Properties of molding compounds to improve package reliability ofSMDs

Package cracking during reflow soldering process is the great problem in the reliability of plastic packages. The technique of lowering the glass transition temperature (Tg) of a molding compound is very effective for improvement of the package cracking resistance because of the properties of low moisture absorption and high adhesion strength for a molding compound. But the package reliability except for the package cracking resistance is also important. In this study, the effects of the Tg for molding compounds on the package reliability was discussed. It was confirmed that decreasing the crosslinking density was an important factor to improve the package cracking resistance. There was no problem in thermal resistance, even if the molding compound has low Tg. However, decreasing the crosslinking density by the lowering of Tg may not satisfy humidity resistance in some cases. It was found to be important to decrease the crosslinking density of molding compounds without lowering Tg in order to improve both the package cracking resistance and the humidity resistance. It was also confirmed that the introduction of rigid structural segments into the matrix resin molecules of the molding compound was a useful technique for achieving excellent package reliability     

LQFP和eLQFP热机械可靠性的有限元分析

封装形式的差异性对产品可靠性具有重要影响。基于有限元法,对比分析了薄型四方扁平封装(LQFP)和载体外露薄型四方扁平封装(eLQFP)在室温和回流焊温度下的翘曲、芯片和粘片胶的应力水平以及各材料界面应力分布。研究表明,LQFP的翘曲比eLQFP的大,但芯片和粘片胶上的最大应力无明显差别;eLQFP在塑封材料与芯片有源面界面的应力水平比LQFP的大;eLQFP在芯片与粘片胶界面、粘片胶与芯片载体界面的剪切应力比LQFP的大,但eLQFP在芯片与粘片胶界面、粘片胶与芯片载体界面的剥离应力比LQFP的小;eLQFP在塑封材料与芯片载体镀银区界面的应力水平高于LQFP的应力水平,由于塑封材料与镀银芯片载体的结合强度弱,eLQFP更易发生界面分层。     

Characterization of chip scale packaging materials

The mechanical stability of Chip Scale Packages (CSP) used in surface mount technology is of primary concern. The dominant issues are package warpage and solder fatigue in solder joints under cyclic loads. To address these issues, molding compound and die attach film were characterized with finite element method which employed a viscoelastic and viscoplastic constitutive model. The model was verified with experiments on package warpage, PCB warpage and solder joint reliability. After the correlation was observed, the effect of molding compound and die attach film on package warpage and solder joint reliability was investigated. It was found that package warpage tremendously affected solder joint reliability. Furthermore, a die attach film was developed based on results of the modeling. CSP with the developed die attach film are robust and capable of withstanding the thermal stresses, humidity and high temperatures encountered in typical package assembly and die attach processes. Also, a lead free solder is discussed based on the results of creep testing. This paper presents the viscoelastic and viscoplastic constitutive model and its verification, the optimum material properties, the experimental and simulated reliability and performance results of the u*BGA packages, and the lead free solder creep.     

Exposed Die-Top Encapsulation Molding for an Improved High- Performance Flip Chip BGA Package

The recent advancement in high- performance semiconductor packages has been driven by the need for higher pin count and superior heat dissipation. A one-piece cavity lid flip chip ball grid array (BGA) package with high pin count and targeted reliability has emerged as a popular choice. The flip chip technology can accommodate an I/O count of more than five hundreds500, and the die junction temperature can be reduced to a minimum level by a metal heat spreader attachment. None the less, greater expectations on these high-performance packages arose such as better substrate real estate utilization for multiple chips, ease in handling for thinner core substrates, and improved board- level solder joint reliability. A new design of the flip chip BGA package has been looked into for meeting such requirements. By encapsulating the flip chip with molding compound leaving the die top exposed, a planar top surface can be formed. A, and a flat lid can then be mounted on the planar mold/die top surface. In this manner the direct interaction of the metal lid with the substrate can be removed. The new package is thus less rigid under thermal loading and solder joint reliability enhancement is expected. This paper discusses the process development of the new package and its advantages for improved solder joint fatigue life, and being a multichip package and thin core substrate options. Finite-element simulations have been employed for the study of its structural integrity, thermal, and electrical performances. Detailed package and board-level reliability test results will also be reported     

PBGA封装的可靠性研究综述

通过传统BT类型的PWB材料与独特的PWB材料来PBGA封装的可靠性。相关的研究结果表明,后者同样具有相同的热循环稳定性和回流焊期间的疲劳强度,并具有较低的封装翘曲特点;模塑料的低吸湿性及粘片材料的高粘附强度和高断裂强度特性,有利于提高回流焊期间的疲劳强度和防止剥离现象的扩散。     

Reliability of plastic ball grid array package

The reliability of plastic ball grid array (PBGA) package is studied for different materials. The reliability of the PBGA packages using conventional Bismaleimide-Triazine type PWB and our original product PWB that is made of high Tg epoxy resin is evaluated. The PBGA package using our original PWB has a feature of lower warpage for the package, and has similar performance regarding the thermal cycling stability and the endurance during reflow soldering as compared with the PBGA using the conventional PWB. The endurance during reflow soldering for each PBGA is JEDEC STD Method A112 level 3. In order to improve the endurance during reflow soldering, not only PWB materials but also other factors are investigated. As a result, the molding compound with the property of low moisture absorption and the die attach material with the properties of high adhesion strength and fracture strength are effective to improve the endurance property during reflow soldering. The package crack mechanism during reflow soldering is briefly described as follows     

A simple model for the mode I popcorn effect for IC packages

A simple model for the Mode I popcorn effect is presented here for packages with rectangular die pad (P-DSO). A package “stability parameter”, relating to its moisture sensitivity, is derived from the popcorn model. It describes the critical factors for a robust package - molding compound properties and package, leadframe design for a given preconditioning and soldering process. Furthermore, nomograms generated from the model enable an easy estimation of moisture sensitivity levels (between 1 and 5) of packages with different die pad sizes and molding compound underpad thicknesses and for different soldering temperatures ranging from 220°C to 260°C (Pb-free soldering).     

环氧塑封料中填充剂的作用和发展

环氧塑封料占据整个半导体封装市场的90％左右,而填充剂含量占环氧塑封料的60％～90％,因此填充剂的性能直接影响环氧塑封料的加工性能、机械性能、导热性能和半导体器件的封装工艺性能、导热性能、可靠性能等。另外,当今社会电子技术日新月异,集成电路正向着超大规模、超高速、高密度、大功率、多功能、绿色环保化的方向发展,因此对环氧塑封料的性能要求愈来愈高,相应的填充剂性能也有了许多新的要求,并且也出现了许多新型填充剂。文中详细地介绍了环氧塑封料中填充剂的作用,各种填充剂对环氧塑封料和封装器件的性能影响以及环氧塑封料中填充剂的分类和发展。     

Computational parametric analyzes on the solder joint reliability of bottom leaded plastic (BLP) package

The bottom-leaded plastic (BLP) package is a lead-on-chip type of chip scale package (CSP) developed mainly for memory devices. Because the BLP package is one of the smallest plastic packages available, solder joint reliability becomes a critical issue. In this study, a 28-pin BLP package is modeled to investigate the effects of molding compound and leadframe material properties, the thickness of printed circuit board (PCB), the shape of solder joint and the solder pad size on the board level solder joint reliability. A viscoplastic constitutive relation is adopted for the modeling of solder in order to account for its time and temperature dependence on thermal cycling. A three-dimensional nonlinear finite element analysis based on the above constitutive relation is conducted to model the response of a BLP assembly subjected to thermal cycling. The fatigue life of the solder joint is estimated by the modified Coffin-Manson equation. The two coefficients in the modified Coffin-Manson equation are also determined. Parametric studies are performed to investigate the dependence of solder joint fatigue life on various design factors.     

MicroSMD-a wafer level chip scale package

This paper outlines National Semiconductor's concept of wafer level chip scale package-also known as microSMD. This new packaging technology has been demonstrated using an 8 I/O package with 0.5 mm bump pitch, and is ideally tailored for low pin count analog and wireless devices. Product extensions to higher pin count (up to 48) are under various stages of qualification. The package construction, process flow, and package reliability are described, together with board level assembly processes and interconnect reliability     

Reliability Study of High-Density EBGA Packages Using the Cu Metallized Silicon

The reliability of high-density enhanced ball grid array (EBGA) packages using the eight-layer Cu metallization silicon was discussed. The key failure mechanisms included the die cracking (in the vicinity of the edge) and thin film delamination. It was noticed that the failure was unique to the Cu metallization silicon. The large package body size (45 mm$^{2}$) and the die size (approximately 15 mm$^{2}$ ) provided additional manufacturing and reliability challenges. The die-edge defects induced during the wafer sawing process were exhibited to be the culprits of the die cracking and the thin film delamination failures. Additionally, the height of die attach fillets significantly influenced the stresses on the die edge, and the excessive fillet height was found to help extend initial cracks at the edge of the silicon. The results demonstrated the adoption of a dual-step wafer sawing scheme and resin blades would control the defects and reduce the failure rate dramatically. A mixture of low-stress encapsulation and die attach materials would help improve the overall reliability of the packages as well. The solder joint reliability of the package was very robust based on the board-level reliability testing results. The statistical analysis of the test results confirmed that most of the die cracking and thin film delamination failures were early-life failures and random. A good sample screening scheme and the process improvement procedure would help improve the reliability and insure the customer a low failure rate for the lifetime of the product. The predicted reliability of the package met the application life needs for the products with process improvement plans in place.      

Use of C-SAM acoustical microscopy in package evaluations and failure analysis

Acoustical microscopy is gaining wide acceptance in the microelectronic packaging community. C-mode scanning acoustical microscopy, C-SAM, is widely used in package evaluations and for failure analysis. This paper discusses several specific topics. These include: (1) popcorn cracking in SMDs; (2) an evaluation of solder die attach in power packages; (3) an instance of top of die delamination which resulted in electrical failures; and (4) moisture sensitivity of other surface mount power packages and how it resulted in ball bond degradation during a new product qualification.     

Mechanical cycling fatigue of PBGA package interconnects

Increased packaging density in micro-electronic products has advantaged attach of BGA, micro-BGA, CSP, and DCA packages. These area array packages are assembled to circuit boards that are reduced in size and thickness, by necessity. These assemblies would include flexible thin laminate circuit boards with area array components attached by solder balls. In normal use, these assemblies would be subjected to numerous ultra-low frequency mechanical deflections; consider a keypad when the user enters telephone numbers. Most of the reliability studies of area array packages have dealt with temperature cycling induced fatigue. However, less attention has been paid to mechanical bending fatigue of these packages.A test method has been developed to elucidate the mechanical bending fatigue issues of BGA, micro-BGA, CSP, and DCA packages attached to printed circuit boards. Appropriate bending fatigue reliability models and their theoretical basis are being developed. The test method and preliminary mechanical cyclic fatigue data on a PBGA package will be presented as a function of printed circuit board thickness. Consideration will be given to fatigue fracture morphology and its relation to solder joint location and rate of crack growth.     

浅谈IC封装材料对产品分层的影响及改善

IC封装不仅要求封装材料具有优良的导电性能、导热性能以及机械性能,还要求具有高可靠性、低成本和环保性,这也是引线框架、环氧树脂成为现代电子封装主流材料的主要原因,其市场份额约占整个封装材料市场的95%以上。由于环氧树脂封装是非气密性封装,对外界环境的耐受能力较差,尤其是受到湿气侵入时,产品会出现一些可靠性问题,最容易发生的现象是分层。简要分析了框架和环氧树脂对产品可靠性的影响,在此基础上提出一些改善措施。     

Highly reliable flip-chip-on-flex package using multilayered anisotropic conductive film

Anisotropic conductive film (ACF) has been used as interconnect material for flat-panel display module packages, such as liquid crystal displays (LCDs) in the technologies of tape automated bonding (TAB), chip-on-glass (COG), chip-on-film (COF), and chip-on-board (COB). Among them, COF is a relatively new technology after TAB and COG bonding, and its requirement for ACF becomes more stringent because of the need of high adhesion and fine-pitch interconnection. To meet these demands, strong interfacial adhesion between the ACF, substrate, and chip is a major issue. We have developed a multilayered ACF that has functional layers on both sides of a conventional ACF layer to improve the wetting properties of the resin on two-layer flex for better interface adhesion and to control the flow of conductive particles during thermocompression bonding and the resulting reliability of the interconnection using ACF. To investigate the enhancement of electrical properties and reliability of multilayered ACF in COF assemblies, we evaluated the performance in contact resistance and adhesion strength of a multilayered ACF and single-layered ACF under various environmental tests, such as a thermal cycling test (−55°C/+160°C, 1,000 cycles), a high-temperature humidity test (85°C/85% RH, 1,000 h), and a high-temperature storage test (150°C, 1,000 h). The contact resistance of the multilayered ACF joint was in an acceptable range of around a 10% increase of the initial value during the 85°C/85% RH test compared with the single-layered ACF because of the stronger moisture resistance of the multilayered ACF and flex substrate. The multilayered ACF has better adhesion properties compared with the conventional single-layered ACF during the 85°C/85% RH test because of the enhancement of the wetting to the surface of the polymide (PI) flex substrate with an adhesion-promoting nonconductive film (NCF) layer of multilayered ACF. The new ACF of the multilayered structure was successfully demonstrated in a fine-pitch COF module with a two-layer flex substrate.     

SCSP中芯片黏结层在焊接时的可靠性

提出了一个细观力学模型,该模型同时考虑了热膨胀和蒸汽膨胀对叠层芯片尺寸封装(SCSP)中芯片黏结层变形的影响.当初始温度确定时,由该模型可求得给定温度下芯片黏结层内部的蒸汽压力和孔隙率,从而判断芯片黏结层在焊接回流时的可靠性.当温度从100℃升高到250℃时,芯片黏结层的饱和蒸汽压、等效弹性模量及孔隙率分别从0.10 ...     

晶圆级芯片规模封装——微型表面贴装元器件

概述了美国国家半导体的晶圆级芯片规模封装技术——也就是微型表面贴装元器件(Mi-croSMD)。采用8I/O数、凸点节距为0.5mm封装论证此新型封装技术,该技术满足于低管脚数模拟和无线元器件。较高管脚数(多达48)产品扩展在各种范围的限定条件之内。论述了封装结构、工艺流程及封装可靠性,并阐述了板级组装工艺过程和互连可靠性。     

应用于MEMS气密性封装测试的微型湿度传感器的设计与制作

MEMS器件的封装一直是MEMS技术的难点之一 ,在封装设计中 ,如何测试封装的有效性就显得尤为重要。本文叙述了一种基于MEMS技术的微型湿度传感器的原理、设计以及工艺流程。在其上进行气密性封装 ,则可通过对封装内的湿度测量来判断该封装的气密性能。在设计中 ,充分考虑了尺寸、工艺以及灵敏度等各方面要求。制作采用的是传统的光刻、刻蚀工艺。该湿度传感器结构简单 ,易于制作 ,其性能能够满足气密性封装测试的要求     

Molding compound trends in a denser packaging world: qualificationtests and reliability concerns

This work benchmarks the current reliability tests used by the electronics industry, examines those tests that affect and are affected by molding compounds, discusses the relevance of accelerated testing, and addresses the major reliability issues facing current molding compound development efforts. Six compound-related reliability concerns were selected: moldability; package stresses; package cracking; halogen-induced intermetallic growth at bond pads; moisture-induced corrosion; and interfacial delamination. Causes of each failure type are surveyed and remedies are recommended     

Measurement of water absorption in ball grid array package

The plastic ball grid array (BGA) package has poor resistance to popcorn cracking, which occurs when high temperatures involved in soldering cause water vapor to expand rapidly. Popcorn cracking occurs at die attach paste, and therefore water absorption and desorption occurring in the vicinity of die attach paste must be studied. We examine the mechanism of popcorn cracking in a BGA, particularly from the aspect of water absorption distribution. Water absorption was simulated by use of deuterium oxide, because the absorption performance of deuterium oxide approximates that of water. Deuterium oxide absorption distribution was measured by time of flight secondary ion mass spectroscopy (ToF-SIMS). We found that the water is absorbed mainly through the upper side of molded portion of the BGA package, and that absorption through the substrate is small. A BGA substrate has a laminated structure, and therefore water cannot penetrate the substrate first. On the basis of the results obtained in our study, we designed a BGA package system that is not prone to popcorn cracking