Ball Bond Research Articles

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.

The influence of mineralogical, chemical and physical properties on grindability of commercial clinkers with high MgO level

This research investigates various methods able to identify possible mineralogical, physical and chemical influences on the grindability of commercial clinkers with high MgO level. The aim of the study is to evaluate the hardness and elastic modulus of the clinker mineral phases and their fracture strength during the comminution processes, comparing samples from clinkers with low MgO level (0.5%) and clinkers with elevated MgO levels (> 5.0%). The study of the influence of mineralogical, chemical and physical properties was carried out using several analytical techniques, such as: optical microscopy, X-ray diffraction with Rietveld refinement (XRD) and X-ray fluorescence (XRF). These techniques were useful in qualifying the different clinker samples. The drop weight test (DWT) and the Bond ball mill grindability test were performed to characterize the mechanical properties of clinkers. Nanoindentation tests were also carried out. Results from the Bond ball mill grindability test were found to be related to the hardness of the mineral phase and to mineralogical characteristics, such as type and amount of inclusions in silicates, belite and alite crystals shape, or microcracked alites. In contrast, the results obtained by the DWT were associated to the macro characteristics of clinkers, such as porosity, as well as to the hardness and mineralogical characteristics of belite crystals in clusters. Hardness instrumented tests helped to determine the Vickers hardness and elastic modulus from the mineral phases in commercial clinkers and produced different values for the pure phases compared to previous publications.

Microelectronic Wire Bonding with Insulated Au Wire: Effects of Process Parameters on Insulation Removal and Crescent Bonding

The characteristics of the crescent bond process with insulated Au wire are investigated. Au wire with a sub-micron thick insulation coating is bonded on standard Ag plated leadframe diepads at 493 K. The wire loops are oriented perpendicular to the ultrasonic horn. The pull force obtained with a basic bonding process of insulated Au wire and bare Au wire are optimized by iteration and compared. Subsequently, the process is modified for the application with insulated wire. To increase the pull force an insulation layer removing stage (cleaning stage) is inserted before the bonding stage. The cleaning stage consists of a scratching motion (shift) toward to the ball bond in combination with ultrasound. The pull force obtained in this way with the insulated wire is 90:1 � 7:9 mN which is 2:4 � 2:0 mN larger than that obtained with bare Au wire. [doi:10.2320/matertrans.MRA2008087]

Impact work index prediction from continuum damage model of particle fracture

The traditional Bond work index proposed by F.C. Bond over 60 years ago is still today a useful tool for characterizing material crushability and grindability in the minerals industry. Although not as popular as the Bond ball mill or the Bond rod mill work indices, the impact work index, also called Bond crushability index, continues to be much used for crusher selection. The present work shows the application of a model that combines measurements of particle fracture energies and a parameter that characterizes material amenability to breakage by repeated impacts in the prediction of the Bond crushability index. Results show that a reasonably good agreement exists between measured and predicted results, as long as losses in the pendulum apparatus, used to determine the Bond crushability index, are taken into account in the simulations.

Sequential environmental stresses tests qualification for automotive components

The purpose is to create a new qualification methodology for plastic encapsulated electronic components used in an automotive environment at high temperature. It is based on the acceleration of failure mechanisms like ball bond lift (due to intermetallic Au–Al thickness growth), by combination of environmental stresses. The delamination measurement was used as an indicator of potential assembly weaknesses. An optimized package sequential qualification test flow is proposed.

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 cm 2/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 Cu 9Al 4, and CuAl 2 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.

Improvement of High-Temperature Reliability of Ball Bond Using Platinum-Modified Gold Alloy Wires

For superior Au/Al ball-bond reliability, we have developed the 3N (>99.9 wt% Au), 2N5 (>99.5 wt% Au) and 2N (>99 wt% Au) Au alloy bonding wires which contain less than 0.1, 0.5 and 1 wt% Pt, respectively. In the thermal aging test of squashed balls (40 μm in diameter), which consists of storage in air at 250°C and evaluation with wire pull and ball shear, it has been found that there are interesting differences in the ball-bond degradation as a function of wire type. Even the samples bonded using the 3N wire with a very small amount of Pt show superior ball-bond reliability compared with those using a 4N (>99.99 wt% Au) wire without any addition of Pt. The ball-bond degradation in the thermal aging test is reduced significantly with increasing Pt content of a wire. From microstructural analysis of the aged 2N ball bonds, the Pt-containing layer, which seems to play a crucial role in improvement of the ball-bond reliability, have been found in the upper part of intermetallic compounds.

Mechanical Cross-Sectioning of Au/Cu Wire Ball Bond with Underneath Cu/low-k Interconnect Layers

Abstract Standard metallographic cross-sectioning of copper and gold wire bonds with underneath Cu/low-k interconnect layers are difficult, easily they fracture during polishing. It can be overcome by appropriate liquid molding methodology and polishing using semi-automatic system at very low frictional forces and using finer alumina or diamond lapping films. SEM observation of the polished microstructures up to 5000–8000× magnification does not reveal micro cracks or smearing of copper.

Effects of die‐pad resonance on ultrasonic bond quality of wire bonds

PurposeWire bonding is an important method of interconnection in microelectronics. Ultrasonic energy is known to soften metallic materials and hence when used in the wire bond process it is effective to decrease the flow stress similar to thermal energy. The paper aims to address this issue.Design/methodology/approachDetailed resonance studies show that some designs have closeby resonance compared to the bonding frequency which causes enhanced vibration resulting in such over squashed bonds. Hence, precautions in the design stage are necessary to understand the closeby resonance frequencies and corresponding mode shapes of the leadframe that are in‐plane in nature along the transducer axis that can be excited by the capillary's motion. This can be determined a priori using numerical methods such as finite element method. In this paper, one such case study has been dealt in detail to explain the overall methodology.FindingsTo minimize the effects of resonance, damping should be increased by bonding polyimide tapes to enhance damping and stiffness which results in better ball bonds with optimum bell shape.Originality/valueThe paper explains the methodology of wire bonding in microelectronics.

Study of factors affecting the hardness of ball bonds in copper wire bonding

Copper wire bonding has gained popularity due to its economic advantage and superior electrical performance. However, copper is harder than gold, and replacing gold wire with copper wire introduces hardness related issues. This article reports investigations of the properties including microhardness of the copper balls bonded using ϕ25.4-μm copper wire and different combinations of electronic-flame-off (EFO) current and firing time settings with forming gas (5%H 2 and 95%N 2) as the inert cover gas. FABs with an identical diameter, obtained under different EFO firing conditions, were ball bonded with the same wire bonding parameters established using design of experiments. Microhardness tests were then performed on the cross-section of the bonded balls. The study revealed that ultrasonic generator current is the most significant factor to increase the bonded mashed ball diameter, ball shear and shear per unit area and to decrease the ball height. The microhardness of bonded copper balls is related to the EFO parameters, with FABs obtained by higher EFO current being softer. The lower hardness is attributed to the higher maximum temperature during the FAB melting state.

Fundamentals of thermo-sonic copper wire bonding in microelectronics packaging

Fine copper wire bonding is capable of making reliable electrical interconnections in microelectronic packages. Copper wires of 0.8–6 mil diameter have been successfully bonded to different bond pad metallized and plated substrate materials such as Al, Cu, Ag, Au and Pd. The three metallurgical related factors; solid-solubility and diffusion of dissimilar contact metals, oxide film breakage and plastic deformation of asperities play a critical role in the bonding. Plastic deformation of an asperity is the most significant factor one has to consider to attain good bonding. Soft aluminum metal (30–40 VHN), with a lower % asperity threshold deformation is easier to wire bond than harder metallic surfaces (Ni, W, Mo, Cr, Co, Ta) of 150–500 VHN. Good adhesion of wire bonding is achieved for the surface roughness (Ra) of 0.01–0.15 μm and 0.02–0.6 μm of bare and plated surfaces respectively. It is rationalized that the application of ultrasonic energy principally breaks the oxide film and deform the asperities, while a compressive force increases the proximity of asperities. Hence wire welds to bond pad surface by molecular attraction and inter diffusion. Storage of copper ball bonds at 175 °C for 100–1,000 h forms copper aluminide at the interface. EDAX and Auger analysis reveal 22 at% Al + 78 at% Cu composition of the aluminides and Cu3Al2 empirical formula is calculated, which, does not match with any of the reported copper aluminides. Hardness of the copper ball bonds and stitch bonds are higher than wire exhibiting work hardening of the bonds on processing.

Roughness Enhanced Au Ball Bonding of Cu Substrates

Process development studies of Au ball bumping on metallographically polished Cu substrates at ambient temperature were conducted by investigating the effect of process parameters on the ball bond shear force and the extent of bonding. These studies were performed on substrates polished with 0.06-mum or 1-mum abrasive solutions so as to assess the effect of surface roughness on bondability. Response surfaces were generated to illustrate the effects of ultrasonic power, bonding force, and time on bond shear force, and process windows were defined as those parametric combinations that yielded bond shear forces of 25gf or higher. After dissolving the Cu substrate away, the etched surfaces of the Au bumps were examined for bonded areas. Au-Cu ball bonds of about 65mum diameter with bond shear force values higher than 25gf were obtained on 0.06-mum polished substrates, but at an optimum bonding time of 1000ms. Increase in surface roughness, however, reduced the bonding time considerably, and values as low as 200ms were sufficient to yield bond shear force values higher than 25gf on 1.0-mum polished substrates. Bonding on 1.0-mum polished substrates not only reduced the bonding time, but also increased the maximum bond shear force and reduced the localization of bonded areas. These results suggest that a greater number of surface asperities of sufficient height on rougher substrates provide more bonding sites and hence improve the bondability

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.

Application of Box–Behnken design and response surface methodology for modeling of some Turkish coals

The aim of our research was to apply Box–Behnken experimental design and response surface methodology for modeling of some Turkish coals. As a base for this study, standard Bond grindability tests were initially done and Bond work indexes ( W i ) values were calculated for three Turkish coals. The Box–Behnken experimental design was used to provide data for modeling and the variables of model were Bond work index, grinding time and ball diameter of mill. Coal grinding tests were performed changing these three variables for three size fractions of coals (−3350 + 1700 μm, −1700 + 710 μm and −710 μm). Using these sets of experimental data obtained by mathematical software package (MATLAB 7.1), mathematical models were then developed to show the effect of each parameter and their interactions on product 80% passing size ( d 80). Predicted values of d 80 obtained using model equations were in good agreement with the experimental values of d 80 ( R 2 value of 0.96 for −3350 + 1700 μm, R 2 value of 0.98 for −1700 + 710 μm and R 2 value of 0.94 for −710 μm). This study proved that Box–Behnken design and response surface methodology could efficiently be applied for modeling of grinding of some Turkish coals.

Impact of Geometric Parameters of Capillary Design on Quality of Gold Ball Bonding

0.13μm or smaller critical dimension of integrated circuits has driven the bonding pad pitch down to 50μm or even smaller. It has also had quite an impact on the adhesive force between the bonding pad and the gold ball. This study focuses on the correlation between the capillary geometric parameters (i.e. tip diameter, chamfer diameter, inner chamfer angle, and cone angle) and the bonding quality (i.e. ball size, ball thickness, ball shear force, specific ball shear force, and capillary vibration). An experimental design is utilized and the response is analyzed statistically. The results show that as the tip diameter and ball size increase, the ball thickness decreases, and the ball shear force increases first and then decreases afterward. As the cone angle decreases, the ball size increases, the ball thickness decreases, and the ball shear force increases. Therefore, both tip diameter and cone angle are significant factors that affect the bond quality. The empirical model, i.e. the correlation between the capillary geometric parameters and the bonding quality is also derived, and is quite useful in predicting the bonding pad size or bonding pad pitch required during process development. The predicted values and the experimental results are shown to have good agreement.

Mechanical reliability of Au and Cu wire bonds to Al, Ni/Au and Ni/Pd/Au capped Cu bond pads

This work is an assessment of the mechanical reliability of Au and Cu ball bonds to Al, Ni/Au and Ni/Pd/Au surfaces in terms of high temperature storage. All systems show very good shear strength after thermal storage for up to 120 days at 150 °C. The Au ball bonds on Al surface show Kirkendall voiding starting from 60 days. This did not decrease their mechanical strength but it is expected to become a reliability issue in the long run. The Cu wire bonds on Al caps show a higher initial strength, much slower intermetallics formation and no Kirkendall voiding. This makes them a potentially better industrial solution. Excellent bond strength was found for Cu- and Au-bonds on Ni/Au and Ni/Pd/Au caps. No intermetallics formation or other microstructural change have been found on these interfaces up to 120 days at 150 °C, which was related to the full solubility of the materials along these interfaces. This result suggests that they can be a successful industrial solution for the next generation of packages.

An Evaluation of Gold and Copper Wire Bonds on Shear and Pull Testing

In microelectronic packaging technology wire bonding is a common interconnect technique. The quality and reliability of wire bonds are generally evaluated by ball shear and stitch pull testing. From the load versus time and load versus tool tip displacement plots of the shear test, three regions can be observed. Region I primarily exhibits elastic-plastic deformation occur, while crack nucleate in region II which propagates in region III which finally ends in a catastrophic failure. Fractographs reveal in the case of gold ball bonds shows fracture occurs in Al bond pad metallization close to Au-Al intermetallics. In Cu ball bonds of 1, 2, and 4ml wire sizes also Al bond pad metallization cracks but penetrate deeper into the pad which indirectly shows that the bonding layer is stronger than that of gold ball bonds. Optical microscopic observation of the sheared copper bond surfaces reveal sticking of Al which provides qualitative information of the area of the bond between the ball bond and the bond pad. In thermally aged gold ball bonds, the gold above the intermetallic layer fractures. The energy required to fracture a gold or copper ball bond of 1ml wire size is around 370J∕m2, while an aged gold ball bond consumes about 520J∕m2. Void nucleation and coalescence mechanism of ductile fracture takes place in the ball and stitch bonds, however, silicon particles may be the preferential void nucleation sites in bond pad aluminum metallization failures. To understand the second bond strength, a stitch pull test was conducted and the results showed the neck of the stitched wire cracks thus leaving behind a tail bond on the lead finger.

Numerical and experimental correlation of high temperature reliability of gold wire bonding to intermetallics (Au/Al) uniformity

A study of the Au/Al intermetallics compound (IMC) in gold wire bonds on aluminum pads is presented. A test vehicle has been deliberately built for high temperature storage (HTS) test. There are two kinds of samples, i.e., one is with uniform intermetallics in the ball bond at time zero, and another one is with non-uniform intermetallics. It is found that the package with initial uniform intermetallics passed the HTS test, while the package with initial non-uniform intermetallics failed with ball lift failure after HTS storage. Unique thermo-mechanical simulation is performed to understand the failure mechanism for HTS. Two types of intermetallics model have been established correspondingly, i.e., one with uniform intermetallics and another one with non-uniform intermetallics. The volume-induced stress under HTS has been simulated. It is found that the difference between IMC stresses in uniform and non-uniform models is not obvious, while there is a large difference between stresses in gold area of uniform and non-uniform model. For the non-uniform model, high normal tensile stress exists in the inner convex areas of gold bond, which connect two pieces of separate intermetallics, while for the uniform model, there is no such high stress area. The interfaces between Au and IMC in non-uniform model are the critical interfaces with high stress. It is indicated from these results that HTS life is much longer for the bond with uniform IMC than that with non-uniform IMC. The thermo-mechanical stress model correlates well with the test results.

Effect of wire diameter on the thermosonic bond reliability

Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6 mil and 0.8 mil diameter wires) shows formation of voids apart from intermetallic compound growth. While, with 1-mil and 2-mil diameter gold wire bonds the void growth is less significant and reveal fine voids. Studies also showed void formation is absent in the case of thicker 3 mil wire bonds. Similar tests on copper ball bonds shows good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as any voids on the microscopic scale and thus exhibits to be a better design alternative for elevated temperature conditions.

An unusual mechanical failure mode in gold ballbonds at 50 μm pitch due to degradation at the Au–Au 4Al interface during ageing in air at 175 °C

While investigating the bake test reliability of gold ball bonds in air at 175 °C an unusual failure mode was encountered that can strictly be classified as a ball lift but which has pull strengths as high as normal neck breaks. The failure mode is termed a high strength ball lift (HSBL) and is distinguished by partial de-bonding between the gold ball and intermetallic compound. The partial de-bonding occurs as a ring or annulus that starts from the outside of the ball and grows inwards toward the ball centre, occurring within the Au 4Al compound layer and between the Au 4Al and Au ball while the remaining part of the gold ball is well-bonded to the Au 4Al at the centre of the bondpad. During pull testing, the de-bonded outer ring acts as a notch that propagates a blunt crack into the gold ball resulting in high pull strengths. As ageing time increases the de-bonded region grows larger and advances inwards towards the ball centre. EDX of both types of ball-lifts indicates higher levels of oxygen on the Au 4Al surface that suggests annulus growth may be partially due to oxidation of the intermetallic. The number of HSBL failures and the point in time at which they occur is observed to depend on the bonded ball squash height, a relatively easily controlled parameter in gold ballbonding. When devices with targeted squash heights (TSH) of 6 μm, 7 μm and 8 μm are isothermally aged, zero ball lifts are only obtained at 7 μm squash height. HSBL failures are encountered earlier at the 6 μm TSH than at 8 μm TSH and as the de-bonded region progresses further inwards during baking, the HSBL’s are observed to transition to low strength ball lifts (LSBL) as the load-bearing area of the ball is reduced. The dependence of failure mode on squash height is believed to be related to interactions between bonded ball stress, intrinsic intermetallic growth stresses and thermo-chemical degradation of the Au 4Al intermetallic compound.