Two-Layer Solder Pillars for Low-Temperature Flip-Chip Bonding
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Solution Overview
Problem
High reflow temperatures in solder bonding for flip chips lead to mechanical reliability issues due to thermal stress and increased formation of intermetallic compounds, which can cause strain and damage in fine pitch solder bumps and chip-package interactions.
Innovation Solution
The use of a two-layer solder structure with a higher melting point solder closest to the semiconductor die and a lower melting point solder at the joining interface, where only the lower melting point solder reflows to form a bond, reducing mechanical stress and maintaining structural integrity by minimizing intermetallic compound formation at the interface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high reflow temperature is used for solder bonding, then complete bonding is achieved, but mechanical reliability deteriorates due to thermal stress and intermetallic compound formation
Solution Approach 1:
The solder structure is segmented into two distinct layers: a first solder layer with higher melting point material and a second solder layer with lower melting point material. This segmentation allows each layer to serve different functions - the first layer provides structural support and strain absorption, while the second layer enables bonding at lower temperatures, thereby resolving the contradiction between achieving complete bonding and avoiding thermal stress damage.
Solution Approach 2:
Different regions of the solder structure are assigned different material properties. The first solder layer (closer to the die) uses higher melting point material for strength and strain absorption, while the second solder layer (at the bonding interface) uses lower melting point material for low-temperature bonding. This local differentiation of material properties allows the system to achieve both strong bonding and low thermal stress.
2Reliability
If high reflow temperature is used, then solder bonding is achieved, but intermetallic compound formation increases causing strain and damage
Solution Approach 1:
The melting point parameter of the solder material is changed by using a composite structure with two different solder materials. The second solder layer has a specifically selected lower melting point that enables bonding at temperatures below 200°C, which significantly reduces intermetallic compound formation while the first solder layer's higher melting point material prevents excessive softening and maintains structural integrity.
3Strength
If single-layer solder is used, then structure is simple, but strain absorption capability is insufficient under thermal stress
Solution Approach 1:
The solder is segmented into two functional layers with different melting points. The first solder layer (higher melting point) provides structural support and strain absorption capability, while the second solder layer (lower melting point) enables low-temperature bonding. This segmentation directly addresses the need for enhanced strain absorption without requiring complex external support structures.
Solution Approach 2:
The solder structure uses a composite of two different solder materials with complementary properties. The higher melting point material provides strength and strain absorption, while the lower melting point material provides bonding capability at reduced temperatures. This composite approach enhances overall performance while maintaining a relatively simple layered structure.
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 reduces mechanical stress and damage in solder joints and under-bump-metallurgy structures by allowing the higher melting point solder to absorb strain while maintaining effective bonding, thus enhancing the reliability of flip chip connections.
Implementation Method 1
the first solder layer has a higher melting point than the second solder layer... allowing the higher melting point solder to absorb strain while maintaining effective bonding
Implementation Method 2
only the lower melting point solder reflows to form a bond
Implementation Method 3
attaching the second solder layer of each pillar in the plurality of pillars to a respective bonding pad of a plurality of bonding pads located on a surface of the second semiconductor structure
Data Source
AI summary
A semiconductor device and formation thereof. The semiconductor device includes a first semiconductor structure, a second semiconductor structure, and a plurality of pillars interconnecting the first semiconductor structure and the second semiconductor structure. The plurality of pillars include a first solder layer and a second solder layer, wherein the first solder layer has a higher melting point than the second solder layer.