芯片堆叠封装耐湿热可靠性

采用Abaqus软件模拟了CPU和DDR双层芯片堆叠封装组件在85℃/RH85%湿热环境下分别吸湿5,17,55和168 h的相对湿气扩散分布和吸湿168 h后回流焊过程中湿应力、热应力和湿热应力分布,并通过吸湿和回流焊实验分析了该组件在湿热环境下的失效机理。模拟结果表明,在湿热环境下,分别位于基板和CPU、CPU和DDR之间的粘结层1和2不易吸湿,造成粘结层的相对湿度比塑封材料低得多,但粘结层1的相对湿度比粘结层2要高。吸湿168 h后,在回流焊载荷下湿应力主要集中在芯片DDR远离中心的长边上,而最大湿热应力和热应力一样位于底层芯片CPU的底角处,其数值是单纯热应力的1.3倍。实验结果表明,界面裂纹及分层集中在底层CPU芯片的边角处和芯片、粘结层和塑封材料的交界处,与模拟结果相一致。     

无铅倒装焊点的热应力与湿热应力分析

板上倒装芯片（FCOB）作为一种微电子封装结构形式得到了广泛的应用。微电子塑封器件中常用的聚合物因易于吸收周围环境中的湿气而对封装本身的可靠性带来很大影响。文章采用有限元软件分析了潮湿环境下板上倒装芯片下填充料在湿敏感元件实验标准MSL-1条件下（85℃／85％RH、168h）的潮湿扩散分布,进而分别模拟计算出无铅焊点的热应力与湿热应力,并加以分析比较。论文的研究成果不仅对于塑封电子元器件在潮湿环境中的使用具有一定的指导意义,而且对于FCOB器件在实际应用中的焊点可靠性问题具有一定的参考价值。     

湿热环境下EMC尺寸对PBGA器件可靠性的影响

塑料封装器件由于湿热导致层间开裂是影响器件可靠性的关键问题之一。采用有限元分析软件模拟和计算了不同的模塑封材料(EMC)尺寸的PBGA(塑封焊球陈列)器件在358.15K、RH60%条件下吸潮168h和398.15K、RH0环境下干燥50h后器件内部的应力对界面可靠性的影响。结果表明,在吸潮情况下,EMC厚度为0.85mm的器件的界面可靠性最低,最大湿应力为0.528MPa;在干燥阶段,EMC厚度为1.25mm的器件的界面可靠性最高,最大湿应力仅为0.124MPa。     

SiP器件回流焊时湿热应力的分析

应用有限元方法分析了QFN形式的SiP封装器件在回流焊中的热应力与湿热合成应力。结果表明,在回流焊过程中,由于其结构特点与湿气的扩散不均引起湿热应力变化梯度加大,在其材料交界处应力集中现象明显。最大湿热应力是单纯考虑热应力的情况1.66倍左右。通过比较得知湿热环境对这种SiP器件的影响比一般的封装器件要大,更可能导致器件失效。     

PBGA封装的热应力与湿热应力分析比较

塑封球栅平面阵列封装作为一种微电子封装结构形式得到了广泛的应用。本文采用有限元软件分析和计算了在潮湿环境下塑封球栅平面阵列封装的潮湿扩散分布，进而分别模拟计算了它的热应力与湿热应力，并且加以分析比较。     

湿热载荷对含底充胶的FCOB器件界面分层影响

分析了湿气在FCOB器件中的扩散行为,然后分别对湿热集成载荷和热载荷作用下的FCOB器件进行了有限元仿真研究。结果表明,硅芯片与底充胶界面拐角处吸湿最多,应力集中最为严重;吸潮后,低温保温结束时刻,该处同时湿热载荷影响,比单纯热载荷影响下,等效Von Mises应力和等效总应变分别增大了58%和60%,说明该处是底充胶分层萌生的潜在位置。     

塑封器件回流焊与分层的研究

由于无铅焊料的应用,回流焊的温度提高影响了塑封器件的质量和可靠性。针对实际的LQFP器件,利用有限元软件建立三维模型,分析了塑封器件在潮湿环境中的湿气扩散及回流焊中的形变和热应力分布,并讨论了塑封料参数及细小裂纹对分层的影响。结果表明,在湿热的加载下,塑封器件的顶角易发生翘曲现象;芯片与塑封料界面处易分层,导致器件失效。     

PBGA器件潮湿扩散和湿热应力的有限元分析

塑料封装器件暴露在一定的潮湿环境下将会吸收潮湿的现象已经得到广泛的认同。针对实际的PBGA器件,采用通用有限元软件分析和计算了器件在潮湿环境下的潮湿扩散。并且计算了由于吸潮使器件在高温下产生的湿热应力。有限元模拟计算表明,不同潮湿环境下器件的潮湿扩散状态是不一样的,这导致在其后的高温焊接过程中器件内部产生的湿热应力的不同。     

湿热环境下吸潮对QFN器件可靠性的影响研究

由吸潮引起的微电子塑封器件失效已经越来越多地引起人们的关注.选用QFN器件作为研究对象,首先进行QFN器件在高温高湿环境下吸潮17 h、50 h、96 h试验;然后利用有限元软件分析和模拟潮湿在QFN器件中的扩散行为,并建立湿气预处理阶段应力计算模型;最后,通过试验与仿真相结合,分析潮湿对封装可靠性的影响.研究表明:微电子塑封器件的潮湿扩散速度与位置有着重要的关系;在高温高湿环境下,微电子器件吸潮产生的湿热应力在模塑封装材料(EMC)、硅芯片(DIE)和芯下材料(DA)的交界处最大;QFN器件在高温高湿环境下吸潮产生的裂纹主要出现在硅芯片与DA材料交界面的边界.     

基于COB技术的SiP模块可靠性分析

针对COB形式的SiP模块,应用有限元分析方法模拟了该模块在湿热环境下的湿气扩散和湿应力分布,以及回流焊过程中的热应力分布,并通过吸湿实验和回流焊实验分析了该模块失效模式.结果表明,在湿热环境下,粘接材料夹在芯片和焊盘中间不易吸湿,造成粘接材料的相对湿度比塑封材料的相对湿度低得多.塑封材料相对湿度较高,产生较大的湿膨胀,使湿应力主要分布在塑封材料与芯片相接触的界面上.由于材料参数失配,回流焊过程产生的热应力主要分布在粘接材料和铜焊盘的界面,以及塑封料和铜焊盘的界面,在经过吸湿和回流焊实验后观察到界面分层沿着这些界面扩展.     

Interface delamination analysis of TQFP package during solder reflow

Interface delamination during solder reflow is a critical reliability problem for the plastic IC packages. The main objective of this study is to apply modified virtual crack closure method (MVCCM) for the analysis of interface delamination between the leadframe pad and the encapsulant during a lead-free solder reflow after the level 1 moisture preconditioning. In this study, the moisture diffusion parameters and the coefficient of moisture expansion (CME) of two different epoxy molding compounds (EMC) are characterized for moisture diffusion analysis and the deformation analysis due to hygroscopic swelling. At the same time, the entire thermal and moisture history of Thin Quad Flat Pack (TQFP) package is simulated from the start of level 1 moisture preconditioning (85 °C/85%RH for 168 h) to subsequent exposure to a lead-free solder reflow process. Finally, the transient development of the stress intensity factors due to thermal stress only K_t, hygrostress only K_h, vapor pressure only K_p and combined energy release rate G_tot are computed and studied by using MVCCM. Based on the calculated stress intensity factors and energy release rates, it seems that for the EMC, the Young’s modulus, moisture diffusion coefficient, CME and adhesion strength with leadframe at high temperature appear to be the most significant variables for the MSL performance of TQFP package and this matches well with the experimental finding.     

湿热环境下倒装焊无铅焊点的可靠性

对板上倒装芯片底充胶进行吸湿实验,并结合有限元分析软件研究了底充胶在湿敏感元件实验标准MSL—1条件下吸湿和热循环阶段的解吸附过程,测定了湿热环境对Sn3.8Ag0.7Cu焊料焊点可靠性的影响,并用蠕变变形预测了无铅焊点的疲劳寿命。结果表明:在湿热环境下,底充胶材料内部残留的湿气提高了焊点的应力水平。当分别采用累积蠕变应变和累积蠕变应变能量密度寿命预测模型时,无铅焊点的寿命只有1740和1866次循环周期。     

A simple moisture diffusion model for the prediction of optimal baking schedules for plastic SMD packages

The moisture concentration at the chip surface is the important parameter for the moisture sensitivity of the P-MQFP80 product considered here. When the critical moisture concentration at the die surface is reached, delamination occurs after soldering shock, e.g at 240°C. This critical moisture concentration, which can be determined by experiments conducted at 30°C/60% relative humidity (RH) followed by soldering shock, allows to predict the product’s moisture performance at other ambient conditions. In the case studied here, prediction was done at a customer use condition of 30°C/85% RH. Furthermore, this work showed that preconditioning of plastic packages not only induces the onset of delamination at the die surface but it appears to weaken the adhesion at this interface as well. As a result, delamination failure starts to occur earlier (i.e. within shorter moisture exposure time) in the devices tested after subsequent thermal cycling stress test. A simple moisture diffusion analytical model is proposed here for predicting the optimal baking schedules for plastic SMD packages.     

Integrated vapor pressure, hygroswelling, and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis

In this paper, a comprehensive and integrated package stress model is established for quad flat non-lead package with detailed considerations of effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress and vapor pressure induced during reflow. The critical plastic materials, i.e., moldcompound and die attach are characterized for hygroswelling and moisture properties, which are not easily available from material suppliers. The moisture absorption during preconditioning at JEDEC Level 1, and moisture desorption at various high temperatures are characterized. The moisture diffusivity is a few orders higher at reflow temperature than moisture preconditioning temperature. Due to coefficient of moisture expansion mismatch among various materials, hygro-mechanical stress is induced. The concept is analogous to coefficient of thermal expansion mismatch which results in thermo-mechanical stress. Thermal diffusivity is much faster than the moisture diffusivity. During reflow, the internal package reaches uniform temperature within a few seconds. The vapor pressure can be calculated based on the local moisture concentration after preconditioning. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure introduces additional strain of the same order as the thermal strain and hygrostrain to the package. Subsequently, the interfacial fracture mechanics model is applied to study the effect of crack length on die/mold compound and die/die attach delamination.     

Mechanism and thermal effect of delamination in light-emitting diode packages

This work reports on the mechanism of delamination in light-emitting diode (LED) packages and its effects on thermal characteristics of LEDs. The LED samples were subjected to moisture preconditioning followed by heat block testing. Transient thermal measurements were performed to investigate the thermal behavior of the delaminated LEDs. Increase of thermal resistance with the degree of delamination was observed from the transient measurement. The thermo-mechanical and hygro-mechanical stress distributions calculated from coupled-field FEA simulation agree well with the micrographical evidence. It was found that the thermo-mechanical stress plays more important role than the hygro-mechanical stress for the development of delamination in the LED packages. Moisture preconditioning for 3 and 6 h under 85 °C/85RH conditions was found to make little contribution to the delamination between the chip and lead frame.     

Effect of Hygrothermal Aging on Interfacial Reliability of Silicon/Underfill/FR-4 Assembly

The reliability issues have been converted to the underfill adjacent interfaces since the introduction of the underfill to flip chip package in 1990's. Both thermal cycling and hygrothermal conditioning severely attack the interfaces to de- laminate. The moisture migrating into the underfill decreases the adhesion strength, swells to deform the assembly, and weakens the mechanical and thermal properties of the material. In this study, interfacial reliability of a silicon/underfill/FR-4 assembly exposed at 85degC/85%RH was studied using moire interferometry and micro-digital image speckle correlation (mu-DiSC) techniques. A thermal aging study was simultaneously performed to understand the long-term reliability of the assembly. The results showed that the thermal aging relieved the stresses induced by hygrothermal swelling mismatch between dissimilar materials involved, whereas increased the strains induced by hygrothermal swelling. It indicated the time effect is not negligible when the assembly is subjected to the moisture conditioning, otherwise, the deformation induced by the swelling could be overestimated. The mu-DiSC technique was applied to measure the critical interfacial fracture toughness of the silicon/underfill interface. The results showed that the moisture could significantly reduce the interfacial strength due to the break of hydrogen bonding. By combining the moire and mu-DiSC results, it was concluded that the hygrothermal loading could increase the possibility of interfacial delamination in a flip chip package. Finally, the morphologies of the fractured surface were studied with the aid of scanning electron microscope. Remarkable changes of the failure mode were observed.     

叠层芯片封装元件热应力分析及焊点寿命预测

研究了温度循环载荷下叠层芯片封装元件(SCSP)的热应力分布情况,建立了SCSP的有限元模型。采用修正后的Coffin-Masson公式,计算了SCSP焊点的热疲劳寿命。结果表明:多层芯片间存在热应力差异。其中顶部与底部芯片的热应力高于中间的隔离芯片。并且由于环氧模塑封材料、芯片之间的热膨胀系数失配,芯片热应力集中区域有发生脱层开裂的可能性。SCSP的焊点热疲劳寿命模拟值为1 052个循环周,低于单芯片封装元件的焊点热疲劳寿命(2 656个循环周)。