The bond pad geometries for MESA chips are shrinking in size progressively as technology advances. Though beneficial for cost reduction and mass production, smaller MESA chip leads and bonding sites exacerbate the failure modes common to a wire bond process. This means that wire bond process developers must be exacting in their consideration of the multitude of wire bonding variables and the configuration and reliability of their wire bond tool performance. Though high quality wire bond tools are available additional tool customization and optimizations may be necessary to meet speed and yield requirements of a wire bond process.
Wire Bond Tool Variables
The wire bond process variable categories associated with the wire bond tool include the tool, bonder, and work holder. The tool is the mechanism that delivers the wire bond wire to the bonder, which applies the bonding heat and ultrasonic energy to complete a wire bond. The bonding surface is the area in which the wire bond is made with the connecting assembly, and the work holder is the device that provides a platform for the bonding surface and assembly, as well as the temperature and reliable support for the bonding tool and bonder.
The majority of the process variables for temperature, pressure, bonding energy, position, stability, and operator interaction impact the way the wire bond tool, bolder, and work holder are chosen and configured. For example, the wire bond tool can be adjusted to enable different clamping forces and controls the position where the bond actually occurs. The bonder can be calibrated to provide an optimum application of temperature, ultrasonic energy, and bond pressure, which can be developed into a schedule over time to deliver the optimum bond profile. Moreover, the work holder can be used to heat up the bonding surface, counteract external environmental factors, contribute to position accuracy, and stabilization of the bonding surface.
Wire Bond Bonding Surface Variables
Though often overlooked during the development of a wire bond process, the bonding surface condition is also vital in determining the optimum wire bond tool configuration. Things to consider about the bonding surface include the metal purity of bond surface material, the uniformity of material, its thickness and tolerance, and the surface texture. As exposure to particulates and other environmental factors can change the bond surface condition, understanding the surface contamination and metal bondability over time and temperature exposure must also be accounted for to develop a reliable and repeatable wire bond process.
Wire Bond Wire Variables
The wire bond wire itself and the condition of the bonded wire are also critical in developing and refining a bond wire process. Bond wires come in a variety of sizes, types, purities, and uniformities, and exhibit different breaking loads, elongation factors, twist/curl tendencies, have different surface conditions, and even a variety of aging profiles. Some of these factors are described when purchasing wire bond wire. However, some must be discovered through while developing a wire bond process.
Analysis of the bonded wire during wire bond process development provides the key indicators in fine tuning the process variables. Typical physical areas of examination include the ball formation, bonded wire geometry, location, and wire curl. Further aspect of the metallic structure that can be determined are the wire recrystallization, intermetallics that are developed post-bond, and the bond pull strength. The environment the wire bond is made in, the bonding schedule, and bond surface condition will also contribute to the end bonded wire features.
Overview of Wire Bond Tool Optimization
When selecting and beginning to configure a wire bond tool for a specific wire bond process, the initial step is to select a tool based on the best fitting footprint to fit the application. The next set of configurations to consider are the spacing, opening, and angle of the bond tool, which are all critical factors in determining the speed and reliability of the wire bond. Properly configuring these wire bond tool parameters can be the deciding factors in the consistency of the bonds, and number of wire breaks, and the amount of rethreading of the wire bond machine necessary.
Trial and error experimentation with different wire bond tool customizations is important to fine tuning the wire bond tool modification and process variables. Ultimately, to get a wire bond tool configuration and balance between the process variables close enough for fine tuning, experience in successfully handling small MESA chips can be the deciding factor. This is often the reason why suppliers will often seek out a subcontractor with expertise in wire bonding small MESA chips to develop an optimum process for their application.
Looking for more? Download our tech brief, "Tips for Increasing Yields when Wire Bonding Small MESA Chips."