Wire bonding
The wire bonding (of Engl. Bond - "compound", "adhesion") referred to in the assembly and interconnection technology a process step in which by means of thin wires ( bonding wire ), the terminals of an integrated circuit or a discrete semiconductor (for example, transistor , Light-emitting diode or photodiode ) are connected to the electrical connections of other components or the housing . In contrast, the process of soldering the rear contacts of a chip without a wire is called chip bonding .
Intended use
The connections ( pins ) visible on the outside of an electronic circuit are connected to the chip connections (bonding islands or pads ) inside the housing via bonding wires . The pads are in turn metallic contacts, which are electrically connected to the semiconductor by means of ohmic contacts . The task of the bonding wire is the electrical connection between the actual integrated circuit or the bare component and the wiring carrier .
The bonding wire is pulled from the connection surface (bonding island) of the chip to the inner part of the connection leg and welded at both points. After bonding, the components are capped, that is, hermetically enclosed in a housing or cast in plastics or synthetic resin. The two process steps are referred to as cycle 2 or "backend" of semiconductor production. The two methods thermosonic bonding and ultrasonic bonding are mainly used.
Bond wire
In microelectronic assembly and connection technology , bonding wire usually consists of gold , aluminum or copper . Gold is used in various degrees of purity, possibly alloyed or doped with other substances. Gold wires are usually in the range from 15 µm to 50 µm, the thinnest possible wires being used for reasons of cost. For fine aluminum wires (approx. 18 µm to 100 µm), high-purity aluminum doped with silicon or magnesium is used. High-purity aluminum wire with diameters of approx. 100 µm to 500 µm is available for power applications. Copper wire was originally more difficult to process than bonding wire, due to the cost savings that can be achieved compared to gold and the very good electrical and mechanical properties as well as the high reliability of the intermetallic connections during bonding, copper has established itself as a common material for bonding wires. If the load capacity of individual bond connections is insufficient, multiple bonds are made per electrical connection. For power semiconductors with high current loads, thick wires or thick wire ribbons are used.
Procedure
The various process variants for sequential contacting of semiconductor components are thermocompression bonding (TC bonding for short), thermosonic ball wedge bonding (TS bonding) and ultrasonic wedge wedge bonding (US bonding). TC bonding is rather untypical for wire bonding, as the high forces and temperatures required for a connection can damage the semiconductor. Gold or copper wires are usually used for TS bonding . US bonding is carried out with aluminum or aluminum- silicon wire (AlSi1).
Thermosonic ball wedge bonding
With TS bonding, the gold wire is passed through a capillary made of sintered metal or ceramic . By means of a flame or nowadays typically by means of a small electric discharge (EFO e lectronic f lame o ff) the protruding bottom end of the wire is melted, so that by the surface tension of a ball (ball) forms. The already solidified ball is bonded to the contact surface ( ball bond) under pressure, heat and ultrasound . The ball is deformed by the capillary. The shape of this contact is reminiscent of a nail head (this is why nailhead bonding is often used). The wire is first guided upwards to form the loop (arch), then guided to the second contact point and contacted again using ultrasound, heat and pressure. The geometry of the capillary then creates the wedge bond (wedge) and the tail bond (tail). The wedge bond forms the end of the wire, the tail bond staples the wire onto the contact surface, so that a wire end protruding from the capillary can be produced again. To do this, the capillary is moved a little upwards, the wire clip attached above it closes and the tailbond is torn off during the subsequent upward movement of the capillary. A new ball can now be melted.
Since the continuation of the wire after the ball bond is independent of direction, ball-wedge bonding is the fastest and most flexible method. The disadvantage is the necessary temperature of approx. 120 to 300 ° C. Since gold, in contrast to aluminum, does not oxidize at all or only slightly, the hard and brittle aluminum oxide layer typical of aluminum wire is missing, which cleans the surfaces with an "emery" effect when bonding with aluminum wire. The oxide particles are mostly transported out of the bonding zone and incorporated to a lesser extent. Due to the higher temperatures when bonding with gold wire, the surfaces are activated before the actual bonding process, so that the flow of material through the deformation of the wire is sufficient to form a bond. Ball-wedge bonds with aluminum wire are only possible to a limited extent, as the oxide skin has a higher melting point than the aluminum itself. When the ball is melted, there is always the risk that parts of the oxide skin will destroy the ball geometry, so that a reproducible bond quality is only possible with a high level of quality equipment ( protective gas atmosphere ) is possible.
Ultrasonic wedge-wedge bonding
This process is mainly used for bonding aluminum wires. However, because of the electrical and mechanical advantages, copper wires and ribbons are increasingly being used, especially in power applications. On the basis of wire thickness, a distinction is made between thin wire bonds with typical wire diameters between 15 µm and 75 µm and thick wire bonds with wire diameters of around 75 µm up to the technological maximum of currently around 600 µm. Wires with a rectangular cross-section can also be processed.
The process of wedge-wedge bonding runs schematically as follows:
- Step 1 : The end of the bond wire (shown in red), which sits under the bond tool (shown in blue) and is called the tail, is pressed onto the surface to be contacted (bond island or bond pad , shown in black) with a defined pressure (step 2 starts at the same time a).
- Step 2 : Applied pressure (bond force) and applied ultrasonic vibrations lead to diffusion processes between the wire and pad material. This results in a firm weld. This process only takes a few milliseconds with thin wire.
- Step 3 : The bonding tool is moved to the second contacting location, the bonding wire being fed through the bonding tool. There the wire is also connected as described in steps 1 and 2.
- Step 4 : In the case of thin wire bonding, the bonding process is completed by removing the bonding tool in a defined tearing movement, whereby the wire tears off due to the weakening that has arisen at the second bond point due to the pressing of the wire. With thick wire bonding, the wire is cut with a knife before it is torn off.
Since the direction of the wire continuation is already specified by the first bond, this method is less flexible than ball-wedge bonding. The advantage of wedge-wedge bonding is the low space requirement for a contact. This is around two to three times less than a comparable ball bond. Against the background of the constantly increasing number of connections of integrated circuits and the resulting space problems for contacting, US bonding shows clear advantages here. Another advantage of this process is that no heat has to be supplied for contacting. Since temperature-sensitive plastics and adhesives are increasingly being used in the manufacture of integrated circuits for reasons of cost and processing, a certain temperature must not be exceeded in the manufacture of the ICs. Furthermore, the heating and cooling time within the contacting process takes up a significant amount of time. By reducing the temperature required for bonding, a considerable increase in productivity can be achieved here.
Device categories
Manual & semi-automatic wire bonders
These devices are used for prototypes and small batch production.
The wire is placed using a microscope.
To make aiming easier, the aiming mechanism has a gear ratio and the bonding needle moves to the "search height" which is approx. 150 µm above the surface.
The wire bonder always needs a user who is responsible for aiming the bonding needle and aligning the microchip.
The process per placed wire takes approx. 8 seconds, so these devices are not suitable for production.
Fully automatic wire bonders
These devices are used for high volume production.
Once the application has been programmed, the bonding takes place automatically; no employee is required.
Aiming is carried out automatically using a camera and recognition software.
Up to seven wires can be placed per second.
These devices can be used in an "inline production line".
See also
literature
- Wolfgang Scheel: Module technology in electronics . 1st edition. Verlag Technik, 1997, ISBN 3-341-01100-5 .
Individual evidence
- ^ Bonding Wires for Semiconductor Technology. (pdf) Heraeus , September 2017, accessed on June 24, 2020 (English).
- ↑ More power in electronics - innovation project: intelligent production of copper bond connections . Technology network Intelligent Technical Systems OstWestfalenLippe. ( Memento of the original from December 8, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved November 30, 2015.
- ↑ Manufacturer website "Hesse Mechatronics". Retrieved February 20, 2016 .
