July 19, 2010 Gale Lockwood

Key elements to successful wire bonding at Birck Theory Bonding Machines Metallization Metals Substrates Process Adhesion Geometry Pad Size (for bond feet) Etching Design Cleanliness of procedure Packages Contact information

Wire Bonding Theory Bonding is achieved by the interaction of: Static force Ultrasonic energy Temperature Time Metallurgical bonding (or cold welding) is initiated between the wire and the bond pad by placing these two metals in intimate contact with each other. A static force is applied to the wire against the pad. In the case of gold ball bonding, the ball at the end of the wire is brought into contact with the bond pad by a needle-like tool known as a “capillary”, presses the ball against the bond pad. Wedge bonding is similar - a wire threaded through the wedge makes contact with the pad, pressing the wire to the bond pad. Ultrasonic energy is then applied by the bonding tool to the wire which scrubs the wire against the bond pad. This scrubbing action cleans the bond pad of debris and oxides, exposing a fresh bond pad surface. The metallurgical bond between the wire and the bond pad is further enhanced by the continued application of ultrasonic energy, resulting in plastic deformation of the wire and bond pad against each other. Heat can also be applied to the wire and bond pad (by heating the substrate) to soften the metals. Aside from the physical contact and deformation of the metals onto each other, diffusion of the wire and bond pad metal atoms also occurs to further enhance the bond. See

Wire Bonding Machines West Bond 1400 – manual wire bonder K&S Ribbon bonder K&S Ball bonder Birck has three types of wire bond machines available for use:

Westbond Wire Bonder

Ultrasonic Wedge Wire Bonding Wire is typically 25 micron diameter. The bonding tool is a wedge tool. The wire is gold or aluminum alloy.

K & S Ribbon Bonder

Ribbon Wire Bonds Ribbon wire is 125 micron wide by 25 micron thick. The bonding tool is a capillary feed wedge. The wire is gold.

K & S Ball Bonder

Ball Wire Bonds Wire is typically 25 micron diameter. The wedge is a capillary feed. The wire is gold.

Metallization Metals The ideal metal for bond pads is the same as the wire metal, i.e. gold to gold, aluminum to aluminum... In the real world, this is often not possible, but the mechanical properties (i.e. hardness) of metals used for pads and wire should be as similar as possible. The closer the match, the more likely that wires will bond. The process of bonding is facilitated by three elements: heat, power and force. The wires are molecularly bonded to the pads. If the wire and pad metals are dissimilar, they may not bond. The metals for wire bond pads most commonly used are gold, aluminum, and sometimes copper. copper usually needs additional scrubbing to remove oxidization before bonding The wire metal most commonly used for bonding is: gold at Birck aluminum at Physics

Substrates Many different kinds of material can be used for wire bonding stubstrates. (glass, alumina, silicon, quartz, ceramic) The substrate must be non-carbon based. (There are exceptions…) The most common substrate is quartz (glass) and silicon.. Hydro-Gel has been used with limited success Success varies in regard to the geometry, layout, and metallization process. Check with the appropriate authority before using a substrate that is not commonly used.

Metallization Process There are primarily two processes used at Birck to deposit metal on your substrate, sputtering and evaporation. The bondability of your metal depends upon many variables: the metal thickness, cleanliness of processing, and handling. Sputtering – requires more metal to make wire bonds stick reliably. A 500 nanometer metal adhesion layer covered by 1000 nanometers of gold is desirable. Evaporation – requires less metal on the substrate to make bonds stick reliably. An absolute minimum 35 nanometer metal adhesion layer (Ag, Ti, chrome) covered by 50 nanometers of gold is required.

Metallization Adhesion Two metal layers are recommended for wire bonding: The top layer – barrier metal thickness Sputtering – minimum 1 micron of gold Evaporation – minimum 50 nanometers of gold The bottom layer - adhesion layer This layer is usually chrome, titanium, or nickel Sputtering - minimum of 500 nanometers Evaporation - can be as thin as 35 nanometers

Geometry The geometry of wire bonds is important to consider when designing your device. Wire bonds like to be straight, point A to point B. When creating your layout, you should have the device pads in- line with the package pads if at all possible. You must consider the wire bond machine (bondhead and tool) access when creating your layout, particularly if you have other structures on your device. For a step bond, the uppermost pad is usually bonded first and the bond foot is slightly larger than the second, lower bond. Please contact the wirebond machine operator to check your layout before you fabricate your device!

Wire Bond Pad Dimensions The wire bond foot is typically 50 microns by 100 microns, so the pad should be (at least) 100 x 100 microns for best results. Bond pads can be smaller, but this will decrease the bond yield and strength of the bonds. For example, the bonding force, power and wedge tool size can be reduced to bond to a 25 microns x 50 microns pad, but this can cause problems. The minimum distance between pads (minimum pitch) is critical to ensure the bonded wires do not cause electrical shorts. The location of the device (your sample) on the package is important to ensure that the package pads and the device pads are in-line and you do not have wires that cross over each other.

Etching This procedure can have an effect on the ability to bond wires. If you over-etch, the wire bond will pull the metallization off the substrate. If you do not etch enough, the wires will not stick to the metallization.

Design When designing the device, it is important to plan the entire process. Please contact everyone who is involved with design, fabrication, and handling before you complete your design. Testing the ability to bond wires to your substrate, device, and package components before you put them together is the best way to ensure success.

Cleanliness The cleanliness of handling your device and package is important at all times. The flux from solder is a problem for wire bonding, so you need to protect all pads to be wire bonded (for example, with Kapton tape) before soldering on the device or package. Any biological material on the substrate will make it very hard for the gold to adhere to the surface. If you have bare copper traces, they need to be scrubbed before wire bonding. It is preferable to do this before you attach your device to your package and then wire bond them as soon as possible.

Packages You can find companies that make packages for your device. Before you order, it is important that you ensure that the package traces are compatible with wire bonding. Students have used packages from these companies in the past: Chelsea Technology, Inc. (978) ext: 236 Lynne Souza Global Chip Materials, (916) htm http ://

Wire Bonding Packages

Contact Gale Lockwood, Lab: Birck 2261 Lab regular hours Monday & Thursday 12:00-3:00pm