微波产品焊点典型失效模式可靠性研究

微波元器件引线和基板的互连方式较多,为了保证产品性能,最理想的方式是将其引线直接与基板搭焊,但这种方式仅靠焊料和引线的形变来释放应力,有一定的局限性,因此常常出现焊点开裂失效的问题。为了评估微波产品焊点释放应力的能力,介绍了在微波产品中典型的三种高热应力焊点失效模式,分析了焊点开裂的机理。使用ANSYS软件对其应力进行了仿真,结合焊点位移量及其所带来的焊点剪切应变来评估焊点热疲劳寿命,并验证了这些方法的可行性和实用性,最后提出了高应力焊点相应的工艺设计优化措施,对提高焊点可靠性具有重要的指导作用。     

无铅焊锡膏性能评估

焊锡膏是SMT组装工艺必须的工艺材料。将锡膏印刷在PCB板焊盘上,在常温下锡膏具有的黏性可以把电子零件暂时固定在既定的贴片位置上。在回流焊炉内,加热到一定的温度锡膏中的合金粉末熔融再流动,液体焊料浸润元器件的引脚与PCB焊盘,冷却后将元器件与PCB通过机械的方式固定在一起,从而实现电气连接的焊点。焊锡膏的物理化学性能,工艺性直接影响SMT的焊接质量。     

基于微电子互连应用的无铅高频感应加热重熔技术研究

本文讨论了一种新的高密度微电子面阵封装与组装互连的重熔方法,高频感应加热重熔技术。通过感应加热使得无铅钎料熔化并在焊盘上润湿形成成型良好的的钎料凸台和互连焊点。采用该方法钎料凸台可以在较大的工艺范围内成型,并且可以较容易地实现对钎料凸台和互连焊点的高度控制。通过红外温度测量证明了在合适的工艺区间内树脂基板的温度远低于焊点。实验证明该方法具有局部发热、较高的加热／冷却速率和易于控制焊点形态等特点。     

Sn42Bi57Ag1无铅焊膏用活性剂的优化设计

针对Sn42Bi57Ag1无铅焊料开发出一种低温无铅焊锡膏,着重研究助焊剂中活性剂对焊锡膏润湿性能的影响。以铺展性能作为主要评价指标,通过回流焊接实验,对用于助焊剂的7种有机酸进行了筛选。最终选取了性能较好的甲基丁二酸、己二酸、水杨酸与丁二酸四种有机酸进行正交试验,通过量化分析选取正交试验水平,根据方差分析确定助焊剂中活性剂的最佳组合。结果表明:有机酸的复配能显著改善焊点铺展情况,当甲基丁二酸、己二酸、水杨酸与丁二酸质量比为2∶3∶5∶2时,Sn42Bi57Ag1无铅焊膏的润湿性能优异,焊点铺展率达86.52%。     

微波电路基板接地技术研究

为了探讨微波电路装配过程中的微带电路基板接地方法对微波电路性能影响情况，先通过对接地缺陷的影响结果的仿真分析，再用实验手段验证了仿真分析结果的正确性；并对微波部件采用接触式微件焊接技术进行大面积接地焊接的方法进行了研究和分析，采用大面积接地焊接的工艺方法，低温焊接实验的结果是可行的。     

楔焊焊点间的高度差对引线强度的影响

作为红外探测器封装中的关键工艺,引线键合的质量直接影响红外探测器组件的性能及使用寿命.通过在陶瓷基板与读出电路之间粘接不同厚度的陶瓷垫片来调整焊点间的高度.在键合后使用DAGE-SERIES-4000-PXY拉力测试仪得到不同高度差下的引线拉力.实验结果表明,当陶瓷基板与读出电路间未粘接陶瓷垫片时,引线强度最高.随着陶...     

小型无线场强探测传感器研究

近年来,微波加热因其高效性和清洁无污染等优点广泛应用于各个领域。然而,微波加热的不均匀性限制了微波作为高效加热能源的应用。通过测量和分析加热腔中的电场分布情况可以帮助设计人员改进微波加热腔体设计,提高微波加热的均匀性。现有的场强测量设备均为有线设备,应用场景极为有限。因此,本文提出了一种由探头、接收机和上位机三部分组成的无线场强探测传感器。介绍了无线场强探测传感器的结构和原理,采用横电磁波小室进行校准。通过一系列测量实验表明实测值与标准场强仪测量值一致性较好,可满足工程测量需求。     

溶剂对细间距用无铅焊锡膏抗热塌性的影响

焊接后出现桥连、短路是高密度电子组装中经常遇到的问题。通过回流焊接实验和抗热塌性实验,研究了助焊剂中溶剂的复配对焊锡膏抗热塌性的影响。结果表明:四氢糠醇与丙二醇苯醚按质量比3:2复配得到的焊锡膏焊点光亮饱满,铺展率达到84.83%,抗热塌性(150℃)优异。四氢糠醇与聚乙二醇单甲醚250(MPEG250)按质量比1:1复配得到的焊锡膏助焊性良好,焊点铺展率高达86.55%,具有优异的抗热塌性(150℃),并在180℃下仍表现优异,且最小不桥连间距为0.06 mm,满足超细间距条件下的焊接要求。     

LTCC电路基板金层表面斑点问题研究

针对LTCC电路基板金层表面斑点问题开展研究,观察了LTCC电路基板表面斑点显微形貌,对斑点组成成分进行定性分析,推断出LTCC电路基板表面斑点形成的原因。分别采用化学清洗和重烧法去除基板表面斑点,对比了两种方法的实验结果,确定了一种去除斑点的有效方法。实验结果表明:LTCC电路基板金层表面红色斑点组成成分为Ag2S,采用重烧法可以消除表面斑点,不损害基板性能,斑点消除区金层金丝键合强度没有减弱并满足国军标使用要求。建议通过加强印刷、烧结环节过程把控,以及采用真空或氮气密封LTCC基板的方式,有效减少金层斑点的形成。     

基板焊盘的金属层结构对焊接点强度的影响

球栅阵列封装具有高密度、低成本的特点被内存领域广泛采用。在实际的使用过程中由于遭受外界各种形式的机械负载和冲击造成器件失效,典型的失效模式有5类,突出问题是焊点失效,而焊点破裂是失效的主要形式。为了改善焊点的强度,提出了焊球的快速剪切测试以及BGA器件的快速剪切测试方法来检测焊点强度,设计了4种不同金属层结构的基板焊盘,对比了不同基板焊盘的快速剪切强度和失效模式。通过数据分析和失效模式研究,证明含较厚铜层或者镍层的焊盘具有更强的焊接强度。     

Lead-free interconnect technique by using variable frequency microwave

A novel lead-free interconnect technique using variable frequency microwave (VFM) was investigated. The lead-free solder interconnection between the component chips and the metal pads through VFM was first demonstrated. Comparison between the microstructures of the lead-free solder joints on Cu and Sn surfaces formed by a conventional thermal reflow process and VFM was conducted. The VFM heating technique successfully created the lead-free solder/Cu and Sn joints through their intermetallic compounds (IMCs), while maintaining the substrate temperature as low as 210°C.     

Local melting induced by electromigration in flip-chip solder joints

A new electromigration failure mechanism in flip-chip solder joints is reported. The solder joints failed by local melting of a PbSn eutectic solder. Local melting occurred due to a sequence of events induced by the microstructure changes in the flip-chip solder joint. The formation of a depression in the current-crowding region of a solder joint induced the local electrical resistance to increase. The rising local resistance resulted in a larger Joule heating, which, in turn, raised the local temperature. When the local temperature rose above the eutectic temperature of the PbSn solder, the solder joint melted and consequently failed. The results of this study suggest that a dynamic, coupled simulation that takes into account the microstructure evolution, current density distribution, and temperature distribution may be needed to fully solve this problem.     

A testing method for assessing solder joint reliability of FCBGA packages

A brittle solder joint at the interface between a solder ball and a substrate metal pad could result in an open joint failure after the solder ball is attached to a motherboard. Existing metrologies, such as ball shear, do not always detect the weak solder joint strength. A peel test for solder joint strength evaluation of substrate was developed. Several problems were observed during the initial stages of the development for this metrology. These included solder bridging and inconsistency of the test results. Solder bridging was reduced by changing the stencil design to have stencil openings that match the solder ball layout and using a jig to align the coupon and substrate while assembling the peel test samples. The paste printing and peeling methods were modified and the coupon quality was carefully monitored to make the sample preparation and testing more consistent. The impact of the physical aging and reflow on the solder joint strength was investigated. It showed that the peel strength did not decrease with physical aging, but did decrease with an increased number of reflows. The peel test results correlated with the performance of the substrates. The substrate with low peel strength had weak solder joints and failed under use conditions. The substrates with high peel strength demonstrated stronger solder joints and performed well under use conditions. The metrology provides a gross check about the presence of brittle solder joints on flip chip ball grid array substrates and is successfully used in monitoring the solder joint strength.     

Interconnections based on Bi-coated SnAg solder balls

To decrease the bonding temperature required for eutectic SnAg solder, SnAg solder bumps were chemically coated with a pure Bi layer. During heating, a low melting eutectic forms between the Bi coating and the SnAg, enabling bonding at temperatures below the melting points of either pure Bi or SnAg solder. As the composition of the molten solder changes toward more dilute Bi concentrations, the melting point in the joint region increases and the joint solidifies. After solidification the joints will no longer melt at the original bonding temperature. Bi-coated SnAg solder balls were joined to metallized substrates at temperatures ranging from 180°C to 250°C. The microstructure at the joint interface was characterized by the SEM/EDS technique. As expected, at 180°C the Bi-coated SnAg solder balls melted only locally at the interfacial regions between the ball and the substrate and so retained their spherical shape during bonding. After solidification there were a lot of small Bi precipitates in the joint region. At higher temperatures, the wetting was evidently better, and there were less Bi precipitates, because the melt was more dilute in bismuth. In all cases, Bi formed relatively small, equi-axed precipitates instead of the eutectic structure found in eutectic Sn-Bi solder joints     

Microstructure characterization of Sn-Ag solder joints between stud bumps and metal pads

The microstructure of the flip-chip solder joints fabricated using stud bumps and Pb-free solder was characterized. The Au or Cu stud bumps formed on Al pads on Si die were aligned to corresponding metal pads in the substrate, which was printed with Sn-3.5Ag paste. Joints were fabricated by reflowing the solder paste. In the solder joints fabricated using Au stud bumps, Au-Sn intermetallics spread over the whole joints, and the solder remained randomly island-shaped. The δ-AuSn, ε-AuSn₂, and η-AuSn₄ intermetallic compounds formed sequentially from the Au stud bump. The microstructure of the solder joints did not change significantly even after multiple reflows. The AuSn₄ was the main phase after reflow because of the fast dissolution of Au. In the solder joints fabricated using Cu stud bumps, the scallop-type Cu₆Sn₅ intermetallic was formed only at the Cu interface, and the solder was the main phase. The difference in the microstructure of the solder joints with Au and Cu stud bumps resulted from the dissolution-rate difference of Au and Cu into the solder.     

低银无铅焊膏板级封装工艺和焊点可靠性试验

针对低Ag无铅焊膏的市场需求,研究开发了一种适用于99.0Sn0.3Ag0.7Cu低Ag无铅焊膏用松香型无卤素助焊剂(WTO—LF3000),配制了相应的无铅焊膏(WTO—LF3000—SAC0307),并对其板级封装工艺适应性及焊点可靠性进行了考察,用测试后样品的电气可靠性作为接头可靠性评价条件。结果表明:所开发的低Ag无铅焊膏熔点和润湿性符合产品实际要求。配制的焊膏印刷质量良好,焊点切片观察其孔隙率<25%,满足行业标准IPC—A—610D之要求。样品分别经跌落、震动和温度循环试验后,无焊点脱落等现象,电气功能正常。     

Effect of bump size on current density and temperature distributions in flip-chip solder joints

Three dimensional thermo-electrical analysis was employed to simulate the current density and temperature distributions for eutectic SnAg solder bumps with shrinkage bump sizes. It was found that the current crowding effects in the solder were reduced significantly for smaller solder joints. Hot-spot temperatures and thermal gradient were increased upon reducing the solder. The maximum temperature for solder joint with 144.7 μm bump height is 103.15 °C which is only 3.15 °C higher than the substrate temperature due to Joule heating effect. However, upon reducing the bump height to 28.9 μm, the maximum temperature in the solder increased to 181.26 °C. Serious Joule heating effect was found when the solder joints shrink. The higher Joule heating effect in smaller solder joints may be attributed to two reasons, first the increase in resistance of the Al trace, which is the main heating source. Second, the average and local current densities increased in smaller bumps, causing higher temperature increase in the smaller solder bumps.     

Controlling intermetallic compound growth in SnAgCu/Ni-P solder joints by nanosized Cu6Sn5 addition

Nanosized Cu₆Sn₅ dispersoids were incorporated into Sn and Ag powders and milled together to form Sn-3Ag-0.5Cu composite solders by a mechanical alloying process. The aim of this study was to investigate the interfacial reaction between SnAgCu composite solder and electroless Ni-P/Cu UBM after heating for 15 min. at 240°C. The growth of the IMCs formed at the composite solder/EN interface was retarded as compared to the commercial Sn3Ag0.5Cu solder joints. With the aid of the elemental distribution by x-ray color mapping in electron probe microanalysis (EPMA), it was revealed that the SnAgCu composite solder exhibited a refined structure. It is proposed that the Cu₆Sn₅ additives were pinned on the grain boundary of Sn after heat treatment, which thus retarded the movement of Cu toward the solder/EN interface to form interfacial compounds. In addition, wetting is an essential prerequisite for soldering to ensure good bonding between solder and substrate. It was demonstrated that the contact angles of composite solder paste was <25°, and good wettability was thus assured.     

Lead-Free Bumping Using an Alternating Electromagnetic Field

A novel lead-free bumping technique using an alternating electromagnetic field (AEF) was investigated. Lead-free solder bumps reflowed onto copper pads through AEF have been achieved. A comparison was conducted between the microstructures of the lead-free solder joints formed by the conventional thermal reflow and AEF reflow. Keeping the substrate temperature lower than that of the solder bumps, AEF reflow successfully created metallurgical bonding between the lead-free solders and metallizations through an interfacial intermetallic compound (IMC). The AEF reflow could be finished in several seconds, much faster than the conventional hot-air reflow. Considering the morphology of the interfacial Cu₆Sn₅ IMC, a shorter heating time above the melting point would be a better choice for solder joint reliability. The results show that AEF reflow is a promising localized heating soldering technique in electronic packaging.     

Forming solder joints by sintering eutectic tin-lead solder paste

It is possible to form solder joints with mechanical integrity, but not mechanical strength comparable to that achieved by melting the solder, by sintering eutectic tin-lead solder paste where small amounts of eutectic Sn-Bi powder are added to the paste. This increases the rate of sintering through liquid-phase sintering.