钎料稀土相表面Sn晶须生长的研究

为了解对钎料可靠性影响极大的Sn晶须的生长机理,系统研究了钎料稀土相CeSn3、LaSn3及ErSn3表面Sn晶须生长的影响因素.结果表明:稀土相的氧化倾向与时效温度共同影响其表面Sn晶须的生长.室温时效条件下,在稀土相CeSn3与LaSn3表面易出现小尺寸的线状Sn晶须,直径为0.1～0.2 μ.m,而在稀土相ErS...     

稀土相表面Sn晶须生长的研究

研究了Sn-3.8Ag-0.7Cu-RE(RE为Ce、Er或Y)焊料在空气中室温与高温时效过程中稀土相CeSn3、ErSn3与YSn3表面Sn晶须的生长情况。结果表明,Sn晶须的开始生长时间、形态及数量与稀土相的种类及时效条件有密切关系。稀土相因氧化产生的体积膨胀提供了Sn晶须生长的驱动力。稀土与氧的化学亲和力参数及时效温度共同影响稀土相表面Sn晶须的生长。室温实效条件下,三种稀土相表面Sn晶须的直径为0.1~2.0μm,长度可达几百微米;150℃时效条件下,Sn晶须的直径为0.1~0.2μm,长度也可达上百微米。     

SnAgCuY钎料表面Sn晶须的旋转生长现象

研究了Sn3.8Ag0.7Cu1.0Y钎料表层上YSn3稀土相表面Sn晶须的生长行为。结果表明:室温时效条件下在YSn3的表面会出现Sn晶须的快速生长现象,生长速度最快可达10–10m/s,长度最长可达200μm。YSn3稀土相氧化的不均匀性是导致Sn晶须在生长时产生各种旋转现象的主要原因。     

时效对Sn3.0Cu0.15Ni/Cu界面组织的影响

研究了150℃时效0,200,500h对Sn3.0Cu0.15Ni/Cu界面组织结构的影响.结果表明:界面金属间化合物层由Cu6Sn5层和Cu3Sn层组成,质量分数为0.15％的Ni的加入会使IMC层最初变厚,但在时效过程中,热稳定性强的界面化合物(Cu,Ni)6Sn5的生成,会抑制Cu3Sn化合物层的生长;同时Ni的加入会降低Cu6Sn5颗粒的长大速度,并且随着时效时间的延长,Cu6Sn5颗粒的形貌呈多面体结构.     

SnAgCuLa钎料合金表面Sn晶须的生长研究

在钎料中添加微量稀土可以显著改善钎料合金的综合性能。但添加过量的稀土时,将会出现Sn晶须的快速生长。研究发现,如果将Sn3.8Ag0.7Cu1.0La钎料内部的稀土相暴露于空气中,稀土相将发生氧化而产生体积膨胀,钎料基体对体积膨胀的抑制作用将使稀土相内部产生巨大的压应力从而加速Sn晶须的生长。晶须直径约1μm,长度可达300μm。     

Au20Sn/Au微焊点抗时效性能的研究

通过加速时效方法研究Au20Sn/Au微焊点钎焊工艺参数与镀Au层的消耗和Au20Sn/Au界面化合物(IMC)生长速率的关系。结果表明:焊点在150℃时效条件下,钎焊温度一定时,高温液态停留时间由30 s增至90 s,镀Au层消耗速率变化速度和界面IMC层生长速率变化速度均逐渐增加。在高温液态停留时间90 s时,相比于钎焊温度300℃的焊点,320℃时镀Au层消耗速率变化速度降低了24.50%,界面IMC层生长速率的变化速度提高了56.09%。同时随时效时间的延长,热沉侧出现一层(Ni,Au)3Sn2相,但芯片侧和热沉侧界面IMC的类型并没有发生变化。     

铜焊盘与锡合金焊点界面物相分析及可靠性探讨

Sn60Pb40焊料与铜焊盘的焊接界面中金属间化合物Cu6Sn5的形成与长大以及在热循环过程中的组织粗化是影响焊点可靠性的重要因素。作者根据SMT工艺的实际情况，使用Au—Sn共晶焊料、Sn60Pb40焊料分别涂覆在铜合金基板表面，并分别在320℃、240℃下保温1min，冷却形成焊点，利用X射线研究分析了两种不同焊盘基材与Sn60Pb40钎料、Au—Sn共晶钎料的钎焊界面的物相。运用经典相变理论、低周疲劳失效的机理以及“柯肯达尔”效应，就优异焊点的形成、物相产生、温度循环后组织粗化与增加Ni阻挡层，对提高焊接接点的温度循环可靠性的作用进行了分析与探讨。     

SnAgCuY钎料高温时效过程的显微组织演化

研究了无铅钎料合金Sn3.8Ag0.7Cu高温时效过程中显微组织,特别是金属间化合物(IMC)的演化规律,以及稀土Y的添加对其产生的影响。结果表明:在高温时效过程中合金内部组元发生扩散与重组,伴随着共晶组织的逐渐溶解,新的IMC在组织内部呈球形弥散析出。结晶初期形成的具有规则形状的较粗大的IMC逐渐发生解体,树枝状富Sn相逐渐取代共晶组织成为受腐蚀的对象。随着时效时间的延长,合金内部各组元的成分也在不断发生变化。     

空位对Cu/Sn焊点中Cu3Sn层元素扩散的影响

为了探究温度和空位浓度对Cu3 Sn层中各元素扩散行为的影响,基于分子动力学方法,使用LAMMPS软件模拟了Cu3 Sn层上空位浓度以及温度对各元素扩散系数的影响.结果表明,Cu3 Sn层各元素的扩散系数均随温度的升高而增大.与不含空位相比,当Cu3 Sn层中存在10％空位时,原子运动更加剧烈,扩散系数增大.进一步研究...     

烧结温度对AuSn焊料薄膜及封装激光器性能的影响

采用不同温度对Au80Sn20共晶合金焊料进行烧结实验,研究了AuSn焊料薄膜在烧结后的形貌、物相组成以及对封装激光器的性能影响等.焊料在烧结后形成ξ相Au5Sn和δ相AuSn两种金属间化合物,随着烧结温度的上升,两相晶粒均明显长大,而ξ相Au5Sn趋向于形成枝晶.较低温度下烧结的焊料表面粗糙度较高,不利于激光器管芯的贴装.高温过烧焊料薄膜的导电导热性能有少许提升,对封装激光器管芯的功率没有明显影响,但焊料薄膜中残余应力较高,使激射波长有所蓝移.该结果将为AuSn焊料的烧结参数优化和硬焊料封装激光器的性能分析提供参考和指导.     

A Model for Rapid Tin Whisker Growth on the Surface of ErSn3 Phase

Spontaneous growth of tin whiskers on the finish of leadframes is an extremely slow process under moderate temperature conditions. It therefore becomes difficult to track the continuous growth of tin whiskers and to vary the experimental conditions to determine their root causes. Accordingly, the fundamental growth behaviors of tin whiskers are still not fully understood. In this study, rapid tin whisker growth was achieved by adding 1 wt.% Er to Sn-3.8Ag-0.7Cu solder alloy. The results showed unique tin whisker morphology with nonconstant cross-section. An explanation is proposed by adding kinetic energy to the conventional energy balance equation. In addition, a double compressive stress zone is proposed to demonstrate the driving force for tin whisker growth in rare-earth-bearing phases.     

Effects of POSS-Silanol Addition on Whisker Formation in Sn-Based Pb-Free Electronic Solders

Previous studies have indicated that silanol in the form of polyhedral oligomeric silsesquioxane (POSS) trisilanol could form strong bonds with solder matrix without agglomeration, and inhibit diffusion of metal atoms when subjected to high ambient temperature and/or high current density. Addition of POSS-trisilanol has also been shown to improve the comprehensive performance of Sn-based Pb-free solders, such as shear strength, resistance to electromigration, as well as thermal fatigue. The current study investigated the whisker formation/growth behaviors of Sn-based Pb-free solders (eutectic Sn-Bi) modified with 3 wt.% POSS-trisilanol. Solder films on Cu substrates were aged at ambient temperature of 125°C to accelerate whisker growth. The microstructural evolution of the solder films’ central and edge areas was examined periodically using scanning electron microscopy. Bi whiskers were observed to extrude from the surface due to stress/strain relief during growth of Sn-Cu intermetallic compounds (IMCs). Addition of POSS-trisilanol was shown to retard the growth of Bi whiskers. The IMCs formed between POSS-modified solders and the Cu substrate showed smoother surface morphology and slower thickness growth rate during reflow and aging. It was indicated that POSS particles located at the phase boundaries inhibited diffusion of Sn atoms at elevated temperatures, and thus limited the formation and growth of IMCs, which resulted in the observed inhibition of Bi whisker growth in POSS-modified solders.     

Abnormal growth of tin whiskers in a Sn3Ag0.5Cu0.5Ce solder ball grid array package

It has been discovered for the first time that Sn whiskers appeared in Sn3Ag0.5Cu0.5Ce solder joints of ball grid array (BGA) packages after storage at room temperature (natural aging) for less than 3 days and they grew at a high rate of 2.9 ?/sec. In one particular case, whiskers even formed after 1 day of storage at an extremely high growth rate of 8.6 ?/sec. Experimental investigations showed that a number of CeSn₃ clusters existed in the Sn3Ag0.5Cu0.5Ce solder matrix after the reflow process. Further natural aging in air for several days caused the CeSn₃ phases to oxidize rapidly, from which many Sn whiskers sprouted and grew to a length of hundreds of micrometers. The most commonly observed whiskers have been long fiber-shaped ones of 0.1 μm to 0.3 μm in diameter (type I), while short whiskers larger than 1 μm in diameter can also be found (type II). Here in our case, the surface oxide of the CeSn₃ phase possessed a higher content of Ce than of Sn, which implied that a Ce-depleted region (nearly of pure Sn) was left beneath the oxide layer. The abnormal whisker growth was attributed to the compressive stress squeezing the Sn atoms in the Ce-depleted region of CeSn₃ phase out of the oxide layer.     

Interfacial microstructure and bonding strength of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn-3Ag-0.5Cu-0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn₃ intermetallics and Zn atoms, formed in the Sn-3Ag-0.5Cu-0.5Ce-xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu-Sn intermetallics growth in Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn-Ag-Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.     

Inhibition of Whisker Growth on the Surface of Sn-3Ag-0.5Cu-0.5Ce Solder Alloyed with Zn

Solder alloys doped with rare-earth elements have been reported to show many beneficial effects. However, tin whiskers have been observed to appear on the surface of Sn-3Ag-0.5Cu-0.5Ce solder joints after air exposure for short periods. Such a phenomenon of abnormal whisker growth may significantly degrade the reliability of electronic packaging. The present study shows that the tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn-3Ag-0.5Cu-0.5Ce solder. The inhibition effect on the whisker growth is correlated to the refining of (Ce_0.9Zn_0.1)Sn₃ intermetallics in this Sn-3Ag-0.5Cu-0.5Ce-0.5Zn alloy.     

Tin Whisker Nucleation and Growth on Sn-Pb Eutectic Coating Layer Inside Plated Through Holes With Press-Fit Pins

It has been long believed that residual stress is the root cause for tin whisker formation on pure tin-plated component leads. However, tin whisker formation could be observed on the surface of other tin-based alloys under certain conditions. In this study, the whisker formation was reported on a coating layer of Sn-Pb eutectic hot air solder leveling (HASL), which was under compression stress conditions due to the inserted compliant pins. In-Situ scanning electron microscopy was used to monitor the nucleation and growth of whiskers. In addition, a mechanical experiment and non-linear contact finite element analysis were used to estimate the magnitude of the stress in the HASL coating layer. It was found that the tin whisker formation with whisker size of more than 10 mum could occur on the surface of 60Sn-60Pb plating within less than 30 min at an ambient temperature under compressing stress conditions. The tin whisker initiation and growth were further studied at an elevated temperature of 70 degC to check if a higher temperature effects Tin whisker formation. It is believed that establishment of a quantitative relationship of whisker formation/growth under compressive stress and elevated temperature conditions could lead to better scientific methods for risk and reliability assessment and smooth transitions to lead-free assemblies.     

Effects of Ce and La Additions on the Microstructure and Mechanical Properties of Sn-9Zn Solder Joints

The effects of rare-earth elements on the microstructure and mechanical properties of Sn-9Zn alloys and solder joints in ball grid array packages with Ni/Au(ENIG) surface finishes have been investigated. Metallographic observations showed that (Ce_0.8Zn_0.2)Sn₃ and (La_0.9Zn_0.1)Sn₃ intermetallic compounds appeared in the solder matrix of Sn-9Zn-0.5Ce and Sn-9Zn-0.5La alloys, respectively. Both fiber- and hillock-shaped tin whiskers were inhibited in the Sn-9Zn-0.5Ce solder, while tin fibers were still observed on the surface of oxidized (La_0.9Zn_0.1)Sn₃ intermetallics in Sn-9Zn-0.5La after air exposure at room temperature. Mechanical testing indicated that the tensile strength of Sn-9Zn alloys doped with Ce and La increased significantly, and the elongation decreased, in comparison with the undoped Sn-9Zn. The bonding strengths of the as-reflowed Sn-9Zn-0.5Ce and Sn-9Zn-0.5La solder joints were also improved. However, aging treatment at 100°C and 150°C caused degradation of ball shear strength in all specimens. During the reflowing and aging processes, AuZn₈ intermetallic phases appeared at the interfaces of all solder joints. In addition, Zn-rich phases were observed to migrate from the solder matrix to the solder/pad interfaces of the aged specimens.