摘要
在电子市场的需求下，多功能、高密度、低成本以及微型化电子产品成为发展的主要方向。裸芯片通过焊点直接组装到电子产品的结构中，省去传统组装工艺的封装过程，即直接进行高密度的板卡组装，这一过程缩小了组装体积，减少了工艺步骤，因此能够减小电子产品尺寸和降低工艺成本，特别是细间距倒装芯片组装。Ni\Au和OSP是目前工业界应用较广泛的焊盘界面保护层，在传统的封装工艺中，由于芯片在封装、组装及返修过程中需要经历多次回流，因此多选用Ni\Au；而OSP较之Ni\Au简化了工艺流程和降低了成本，因此是电子组装中基板侧Cu焊盘的理想选择。
尽管倒装芯片组装具有上述优点，但裸芯片和PCB的CTE相差较大，在运行及热循环环境中易因热失配而导致焊点失效，特别是细间距倒装焊点承受着更高的应力和应变。此外，倒装焊点尺寸小，焊点的微观组织在运行和可靠性环境中会发生较大改变，而焊点的微观组织改变会导致新的可靠性问题出现。
本论文的研究目的：为细间距（100μm）倒装芯片通过微小焊点直接组装到PCB基板上，且基板侧和芯片侧焊盘保护层分别为OSP和Ni\Au，做一定的基础理论研究；同时选用较大间距（200μm）倒装芯片，采用相同的保护层、工艺、组装结构以及可靠性试验，与细间距倒装组装焊点对比，评估倒装组装焊点尺寸效应对焊点微观组织和可靠性的影响；最后通过基础理论研究、可靠性试验以及对比大间距倒装芯片组装的可靠性评估100μm倒装芯片组装在工业上应用的可行性。研究思路和内容为：采用焊点间距分别为100μm和200μm的倒装芯片组装，研究在回流过程、热时效和热冲击过程中焊点界面和基体中IMC生长和演变，探讨尺寸效应对焊点IMC生长和演化的影响；研究异质焊盘界面原子的交互作用对IMC生长和演化影响；研究焊点在热冲击条件下裂纹的萌生和生长、裂纹生长机理以及尺寸效应对焊点裂纹生长机影响，并评估IMC对焊点可靠性的影响；建立三维有限元模型对焊点承受的力学行为进行模拟，获得评估焊点失效的损伤尺度。此外，利用威布尔分布概率统计，评估两种倒装芯片组装在热冲击条件下的可靠性寿命；利用Darveaux理论寿命预测模型评估焊点寿命,并建立寿命预测模型。最后评估细间距（100μm）倒装芯片组装，且基板侧Cu/OSP，应用到工业上的可行性。研究内容和结果如下：
研究凸点制备回流及组装互连回流过程结果显示：在凸点制备回流过程中，由于凸点的尺寸效应，在100μm间距凸点Ni焊盘界面上形成的主要IMC为(Ni,Cu)_3Sn_4，呈长针状；而在200μm间距凸点Ni焊盘界面上形成的主要IMC为(Cu,Ni)_6Sn_5，呈标准的钻石状，在其下面形成了一薄层(Ni,Cu)_3Sn_4，呈团聚的颗粒状。通过反应前后Cu原子的质量守恒公式可算出，在100μm和200μm间距焊点中Cu原子在回流反应后的残余量分别为0.15%和0.31%，结合相图分析可知焊点中Cu原子的残余含量决定着界面主要IMC的种类和形貌。互连回流过程中，在Ni焊盘界面上，由于Cu焊盘界面Cu原子的交互作用，100μm间距焊点界面上的主要IMC(Ni,Cu)_3Sn_4转化成(Cu,Ni)_6Sn_5，晶粒形貌转化成细棒状；200μm间距焊点界面上IMC种类没有发生变化，但主要IMC(Cu,Ni)_6Sn_5厚度和晶粒数目增加明显；在Cu焊盘界面上，两种尺寸焊点中均形成了(Cu,Ni)_6Sn_5，但200μm焊点中的棒状(Cu,Ni)_6Sn_5更长，且(Cu,Ni)_6Sn_5中存在着很多微孔。
热时效过程结果显示：在Ni焊盘界面上，对于100μm间距焊点，短的互连高度缩短了Cu原子从Cu焊盘界面扩散到Ni焊盘界面的扩散距离，导致了Ni焊盘界面上的(Ni,Cu)_3Sn_4全部转化成(Cu,Ni)_6Sn_5。在Cu焊盘界面上，对于两种尺寸焊点，由于Cu焊盘界面(Cu,Ni)_6Sn_5中Ni原子对Cu_3Sn的抑制作用，在时效过程中Cu_3Sn生长很慢。同时两种尺寸焊点两侧界面IMC生长速率先快后缓，Cu焊盘界面IMC生长速率远高于Ni焊盘界面，100μm间距焊点界面IMC生长速率高于200μm间距焊点界面。IMC生长动力学研究表明，两种尺寸焊点界面IMC的生长均为界面原子扩散控制，100μm和200μm间距焊点Cu焊盘界面(Cu,Ni)_6Sn_5生长的热激活能分别为67.89kJ/mol和77.68kJ/mol。
热冲击过程结果显示：热冲击过程中两种尺寸焊点双侧焊盘界面IMC的生长和演化趋势与热时效过程相似，然而界面和焊料基体中IMC的尺寸明显增大，通过有限元模拟可知，热冲击过程中焊点承受着更高应力，高应力促进了IMC的生长。焊点的尺寸效应对IMC影响研究发现，100μm间距焊点界面和焊料基体中IMC的生长速率要明显高于200μm间距焊点，通过有限元模拟可知，小尺寸焊点承受更大的应力，因此促进了IMC的生长。IMC生长动力学研究可知：100un间距焊点界面IMC生长的动力学时间常数n和生长系数D分别为2.47和0.203μm/h~(1/2)，200μm间距分别为2.18和0.104μm/h~(1/2)，由此可知热冲击条件下Cu焊盘界面IMC生长受界面原子扩散控制。
热冲击条件下焊点的可靠性研究结果显示：对于100μm间距焊点，焊点失效属于疲劳裂纹失效，通过有限元模拟读出焊点的损伤尺度，利用损伤尺度在焊点上的分布分析了裂纹的生长，与实验结果非常一致。此外，从金属学原理的位错和回复与再结晶角度分析了100μm间距焊点的失效机理，可以推断热冲击条件下大量位错在芯片侧焊盘和焊料基体间的界面处聚集以及回复再结晶导致Sn晶粒的增多是导致裂纹萌生和扩展的主要原因。而200μm间距焊点，失效属于提前脆性裂纹失效，机理分析可知Ni焊盘上大量颗粒状团聚的(Ni,Cu)_3Sn_4增加了界面能，为裂纹的生长提供能量；P等杂质扩散至(Ni,Cu)_3Sn_4界面降低了界面的粘结性，为裂纹的生长提供位置；芯片侧Ni焊盘界面处于高应力集中状态为裂纹生长提供驱动力。根据威布尔分布寿命统计可知焊点间距为100μm和200μm倒装芯片组装的特征寿命分别是4714次和6217次；根据Darveaux寿命预测模型计算100μm和200μm间距边角倒装焊点的寿命分别为6171次和7227次；100μm间距焊点Darveaux寿命预测模型常数K_1，K_2，K_3和K_4分别为1648.96、-0.2349、0.00479和-0.7004，200μm间距焊点Darveaux寿命预测模型常数K_1，K_2，K_3和K_4分别为551.6975、-1.1095、0.2362和1.5477。
通过研究结果可知，采用100μm细间距倒装芯片组装，并选用OSP和Ni/Au分别作为基板侧和芯片侧焊盘保护层，应用于电子产品的组装中是可行的。需要注意的是，本课题主要关注倒装芯片直接完成与PCB的组装，不需要经过多次回流，因此OSP作为基板侧Cu焊盘保护层可行，但如果焊点需要多次回流，Cu焊盘界面IMC易生长过厚导致焊点失效，因此需要做进一步评估研究。
As the requirement of electronic market, the electronic products with the multi-functonal, higher density and miniaturization have been developed. Meanwhile, how toimprove the products function combined with the price reduction become the aim for theelectronic company. The sillicon die can assembly into the electronic product through thesolder joints as a result of the reduction of both assembly size and price, especially for thefine-pitch flip chip assembly. Ni\Au and OSP are common surface finish used in electronicindustry. In typical assembly process, the solder joints as interconnection usuallyexperienced several reflows, if OSP was used, it can make IMC much thicker as a result ofnagative influence for solder joint reliability. While for Ni\Au, the interfacial IMC growth isslow, and thus suitable for milti-reflow process of typical electronic assembly. However,compared to Ni\Au, OSP can reduce the expense and and simplify the process, thereforeOSP is a ideal choice as PCB Cu surface finish for the electronic assembly with one reflowprocess.
The flip chip has everal advantages as mentioned above. However, due to the CTEdifference between silicon die and PCB, the solder joints are easier to fail under theoperation and thermal cycle, especially for the fine pitch solder joints. In addition, smallersize can cause the great microstructure change of solder joints under the operation andthermal cycle condition, therefore the solder joints of flip chip assembly have to face thenew reliability problem.
The aim of the project: the100μm pitch flip chip assembly with OSP on the substrateCu pad and Ni/Au on the chip Ni pad was evaluated to be used on the real electronic productby the study of basic theory, reliability experiments and the comparison with the big pitchassembly. In this thesis,100μm and200μm pitch of flip chip assembly were used toinvestigate the microstructure evolution under reflow, thermal aging and thermal shock,analyze the size effect of microstructure evolution, and evaluate the influence on the solderjoint reliability. The3-D element model was built to anslyze the mechanics behavior. Andmeanwhile, the failure mechanism of solder joint and the influence of size effect on failurewere analyzed. Additionally, the theoretical model was used to evaluate the solder jointlifetime, and the lifetime model of solder joint was built according to the parameter oftheoretical model. At last, the100μm pitch flip chip assembly with Cu/OSP on substrate sidewas evaluated to be used in real electronic product。The detail study result followed asbelow:
The IMC growth and evolution were investigated on the single Ni pad surface for bothsizes of solder bumps after bump making reflow, and they were still studied on both surfacesof Ni and Cu pad side. The results show that due to the size effect of solder joint, thedifferent types of IMC formed on the Ni pad interface for200μm and100μm pitch, they areseparately (Cu,Ni)_6Sn_5and (Ni,Cu)_3Sn_4, and the morphology of IMC are diamond shapedand needle shaped correspondingly. After interconnection reflow, based on the Cu inter-diffusion effect on the Cu pad, the (Ni,Cu)_3Sn_4transit to (Cu,Ni)_6Sn_5, and the morphologywas exchanged to rod shaped, however the IMC thickness and IMC grain numbers wereincreased obviously. On the Cu pad interface, the (Cu,Ni)_6Sn_5was formed in both size ofsolder joints, however the rod shaped of (Cu,Ni)_6Sn_5is much longer for200μm solder joint,and there are a lot of micro-voids existed on the (Cu,Ni)_6Sn_5.
The IMC growth and evolution were investigated on both side surfaces of both sizes ofsolder bumps under thermal aging. The results show that the stand-off height has a greatinfluence on the Cu diffusion towards Ni pad interface as a result of all (Ni,Cu)_3Sn_4transition, while for200μm pitch solder joint, even aging to650h, the (Ni,Cu)_3Sn_4stillexisted on the Ni pad interface. Due to the effect suspession of Ni on Cu_3Sn growth, theCu_3Sn growed slowly. Forthermore, the IMC growth rate become slower and slower as theaging time increase for both size of solder joints, and it is quicker on the Cu surface than Nisurface. The IMC growth dynamics shows that the IMC growth is controled by theinterfacial atoms diffusion mechanism for two sizes of solder joints, and thermal activationenergy of interfacial IMC are67.89kJ/mol and77.68kJ/mol separately.
The IMC growth and evolution were investigated on both side surfaces of both size ofsolder bumps under thermal shock. The results show that the trend of IMC growth andevolution under thermal aging were similar with that under thermal shock, however the IMCon pad surface and solder matrix was bigger obviously, more stress promoted the IMCgrowth under thermal shock combined with the simulation analysis. Compared the IMCgrowth between the two size solder joints, the IMC growth rate of small solder joint wasbigger obviously, the more stress small solder joint underwent is the reason as the simulationresult. The IMC growth dynamics result shows that the IMC growth was controlled byinterfacial atoms diffusion mechanism, the constants of IMC growth rate under thermalshock was bigger that that under thermal shock. The dynamics parameters of n and D were2.47and0.203μm/h~(1/2)for100μm pitch solder joint,2.18and0.104μm/h~(1/2)for200μm pitchsolder joint. While for200μm pitch solder joints, its failure mechanism belong to prematurebrittle crack. The interfacial energy increased due to a great of particle shaped (Ni,Cu)_3Sn_4 gathered on the Ni surface, it provided the energy for crack growth; The impurities diffusedon the interface as a result of interfacial adhesion reduction, which provided the situation forcrack growth, while the higher stress concentration on the Ni interface caused by the CTEdifference between the chip and PCB worked as driving force for crack growth. Accordingto Weibull distribution, the characteristic lifetime of100μm and200μm pitch assembly are4714and6217separately. Based on the Darveaux lifetime model, the life time of100μmand200μm pitch solder joints are6171and7460. Darveaux lifetime model parameters ofK_1,K_2,K_3and K_4are1648.96、-0.2349、0.00479and-0.7004for100μm pitch solderjoint,while are551.6975、-1.1095、0.2362and1.5477for200μm pitch solder joint.
As a result, the100μm fine pitch solder joint of flip chip assembly with Cu/OSP andNi/Au was feasible to be used in electronic assembly. However, this project was focused onthe flip chip assembly with solder joints as interconection, this case is different with thetypical electronic assembly. If the assembly need multi-reflow to complete assembly, theIMC should be much thicher to damage the solder joint on Cu/OSP surface finish structrure,therefore this case should be evaluated in the future research.
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