Copper Post Solder Bumps via Injection Molding
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Solution Overview
Problem
Current flip chip assembly technologies face challenges in achieving high current capability and reliability due to stress transmission to the BEOL structure, electromigration, and thermal conductivity issues, particularly with Cu/Sn ratio imbalances and substrate warpage, which lead to assembly failures and reliability concerns.
Innovation Solution
The use of injection molded solder (IMS) technology to form solder caps or bumps on conductive pillars, specifically Cu posts, on substrates, reducing the size and volume of solder bumps on wafers and eliminating them, while combining solder with Cu posts to decrease stress and enhance reliability, using tapered posts and reservoirs to minimize solder leakage and facilitate easy alignment and underfill flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If traditional solder paste stencil printing method is used to form solder bumps on substrates, then the process is simple and low cost, but the Cu/Sn ratio is imbalanced leading to stress transmission to BEOL structure and reliability issues
Solution Approach 1:
The invention changes the physical state of solder from paste to molten injection molding material, and controls the Cu/Sn ratio by separately forming Cu posts and injecting solder material. This parameter change resolves the contradiction by achieving both process simplicity and improved reliability through better Cu/Sn ratio control that reduces stress transmission to BEOL structure
Solution Approach 2:
The invention creates a composite structure with Cu posts and solder caps/bumps, where the Cu post provides structural support and the solder provides electrical connection. This composite approach resolves the contradiction by optimizing the Cu/Sn ratio distribution, with Cu posts reducing stress transmission while solder caps maintaining electrical connectivity, thereby improving assembly reliability without complicating the manufacturing process
2Reliability
If Cu posts with high stand-off height are used on substrates, then electromigration resistance improves, but stress on BEOL structure increases due to Cu/Sn ratio imbalance
Solution Approach 1:
The invention applies local quality by creating Cu posts with specific heights at different locations to achieve electromigration resistance where needed, while controlling the overall Cu/Sn ratio to minimize stress on BEOL structure. The localized Cu post formation with optimized dimensions resolves the contradiction by providing electromigration protection without excessive stress transmission
Solution Approach 2:
The invention changes the Cu post dimensional parameters (height, diameter) and Cu/Sn ratio to optimize both electromigration resistance and stress reduction. By precisely controlling these parameters, the invention achieves high stand-off height for electromigration protection while maintaining appropriate Cu/Sn balance to reduce stress on BEOL structure
3Stress or pressure
If solder bump volume is reduced on wafers by using IMS technology, then stress on BEOL structure decreases, but manufacturing precision requirements increase
Solution Approach 1:
The invention replaces the traditional mechanical stencil printing process with injection molding technology to form solder bumps. This substitution resolves the contradiction by using molten solder injection under controlled pressure and temperature, which naturally fills the mold cavity to precise dimensions, reducing stress on BEOL structure while maintaining high manufacturing precision through the injection molding process itself
4Ease of operation
If tapered posts and reservoirs are used in IMS process, then solder leakage is minimized and alignment is easier, but device complexity increases
Solution Approach 1:
The invention uses asymmetric tapered post and reservoir geometries where the taper angle and shape are specifically designed to guide solder flow and facilitate alignment. This asymmetric design resolves the contradiction by making alignment easier through the self-aligning tapered geometry while minimizing solder leakage, with the added structural complexity being offset by the operational benefits
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 stress on the BEOL structure, improves electromigration resistance, and maintains high stand-off heights, making it easier to clean flux residue and prevent void formation, thereby enhancing the reliability and assembly yield of flip chip connections.
Implementation Method 1
injection molded solder (IMS) technology to form solder caps or bumps on conductive pillars
Implementation Method 2
solder the solder bumped substrate to the sites where the distal ends extend into the mask
Implementation Method 3
using tapered posts and reservoirs to minimize solder leakage
Data Source
AI summary
A method of assembling a semiconductor flip chip comprising a wafer having solderable electrical conducting sites and a substrate having electrical connecting pads and electrically conductive posts operatively associated with the pads and extending away from the pads to terminate in distal ends, comprises the pre-assembly steps of solder bumping the distal ends through openings in a solder mask by injection molding solder onto the distal ends so that the distal ends extend into the mask through the openings to produce a solder bumped substrate, and soldering the solder bumped substrate to the sites.