Wire bonding

Wire bonding (of English bond -. "Connection", "Liability" ) referred to in the construction and connection technology a step in the means of thin wires ( bonding wire ) terminals of an integrated circuit or discrete semiconductor ( eg, transistor, LED or photodiode ) can be connected to the electrical terminals of the chip package. The process of wire - bonding is referred to as wire bonding, the operation of soldering of the back of the chip as the chip bonding. Wire bonding but is also applied in the hybrid technology and discrete components.

Purpose

The case of an electronic circuit outside the visible terminals (pins) are connected by bonding wires inside the housing with the chip terminals ( bonding pads or pads). The pads in turn are metallic contacts, which are connected by means of ohmic contacts being electrically connected to the semiconductor. The task of the bonding wire, the electrical connection between the actual integrated circuit and the bare element and the wiring substrate.

The bonding wire is pulled from the pad ( bonding pad ) of the chip to the inner portion of the connecting leg and welded at two points. After bonding the components are capped, i.e., hermetically sealed in a housing or molded into the plastics or resin. The two process steps are referred to as the cycle 2, or " back " of the semiconductor production. There the two procedures thermo-sonic bonding and ultrasonic bonding are mainly applied.

Bonding wire

In the microelectronic assembly and interconnection technology bonding wire is usually made ​​of pure or alloyed or doped gold, but also aluminum with ( small ) percentage of silicon ( AlSi1 ) and copper are used. The minimum diameter round thin wires are usually at 12.5 microns of gold, 18 microns for aluminum and 20 microns of copper .. Thinner wires allow closer pad geometries and therefore higher packing densities. In the field of power electronics pure ( 99.99 % Al content and higher) aluminum materials are used in discrete semiconductors (diodes, transistors ), and integrated circuits is highly pure gold, or for reasons of cost, more recently, copper is used. When power semiconductors with high current loads Dick wires are used with diameters between 100 microns and 500 microns thick wire or ribbon. That is not sufficient, is bonded repeatedly.

Method

The different process variants in the sequential contact of semiconductor devices are thermo-compression bonding (abbreviated TC- bonding), the thermosonic ball-wedge bonding ( TS- bonding ), and the ultrasonic wedge-wedge bonding ( U.S. bonding). The TC bonding is rather atypical in wire bonding, as required for a connection high forces and temperatures can lead to damage of the semiconductor. The TS - bonding a gold or copper wires are used as a rule. U.S. - bonding is carried out with aluminum or aluminum -silicon wire ( AlSi1 ).

Thermosonic ball - wedge bonding

The TS - bonding, the gold wire is fed through a capillary tube of sintered metal or ceramic. By means of a flame or nowadays typically by means of a small electric discharge is below the protruding end of the wire melted, so that by the surface tension of a sphere ( ball ) forms. The already froze ball is bonded under pressure, heat and ultrasound on the contact surface ( ball bond ). In this case, the ball is deformed by the capillary. The shape of this contact is reminiscent of a nail head (hence is often spoken by Nailhead bonding ). The wire is first passed upwards around the loop auszuformen (sheet ), then led to the second contact point and contacted again by means of ultrasound, heat, and pressure. Due to the geometry of the capillary then the wedge bond is formed ( Wedge = engl. Wedge for ) and the Tailbond ( Tail = eng for tail). The wedge bond forms the end of the wire, the Tailbond attached the wire to the contact surface, so again standing out from the capillary end of the wire can be produced. To this end, the capillary is made ​​up a little, which closes over her mounted wire bracket and in the subsequent movement of the capillary to the top of the Tailbond will be demolished. Now a new ball can be melted.

As the continuation of the wire is independent of direction after the ball bond, the ball - wedge bonding is the fastest and most flexible method. The disadvantage is the necessary temperature of about 120 to 300 ° C. Since gold as opposed to aluminum not or only slightly oxidized, lacks the typical for the aluminum wire brittle-hard aluminum oxide layer, the surfaces cleaned by a "sandpaper " when bonding with aluminum wire. The oxide particles are thereby transported out for the most part from the bonding zone and incorporated to a lesser extent. Due to the higher temperatures, the surfaces are activated before the actual bonding process, so that only the material flow sufficient by the deformations of the wire to bond formation during bonding with gold wire. Ball-wedge bond with aluminum wire are difficult to make, since the oxide layer has a higher melting point than the aluminum itself In the melting of the ball, therefore there is always the risk that parts of the oxide skin destroy the ball geometry so that a reproducible bond quality only with high complex apparatus (protective gas atmosphere) is possible.

Ultrasonic wedge-wedge bonding

This method is mainly used for the bonding of aluminum wires. Because of the electrical and mechanical advantages, however, copper wires and ribbon- used especially in performance applications increasingly. By means of the wire thickness is made between the thin wire bonding with typical wire diameters between 15 microns and about 100 microns thick and wire bonding with wire diameters of approximately 100 microns to a technological maximum of currently about 500 microns.

The process of the wedge-wedge bonding runs schematically as follows:

• Step 1: The end of the bonding wire ( shown in red ), which under the bonding tool ( shown in blue ) sits and tail is called (shown Bond Island or bond pad, black) to be contacted surface is pressed with a defined pressure ( time equates Step 2 a ).
• Step 2: Agitated pressure (bond strength ) and applied ultrasonic vibrations cause to diffusion processes between wire and pad material. This results in a solid weld. This process takes only a few milliseconds for thin wire.
• Step 3: The bonding tool is moved to the second Kontaktierungsort, wherein the bonding wire is fed through the bonding tool. There, the wire is also coupled as in step 1 and 2.
• Step 4: The bonding process is completed at the thin wire bonding by removing the bonding tools in a defined Abreißbewegung, wherein the wire due to the weakening that has emerged at the second bonding point by the clinching of the wire breaks off there. Beimdick wire bonding, the wire is cut with a knife before it is demolished.

Since the direction of the wire is determined by the continuation of first bond already, this method is less flexible than the ball-wedge bonding. The advantage of the wedge-wedge bonding is in the low space requirement for a contact. This is about two to three times lower than that of a comparable ball bonding. Against the backdrop of ever increasing connection speeds of integrated circuits and space problems thus arise for contacting the U.S. Department of bonding shows significant advantages. Another advantage of this method is that heat must be supplied to the contacting. Because of cost and processing grounds temperature-sensitive plastics and glues are used in the manufacture of integrated circuits increasingly allowed a certain temperature in the production of the ICs are not exceeded. Furthermore, taking the heating and cooling within the Kontaktierprozesses up a significant period of time. By reducing the temperature necessary for bonding can be achieved here, a considerable increase in productivity.