July 19, 2010
Gale Lockwood
231-357-2263
Key elements to successful wire
bonding at Birck
 Theory
 Bonding Machines
 Metallization
 Metals
 Substrates
 Process
 Adhesion
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Geometry
Pad Size (for bond feet)
Etching
Design
Cleanliness of procedure
Packages
Contact information
Wire Bonding Theory
Bonding is achieved by the interaction of:
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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 http://www.siliconfareast.com/bonding-theory.htm
Wire Bonding Machines
Birck has three types of wire bond machines available for use:
 West Bond 1400 – manual wire bonder
 K&S 4526 - Ribbon bonder
 K&S 4124 - Ball bonder
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
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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
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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) 682-3000 ext: 236
Lynne Souza Lynne.Souza@chelseatech.com,
 Global Chip Materials, (916)-853-9300
http://www.globalchipmaterials.com/visitors/products_visitors.
htm
 www.adtechceramics.com/design.htm
 www.asice.com/DesignServices.htm
 http://www.icpackages.com/visitors/products_visitors.htm
 http://www.idaxlabs.com/1b.asp
 http://www.aspentechnologies.com
 http://www.spectrum-semi.com/index.html
Wire Bonding Packages
Contact
 Gale Lockwood, 231-357-2263
 Lab: Birck 2261
 Lab regular hours Monday & Thursday 12:00-3:00pm