Silver Bonding Wire for High-Current Power Semiconductor Reliability
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Solution Overview
Problem
Conventional Al wires used in power semiconductor devices face issues with thermal stress, heat generation, and reliability due to low melting point and high thermal resistance, leading to defects and increased resistance at high temperatures.
Innovation Solution
Using Ag wire with a die electrode having multiple layers of different metals like Ni, Cr, Cu, Pd, V, Ti, Pt, Zn, Ag, Au, or W, and employing wedge bonding to secure reliable connections, reducing heat generation and improving thermal resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If Al wire is used for metal wire connection, then bonding connection can be made easily by ultrasonic waves, but thermal resistance is high and heat generation is considerable at high current
Solution Approach 1:
The patent employs a composite wire structure with Cu core and Al cladding. The Cu core provides low thermal resistance and low heat generation due to its superior electrical conductivity, while the Al cladding maintains ease of bonding with Al electrodes through ultrasonic waves. This composite structure resolves the contradiction by combining materials with complementary properties.
2Quantity of substance
If Al wire is used for metal wire connection, then low cost is achieved, but melting point is low and reliability decreases at high temperature
Solution Approach 1:
The Cu-Al composite wire combines the low cost advantage of Al with the high temperature reliability of Cu. The Cu core maintains structural integrity and electrical performance at high temperatures up to its melting point of 1083°C, while the Al cladding keeps bonding costs low and process compatible with Al electrodes.
3Loss of energy
If Ag wire is used for metal wire connection, then heat generation is reduced and thermal resistance is improved, but Al 2 Ag intermetallic compound forms which is brittle and causes bonding failure
Solution Approach 1:
The Al cladding acts as an intermediary layer between the Ag/Cu core and the Al electrode, preventing direct contact and intermetallic compound formation. This intermediary structure allows the wire to benefit from Ag's low heat generation properties while avoiding the brittleness issue of Al-Ag intermetallics.
Solution Approach 2:
The composite wire structure with Cu-Al-Sn combination prevents brittle intermetallic formation while maintaining low heat generation. The Sn element modifies the intermetallic phase formation, and the layered composite structure controls diffusion paths to avoid brittle compound formation at the bonding interface.
4Power
If heavy wire is used for power semiconductor device, then current capacity is increased, but thermal stress causes crack at bonding interface
Solution Approach 1:
The Cu-Al-Sn composite wire structure provides both high current capacity and resistance to thermal stress-induced cracking. The multi-phase composite structure with controlled grain refinement and ductile intermetallic phases (modified by Sn) absorbs thermal stress, preventing crack formation at the bonding interface while carrying high current.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution ensures reliable bonding, reduced heat generation, and enhanced thermal resistance at high temperatures, preventing defects and maintaining bonding integrity under high current and temperature conditions.
Implementation Method 1
the specific resistance of Ag, 1.6 μΩ·cm, is lower than that of Al, 2.7 μΩ·cm, heat generation from a large amount of current flow in the Al wire is considerable
Implementation Method 2
the wire coating layer consists of one or more metals or an oxide or nitride of the metal(s), and each of the metal(s) is one selected from the group consisting of Pd, Au, Zn, Pt, Ni and Sn
Implementation Method 3
Cu wire is not used as a heavy wire for a power semiconductor device. Because Cu wire has high hardness and high work hardening coefficient, use of heavy wire likely damages a die electrode when a bonding connection is made by using ultrasonic waves
Implementation Method 4
This heat generation causes a temperature of the semiconductor die and the metal wire connected thereto to increase and decrease repeatedly. This causes thermal stress in a bonding part depending on the thermal expansion difference between members
Implementation Method 5
This heat generation causes a temperature of the semiconductor die and the metal wire connected thereto to increase and decrease repeatedly. This causes thermal stress in a bonding part depending on the thermal expansion difference between members
Data Source
Figure 1~2
AI summary
A power semiconductor device, operable regardless of thermal stress generation, which reduces heat generation from a wire, and secures the reliability of a bonding portion when the device is used for dealing with a large amount of current and/or under a high temperature atmosphere, a method of manufacturing the device and a bonding wire. The power semiconductor device comprises a metal electrode on a power semiconductor die and another metal electrode connected by a metal wire using a wedge bonding connection, wherein the metal wire is Ag or a Ag alloy wire having a diameter greater than 50µm and not greater than 2mm, and the metal electrode has thereon one or more metals and/or alloy layers, each of the layers being 50Å or more in thickness, wherein the metal for the layer is selected from Ni, Cr, Cu, Pd, V, Ti, Pt, Zn, Ag, Au, W and Al.