Pd-Coated Cu Bonding Wire for HTS Oxidation Resistance
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Solution Overview
Problem
Cu bonding wires used in semiconductor devices face reliability issues in high-temperature and high-humidity environments due to oxidation and corrosion, leading to degraded bonding reliability, especially in on-vehicle devices where high-temperature storage tests reveal poor performance of ball bonded parts.
Innovation Solution
A Cu alloy core material with a Pd coating layer and the inclusion of elements such as Ni, Zn, Rh, In, and Pt in concentrations of 0.03 to 2% by mass, along with specific crystal orientation and grain size control, enhances bonding reliability by maintaining a strength ratio of 1.1 to 1.6, improving both ball bonded and wedge bonding strengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-purity Cu is used as bonding wire material, then cost is reduced and electrical conductivity is improved, but bonding reliability and oxidation resistance deteriorate
Solution Approach 1:
A Pd coating layer with thickness of 0.01 to 0.06 μm is applied on the Cu core material surface. The Pd layer acts as an intermediary barrier between the Cu core and the corrosive environment, preventing oxidation and sulfur corrosion while maintaining electrical conductivity and bonding reliability in high-temperature storage conditions.
Solution Approach 2:
The bonding wire is constructed as a composite structure with a Cu-based core material containing specific alloying elements (Ni: 0.01-0.5 mass%, Pd: 0.01-0.1 mass%, and at least one of In, Ga, Ge, or Ag) covered by a Pd coating layer. This composite structure combines the high conductivity of Cu with the oxidation resistance of Pd and the beneficial effects of alloying elements.
2Reliability
If alloying elements are added to improve bonding reliability in HTS, then oxidation resistance improves, but wire strength ratio may increase excessively
Solution Approach 1:
The patent specifies precise parameter ranges for alloying element concentrations (Ni: 0.01-0.5 mass%, Pd: 0.01-0.1 mass%, In/Ga/Ge/Ag: 0.003-0.03 mass%) and crystal grain size (0.9-1.3 μm) to control the strength ratio within 1.1 to 1.6. These parameter changes optimize both bonding reliability in high-temperature storage and mechanical properties for wedge bonding.
Solution Approach 2:
Different alloying elements are strategically selected to provide specific local functions: Ni and Pd for oxidation resistance, and In/Ga/Ge/Ag for controlling crystal grain growth and strength characteristics. The Pd coating layer provides localized protection at the surface while the core material composition is optimized for bulk mechanical properties.
3Stability of the object's composition
If sulfur-containing silane coupling agent is added to mold resin for improved adhesiveness, then adhesion between resin and substrate improves, but corrosion of Cu wire increases at high temperature
Solution Approach 1:
The Pd coating layer serves as a protective intermediary between the Cu core and sulfur released from the mold resin's silane coupling agent at high temperatures. This barrier prevents sulfur from reaching and corroding the Cu, allowing the use of sulfur-containing adhesion promoters in the mold resin without compromising wire reliability.
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 significantly improves the bonding reliability of ball bonded parts in high-temperature storage tests at 175°C to 200°C, maintaining favorable wedge bonding strength while reducing the risk of corrosion and oxidation, thus enhancing the longevity and performance of semiconductor devices in harsh environments.
Implementation Method 1
Cu has the drawback of being more susceptible to oxidation than Au and has problems in that bonding reliability, ball formability, and wedge bondability are inferior. As a method for preventing surface oxidation of the Cu bonding wire, a structure that coats a surface of a Cu core material with a metal such as Au, Ag, Pt, Pd, Ni, Co, Cr, or Ti is proposed
Implementation Method 2
When sulfur freed at a high temperature of 175°C or more contacts with Cu, Cu violently corrodes to produce a sulfide (Cu 2 S) or an oxide (CuO). When the corrosion of Cu is produced in the semiconductor device using the Cu bonding wire, the bonding reliability degrades especially of the ball bonded part
Implementation Method 3
A bonding process of the bonding wire includes heating and melting a wire tip with arc heat input, forming a ball (FAB: free air ball) through surface tension
Implementation Method 4
heating and melting a wire tip with arc heat input, forming a ball (FAB: free air ball) through surface tension
Implementation Method 5
A method for bonding the bonding wire is generally a thermal compressive bonding technique with the aid of ultrasound
Implementation Method 6
a thermal compressive bonding technique with the aid of ultrasound
Data Source
AI summary
There is provided a bonding wire for a semiconductor device, the bonding wire including a Cu alloy core material and a Pd coating layer formed on a surface thereof, achieving simultaneously improvement in bonding reliability of a ball bonded part in HTS at 175°C to 200°C and an strength ratio (= ultimate strength/0.2% offset yield strength) of 1.1 to 1.6. Containing one or more of Ni, Zn, Rh, in, Ir, and Pt in the wire in a total amount of 0.03 to 2% by mass improves the bonding reliability of the ball bonded part in HTS, and furthermore, making an orientation proportion of a crystal orientation <100> angled at 15 degrees or less to a wire longitudinal direction among crystal orientations in the wire longitudinal direction 50% or more when measuring crystal orientations on a cross-section of the core material in a direction perpendicular to a wire axis of the bonding wire, and making an average crystal grain size in the cross-section of the core material in the direction perpendicular to the wire axis of the bonding wire 0.9 to 1.3 µm provides a strength ratio of 1.6 or less.