QFN器件焊接缺陷分析与工艺优化

QFN器件具有良好的电气性能,但器件回流焊接过程中极易产生底部热沉焊盘焊接空洞、器件引脚间锡珠、桥连等缺陷,当一个印制板焊接多个QFN器件时,缺陷发生率颇高。在高可靠性要求的航天产品焊接过程中,器件返修次数有限制,且返修会造成器件性能下降、组件可靠性降低等问题,因此亟需对QFN器件一次装配良率和焊接效果进行提升优化。为此,从原理上分析QFN器件热沉焊盘焊接空洞、器件引脚间锡珠缺陷产生机理,并从产品焊盘工艺性设计、钢网模板设计、焊接温度曲线设计等方面开展分析与优化。优化后,QFN器件一次装配良率提高,没有产生锡珠、虚焊等缺陷。     

随机振动加载条件下QFN焊点的优化设计

采用基于最小能量原理和有限元数值分析方法的Surface Evolver软件，建立了四方扁平无引脚器件（QFN：Quad Flat No-lead）焊点三维形态预测模型；选取焊盘长度、焊盘宽度、焊料体积和间隙高度作为四个关键因素，采用水平正交表设计了9种不同的QFN焊点工艺参数水平组合，建立了这9种焊点的三维形态预测模型，得到了不同工艺参数水平组合下的QFN焊点形态；分析在随机振动加载条件下，焊盘长度、焊盘宽度、焊料体积和间隙高度四个工艺参数的改变对QFN焊点的应力应变的影响；通过对因子趋势图分析表明：在随机振动加栽下焊盘长度和焊盘宽度对焊点应力应变影响较大，间隙高度和焊料体积对应力应变影响较小；使QFN焊点应变值最小的参数细合为：焊盘长度为0．8mm；焊盘宽度为0．37mm；间隙高度为0．15mm；焊料体积为0.014mm^3。     

PCB粘弹性对QFN焊点可靠性的影响

基于动态拉伸DMA实验所获得的FR—4PCB的蠕变柔量曲线,用广义Maxwell模型表征了PCB的粘弹性蠕变松弛特性。通过有限元软件MSC Marc分别模拟了基于PCB弹性和粘弹性两种不同性质下,QFN器件在–55～+125℃热循环条件下的应力应变,并利用修正后的Coffin-Masson方程分别计算了它们的热疲劳寿命。结果表明,基于粘弹性条件下QFN焊点可靠性模拟结果更接近实际情况。     

QFN返修工艺技术

QFN器件体积小、质量轻,对返修温度敏感性大,其周边器件排布密集,容易因高温使可靠性受到影响,因此对于返修温度曲线提出了更高的要求,同时QFN器件的潮湿敏感性提出了对器件的烘烤需求,拆焊后焊盘残留物的清理要注意吸锡编带和烙铁头的选用等.对于上述问题给出了解决方案,对于QFN的返修工艺技术进行了全面的阐述.     

回焊炉内氧气体积分数对焊接可靠性的影响

SMT(表面贴装技术)回焊炉工作时,炉中氧气体积分数的差异很大,从100×10–6至10000×10–6不等,在已验证氧气体积分数[(O2)]超过4000×10–6会造成产品不良的前提下,针对500×10–6,1000×10–6,3000×10–6以及4000×10–6这四种不同的氧气体积分数,分别对回焊炉焊接的铜片上的锡焊点进行润湿角、EDS分析,对组装印制电路板(PCBA)上的锡焊点进行X射线、推力、通孔填充量等测试。结果显示,(O2)在4000×10–6以下,元件的焊接可靠性并无明显差异。选择(O2)=4000×10–6可降低回焊炉的氮气用量,节约成本。     

SMT中焊点形状对焊点可靠性的影响

根据SMTSnPb焊料焊点在热疲劳过程中的受载特点,采用有限元方法,建立了用于预测该种类型焊点热疲劳寿命的数值模型,并借热疲劳试验验证了模型的有效性。结果表明:热疲劳试验中,焊盘(焊盘1)伸出较短的SMT焊点在1440~1680周期之间发生大量破坏,且其热疲劳寿命要低于焊盘(焊盘2)伸出较长的焊点,这与有限元模拟预测的寿命结果基本一致。进而采用该模型研究了焊点形状对其热疲劳寿命的影响规律,在一定条件下,其结果可应用于无铅焊料。     

QFN器件的组装工艺

QFN器件的侧面若未形成良好的焊点,则无法通过目视检查判断其焊接的质量问题,并且QFN组装技术目前没有统一标准规范,为后期出现组装缺陷埋下隐患.针对上述问题,通过分析印制板焊盘及钢网设计对后期组装具有影响的主要因素,拟定工艺试验方案,设计试验样件,进行工艺试验.试验结果表明,通过对QFN器件焊盘设计以及钢网开孔设计技术...     

QFN器件组装工艺缺陷分析及预防措施

QFN器件在组装过程中容易出现信号焊盘桥连、散热焊盘空洞和侧面焊点爬锡高度不足等工艺缺陷。分析了缺陷形成机理,并从PCB焊盘设计、焊膏印刷模板设计提出了有效预防措施。     

QFN侧面焊点爬锡的影响因素分析

采用六西格玛的分析方法,通过二水平全因子试验设计,研究不同影响因素与具有非连续可焊端QFN侧面焊点爬锡的关系。试验结果表明:焊膏印刷质量对其影响最显著,其次为侧面焊盘是否预上锡。大量的验证试验证明,通过确保稳定的焊膏印刷质量及对老化筛选的QFN器件侧面焊盘进行良好的预上锡处理可以获得润湿良好、可靠的QFN焊点。     

集成湿热及蒸汽压对塑封QFN器件的层裂影响

针对微电子封装器件的界面层裂失效问题,利用有限元法,集成湿热及蒸汽压力的作用对塑封QFN器件的界面层裂失效问题进行了建模分析,探索了塑封QFN器件参数的优化组合。结果表明,通过对各参数的优化,如EMC的αv为12×10–6/K,引线框架的E为110GPa等,界面危险裂纹尖端点的J积分降低到优化前的1/10~1/100。研究中还发现,封装器件的参数优化组合不唯一,很有必要探讨并选择一种能适合于这种多优化组合设计的方法。     

Comprehensive board-level solder joint reliability modeling and testing of QFN and PowerQFN packages

For quad flat non-lead (QFN) packages, board-level solder joint reliability during thermal cycling test is a critical issue. In this paper, a parametric 3D FEA sliced model is established for QFN on board with considerations of detailed pad design, realistic shape of solder joint and solder fillet, and non-linear material properties. It has the capability to predict the fatigue life of solder joint during thermal cycling test within ±34% error. The fatigue model applied is based on a modified Darveaux’s approach with non-linear viscoplastic analysis of solder joints. A solder joint damage model is used to establish a connection between the strain energy density (SED) per cycle obtained from the FEA model and the actual characteristic life during thermal cycling test. For the test vehicles studied, the maximum SED is observed mostly at the top corner of peripheral solder joint. The modeling predicted fatigue life is first correlated to thermal cycling test results using modified correlation constants, curve-fitted from in-house QFN thermal cycling test data. Subsequently, design analysis is performed to study the effects of 17 key package dimensions, material properties, and thermal cycling test condition. Generally, smaller package size, smaller die size, bigger pad size, thinner PCB, higher mold compound CTE, higher solder standoff, and extra soldering at the center pad help to enhance the fatigue life. Comparisons are made with thermal cycling test results to confirm the relative trends of certain effects. Another enhanced QFN design with better solder joint reliability, PowerQFN, is also studied and compared with QFN of the same package size.     

QFN焊盘设计及工艺组装

随着电子产品向更轻、更薄、更小、高密度化和高可靠性的发展,QFN（方形扁平无引脚）封装由于具有良好的电和热性能、体积小、质量轻,在电子产品中被越来越广泛的推广和应用。文章对QFN器件的焊盘设计,网板设计及组装工艺作了详细的介绍。     

LGA焊点形态对焊点寿命影响的有限元分析

通过Surface Evolver软件对LGA焊点进行了三维形态预测,利用有限元数值模拟对LGA焊点在热循环条件下寿命进行了分析。研究了热循环条件下LGA焊点的应力应变分布规律,随着焊点远离元件的中心位置焊点所受到的等效应力、等效应变和塑性应变能密度逐渐增大,从而得出处于外面拐角的焊点最先发生失效的结论。基于塑性应变范围和Coffin-Manson公式计算了焊点热疲劳寿命;找出了LGA焊点形态对焊点寿命的影响规律,模板厚度一定时PCB焊盘尺寸小于上焊盘时LGA焊点的热疲劳寿命与PCB焊盘尺寸成正比,大于上焊盘时成反比,大约相等时焊点寿命最大。当PCB焊盘和模板开孔尺寸固定时,通过增大模板厚度来增加焊料体积在一定程度上可提高LGA焊点的热疲劳寿命,但是模板厚度增大到一定值时LGA焊点寿命会逐渐降低。     

QFN封装元件组装工艺技术研究

QFN是一种焊盘尺寸小、体积小、以塑料作为密封材料的新兴的表面贴装芯片封装技术。由于底部中央的大暴露焊盘被焊接到PCB的散热焊盘上,这使得QFN具有极佳的电和热性能。QFN封装尺寸较小,有许多专门的焊接注意事项。介绍了QFN的特点、分类、工艺要点和返修。     

QFN封装元件组装工艺技术的研究

QFN(Quad Flat No-lead Package,方形扁平无引脚封装)是一种焊盘尺寸小、体积小、 以塑料作为密封材料的新兴表面贴装芯片封装技术。由于底部中央大暴露焊盘被焊接到PCB的散热焊 盘上,这使得QFN具有极佳的电和热性能。QFN封装尺寸较小,有许多专门的焊接注意事项。文章 介绍了QFN的特点、分类、工艺要点和返修。     

粘结剂形态及尺寸对QFN器件热应力的影响

使用有限元软件MSC.Marc分析了芯片粘结剂的形态、厚度和宽度对典型微电子封装QFN(四方扁平无引脚封装)器件热应力的影响。结果表明:在有限元网格密度相同的条件下,粘结剂形态的不同会对QFN器件的热应力产生较大影响,粘结剂无溢出形态的最大热应力为85.87MPa,而粘结剂有溢出形态的最大热应力为77.84MPa,并且最大热应力出现的位置也不同;粘结剂的厚度和宽度对热应力的影响不明显;由于粘结剂形态的不同界面热应力的分布会有较大差别。     

Reliability Analysis of Lead-Free Solder Castellations

In this paper, the reliability of lead-free solder castellations is considered. The newly developed stress-dependent Engelmaier's solder fatigue model is utilized in this task. Based on this model, it is possible to interpret the thermal cycling test results. A very good agreement between the test results and the lifetime predictions is obtained. Using the lifetime prediction model, optimal solder castellation shape is investigated. Based on the findings, the fatigue life can be improved by up to 30% simply by solder pad length optimization. Further increment in lifetime length can be expected if the solder joint shape is optimized with the help of modeling tools presented here. Understanding how the crack propagates in solder material is vital if optimal lifetime behavior is expected.     

An optimal structural design to improve the reliability of Al2O3–DBC substrates under thermal cycling

An optimal structural design of direct bonding copper (DBC) substrate with ladder shaped copper layers was proposed through numerical optimization approach in this paper. In order to study the fatigue mechanism and life, thermal–mechanical stress and strain distributions of DBC substrates under thermal cycling ranged from − 55 °C to 150 °C were simulated and analyzed by finite element (FE) method. Improved Coffin–Mason law was applied to calculate the fatigue life under thermal cycling, Chaboche constitutive model with non-linear kinematic hardening was used to describe the elastic–plastic behavior of ductile copper. Design of experiment method was applied to investigate the sensitivity of geometric parameters on the thermal–mechanical performance. The ladder shaped DBC substrate design with specific designed copper ladder thickness was demonstrated to be robust. The fatigue life of proposed design was about three times longer than the traditional one under thermal cycling test. Moreover, the length of edge tail of the second copper ladder played an important role on the fatigue life. The lifetime of the substrate can be improved by the increase of edge tail length. The simulated results of the DBC substrate fatigue life were verified by experimental results.     

Thermosonic Wire Bonding Process Simulation and Bond Pad Over Active Stress Analysis

In this paper, a transient nonlinear dynamic finite element framework is developed, which integrates the wire bonding process and the silicon devices under bond pad. Two major areas are addressed: one is the impact of assembly 1st wire bonding process and another one is the impact of device layout below the bond pad. Simulation includes the ultrasonic transient dynamic bonding process and the stress wave transferred to bond pad device and silicon in the 1st bond. The Pierce strain rate dependent model is introduced to model the impact stain hardening effect. Ultrasonic amplitude and frequency are studied and discussed for the bonding process. In addition, different layouts of device metallization under bond pad are analyzed and discussed for the efforts to reduce the dynamic impact response of the bond pad over active design. Modeling discloses the stress and deformation impacts to both wire bonding and pad below device with strain rate, different ultrasonic amplitudes and frequencies, different friction coefficients, as well as different bond pad thickness and device layout under pad. The residual stress, after cooling down to a lower temperature, is discussed for the impact of substrate temperature.     

Reliability of thick Al wire bonds in IGBT modules for tractionmotor drives

Reliability enhancement of thick Al wire bonds during thermal fatigue test has been investigated from a metallurgical viewpoint. Al wire bonds degrade with the increase of crack length during thermal fatigue tests with high ΔT_j due to the tensile stress generated by the thermal expansion coefficient mismatch between Al wires and Si. It is also found that cracks propagate along the small grain boundaries of Al wires at the bonding interface. It is predicted that the Al wire bonds may not degrade due to thermal fatigue if ΔT_j is controlled below 40 K, i.e., keeping it within the actual temperature fluctuation range in IGBT modules for traction motor drives. The reliability of Al wire bonds can be enhanced by increasing the grain size of the Al wire at the bonding interface. The high temperature bonding is considered to be a good candidate for enhancing the reliability of Al wire bonds