Effects of metallization characteristics on gold wire bondability of organic printed circuit boards

Organic printed circuit boards (PCBs) with Au/Ni plates on bond pads are widely used in chip-on-board (COB), ball grid array (BGA), and chip-scale packages. These packages are interconnected using thermosonic gold wire bonding. The wire bond yield relies on the bondability of the Ni/Au pads. Several metallization parameters, including elemental composition, thickness, hardness, roughness, and surface contamination, affect the success of the solid state joining process. In this study, various characterization and mechanical testing techniques are employed to evaluate these parameters for different metallization schemes with varying Ni and Au layer thicknesses. The pull force of Au wires is measured as a function of plasma treatment applied before wire bonding to clean the bond pads. Close correlations are established between metallization characteristics and wire bond quality.     

Effect of plasma treatment of Au-Ni-Cu bond pads on process windows of Au wire bonding

The wire bondability of Au-Ni-Cu bond pads with different Au plating schemes, including electrolytic and immersion plates, are evaluated after plasma treatment. The plasma cleaning conditions, such as cleaning power and time, are optimized based on the process window and wire pull strength measurements for different bond pad temperatures. Difference in the efficiency of plasma treatment in improving the wire bondability for different Au plates is identified. The plasma-cleaned bond pads are exposed to air to evaluate the recontamination process and the corresponding degradation of wire pull strength. The changes in bond pad surface characteristics, such as surface free energy and polar functionality, with exposure time are correlated to the wire pull strength, which in turn provides practical information about the shelf life of wire bonding after plasma cleaning.     

Process windows for low-temperature Au wire bonding

The process windows are presented for low-temperature Au wire bonding on Au/Ni/Cu bond pads of varying Au-layer thicknesses metallized on an organic FR-4 printed circuit board (PCB). Three different plating techniques were used to deposit the Au layers: electrolytic plating, immersion plating, and immersion plating followed by electrolytic plating. Wide ranges of wire bond force, bond power, and bond-pad temperature were used to identify the combination of these processing parameters that can produce good wire bonds, allowing the construction of process windows. The criterion for successful bonds is no peel off for all 20 wires tested. The wire pull strengths and wire deformation ratios are measured to evaluate the bond quality after a successful wire bond. Elemental and surface characterization techniques were used to evaluate the bond-pad surfaces and are correlated to wire bondability and wire pull strength. Based on the process windows along with the pull strength data, the bond-pad metallization and bonding conditions can be further optimized for improved wire bondability and product yields. The wire bondability of the electrolytic bond pad increased with Au-layer thickness. The bond pad with an Au-layer thickness of 0.7 μm displayed the highest bondability for all bonding conditions used. The bondability of immersion bond pads was comparable to electrolytic bond pads with a similar Au thickness. Although a high temperature was beneficial to wire bondability with a wide process window, it did not improve the bond quality as measured by wire pull strength.     

Study of nickel contamination on the ultrasonic bondability of gold bond finger of SD-48L packages

Contamination is a major barrier to the adhesion of solid-phase metal couples. If me-tallic impurities are present on a gold (Au) bonding surface, it can easily react with oxygen molecules in the atmosphere to form oxides. The oxides formed prevent intimate metal-metal contact which is important for bond formation. A study is carried out to investigate whether nickel (Ni) contamination can affect the ultrasonic bondability of Au bond finger of side-braze 48-lead ceramic package. Two sets of samples are used for this study. The first set comprising uncontaminated packages serves as a control group. The second set are Ni contaminated packages. The results of the study show that Ni contamination does not affect the ultrasonic bondability of the Au bond finger. A second study to ascertain the quality of the wire bonds however shows that the mean bond tensile strength of the Ni contaminated packages has weakened slightly.     

The development of Cu bonding wire with oxidation-resistant metal coating

Although Cu bonding wire excels over Au bonding wire in some respects such as production costs, it has not been widely used because of its poor bondability at second bonds due to surface oxidation. We conceived an idea of electroplating oxidation-resistant metal on the Cu bonding wire to prevent the surface oxidation. The electroplating of Au, Ag, Pd, and Ni over Cu bonding wire all increased bond strengths as expected, but it caused problematic ball shapes except Pd-plated Cu bonding wire. The wire could produce the same ball shape as that of Au bonding wire. It was also proved to have excellent bondability sufficient to replace Au bonding wire. That is, it excelled in bond strengths, defective bonding ratio, and wideness of "Parameter Windows". It also showed the same stability as Au bonding wire in reliability tests, while bonds of Cu bonding wire were deteriorated in a few of the tests. In short, the Pd-plated Cu bonding wire can realize excellent bonding similar to Au bonding wire, while having much lower production costs.     

表面特性对PBGA软金丝键合点可焊性的影响

一级封装中最流行的互连技术仍为丝焊。引线键合的效率主要依赖于受表面特性影响的键合点的可焊性。在最近的研究中,我们调查了表面特性对金-金超声压焊系统的影响。表面特性包括金层厚度,表面硬度和粗糙度、有机物杂质及金属杂质。对两个样本间的不同特性进行比较。确定金表面特性的粗糙度依赖于镍层的外形结构。焊料掩膜逸出气体对可焊性具有负面影响,等离子清洗能够有效地去除有机物杂质。金层中的杂质将导致不良的可焊性。     

金丝球焊复合键合工艺可靠性研究

对芯片铝焊盘上不同重叠面积的金丝球焊复合键合的可靠性进行研究,并与非复合键合进行对比。结果表明,随着复合键合重叠面积的减少,键合拉力和界面生成的合金化合物面积均无明显变化,而剪切强度呈下降趋势。高温储存结果表明,复合键合拉力值满足国军标要求。复合键合有掉铝和弹坑缺陷隐患。经分析,原因是复合键合时施加的超声能量破坏了硅及金属化的结合态,在硅及金属化的结合界面上形成微裂纹。复合键合在高可靠电路中应进行键合参数优化并验证充分后使用。     

Deformation and bonding processes in aluminum ultrasonic wire wedge bonding

The ultrasonic wire bonding (UWB) process has been examined using transmission electron microscopy (TEM) and standard wire pull testing techniques. Al-0.5 wt.% Mg wires 75 μm in diameter were bonded to pure and alloyed Al substrates. The bonding parameters, surface roughness, and surface contamination levels were variables in the experiments. Cross-section TEM specimens were made from these samples. TEM analysis was conducted on the wire, wire/substrate interface and substrate. Pull tests showed that for the Al substrates the surface roughness or the presence of contamination did not effect the bond strength, whereas for contaminated stainless steel substrates, a three μm surface finish resulted in the highest bond pull strength. The TEM observations revealed features such as low-angle grain boundaries, dislocation loops and the absence of a high dislocation density, indicating that the wire and substrate were dynamically annealed during bonding. Based on the width of a zone near a grain boundary in the wire which was depleted of dislocation loops, it was estimated that local heating equivalent to a temperature of 250° C for 90 msec was achieved in the wire during bonding. No evidence was found for melting along the bond interface, indicating that UWB is a solid-state process. Based on the TEM observationsof the bond interface and the pull tests, it is concluded that the ultrasonic vibrations clean the surfaces to be joined to the extent that a good bond can be obtained by intimate metal-metal contact in the clean areas.     

Nickel-palladium bond pads for copper wire bonding

The semiconductor packaging industry is undergoing a step-change transition from gold to copper wire bonding brought on by a quadrupling of gold cost over the last 8 years. The transition has been exceptionally rapid over the last 3 years and virtually all companies in the industry now have significant copper wire bonding production. Among the challenges to copper wire bonding is the damage to bond pads that had been engineered for wire bonding with the softer gold wire. This paper presents an extensive evaluation of electroless NiPd and NiPdAu bond pads that offer a much more robust alternative to the standard Al pad finish. These NiPd(Au) bond are shown to outperform Al in virtually all respects: bond strength, bond parameter window, lack of pad damage and reliability.     

引线键合中第一点勾线力的影响因素

勾线拉力测试是评估引线键合质量的一种主要方法,同时影响测试结果的因素有很多.文章通过理论分析和实际测量,发现封装结构、线弧高度和勾线位置对第一焊点的勾线力有显著影响.Non-EP结构中线弧的第一点勾线力比EP结构中的大;随着线弧高度的增加,线孤的第一点勾线力将逐渐变大;在勾线位置由第一焊点逐渐移至线弧中间位置时,一般高...     

LTCC基板金丝热超声楔焊正交试验分析

引线键合是微组装技术中的关键工艺,广泛应用于军品和民品芯片的封装。特殊类型基板的引线键合失效问题是键合工艺研究的重要方向。低温共烧陶瓷(LTCC)电路基板在微波多芯片组件中使用广泛,相对于电镀纯金基板,该基板上金焊盘楔形键合强度对于参数设置非常敏感。文章进行了LTCC基板上金丝热超声楔焊的正交试验,在热台温度、劈刀安装长度等条件不变的情况下,分别设置第一键合点和第二键合点的超声功率、超声时间和键合力三因素水平,试验结果表明第一点超声功率和第二点超声时间对键合强度影响明显。     

Effect of the plasma cleaning process on plastic ball grid array package assembly reliability

Interface delamination is recognized as one of the major failures of microelectronics packaging. It can result from various factors, including stresses from mismatch of adherent materials, hygrothermal stress from the release of vapor pressure of moisture during soldering reflow process, and interface material adhesion strength. The failure mechanisms are associated with cyclic loads, temperature and moisture condition as well as interface adhesion strength degradation. This paper focuses on the evaluation of plasma cleaning on PBGA assembly, including resistance to interface delamination. Two different plasma systems, powered by radio frequency (RF) and microwave (MW) energy, are studied. The optimized plasma cleaning process parameters are obtained by surface contact angle measurements. The plasma cleaning results are also verified by scanning electron microscopy (SEM) as well as physical pull and shear tests. The test vehicles are 27/spl times/27 mm 292-ball PBGAs. The results from encapsulation peel tests, die and encapsulant pull tests, bonding wire pull tests and C-Mode SAM (C-SAM) examination are presented. It is clear that an optimal plasma cleaning process can be achieved with different plasma systems. The experimental results also demonstrate that plasma cleaning has little effect on wire bonding process and die attach pull strength for given substrates and assembly materials. In all the cases, optimal plasma cleaning steps improve PBGA resistance against interface delaminations for cases where plasma cleaning is carried out before encapsulation process. Moreover, different plasma cleaning techniques would affect the assembly productivity, investment and yield. This paper demonstrates that the optimized plasma cleaning process would enhance PBGA package qualification level and improve the process yields and productivity.     

剪切推球测试条件对Au球焊点推球结果的影响

Au引线键合是电子封装中应用广泛的芯片互连技术,剪切推球测试是评价引线键合球焊点质量和完整性的主要方法之一.利用未热老化和热老化不同时间的Au球键合试样,通过试验和数值模拟研究了推球高度和推球速度对推球值和推球失效界面形貌的影响.     

Effects of Cu and Pd addition on Au bonding wire/Al pad interfacial reactions and bond reliability

Finer pitch wire bonding technology has been needed since chips have more and finer pitch I/Os. However, finer Au wires are more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and Pd were added to Au bonding wire as alloying elements. These alloyed Au bonding wires—Au-1 wt.% Cu wire and Au-1 wt.% Pd wire—were bonded on Al pads and then subsequently aged at 175°C and 200°C. Cu and Pd additions to Au bonding wire slowed down interfacial reactions and crack formation due to the formation of a Cu-rich layer and a Pd-rich layer at the interface. Wire pull testing (WPT) after thermal aging showed that Cu and Pd addition enhanced bond reliability, and Cu was more effective for improving bond reliability than Pd. In addition, comparison between the results of observation of interfacial reactions and WPT proved that crack formation was an important factor to evaluate bond reliability.     

Iterative optimization of tail breaking force of 1 mil wire thermosonic ball bonding processes and the influence of plasma cleaning

An online tail breaking force measurement method is developed with a proximity sensor between wire clamp and horn. The wire under the tensile load measures about 1.5 cm extending from the bond location to the wire clamp. To increase the sensitivity, the bondhead speed is reduced to 2 mm/s during breaking the tail bond. It takes roughly 10 ms to break the tail bond. The force resolution of the method is estimated to be better than 5.2 mN. An automatic wire bonder used to continuously bond up to 80-wire loops while recording the on-line proximity signals. All wires are directed perpendicular to the ultrasound direction. The tail breaking force for each bond is evaluated from the signal and shown automatically on the bonder within 2 min after bonding.Results are obtained for a typical Au wire and a typical Cu wire bonding process. Both wires are 25 mm in diameter and bonded on Ag plated diepads of standard leadframes at 220 °C. An average Cu tail breaking force of higher than 50 mN is obtained if the leadframe is plasma cleaned before the bonding with 100% Ar for 5 min. This result is comparable to that obtained with Au wire. The standard deviation of the Cu tail breaking force is about twice that obtained with Au wire. The tail breaking force depends on the bonding parameters, metallization variation, and cleanliness of the bond pad. The cleanliness of the bonding pad is more important with Cu wire than with Au wire.     

Plasma cleaning of plastic ball grid array package

An investigation of O₂, Ar and Ar/H₂ plasma cleaning was carried out on plastic ball grid array (PBGA) substrates to study its effects on surface cleanliness, wire bondability and molding compound/solder mask adhesion. Optimization of the plasma cleaning process parameters was achieved using the contact angle method and verified by auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and wedge pull tests. It was found that both the wedge bond quality and moisture sensitivity of a 225 I/O PBGA package were improved after plasma cleaning. Furthermore, atomic force microscopy (AFM) characterization and XPS analysis revealed that the solder mask has undergone plasma-induced surface modification. Cross-contamination of Au and F traces on the solder mask that has occurred during plasma cleaning was identified by XPS. This study has demonstrated the benefits and consequences of plasma cleaning for a PBGA package.     

Power Device Packaging Technologies for Extreme Environments

Silicon carbide is a wide-bandgap semiconductor capable of operation at temperatures in excess of 300degC. However, high-temperature packaging to interface with the other elements of the electrical system is required. Die attach, wire bonding, and passivation materials and techniques have been demonstrated for use at 300degC. Transient liquid phase bonding has been developed with Au:Sn/Au, yielding high die shear strength after 2000 h at 400degC. Large diameter (250 mum) gold and platinum wire bonding was evaluated for top side electrical contact. Au wire was reliable after 2000 h at 300degC with Ti/Ti:W/Au pads over passivation on the SiC. However, Au wire on Ti/Pt/Au and Pt wire on both Ti/Tl:W/Au and Ti/Pt/Au exhibited passivation fracture with aging. Polyimide has been demonstrated for 2000 h at 300degC in air as a high-voltage passivation layer.     

金丝楔形键合强度的影响规律分析

金丝楔形键合是一种通过超声振动和键合力协同作用来实现芯片与电路引出互连的技术。现今,此引线键合技术是微电子封装领域最重要、应用最广泛的技术之一。引线键合互连的质量是影响红外探测器组件可靠性和可信性的重要因素。基于红外探测器组件,对金丝楔形键合强度的多维影响因素进行探究。从键合焊盘质量和金丝楔焊焊点形貌对键合强度的影响入手,开展了超声功率、键合压力及键合时间对金丝楔形键合强度的影响研究。根据金丝楔焊原理及工艺过程,选取红外探测器组件进行强度影响规律试验及分析,指导实际金丝楔焊工艺,并对最佳工艺参数下的金丝键合拉力均匀性进行探究,验证了金丝楔形键合强度工艺一致性。     

InSb红外探测器芯片金丝引线键合工艺研究

InSb红外探测器芯片镀金焊盘与外部管脚的引线键合质量直接决定着光电信号输出的可靠性,对于引线键合质量来说,超声功率、键合压力、键合时间是最主要的工艺参数。从实际应用出发,采用KS公司4124金丝球焊机实现芯片镀金焊盘与外部管脚的引线键合,主要研究芯片镀金焊盘第一焊点键合工艺参数对引线键合强度及键合区域的影响,通过分析键合失效方式,结合焊点的表面形貌,给出了适合InSb芯片引线键合质量要求的最优工艺方案,为实现InSb芯片引线键合可靠性的提高打下了坚实的基础。     

Silver pick up during tail formation in thermosonic wire bonding process

Cu ball bonding processes are significantly less robust than Au ball bonding processes. One reason for this are higher variations observed, e.g. in the free-air ball (FAB) formation process. There is a strong influence the tail bond process has on subsequent FAB formation. Tail tips and bond fractographs made with Cu and Au wires are investigated using scanning electron microscopy. Au and Cu wires pick up Ag material (Ag pick up) from the Ag metallization of Cu leadframe diepads during wire tail breaking. Ag pick up by Cu wire is more dominant than that by Au wire. Ultrasonic friction energy is necessary in order for Ag material pick up to occur. The impact force plays an important role for Ag pick up; less pick up is observed with a higher impact force. The hardness of the free-air ball (FAB) with Ag pick up is reduced by up to 4% compared to that of a FAB made from a tail broken at the neck of a ball bond and therefore having no Ag pick up. No significant change of FAB diameter is observed in these two cases.