WIRE BONDING

At CIDESI Queretaro clean room we have an apparatus wire bonder “iBond5000 Dual bond model” of the company Kulicke&Soffa with which wired process are carried on like in the semiconductors based devices with their packaging. This process is used mainly to verify the funcionality of the N-MOS devices.

The wire bonding technique is the most used to make interconnections between integrated circuits (IC), semiconductors based devices and their packaging during the manufacturing of microelectronic devices. With this technique is also possible to interconnect Printed CIrcuit Boards (PCB). the wire bonding technique is considered the most effective technology by its flexibility and cost-beneffit to assemble a wide variety of semiconductor based devices. Also it can be used with frequencies up to 100GHz.

Fig. 1. In the image the ultrasonic ball bonding process is observed with a gold cable and carrier of chips tha contain a NMOS.

The  cables or wires in these apparatus are made of materials as Aluminum, Copper, Silver and Gold. The wire diameters go from 10 micrometers to hundreds of micrometers for applications in high power elecronics. From the materials before mentioned, the copper wires have become in the preferred ones for electric interconnections in several semiconductors and microelectronic applications, the last one is due the copper wires cost is way cheaper in comparison with the Gold wires.

In the wire bonding process, there exist three operation physical principles:  thermosonic, thermocompression and ultrasonic.

The apparatus we have at the CIDESI Queretaro clean room performs the bonding or thermosonic and ultrasonic wired by the ball bonding and wedge bonding technique, this is obtained with a simple adaptation that the apparatus has. Next we tell you more about the operation principles of these processes.

Thermosonic wired, this technique is implemented using heat, force and ultrasonic energy to bond a Gold wire to a substrate with gold or aluminum surface. In this technique, the heat is applied putting the substrate in a hot holder. Some systems heat up the tip, that improves the bond functioning, then, a force is applied when exercising pressure on the cable in contact with the surface of the bond. The ultrasonic energy is delivered vibrating the tip of the cable when it is in contact with the surface.

The strong>Ultrasonic wired is performed in room temperature conditions and it is implemented with a combination of force and ultrasonic energy. The pressure used and time in the joint in the ultrasonic and thermosonic bond techniques are way lower and shorter than the required in thermocompression wired. Although the gold wired can be used to bond to surfaces of gold in ultrasonic bonding, this is used primarily to bond aluminum wires on gold or aluminum substrates, being the dominating technique in electronic of power when aluminum wires of large diameter are implemented.

Thermocompression wired is implemented using a combinatio of force and heat to deform the wire and make the joints or connections. The important parameters with this technology are time, temperature and union strenght. In this technique a high temperature is required (higher than 300°C) and a long wait time to make the joints. We have to consider that these high temperatures may damage sensitive integrated circuits and the process is sensible to contaminants on the joint surface.

As mentioned before there are two modalities of wire bonding that can be used in this process. For  ball bonding, it is restricted to use Gold and Copper wires, usually the quantity of heat is limited to use wires with small diameters, approximately 10 micrometers. For wedge bonding,  are used wires with larger diameters in applications of electronic of high power. With these techniques ball and wedge bonding, the wire is joint from both ends using a combination of pressure, ultrasonic energy and in its case heat to make the connection. The heat is used to make the metal softer. The correct combination of temperature and ultrasonic energy is employed to maximize the reliability and mechanical resistance of the connection. When the wedge bonding is used, the wire is stretched according to the first joint. This increases the wait time of the process due the alignment of the apparatus. In strong>ball bonding, alignment is not necessary as the initial joint does not have a preferred direction, so the wire can be stretched in any direction.

Lastly, in the Microtechnology Management of CIDESI Queretaro we are interested that the facilities that we have are useful in the development of projects as much as of scientific interest as provison of services to the community in general.

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