigue failure under vibration loading has been a great concern in microelectronic industry. High-cycle fatigue failure of lead-free solder joints has not been adequately addressed, especially under random vibration loading. This study aims to understand the lead-free solder joint behavior of BGA packages under different random vibration loadings. At first, non-contact TV Laser holography technology was adopted to conduct experimental modal analysis of the test vehicle (printed circuit board assembly) in order to understand its dynamic characteristics. Then, its first order natural frequency was used as the center frequency and narrow-band random vibration fatigue tests with different kinds of acceleration power spectral density (PSD) amplitudes were respectively carried out. Electrical continuity through each BGA package is monitored during the vibration event in order to detect the failure of package-to-board interconnects. The typical dynamic voltage histories of failed solder joints were obtained simultaneously. Thirdly, failed solder joints were cross-sectioned and metallurgical analysis was applied to investigate the failure mechanisms of BGA lead-free solder joints under random vibration loading. The results show that the failure mechanisms of BGA lead-free solder joint vary as the acceleration PSD amplitude increases. Solder joint failure locations are changed from the solder bump body of the PCB side to the solder ball neck, finally to the Ni/intermetallic compound (IMC) interface of the package side. The corresponding failure modes are also converted from ductile fracture to brittle fracture with the increase of vibration intensity.     

SnAgCu体钎料及BGA焊球焊点纳米级力学性能

为了研究低银Sn-0.3Ag-0.7Cu无铅体钎料、BGA焊料小球和BGA焊点的力学行为,基于物理反分析的方法采用纳米压痕仪对其进行实验。从压痕载荷–深度曲线提取出弹性模量、硬度和蠕变速率敏感指数。结果表明:体钎料的杨氏模量和蠕变速率敏感指数大约是BGA焊料小球和BGA焊点的2.5倍,验证了尺寸效应理论。采用纳米压痕仪测出的体钎料维氏硬度(15.101HV)小于显微硬度计的测量结果(20.660HV)。     

黑盘与化镍金PCB焊点失效模式

文章通过多个化镍金PCB焊接失效案例,探讨了黑盘与化镍金PCB焊点失效模式之间的关系。     

废旧BGA器件二次利用的焊接强度研究

BGA器件首先经历2 000 h的温循老化以接近废旧状态,再以自重跌落,然后用锡膏印刷的方法完成焊锡球的修复,最后利用Dage4000测试机对不同状态的器件进行焊锡球推拉力测试。结果发现,随着温循老化的时间增加,失效模式逐渐由韧性失效主导转向脆性失效主导。当焊锡球被修复后,失效模式变为混合模式。通过强度对比可以发现,器件的焊接强度随着温循老化时间的增加而降低。当焊锡球被修复后,焊接强度可以恢复到初始状态的80%左右。修复的焊锡球产生的再生金属间化合物可能是造成焊接强度变化的主要原因。     

BGA混合焊点热循环负载下的可靠性研究

焊料从有铅向无铅转换中,不可避免会遇到二者混合使用的情况,有必要对生成的混合焊点进行可靠性研究。通过对不同工艺参数下形成的混合焊点和无铅焊点进行了外观检测、X射线检测和温度循环测试。结果显示,只要工艺参数控制得当,混合焊点是可行的。在焊球合金、焊料合金、峰值温度、液相线以上时间和焊接环境五个关键因素中,前四项对焊点可靠性比较重要,焊接环境对焊点可靠性的影响不很显著。     

Interfacial microstructure and strength of lead-free Sn-Zn-RE BGA solder bumps

Rare earth (RE) elements, primarily La and Ce, were doped in Sn-Zn solder to improve its properties such as wettability. The interfacial microstructure evolution and shear strength of the Sn-9Zn and Sn-9Zn-0.5RE (in wt%) solder bumps on Au/Ni/Cu under bump metallization (UBM) in a ball grid array (BGA) were investigated after thermal aging at 150 /spl deg/C for up to 1000 h. In the as-reflowed Sn-9Zn solder bump, AuSn/sub 4/ intermetallic compounds (IMCs) and Au-Zn circular IMCs formed close to the solder/UBM interface, together with the formation of a Ni-Zn-Sn ternary IMC layer of about 1 /spl mu/m in thickness. In contrast, in the as-reflowed Sn-9Zn-0.5RE solder bump, a spalled layer of Au-Zn was formed above the Ni layer. Sn-Ce-La and Sn-Zn-Ce-La phases were found near the interface at positions near the surface of the solder ball. Upon thermal aging at 150 /spl deg/C, the concentration of Zn in the Ni-Zn-Sn ternary layer of Sn-9Zn increased with aging time. For Sn-9Zn-0.5RE, the Au-Zn layer began to dissolve after 500 h of thermal aging. The shear strength of the Sn-9Zn ball was decreased after the addition of RE elements, although it was still higher than that of the Sn-37Pb and Sn-36Pb-2Ag Pb-bearing solders. The fracture mode of the Sn-9Zn system was changed from ductile to partly brittle after adding the RE elements. This is mainly due to the presence of the brittle Au-Zn layer.     

Assessment of acceleration models used for BGA solder joint reliability studies

Solder joint reliability was one of the top priorities when evaluating the reliability of electronic packages. In general, an acceleration model would be used to predict solder joint fatigue life in the use conditions. However, the accuracy of the model was difficult to validate. As a result, the fatigue life of the solder joints could be over-designed with added cost or time, or under-estimated with a compromised reliability performance. It was an important goal for engineers to use valid and accurate life models to predict the field life of the solder joints and reduce development cost and time.Many empirical models including Norris-Landzberg model and its modifications usually considered the effects of temperature range, the cycle frequency, and the maximum temperature. No matter what the package structures were or the materials were used, engineers had been using the same model parameters for many years. Moreover, little was done to validate the models for modern packages structures and materials.In this article, a variety of package was studied and the failure data was analyzed through a reliability engineering approach. The results showed that the available model parameters were not suitable to predict the solder joint life of test samples exclusively. A new set of model parameters might be required for certain cases. Also, the acceleration factor models would depend on the solder joint materials and microstructures. The solder joint fatigue life performance was too complicated to be assumed as a fixed empirical model. One of the reasons was there were too many factors affecting the strain which the solder joints would endure.In the future study, critical factors such as materials or structures could be integrated into the current model format. Additionally, the ramp rate could be a concern especially when dealing with cases under thermal shock conditions. The methodology to develop an acceleration factor model and the demonstration of their weakness would help achieve reliable solder connections in the future.     

Investigation of mechanical shock testing of lead-free SAC solder joints in fine pitch BGA package

The lead-free Sn-Ag-Cu (SAC 305/405) solder that replaced the tin-lead eutectic solder tends to be more brittle in nature due to high stiffness and excessive solder interfacial reactions. This leads to higher occurrences of solder joints failure during surface mount assembly and handling operations as a result of PCB bending, shock impact and drop. In this work, mechanical tests simulating the shock impact were conducted on lead-free SAC of different weight percentages. These SAC materials were prepared for use in the solder joints of fine pitch ball grid array (BGA) components which were mounted onto the motherboard. After the mechanical shock tests, strain measurements were performed on the BGA components to gauge the solder joint integrity, which was shown to be related with the formation of intermetallics in the bulk and at the interface of the SAC solder. The ball pull tests were conducted to determine both the bulk and interfacial strength and the solder joint fracture, which was classified as either mode 1, 2 or 3. A correlation was made between the silver (Ag) and copper (Cu) weight percentages with the metallurgical reactions.     

Effect of high-speed loading conditions on the fracture mode of the BGA solder joint

High-speed ball shear test was investigated in terms of the effects of two test parameters, shear speed and shear height, with an experimental and computational simulation utilizing 3-dimensional non-linear finite element modeling for evaluating the solder joint integrity of ball grid array (BGA) packages. A representative Pb-free solder alloy, Sn–0.7Cu, was employed in this study. Far greater shear forces were measured by high-speed shear test than by low-speed shear test. The shear force further increased with shear speed mainly due to the high strain-rate sensitivity of the solder alloy. Brittle interfacial fractures can be more easily achieved by high-speed shear test, especially in higher shear speed. These were discussed with the strengthening mechanism of the solder materials by density and mobility of dislocations. Shear force decreased with shear height, and it could be found that excessively high shear heights unfavorably affected the test results leading to unexpectedly high standard deviation values or shear tip sliding from the solder ball surface.     

回流焊峰值温度对混装BGA焊点的影响研究

针对当前大量使用有铅焊料焊接无铅BGA的实际现状,通过调控有铅制程回流曲线的峰值温度,研究其对混装BGA焊点坍塌高度、空洞率及微观组织的影响。结果表明,峰值温度从210℃提升至225℃,无铅BGA焊球能够全部充分坍塌且高度保持一致;峰值温度为210℃时,混合焊点内的空洞率最低,随着峰值温度的升高,空洞尺寸和空洞率均有所增加;峰值温度为215℃时的微观组织最细小且尺寸分布最均匀,继续提升峰值温度,微观组织尺寸会随之增大。因此使用有铅焊料焊接无铅BGA的最佳峰值温度为215℃,与有铅制程保持一致。     

芯片尺寸封装焊点的可靠性分析与测试

利用ANSYS有限元分析软件,将芯片尺寸封装(CSP)组件简化为了二维模型,并模拟了CSP组件在热循环加栽条件下的应力应变分布;通过模拟发现了组件的结构失效危险点,然后对危险点处的焊点热疲劳寿命进行了预测;最后进行了CSP焊点可靠性测试.结果表明,用薄芯片可提高焊点可靠性.当芯片厚度从0.625 mm减小到0.500 ...     

焊点尺寸对微焊点拉伸断裂强度的影响

采用基于动态力学分析仪(DMA)的精密拉伸试验与ANSYS有限元数值模拟方法研究了“铜引线/Sn-3.0Ag-0.5Cu钎料/铜引线”三明治结构微焊点(直径均为200 μm,高度为75～225 μm)的拉伸断裂行为.结果表明:微焊点直径不变而高度为225,175,125和75 μm时,其拉伸断裂强度分别为79.8,82...     

热疲劳载荷作用下不同成分BGA互连焊点可靠性研究

本文针对微电子组装中常见的BGA 封装形式,对比采用三种不同成分的BGA焊球和焊膏组合（锡铅共晶焊球和锡铅共晶焊膏、Sn3Ag0.5Cu 焊球和锡铅共晶焊膏、以及Sn3Ag0.5Cu焊球和Sn3Ag0.5Cu焊膏）焊接得到的BGA 互连点,经过不同周期的热疲劳试验后,在金相显微镜和电子背散射衍射下观察,发现Sn3Ag0.5Cu焊球和锡铅共晶焊膏混装形成的BGA焊点中黑色的富锡相均匀弥散分布在焊球内,在热循环载荷作用下极难形成再结晶,抗热疲劳性能最好。     

Influence of current density on mechanical reliability of Sn-3.5Ag BGA solder joint

In this study, we investigated the effect of the current density on the interfacial reaction and mechanical reliability of an electroless Ni/immersion Au (ENIG) substrate with Sn-3.5Ag solder. We first evaluated the interfacial reactions of the solder joint under aging for up to 800 h and current stressing with current densities of 3 × 10² A/cm² and 5 × 10³ A/cm². Also, we successfully revealed the correlation between the interfacial reaction behavior and mechanical reliability under current stressing. With increasing aging time, the thickness of the Ni₃Sn₄ layer increased. At both low and high current densities, the thickness of the Ni₃Sn₄ layer increased up to 400 h and decreased thereafter at the cathode, while that of the IMC increased up to 800 h at the anode. After the die shear test, the ductile fracture was observed in the as-reflowed joint without current stressing. The fracture mode changed from ductile fracture to brittle fracture when thermal aging and current flow were simultaneously applied. The combination of the current stressing and isothermal aging at high temperature significantly deteriorated the mechanical reliability of the solder joint.     

Board level solder joint reliability analysis of stacked die mixed flip-chip and wirebond BGA

Stacked die BGA has recently gained popularity in telecommunication applications. However, its board level solder joint reliability during the thermal cycling test is not as well-studied as common single die BGA. In this paper, solder joint fatigue of lead-free stacked die BGA with mixed flip-chip (FC) and wirebond (WB) interconnect is analyzed in detail. 3D fatigue model is established for stacked die BGA with considerations of detailed pad design, realistic shape of solder ball, and non-linear material properties. The fatigue model applied is based on a modified Darveaux’s approach with non-linear viscoplastic analysis of solder joints. Based on the FC–WB stack die configuration, the critical solder ball is observed located between the top and bottom dice corner, and failure interface is along the top solder/pad interface. The modeling predicted fatigue life is first correlated to the thermal cycling test results using modified correlation constants, curve-fitted from in-house lead-free TFBGA46 (thin-profile fine-pitch BGA) thermal cycling test data. Subsequently, design analyzes are performed to study the effects of 20 key design variations in package dimensions, material properties, and thermal cycling test conditions. In general, thinner PCB and mold compound, thicker substrate, larger top or bottom dice sizes, thicker top die, higher solder ball standoff, larger solder mask opening, smaller PCB pad size, smaller thermal cycling temperature range, longer ramp time, and shorter dwell time contribute to longer fatigue life. SnAgCu is a common lead-free solder, and it has much better board level reliability performance than eutectic solder based on modeling results, especially low stress packages.     

Board level solder joint reliability analysis and optimization of pyramidal stacked die BGA packages

Due to requirements of cost-saving and miniaturization, stacked die BGA has recently gained popularity in many applications. However, its board level solder joint reliability during the thermal cycling test is not as well-studied as common single die BGA. In this paper, solder joint fatigue of wirebond stacked die BGA is analyzed in detail. 3D fatigue model is established for stacked die BGA with considerations of detailed pad design, realistic shape of solder ball, and non-linear material properties. The fatigue model applied is based on a modified Darveaux's approach with non-linear viscoplastic analysis of solder joints. The critical solder ball is observed located between the top and bottom dice corner, and failure interface is along the top solder/pad interface. The modeling predicted fatigue life is first correlated to the thermal cycling test results using modified correlation constants, curve-fitted from in-house TFBGA (thin-profile fine-pitch BGA) thermal cycling test data. Subsequently, design analyses are performed to study the effects of 16 key design variations in package dimensions, material properties, and thermal cycling test conditions. In general, smaller top and bottom dice sizes, thicker top or bottom die, thinner PCB, thicker substrate, higher solder ball standoff, larger solder mask opening size, smaller maximum ball diameter, smaller PCB pad size, smaller thermal cycling temperature range, longer ramp time, and shorter dwell time contribute to longer fatigue life. The effect of number of layers of stacked-die is also investigated. Finally, design optimization is performed based on selected critical design variables.     

Experimental and numerical analysis of BGA lead-free solder joint reliability under board-level drop impact

Board-level solder joint reliability is very critical for handheld electronic products during drop impact. In this study, board-level drop test and finite element method (FEM) are adopted to investigate failure modes and failure mechanisms of lead-free solder joint under drop impact. In order to make all ball grid array (BGA) packages on the same test board subject to the uniform stress and strain level during drop impact, a test board in round shape is designed to conduct drop tests. During these drop tests, the round printed circuit board assembly (PCBA) is suffered from a specified half-sine acceleration pulse. The dynamic responses of the PCBA under drop impact loading are measured by strain gauges and accelerometers. Locations of the failed solder joints and failure modes are examined by the dye penetration test and cross section test. While in simulation, FEM in ABAQUS software is used to study transient dynamic responses. The peeling stress which is considered as the dominant factor affecting the solder joint reliability is used to identify location of the failed solder joints. Simulation results show very good correlation with experiment measurement in terms of acceleration response and strain histories in actual drop test. Solder joint failure mechanisms are analyzed based on observation of cross section of packages and dye and pry as well. Crack occurred at intermetallic composite (IMC) interface on the package side with some brittle features. The position of maximum peeling stress in finite element analysis (FEA) coincides with the crack position in the cross section of a failed package, which validated our FEA. The analysis approach combining experiment with simulation is helpful to understand and improve solder joint reliability.