Planarized Under-Bump Metallization for Crack-Resistant Flip-Chip Bonds
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Solution Overview
Problem
Conventional flip-chip bonding techniques induce mechanical stresses and crack formation at the interface between solder bumps and metalized pads, leading to potential performance alterations and device failure in electronic devices.
Innovation Solution
The use of a repassivation material layer with a substantially planar surface and metalized pads surrounded by and coplanar with the repassivation material, which reduces mechanical stresses by allowing for elastic deformation and minimizing peeling stresses during bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional flip-chip bonding techniques are used, then solder bumps can be formed and bonded to substrates, but mechanical stresses and crack formation occur at the interface between solder bumps and metalized pads
Solution Approach 1:
A repassivation material layer is formed over the metalized pad before solder bump bonding, creating a cushioning layer that absorbs and distributes mechanical stresses. This layer prevents direct stress concentration at the solder bump-metalized pad interface, thereby reducing crack formation and improving device reliability during thermal cycling and mechanical loading.
Solution Approach 2:
The mechanical properties of the interface are modified by introducing a repassivation material layer with specific elastic modulus and thickness parameters. This changes the stress distribution characteristics at the bonding interface, transforming the rigid metalized pad-solder bump interface into a more compliant structure that accommodates thermal expansion differences and reduces peeling stresses.
2Reliability
If metalized pads are used for solder bonding, then electrical connectivity is achieved, but peeling stresses cause crack formation at the solder bump-metalized pad interface
Solution Approach 1:
The repassivation material layer serves as an intermediary between the metalized pad and the solder bump, mediating the mechanical stress transfer. This intermediate layer reduces the direct peeling stress at the metalized pad-solder interface by distributing forces across a larger area and providing a compliant transition zone that accommodates differential thermal expansion.
Solution Approach 2:
The bonding interface structure is transformed from a simple metalized pad-solder bump interface into a composite structure consisting of the metalized pad, repassivation material layer, and solder bump. This composite structure leverages the different mechanical properties of each material to reduce peeling stresses, with the repassivation layer providing stress distribution and the metalized pad providing electrical connectivity.
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 configuration significantly reduces peeling stresses and the incidence of crack formation, enhancing the reliability and performance of electronic devices by mitigating stress-related issues at the solder bump-substrate interface.
Implementation Method 1
reduces mechanical stresses by allowing for elastic deformation
Data Source
Figure 1A~1C
Figure 2
Figure 3A
AI summary
An electronic device substrate with a substantially planar surface formed from an electrically non-conductive material is provided with one or more metalized pads on the substantially planner surface. Each of the one or more metalized pads is surrounded by and coplanar with the first electrically nonconductive material along an outer boundary of the metalized pad. The metalized pad is patterned such that portions of the metalized pad form metalized fingers that extend radially from the outer boundary of the metalized pad in an interdigitated arrangement with the first electrically nonconductive material. The metalized pad has a solderable surface.