Westbond :: fit.physics.ucdavis.edu

The Westbond Manual Wire Bonder allows us to very accurately place gold studs on a substrate. This machine is used to create gold stud bumps with 1 mil or 0.5 mil gold wire for all gold stud work done in the lab.

The gold studding process:

Under normal operations, a 1 mil (25.4 μm) gold wire is threaded through the capillary. A high voltage arc, applied to the tip of the wire by a pneumatic arm, melts the gold and forms a small ball at the tip. The capillary, now carrying the gold ball at its tip, is brought to the surface of the device being studded, and after reaching a threshold of pressure, uses ultrasonic energy to create friction between the ball and the substrate to form a metallic bond. The capillary is then retracted, breaking off the wire and leaving the stud on the pad.

sem gold studs — SEM picture taken of several gold studs. While the exact shape of the studs depends on several parameters, several features are common to all: a “tail” extending from a hemispherical “head” (red) that sits atop of a flattened “collar” (blue). Before coining, the head of the stud is consistently 72 μm in diameter, the collar is (104 ± 7) μm in diameter, and can be placed on the pad to a precision of (16 ± 7) μm.

SEM single coined gold stud — SEM picture of a single coined gold stud. Coining is generally used when a double studding method is desirable. In order to coin, a moderate pressure is applied to a studded chip, flattening the gold stud into a “coin” shape. This is desirable when bonding with double gold studs because it allows for a more accurate bond (placing two coins atop of each other is much more accurate than aligning two cones tip to tip). Coining removes all distinguishable features, leaving a flat, wide plateau. After coining, the plateau widens to a diameter of (143 ± 12) μm. Therefore, the minimum pitch we can accurately bond is 183 μm.

An operator places gold studs onto a blank silicon test chip (dark square near the center of the photo). The operator's left hand positions the platform onto which the chip is secured. A capillary tip (just above the chip) dispenses 1 mil gold wire and places gold studs.

Close-up view of capillary and a gold on ceramic substrate for placing test studs. The capillary is the cylindrical white object, surrounded by an optional heating mechanism (the thick wire encircling the capillary). Just bellow the capillary, a thin gold wire hangs with a gold stud at the end of it.

User view when studding. Visible is the capillary tip (foreground, out of focus) and a bonding wafer dummy chip often used for testing bonds.

stud on lincoln — A stud placed on Mr. Lincoln's eye and three in his hair. Useful for a size comparison.

stud on lincoln zoomed — A zoom of the above.