As semiconductor chip production moves to 300 mm wafers, wafer bumping for flip chip assembly faces more demanding requirements. Evaporation and sputtering processes for under-bump metal (UBM) and for bumping face less favorable deposition geometries, and require larger vacuum chambers. Plated bumping processes must maintain the uniformity of plating solutions and of electric current densities over even greater areas.

While gold stud bumping requires no UBM, stud bumping of 300 mm wafers presents three challenges: maintaining the required mechanical placement accuracy over larger distances, accommodating the smaller pad size and finer pitch of high-density chips, and achieving acceptable throughput in a serial, bump by bump, deposition and coining process.

Kulicke & Soffa have responded to these challenges by drawing on their experience in wide area, fine pitch wire bonding. The recently announced K&S WaferPRO ^TM large area ball bonder combines the industry’s highest cycle speed with the largest table travel, to allow efficient bumping of wafers up to 300 mm.

WaferPRO bumping speed is 16 bumps per second, with a complete line of fully automatic and manual handlers for 75 mm to 300 mm wafers available to translate this speed into throughput.

WaferPRO table travel of 400 mm by 330 mm (16" by 13"), the industry’s largest, easily accommodates 300 mm wafers. The table travel has an XY resolution of 0.1 micron, and a Z resolution of 0.3 microns. This allows a minimum production bump pitch of 60 microns, with a bump placement accuracy of +/- 5 microns. Minimum bump diameter is 45 microns.

K&S’s Accubump coining process combines coining in a single operation with bumping, for one-pass processing. Accubump can maintain a coined bump height within +/- 3 microns across a 300 mm wafer. Coining is part of the wire break process, rather than simply a compression of the wire tail into the ball. The SEM photo shows a coined bump, with the small ridge ("beard") characteristic of Accubump coining. The direction of capillary movement during coining can be programmed to maintain fine pitch, i.e. not to bash into the adjacent ball. With Accubump, the WaferPRO can place AND coin up to 12 bumps per second, in a single pass.

The 120 KHz Ultrasonic Transducer is fully programmable for constant current or constant voltage operation. The higher frequency allows lower temperature processing, important for temperature sensitive wafer materials such Lithium Niobate, Lithium Tantalate, Gallium Arsenide, and Indium Phosphide. Programmable dual-chuck thermal ramping is another feature for protecting temperature-sensitive materials. The dual chucks reduce thermal shock by temperature ramping on one chuck while the other chuck is busily bumping.

In summary, with the WaferPRO, K&S has moved the boundary for fast, efficient stud bumping of high density, fine pitch 300 mm wafers.