【作者】 陈继兵；

【导师】 吴懿平；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2013， 博士

【摘要】 电子封装微互连技术正在向小型化,高密度方向快速发展,微焊点不仅起着电气和机械连接的作用,还提供了散热的途径,因此微焊点的可靠性显得尤为重要。本文采用快速热疲劳研究无铅微焊点的界面与表面组织的变化与演化,金属间化合物(IMC)的生长规律以及在极限循环温度下的热疲劳现象；还采用快速热疲劳对LED进行应用研究,研究其对LED性能以及对芯片无铅微焊点的界面微观组织影响,并与在高低温恒湿条件下的常规热疲劳试验结果进行对比。结果表明,快速热疲劳对微焊点的性能和微观组织产生明显的影响,并可用其对微焊点的可靠性和相对使用寿命进行评估。本文首先设计并搭建了一套快速加热的实验研究装置,并采用无铅锡银铜焊料Sn3.0Ag0.5Cu (SAC305)分别制备球栅阵列(BGA)单基板和双基板微焊点。首先研究了快速热疲劳对微焊点界面和表面的性能和微观组织影响；另外还采用快速热疲劳对大功率LED进行测试,除了研究其对LED的性能影响外,还研究了其对LED中无铅微焊点(Au/Sn和Ni/Au)的微观组织影响,并与常规热疲劳下的结果进行比较。研究得出的主要结论如下：研究了快速热疲劳对单基板SAC305微焊点在不同循环时间和循环温度条件下的微观组织影响。在0h至12h循环时间以及在55-125℃和55-180℃循环温度的热疲劳过程中,焊点界面间IMC开始由扇贝状变成大波浪状,其厚度逐渐增厚,而且IMC中由开始单一的Cu6Sn5,后来在Cu6Sn5层与Cu层之间会生成新的IMC层Cu3Sn,且Ag3Sn颗粒由开始的短棒状变成椭圆的颗粒状。此外,还发现在IMC中Cu3Sn层内出现柯肯达尔孔洞,并且随温度升高越来越大。根据Fick扩散定律和分子动力学方程,IMC的生长厚度与累积测试时间的平方根大致呈线性关系,故拟合计算出了与传统热疲劳结果近似的扩散系数,扩散常数和活化能。还采用快速热疲劳对微焊点在60-200℃这种极限温度下分别进行24h和36h的实验时,发现了界面IMC呈锯齿状的生长,而且IMC的边界出现了疲劳裂纹现象,并正在进行萌生和扩展。因此,验证了快速热疲劳对微焊点研究具有一定的可行性。对比研究了快速热疲劳对双基板SAC305和63Sn37Pb两种微焊点在不同循环时间和循环温度条件下的微观组织影响。首先对双基板微焊点进行热模拟分析,分析在热循环过程中其受热分布。然后在循环温度为60-200℃的条件下观察循环时间为Oh,12h,24h,36h和72h的双基板SAC305微焊点微观组织,可以看到焊点界面中IMC开始由只有Cu6Sn5后演变成有Cu6Sns和Cu3Sn。在36小时后,焊点界面出现重结晶的现象,导致产生应力集中和裂纹的产生。在72小时后则发现裂纹已经充满整个截面,并且形成网络化。最后还通过扫描电镜(SEM)和X射线衍射(XRD)观察微焊点表面的形貌和成分,分析得出是由于在表面有氧化锡薄膜的存在,加上热膨胀系数(CTE)差异和氧化锡的脆性促使腐蚀沟形成,导致焊点的网络化龟裂失效的主要原因。观察在循环温度为55-180℃和相同的循环时间条件下的双基板63Sn37Pb微焊点界面和表面的微观组织,看到63Sn37Pb晶粒比SAC305的晶粒细小,而且没有出现重结晶的现象,裂纹从边界处开始萌生和扩展,最终扩展至整个焊点截面。最后根据热变形理论对双基板微焊点的失效过程进行演示,以上结论对评估焊点的热疲劳损伤有重要参考价值。研究了快速热循环对LED在工作状态和非工作状态下的性能影响,并与在高低温恒湿条件下的常规热疲劳试验的LED性能进行对比。在工作状态测试后的LED,以光通量为例,在测试24小时后下降了30%,达到了失效的条件；而非工作状态测试则在440小时下降了15%,在高低温恒湿条件下测试在552小时后仅下降了4%；同时,还对比光谱图,在工作状态的LED蓝光相对光谱提高36%,在非工作状态下提高了17%,而在高低温恒湿条件下只提高了7%,表明红绿光谱出现不同程度地下降。此外,色纯度和辐射功率都是出现了类似下降的情况,其幅度都逐渐降低；但是色温和正向工作电压却逐渐升高,而且工作状态的上升幅度最高,常规热疲劳试验的下降幅度最小。这表明快速热循环对LED的可靠性能影响非常显著,故可作为快速评估LED可靠性和相对使用寿命的一种方法。研究了快速热循环对LED芯片无铅微焊点(Au/Sn和Ni/Au)在工作状态和非工作状态下界面微观组织的影响。首先通过SEM观察在工作状态下测试的微观组织,观察到芯片n电极微焊点(Ni/Au)开始变形,微观组织出现空洞,p电极金属层(Au/Sn)也开始出现分层和剥离现象,随着时间的推移,现象越来越明显,分析原因是由于界面各材料的热膨胀系数不同产生了内应力,在LED工作过程中芯片自身产生的温度和热循环施加的环境温度,导致焊点发生重熔而变形,同时由于电迁移和热迁移耦合的作用,使得微焊点出现空洞,导致接触面积变小,使得其性能快速下降。在观察非工作状态下的微焊点微观组织时,n电极微焊点发生变形和p电极金属层分层的程度没有在工作状态时明显,分析原因是由于n电极微焊点处仅发生了热迁移现象使得发生微焊点晶粒粗化,p电极金属层因热膨胀系数差异而产生的内应力小,使得接触面积减小有限,故性能下降幅度没有工作状态时大。在观察高低温恒湿条件下的测试LED的微观组织时,观察到n电极微焊点仅出现了轻微的裂纹,且p电极金属层仅开始出现分层,两者的幅度均没有前两种状态的明显,这些结果与测试的性能变化结果基本是一致的。因此进一步证实了快速热循环可以作为快速评估LED可靠性和相对使用寿命的方法。更多还原

【Abstract】 Micro-interconnection technology in electronic packaging is rapidly developing to the orientation of miniaturization, high-density, the solder joint not only plays the role of electrical and mechanical connection, also provides a way to heat dissipation, so the reliability of micro-interconnection solder joint is very important. The present paper study the change and evolution of interface microstructure, the growth rhythm of intermetallic compounds (IMC) and thermal fatigue proformance under limiting conditions for ball grid array (BGA) lead-free micro-solder joint by rapid thermal fatigue. In addition, light-emitting diodes (LED) was appliedly researched by rapid thermal fatigue, too. The influences of LED property and interface microstructure of lead-free micro-solder joint in its chip were studied, and compared with results of conventional thermal fatigue experiment, the results shows that rapid thermal fatigue can obviously affects ther preformance and microstructure of micro-solder joint, so it can be used to evaluate reliability and life of micro-solder joint.In this present paper, a rapid heating experimental device was designed. Lead-free Sn3.0Ag0.5Cu (SAC305) solders were used to preparate SAC305micro-solder joints with single/double substrates, respectively. The influence of interface and surface profermances and microstructure of BGA micro-solder joints were investigated by rapid thermal fatigue. In addition, high-power LEDs were tested by rapid thermal fatigue. The influence of microstructures of lead-free micro-solder joint (Au/Sn and Ni/Au) in LED chip were researched except for the influence of LED properties, which results were compared with results of conventional thermal fatigue experiment.The main research results are listed as follows:The study on the single-base substrate SAC305solder joints revealed the influence of rapid thermal fatigue on microstructures of at0hour and rapid thermal cycle12hours at55-125℃and55-180℃. It was found that the intermatellic (IMC) between interface of solder joint starts from the scallop shape to a big wave, and its thickness increases with time, and IMC is only single Cu6Sn5firstly, later a new Cu3Sn layer is generated in the between Cu6Sn5layer and Cu layer, and it is also gradually thickening, and Ag3Sn grain start from short stick shape to elliptical granular. In addition, it was also found that Kirkendall holes appear in the IMC Cu3Sn layer, and it is bigger and bigger with the temperature. According to Fick diffusion law, the growth of the relationship of IMC thickness and the square root of cumulative test time is roughly linear, so the diffusion coefficient, and the diffusion constant and activation energy are calculated by the means of fitting. The single-base plate SAC305/Cu solder joint was tested at extreme temperature60-200℃by rapid thermal cycle24hours and36hours respectively, it was found the IMC grow by the zigzag shape, and fatigue cracks appear at the interface of IMC/Cu, they are initiating and propagating along the boundary, and thread through the cross section of solder joint in the end, which lead to the failure of solder joint.The study compared the influence of rapid thermal fatigue on the interface microstructures of two solder joints with double-base substrates of Cu/Sn3.0Ag0.5Cu/Cu and Cu/63Sn37pb/Cu at cyclic temperature60-200℃and55-180℃and test time for0hour,12hours,24hours,36hours and72hours, respectively. It was found the IMC in SAC305solder joints only haves Cu6Sn5, then evolves into Cu6Sn5and Cu3Sn. After36hours, the interface of solder joint appears recrystallization phenomenon, which causes stress concentration and crack generation. After72hours, it was also found that cracks have filled the entire section, and formed a network. However, the solder in Cu/63Sn37Pb/Cu solder joints happen remelting and deformation, the solder didn’t happen a recrystallization phenomenon, but the tin oxide film exist on the solder surface later, and the reasons of dismatch coefficient of thermal expansion (CTE) and frangibility of tin oxide urge cracks to form corrosion grooves. At last, according to the thermal simulation analysis and thermal deformation theory, the cracking process was explained with a schematic diagram.The study on LED revealed the influence of rapid thermal fatigue on the changes of performances under operating life test and non-operating life test onditions and compared the results of conventional thermal fatigue contidion. It was found that a luminous flux is ones of LED optical properties, as an example, and quickly fell down30%when it was tested24hours in the operating life test and achieve a failure standard, and it fell down15%after440hours in non-operating life test, but it only dropped to4%after552hours test. At the same time, their spectrogram also were studied, the relative spectrum of LED blue light is increased36%in the operating life test, it is increased17%in non-operating life test, and it is only increased7%in the constant humidity and temperature conditions. In addition, the color purity and radiation power appear similar declines, its amplitude are gradually reduced. But color temperature and forward working voltage is increased, and their increasing amplitude in operating life test is the highest among three states and conditions and it is minimum in the constant humidity and temperature conditions. This results show that rapid thermal cycle on LED reliability is remarkable, but the test in the constant humidity and temperature condition effects a little about LED reliability, so it must pay attention to heat dissipation for LED.更多还原