Wire Bonding Semiconductor Packaging

Wirebonding is the process utilized to provide electrical connection between the silicon chip and the external leads of the semiconductor device using very fine bonding wires. At Minco we use either gold (Au) or aluminum (Al) wire for our wire bonding process. There are two common wirebonding processes: Au ball bonding and Al wedge bonding.  

Thermosonic Gold - .7 to 3 mils

Ultrasonic Aluminum – 1 to 25 mils

During gold ball wire bonding, a gold ball is first formed by melting the end of the wire utilizing an electronic flame-off (EFO). This gold ball has a diameter ranging from 1.5 to 2.5 times the wire diameter.  The gold ball is then brought into contact with the bond pad. Adequate pressure, heat, and ultrasonic forces are then applied to the ball for a specific amount of time, forming the initial metallurgical weld between the ball and the bond pad as well as deforming the ball bond itself into its final shape. The wire is then run to the corresponding finger of the package or substrate forming a "loop" between the bond pad and the package lead. Pressure and ultrasonic forces are applied to the wire to form the second bond (known as a wedge bond, stitch bond, or fishtail bond). The wirebonding machine or wirebonder breaks the wire in preparation for the next wirebonding cycle by clamping the wire and raising the capillary.

During aluminum wedge wire bonding, a clamped aluminum wire is brought in contact with the aluminum bond pad. Ultrasonic energy is then applied to the wire for a specific duration while being held down by a specific amount of force, forming the first wedge bond between the wire and the bond pad. The wire is then run to the corresponding finger of the package or substrate, against which it is again pressed. The second bond is again formed by applying ultrasonic energy to the wire. The wire is then broken off by clamping and movement of the wire.

Because it is non-directional, gold ball bonding is much faster than aluminum wedge bonding, which is why it is extensively used in plastic packaging. Unfortunately, gold ball bonding on Al bond pads cannot be used in hermetic packages, primarily because the high sealing temperatures (400-450 deg C) used for these packages tremendously accelerate the formation of Au-Al intermetallics that can lead to early life failures.  Gold ball bonding on gold bond pads, however, may be employed in hermetic packages.

Unlike Al-Al ultrasonic wedge bonding, Au-Al thermosonic ball bonding requires heat to facilitate the bonding process. The Al bond pad is harder than the Au ball bond, making good bonding between them through purely ultrasonic means impossible without causing wire, bond pad, or silicon substrate damage. The application of thermal energy to the Al bond pads 'softens' them, promoting the inter-diffusion of Au and Al atoms that ultimately form the Au-Al bond.  Heat application also improves bonding by removing organic contaminants on the bond pad surface.

The most common reason for insufficient intermetallic formation aside from incorrect parameter settings is the presence of foreign materials or contaminants on the surface of the bond pad.  Frequently-encountered bond pad contaminants include unetched glass; silicon saw dust, and process residues.  Minco uses a high pressure deionized water rinse after a wafer is sawn to remove any residual silicon saw dust.  An argon etch is performed just before wire bonding to remove any oxidation or ionic contaminates. The quality of a bond may be assessed using bond strength tests.

Here are some common reasons for wirebonding failure:

Failure Mechanism

Common Cause

Ball bond lifting is the detachment of the ball bond from the silicon chip; also refers to non-sticking of the ball bond to the bond pad.

Contamination on the bond pad, incorrect wire bond parameter settings, instability of the die during bonding, bond pad corrosion, excessive bond pad probing, Kirkendall voiding, intermetallic spiking due to the Devaney mechanism, excessive thermal stress resulting in excessive intermetallic formation, bond pad metallization/barrier metallization lifting, cratering.

Wedge bond lifting is the detachment of the wedge bond from the silicon chip, bonding post, or lead; non-sticking of the wedge bond to the bond pad, post, or lead.

Contamination on the bond pad or lead, incorrect parameter settings, instability of the die or lead frame during bonding, bond pad or lead corrosion, excessive bond pad probing.

Ball bond neck break is breakage of the wire at the neck of the Au ball bond.

Incorrect wire bond parameter settings, incorrect wire looping, die-to-package delamination, excessive wire sweeping during mold, excessive die overcoat, 'bamboo' grain structure due to excessive thermal treatment.

Midwire breakage along the span of the wire.

Wire nicks or damage, wire corrosion, tight wire looping, excessive wire sweeping, electrical overstress wire nicks or damage, wire corrosion, tight wire looping, excessive wire sweeping, electrical overstress.

Wedge bond heel break is breakage of the wire at the heel of the Al wedge bond.

Incorrect wire bond parameter settings, incorrect wire looping, lead finger-to-package delamination.   

Bond-to-Metal shorting is electrical shorting between the bond and a metal line on the die.

Incorrect wire bond parameter settings, incorrect bond placement, insufficient bond pad-to-metal distance.

Bond-to-Bond shorting is electrical shorting between two bonds.

Incorrect wire bond parameter settings, incorrect bond placement, insufficient bond pad-to-bond pad distance.

Wire-to-Wire shorting is electrical shorting between two wires.

Incorrect wire looping, excessive wires weeping, insufficient wire-to-wire distance.

Cratering is silicon damage under the bond pad, the worst of which is when a chunk of silicon is completely detached from the active circuit.

Incorrect wire bond parameter settings, excessive bond pad probing

 

 

 

 

 

 

About Minco Technology Labs, LLC: Minco Technology Labs, LLC, headquartered in Austin, TX, is a leading provider of mission critical high reliability (Hi-Rel) semiconductor packaged solutions, die processing, and test services.  Minco services advanced technologies with state of the art solutions, as well as hard to find, obsolete or end-of-life products for multiple applications.  Minco Technology Labs, LLC is a Leading Value Added Supplier of High Reliability Components, Semiconductor Die Processing, Assembly, Test Services and Distribution. Minco serves the Military, Aerospace, and Energy, Transportation, Heavy Industrial and other specialty markets.