Direct gold and copper wires bonding on copper

The key to bonding to copper die is to ensure bond pad cleanliness and minimum oxidation during wire bonding process. This has been achieved by applying a organic coating layer to protect the copper bond pad from oxidation. During the wire bonding process, the organic coating layer is removed and a metal to metal weld is formed. This organic layer is a self-assembled monolayer. Both gold and copper wires have been wire-bonded successfully to the copper die even without prior plasma cleaning. The ball diameter for both wires are 60 μm on a 100 μm fine pitch bond pad. The effectiveness of the protection of the organic coating layer starts from the wafer dicing process up to the wire bonding process and is able to protect the bond pad for an extended period after the first round of wire bond process. In this study, oxidization of copper bond pad at different packaging processing stages, dicing and die attach curing, have been explored. The ball shear strength for both gold and copper ball bonds achieved are 5 and 6 g/mil² respectively. When subjected to high temperature storage test at 150 °C, the ball bonds formed by both gold and copper wire bond on the organic coated copper bondpad are thermally stable in ball shear strength up to a period of 1440 h. The encapsulated daisy chain test vehicle with both gold and copper wires bonding have passed 1000 cycles of thermal cycling test (−65 to 150 °C). It has been demonstrated that orientation imaging microscopy technique is able to detect early levels of oxidation on the copper bond pad. This is extremely important in characterization of the bondability of the copper bond pad surface.     

高温老化期间引线键合空洞形成探讨

在金-铝金属间化合物相中形成的空洞,降低了把金丝与焊盘键合的长期可靠性。在该文中,通过一系列微结构研究来评定引线键合中空洞的形成。把形成的空洞分为初始、环形和极小三种类型。形成初始空洞的主要原因是探测标记和铝焊蕊污染,初始空洞阻碍合会扩散并使金属间化合物生长减缓。环形空洞是由热超声引线键合的超声挤摔作用造成的,这些压焊缝隙可能成为腐蚀并降低引线键合的一类卤化物形成的途径。极小的空洞是住Au4l相阶段形成的。由于不同Au4l相形成的反应,或与金球表层上品粒界面影响的关系,在这些空洞中会出现两种Au4l相的纹理。     

Wire-bond void formation during high temperature aging

Voids formed in Au-Al intermetallic phases degrade the long-term reliability of gold wire bonds to aluminum pads. In this study, a series of microstructural studies were performed to evaluate void formation in wire bonds. Voids are classified as initial, annular or minute. Probe marks and Al pad contamination are the main causes of initial voids that block alloy diffusion and slow down intermetallic growth. Annular voids are caused by the ultrasonic squeeze effect of thermosonic wire bonding. These bonding gaps may become pathways for halide species that corrode and degrade wire bonds. Minute voids are formed during the Au/sub 4/Al phase. The two Au/sub 4/Al phase textures in these voids may be due to different Au/sub 4/Al phase formation reactions or be related to grain boundary effects on the surface layer of the Au ball.     

高温老化期间引线键合空洞形成探讨

在金-铝金属间化合物相中形成的空洞,降低了把金丝与焊盘键合的长期可靠性。文中通过一系列微结构研究来评定引线键合中空洞的形成。把形成的空洞分为初始、环形和极小三种类型。形成初始空洞的主要原因是探测标记和铝焊盘污染,初始空洞阻碍合金扩散并使金属间化合物生长减缓。环形空洞是由热超声引线键合的超声挤榨作用造成的,这些压焊缝隙可能导致会腐蚀并降低引线键合的一类卤化物的形成。极小的空洞是在Au4Al相阶段形成的。由于不同Au4Al相形成的反应,或与金球表层上晶粒界面影响的关系,在这些空洞中会出现两种Au4Al相的纹理。     

Growth behavior of Cu/Al intermetallic compounds and cracks in copper ball bonds during isothermal aging

Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. There are many potential benefits for use of copper in these applications, including better electrical and mechanical properties, and lower cost. Usually, wires are bonded to aluminum contact pads. However, the growth of Cu/Al intermetallic compounds (IMC) at the wire/pad interfaces is poorly understood, and if excessive would increase the contact resistance and degrade the bond reliability.To study the Cu/Al IMC growth in Cu ball bonds, high temperature aging at 250 °C for up to 196 h has been used to accelerate the aging process of the bonds. The Cu/Al IMCs growth behavior was then recorded and the IMC formation rate of 6.2 ± 1.7 × 10⁻¹⁴ cm²/s was obtained. In addition to the conventional yz-plane cross-section perpendicular to the bonding interface, a xy-plane cross-section parallel through the interfacial layers is reported. Three IMC layers were distinguished at the Cu/Al interfaces by their different colors under optical microscopy on the xy-plane cross-sections of ball bonds. The results of micro-XRD analysis confirmed that Cu₉Al₄, and CuAl₂ were the main IMC products, while a third phase is found which possibly is CuAl. During the aging process, IMC film growth starts from the periphery of the bond and propagates inward towards the centre area. Subsequently, with increased aging time, cavities are observed to develop between the IMC layer and the Cu ball surface, also starting at the bond periphery. The cavitation eventually links up and progresses toward the centre area leading to a nearly complete fracture between the ball and the intermetallic layer, as observed after 81 h.     

Microstructural study of copper free air balls in thermosonic wire bonding

Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. In this paper, a microstructural study is reported of cross-sectioned free air balls (FABs) made with 23 μm diameter copper bonding wire. It was found that the FAB is comprised of a few columnar grains and a large number of fine subgrains formed within the columnar grains around the periphery of the FAB. It was determined that conduction through the wire was the dominant heat loss mechanism during cooling, and the solidification process started from the wire-ball interface and proceeded across the diameter then outward towards the ball periphery.The microstructure of the Cu ball bond after thermosonic bonding was investigated. The result showed that the subgrain orientations were changed in the bonding process. It is evident that metal flow along the bonding interface was from the central area to the bond periphery during thermosonic bonding.     

Cu wire bond microstructure analysis and failure mechanism

In this study, copper wire bonding samples were aged at 205 °C in air from 0 h to 2000 h. It was found that the bonding of a Cu wire and an Al pad formed Cu₉Al₄, CuAl, and CuAl₂ intermetallic compounds, and an initial crack was formed by the ultrasonic squeeze effect during thermosonic wire bonding. The cracks grew towards the ball bond center with an increase in the aging time, and the Cl ions diffused through the crack into the ball center. This diffusion caused a corrosion reaction between the Cl ions and the Cu-Al intermetallic phases, which in turn caused copper wire bonding damage.     

金、铜丝球键合焊点的可靠性对比研究

金丝球焊是电子工业中应用最广泛的引线键合技术,但随着高密度封装的发展,铜丝球焊日益引起人们的关注。采用热压超声键合的方法,分别实现Au引线和Cu引线键合到Al-1%Si-0.5%Cu金属化焊盘。对焊点进行200℃老化实验的结果表明:铜丝球焊焊点的金属间化合物生长速率比金丝球焊焊点慢的多;铜丝球焊焊点具有比金丝球焊焊点更稳定的剪切断裂载荷,并且在一定的老化时间内铜丝球焊焊点表现出更好的力学性能;铜丝球焊焊点和金丝球焊焊点在老化后的失效模式不同。     

Insulated Cu wire free air ball characterization

Insulated Cu wire technology has immense potential for fine pitch wire bonding interconnection. Understanding the behavior of the insulated Cu free air ball (FAB) formation is crucial for wire bonding process. The FAB formation, size, shape and cleanliness under different conditions for 20 μm insulated Cu wire were investigated using SEM, FESEM and FTIR surface analysis. The results were compared with that of bare Cu wire. Consistently spherical residue free FAB of insulated Cu wire were formed using forming gas. The samples with insulated Cu wire consistently produced larger FAB than that of bare Cu wire, indicating that the energy required for free air ball formation is lower. Basic bonding performances in terms of ball bond strength, intermetallic (IMC) coverage growth and stitch bond strength of insulated Cu wire at time zero are also discussed in the paper.     

Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process

To understand the copper oxide effect on the bondability of gold wire onto a copper pad, thermosonic gold wire bonding to a copper pad was conducted at 90–200 °C under an air atmosphere. The bondability and bonding strength of the Au/Cu bonds were investigated. The bondability and bonding strength were far below the minimum requirements stated in industrial codes. At elevated bonding temperature of 200 °C, the bondability and bonding strength deteriorated mainly due to hydroxide and copper oxide formation on the copper pad. Oxide formation occurred if no appropriate oxide preventive schemes were applied. At lower bonding temperature, 90 °C, poor bondability and low bonding strength were mainly attributed to insufficient thermal energy for atomic inter-diffusion between the gold ball and copper pad.Copper pad oxidation was investigated using an electron spectroscopy for chemical analysis (ESCA) and thermogravimetric analysis (TGA). An activation energy of 35 kJ/mol for copper pad oxidation was obtained from TGA. This implies that different mechanisms govern the oxidation of copper pad and bulk copper. Hydroxide and copper oxide were identified based on the shifted binding energy. Cu(OH)₂ forms mainly on the top surface of copper pads and the underlying layer consists mainly of CuO. The hydroxide concentration increased with increasing the heating temperatures. After heating at 200 °C, the hydroxide concentration on the copper pad surface was approximately six times that at 90 °C. Protective measures such as passivation layer deposition or using shielding gas are critical for thermosonic wire bonding on chips with copper interconnects.     

Fine pitch copper wire bonding in high volume production

Wire bonding is the predominant mode of interconnection in electronic packaging. Gold wire bonding has been refined again and again to retain control of interconnect technology due to its ease of workability and years of reliability data. Economic realities are now enabling fine diameter copper wire to compete with gold wire. It needs to be demonstrated however that known challenges in the assembly process and long term reliability can be managed successfully. Here, a rigorous methodology has been established to introduce copper wire bonding on new as well as converted dice into manufacturing. Manufacturing efficiencies and yields have been driven to be equivalent to gold wires. Reliability testing has passed the usual criteria and has been extended to demonstrate the viability of this technology. Interfacial analysis has confirmed the observations that intermetallic compounds form and grow slowly.     

Impact of Thermal Cycling on Sn-Ag-Cu Solder Joints and Board-Level Drop Reliability

The electronic packaging industry uses electroless nickel immersion gold (ENIG) or Cu-organic solderability preservative (Cu-OSP) as a bonding pad surface finish for solder joints. In portable electronic products, drop impact tests induce solder joint failures via the interfacial intermetallic, which is a serious reliability concern. The intermetallic compound (IMC) is subjected to thermal cycling, which negatively affects the drop impact reliability. In this work, the reliability of lead-free Sn-3.0Ag-0.5Cu (SAC) soldered fine-pitch ball grid array assemblies were investigated after being subjected to a combination of thermal cycling followed by board level drop tests. Drop impact tests conducted before and after thermal aging cycles (500, 1000, and 1500 thermal cycles) show a transition of failure modes and a significant reduction in drop durability for both SAC/ENIG and SAC/Cu-OSP soldered assemblies. Without thermal cycling aging, the boards with the Cu-OSP surface finish exhibit better drop impact reliability than those with ENIG. However, the reverse is true if thermal cycle (TC) aging is performed. For SAC/Cu-OSP soldered assemblies, a large number of Kirkendall voids were observed at the interface between the intermetallic and Cu pad after thermal cycling aging. The void formation resulted in weak bonding between the solder and Cu, leading to brittle interface fracture in the drop impact test, which resulted in significantly lower drop test lifetimes. For SAC/ENIG soldered assemblies, the consumption of Ni in the formation of NiCuSn intermetallics induced vertical voids in the Ni(P) layer.     

Characterization of intermetallic compounds in Cu–Al ball bonds: Mechanical properties,interface delamination and thermal conductivity

In high power automotive electronics copper wire bonding is regarded as the most promising alternative for gold wire bonding in 1st level interconnects and therefore subjected to severe functional requirements. In the Cu–Al ball bond interface the growth of intermetallic compounds may deteriorate the physical and mechanical properties. The layer growth and properties of these intermetallic compounds are crucial in the prediction of the long term behavior. To mimic the growth of intermetallic compounds during and after copper ball bonding, diffusion couples of aluminium and copper were annealed at 225–500 °C and chemically analyzed by SEM/EDS. Also five separate intermetallic compounds were melted together from the pure elements and aged in evacuated quartz ampoules for 240 h at 500 °C. In this work values for the indentation Young’s modulus, load independent hardness, indentation fracture toughness, volumetric densities, interface delamination and thermal conductivity are presented. It can be concluded that the Cu-rich intermetallics Cu₉Al₄ and Cu₃Al₂ are less sensitive to fracture and have lower average densities than the other intermetallic compounds. The volumetric decrease during formation causes internal stress. Interfacial delamination initiates in the Al-rich intermetallics (CuAl, CuAl₂) and propagates easily into other intermetallic layers. The Cu₉Al₄–Cu s.s. interface is also found to be susceptible for delamination fracture. The thermal conductivity for 3 intermetallics is much lower than for pure copper or aluminium and in the range of 26–87 W/m⁻¹ K⁻¹, where Cu₃Al₂ layer has the lowest thermal conductivity (26–33 W/m⁻¹ K⁻¹).     

Acid Decapsulation of Epoxy Molded IC Packages With Copper Wire Bonds

Epoxy molded IC packages with copper wire bonds are decapsulated using mixtures of concentrated sulfuric acid (20%) and fuming nitric acid in an automatic decapping unit and, observed with minimal corrosion of copper wires (0.8–6 mil sizes) and bond interfaces. To attain maximum cross-linking of the molded epoxies, the post mold cured packages (175$^circ$C for 4 h) were further, aged at high temperature of 150$^circ$C for 1000 h. These packages are decapsulated using mixtures of higher ratio of concentrated sulfuric acid (40%) along with fuming nitric acid. The shear strength of copper wire bonds with 1 mil (25$mu$m) diameter of the decapsulated unit is higher than 5.5 gf/mil$^2$. The present study shows copper stitch bonds to Au, Cu, Pd, and Sn alloy plated surfaces are less affected on decapping, with a few grams of breaking load on stitch pull test, while stitch bonds on silver plated surfaces reveal lifting of wire bonds on decapping.     

New encapsulation development for fine pitch IC devices

The continuous reduction of chip size driven by the market demand has a significant impact on circuit design and assembly process of IC packages. Shrinking chip size and increasing I/O counts require finer bond pad pitch and bond pad size for circuitry layout. As a result, serious wire deflection during transfer molding process could make adjacent wires short, and this issue becomes more critical as a smaller wire diameter has to be applied for the finer pitch wire bonded IC devices.This paper presents a new encapsulation process development for 50 μm fine pitch plastic ball grid array package. Since reduced wire diameter decreases the bending strength of bonded wires significantly, wire deflection during molding process becomes quite serious and critical. Experiments on conventional transfer molding were conducted to evaluate wire span threshold with 23.0 μm diameter gold wire. The results show that the wire span threshold is about 4.1 mm, which is much shorter than the wire span threshold of over 5.0 mm for wire with 25.4 μm diameter. Finite element analysis shows there is a significant difference in the wire deflection between 23.0 μm gold wire and 25.4 μm gold wire diameter under the same action of mold flow. A novel encapsulation method is introduced using non-sweep solution. The wire span could be extended to over 5.0 mm with wire sweep less than 1%. Reliability tests conducted showed that all the units passed 1000 temperature cycles (−55 to 125 °C) with JEDEC moisture sensitivity level 2a (60 °C/60% relative humidity for 120 h) and 3 times reflow (peak temperature at 220–225 °C). It is believed that this solution could efficiently overcome the risk of wire short issues and improve the yield of ultra fine pitch wire bonds in high-volume production.     

Reliability and failure analysis of fine copper wire bonds encapsulated with commercial epoxy molding compound

The small outline transistor (SOT) devices which were interconnected with 20 μm copper bonding wire and encapsulated with commercial epoxy molding compound (EMC) have been used in a series of reliability tests which including the thermal shock test, the electrical service life test, and the isothermal aging test. Isolated IMC spots were found at the bonding interface during the thermal shock test. No void or crack was observed even after 1500 cycles thermal shock test. No electrical failure was happened. The isolated IMC spots also occurred at the Cu/Al bonds interface after 500 h electrical operation. After 1000 h electrical operation, the sizes of the IMC spots were about 0.5 μm. No layered IMC was observed. The IMCs were formed at the bonding interface when the aging temperature was between 150 °C and 250 °C. Micro cracks and Kirkendall voids were observed with the aging time of 9 days at 200 °C and the aging time of 9 h at 250 °C. The minor element in the EMC, Sb, has reacted with Cu wire and Cu bond surface at 250 °C when the aging time was more than 16 h. Cu₃Sb was the main product of the diffusion reaction. With the aging time of more than 49 h, the Cu wire was crashed into pieces and the Cu bond periphery has been severely corroded.     

Thermosonic bonding of gold wire onto silver bonding layer on the bond pads of chips with copper interconnects

A copper pad oxidizes easily at elevated temperatures during thermosonic wire bonding for chips with copper interconnects. The bondability and bonding strength of a gold wire onto a bare copper pad are seriously degraded by the formation of a copper oxide film. A new bonding approach is proposed to overcome this intrinsic drawback of the copper pad. A silver layer is deposited as a bonding layer on the surface of copper pads. Both the ball-shear force and the wire-pull force of a gold wire bonded onto copper pads with silver bonding layers far exceed the minimum values stated in the JEDEC standard and MIL specifications. The silver bonding layer improves bonding between the gold ball and copper pads. The reliability of gold ball bonds on a bond pad is verified in a high-temperature storage (HTS) test. The bonding strength increases with the storage time and far exceeds that required by the relevant industrial codes. The superior bondability and high strength after the HTS test were interpreted with reference to the results of electron probe x-ray microanalyzer (EPMA) analysis. This use of a silver bonding layer may make the fabrication of copper chips simpler than by other protective schemes.     

New observations on intermetallic compound formation in gold ball bonds: general growth patterns and identification of two forms of Au4Al

Relatively little information is available on the growth patterns and metallurgy of Au–Al intermetallics in fine-pitch (FP) and ultra-fine pitch (UFP) ball bonding. This paper presents a study of the growth pattern and chemistry of intermetallic compounds formed between a 25 μm 4 N gold wire and aluminium pad metallization after isothermal ageing in air at 175 °C. The data show the intermetallics grow vertically and laterally under the ball and totally consume the Al in the bond pad at <20 h. Then, a third layer of intermetallic grows between Au₄Al and Au₅Al₂. Measurements and observations made with EDX and optical microscopy lead to the conclusion that the new compound is a different form of Au₄Al, most probably a low-temperature version of the α-Au₄Al intermetallic structure. Electrical resistance during intermetallic growth was not measured in this study but wire chemistry and bonding conditions are found to affect the thickness of the intermetallic compounds, which suggests that the resistance of ball bonds during moulding and operation can change.     

A new approach in free air ball formation process parametersanalysis

The shape and size of gold wire ball formation deeply affects the quality of wire bonding. It not only affects the bond-ability of the first bond (ball bond), but also affects the possibility of processing low loop height bonding for thin packaging [such as thin small outline package (TSOP) and thin quad flat package (TQFP)] and high input/output (I/O) fine pitch packaging such as ball grid array. The parameters which affect the gold wire ball formation include: 1) tail length left after second bond; 2) type and shape of capillaries used; 3) material characteristics of gold wire; 4) supplied voltage, current, and time of electrical flame-off (EFO) unit; 5) gap between tail and electrode plate; and 6) relative position between capillary and electrode plate. In this paper, experiments were conducted to find the effect of these parameters on ball formation. Taguchi method together with neural network is applied in this research to find the best parameters setting for gold wire ball formation. It can then be used for wire bonding process parameter adjustment and process monitoring. It can also be used as reference for the development of wire bonders