Copper Wire Bond Research Articles

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 degC for 4 h) were further, aged at high temperature of 150 degC 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 mum) diameter of the decapsulated unit is higher than 5.5 gf/mil2. 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

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.

Microcharacterization of thin copper and aluminium bond wires

Microstructure, texture and geometric dimensions have strong influence on the mechanical properties of thin bond wires. Sensitive mechanical and microscopic test procedures are needed for their characterization. The pretreatment and the tensile and fatigue testing methods are described, as well as the procedures for microcharacterization (SEM, EBSD, TEM). The experimental results reveal the microstructure and the crystallography of the wires after tensile and fatigue testing.

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/mil 2 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.

A comparison of copper and gold wire bonding on integrated circuit devices

A comparative approach was used to develop new copper wire bond technology using the current standard production gold wire process as a baseline. Characterizations were made in the areas of assembly processes and material selection, electrical and thermal performance, intermetallic formation on Al and Al alloy bond pads, and reliability testing. It was demonstrated that copper wire bond strength tested with wire-pull and ball-shear methods can be maintained at higher levels than comparable gold wire bonds. Die shear strengths were degraded by temperature cycling, but not by high-temperature storage. AC/DC parameter characterizations across temperature and power supply exhibited identical performances on copper and gold devices. Input impedance, capacitance, and ESD (electrostatic discharge) showed no deviations between Au and Cu wire bonded devices selected for this investigation. The lower resistivity of Cu wire offers the potential for increased device power ratings. The thermal transient response showed negligible differences between Au and Cu wire bonded devices. The difference in intermetallic growth rates between Au and Cu wire processed devices suggest that Cu wire bonded devices will have greater reliability than Au wire bonded devices. The reliability data indicate that copper wire bonding is at least equal to conventional gold wire bonding. It is concluded that a copper ball bond assembly process can be developed for mass production which will equal and potentially surpass the performance and reliability of the present gold ball bond assembly process. >

Effects of Ambient Atmosphere on Aluminum - Copper wirebond Reliability

The effect of ambient atmosphere on the stability of Al-Cu bonds aged at 150 to 300°C was studied. Data show that atmosphere plays a major role in determining the failure mode of wires bonded and aged under otherwise identical conditions. Bonds aged in air, nitrogen, and vacuum all experienced an initial decrease in pull strength due to annealing of the wire. The mechanical strength of vacuum and nitrogen aged samples continued to degrade with time. A shift in the primary failure mode from wire breaks to bond lifts was noted. The pull strengths of air-aged samples remained stable through 1600 h. The dominant failure mode was wire breaks. Al-Cu intermetallic phases were formed under all conditions. Surface diffusion was suggested by the presence of a groove at the bond perimeter. With time, the groove propagated along the metallic Cu/Cu intermetallic interface, undercutting the bond. The formation of copper oxides in air-aged samples retarded this process. An effective activation energy of 0.45 eV was calculated for the Al-Cu failure mechanism in vacuum.