Copper Pillar Flip-Chip Bonding with Nickel-Gold-Palladium Surface Finish
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Solution Overview
Problem
Conventional semiconductor flip-chip bonding methods face issues with intermetallic compound (IMC) formation, leading to reduced shear strength and potential cracking due to the brittleness of IMCs and excessive volume ratio of IMCs to solder, which affects the reliability of the semiconductor device.
Innovation Solution
The use of a semiconductor device with a copper pillar and a surface finish layer composed of nickel, gold, and palladium, where a solder layer forms both a copper-nickel-tin IMC and a gold-tin or palladium-tin IMC, controlling the volume ratio of these IMCs to less than 80% of the solder layer to enhance mechanical reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the solder reacts with the pad to form intermetallic compounds (IMCs), then the bonding between solder and pad becomes tighter, but the pad may crack if it is thin and the IMC layer becomes too thick
Solution Approach 1:
A nickel barrier layer is introduced as an intermediary between the copper pad and the solder layer. This nickel layer reacts with the solder to form nickel-tin IMCs, preventing direct reaction between copper pad and solder, thereby avoiding pad cracking while maintaining bonding strength
Solution Approach 2:
The nickel barrier layer serves dual functions: it acts as a diffusion barrier to prevent copper migration into the solder, and simultaneously provides a reactive surface that forms strong nickel-tin IMCs with the solder, making the system self-sufficient for both protection and bonding
2Reliability
If a thicker IMC layer forms, then the bonding between solder and pad becomes stronger, but the shear strength of the semiconductor device is reduced because IMCs are brittle
Solution Approach 1:
The invention controls the thickness and composition parameters of the nickel barrier layer and solder layer to regulate IMC formation. By optimizing these parameters, the IMC layer achieves optimal bonding strength without excessive thickness that would compromise shear strength
Solution Approach 2:
The invention creates a composite structure consisting of copper pad, nickel barrier layer, and solder layer with controlled IMC formation. This composite structure combines the advantages of different materials to achieve both strong bonding and adequate shear strength
3Volume of moving object
If the solder layer is very thin (less than 30 μm), then the device structure is more compact, but the volume ratio of IMCs to solder exceeds 80%, resulting in joint cracks
Solution Approach 1:
The invention changes the chemical composition parameters of the solder layer and controls the thickness of the nickel barrier layer to regulate IMC formation rate, ensuring that even in thin solder layers, the IMC volume ratio remains below 80% to prevent joint cracks
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
This approach prevents joint cracking and increases the mechanical reliability of the semiconductor flip-chip bonded device by controlling the IMC formation, ensuring the solder layer maintains sufficient volume and integrity.
Implementation Method 1
performing a reflow process to form a first IMC and a second IMC in the solder layer
Data Source
AI summary
A semiconductor device includes a first circuit layer, a copper pillar disposed adjacent to the first circuit layer, a second circuit layer and a solder layer. The second circuit layer includes an electrical contact and a surface finish layer disposed on the electrical contact, wherein a material of the surface finish layer is a combination of at least two of nickel, gold, and palladium. The solder layer is disposed between the copper pillar and the surface finish layer. The solder layer includes a first intermetallic compound (IMC) and a second IMC, wherein the first IMC includes a combination of two or more of copper, nickel and tin, and the second IMC includes a combination of gold and tin, a combination of palladium and tin, or both.