The challenge to this process is in the application of epoxy in sufficiently small quantities. Gold bump sizes today range from two to four mils in diameter. Placing an equal sized dot of epoxy can be challenging due to viscosity and dispensing equipment limitations. Solutions available today include positive-displacement dispensers, screening, pin transfer, daubing, and gang daubing
Ball bumps can be made with a commercially available wire bonder. In fact, gold ball bumping is an extension of the 50-year-old wire bonding process. The ball bumping process is a variation of this wire bonding operation where the wire is sheared off after the ball is initially connected to the die or substrate.
The optimization of any gold flip chip process is achieved by optimizing the bump shape. New bump formation techniques have been developed that can create a gold bump without the traditional tail. This bump was designed specifically for a thermocompression or thermosonic bonding process. This shape directs the compression forces to assist in the formation of an intermetallic bond. By focusing the applied energy down to a smaller surface area, the other bond parameters (heat, force, and ultrasonics) can be reduced. As this bump continues to be compressed and deformed, the surface area in contact will grow to increase the conductive area.
There are a number of different gold bump processes that are available to meet the different process needs. The gold bump shape should be optimized to ensure the most efficient manufacturability and long-term reliability. Recent advances in bump technology allow the formation of flat-topped bumps without the added step of a coining process, providing a ready-made solution for packaging and process engineers.
Connectivity Benefits and Cost Advantages
Gold bumping also provides superior electrical connectivity, low inductance values, reduced electrical loss and lower power requirements, making it attractive for those seeking to move away from gold wire bonding or solder interconnects. As the demand for lead-free interconnect solutions increases, gold bumping has become an excellent alternative to solder bumping. Gold bump connections offer a cleaner process and eliminate the need for under-bump metallization (UBM) and fluxing.
Gold bumping can be less expensive than solder bumps in many applications. The cost benefit of gold versus solder primarily depends on how many bumps are on each wafer—when using wire to make gold ball bumps, you only pay for the number of bumps placed, so if your application requires less than 140,000 bumps, then it’s more cost effective to use gold.
