Wire Bonding: The Ultrasonic Bonding Mechanism

Wire bonding is a welding process. During both ball and wedge bonding, wire and bond pad are massively deformed between the bond tool and the anvil of the bond pad or substrate. The dominant variables affecting deformation are ultrasonic energy, temperature, bond force and bond time. Deformation exposes new surface material that is clean and has not been exposed to atmospheric contamination and oxidation. As the new wire and bond pad surfaces mix, they form diffusion couples that grow and transform into the intermetallic weld nugget. The initial mixing is not at equilibrium in that it does not initially form the compounds described by the equilibrium phase diagram, but temperature and time very quickly allows diffusion to relax the initial mixture into the equilibrium phase diagram compounds. This paper will discuss the mechanisms behind the formation of ball and wedge bonds.

Process advantages of thermosonic wedge-wedge bonding using dosed tool heating

Thermosonic wire bonding has a number of advantages over “cold” ultrasonic wire bonding. Despite these potential advantages, it is rarely used besides ball-wedge and gold wedge-wedge applications, mostly due to the drawbacks and limitations of available heating technology. A recently introduced novel thermosonic process using a laser-heated bonding tool avoids most of these drawbacks. This contribution presents the results of two series of bonding tests which have used this novel process to bond aluminium and copper heavy wire to sheets of the same metal. The bond test results prove that thermosonic wedge-wedge bonding with a laser-heated tool has a number of significant advantages in both aluminium and copper wire bonding. It can be used to reduce the process time, to decrease the mechanical stress in the substrate by reducing ultrasound vibration amplitude and/or normal force, and to increase bond strength. These advantages are the same as in classic thermosonic wire bonding, but without the major disadvantage of having to heat to whole package. Because of the high thermal conductivity and capacity of the investigated metal sheet substrates, the observed positive effects of a heated tool are expected to be significantly higher on real-world substrates such as power semiconductors.

Ultra-Fine Pitch Wedge bonding for Device Reliability Characterization

Intel's EM (electromigration) package level stress lab has historically used 1mil aluminum wire to bond to pads 53 μm by 60 μm, with a pitch of 86 μm by 88 μm. The lab was challenged to align its wedge bonding capabilities to match the pitch used at wafer level probing with a pad size of 30 μm by 37 μm and pitch of 43 μm by 50.6 μm. In order to achieve the 43 μm by 50.6 μm pitch, 0.7 mil aluminum wire and an ultra-fine pitch wedge was used. In this paper, the benefits or matching the wafer level probe card capabilities are discussed as well as the concerns and considerations of implementing such a small pitch process. The primary concerns are heel shorting and bond placement repeatability but many factors influence these parameters. A brief summary of electrical testing performed to validate the process is discussed as well as new challenges that have arose due to the new testing capabilities.

Aluminum Wedge-Wedge Bonding Using Capillary and Ball Bonder

The paper discusses on the bondability and reliability of aluminum (Al) wedge-to-wedge bonding using fine ceramic capillary and ball bonder. Initial trials revealed aluminum build-up and poor capillary life with touch down of 20K or less. Optimizing the process parameters, switching on air-scrub, reducing shape angle to 20° instead of usual 35° and using Al-1wt%Si wire processed with refined grains revealed better capillary life with touch down of 200K without surface burrs. The method is capable of bonding complex looping and sharp acute bends. The data comprising of 1st and 2nd wedge dimensions, wedge pull, wedge shear and fracture mode for 20μm and 50μm Al-1wt%Si wires are presented. High temperature storage of aluminum wedge bonding to different substrate surfaces such as Al-0.5wt%Cu metallization, bare copper and gold plating revealed stable bond. From the wedge pull and tensile data, floor and shelf life of the wire is recommended to be 7days and 6months respectively. Evaluation of gold, copper and silver base bonding wires by this method showed feasible to bond and needs detailed studies to practice. The fusing current of Al-1wt%Si wire for varying diameter from 0.6 to 3mil and wire length from 1 to 20mm are also stated.