Curable Dielectric Film Underfill for Void-Free IC Bonding
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Solution Overview
Problem
Conventional microelectronic assembly processes face issues with throughput, underfill voiding, and joint misalignment, particularly due to the generation of significant underfill voids and the clam-shell pressing equipment's inability to apply a purely vertical force, leading to lateral shifts and misaligned joints.
Innovation Solution
A method involving the application of a curable dielectric film under vacuum and controlled temperature to align and bond IC dies to a workpiece, using a pressing system with separate heads to apply vertical force and prevent substrate bowing, thereby reducing void formation and improving joint alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional capillary underfill dispensing is used after reflow, then underfill can seal the area between IC die and substrate, but the process requires additional cure oven and flux application steps, increasing process complexity
Solution Approach 1:
The patent combines the underfill dispensing operation with the IC die placement operation into a single step. The underfill is dispensed onto the substrate before the IC die is placed, allowing both operations to occur simultaneously rather than sequentially. This eliminates the need for separate dispensing and placement steps, reducing overall process complexity while maintaining the sealing function of the underfill.
Solution Approach 2:
The underfill is dispensed onto the substrate in advance, before the IC die placement and reflow processes. This preliminary action allows the underfill to be in position ready for the IC die to be placed directly onto it, eliminating the need for subsequent dispensing operations and reducing the number of process steps required.
2Productivity
If no-flow underfill is dispensed before IC die placement, then underfill cures during reflow process, but the process still requires a dispenser and precise timing control
Solution Approach 1:
The patent merges the underfill dispensing function with the IC die placement head into a single integrated tool. The placement head includes both the dispenser for applying underfill and the mechanism for placing the IC die, allowing both functions to be performed by one device rather than requiring separate dispenser and placement equipment. This reduces device complexity while maintaining process integration.
3Device complexity
If thermal compression bonding is used to attach IC die to substrate, then extra processing steps are eliminated, but significant density of underfill voids are generated
Solution Approach 1:
The underfill is dispensed onto the substrate before the IC die is placed and before the reflow process begins. This preliminary positioning of the underfill allows it to be properly distributed and ready to fill gaps as the IC die is pressed onto the substrate during placement, rather than relying on void formation and subsequent filling during compression bonding. This eliminates underfill voids while maintaining the simplified process of combining placement and bonding.
Solution Approach 2:
The patent changes the state of the underfill material by dispensing it in a liquid or paste form that can be easily applied and positioned before the reflow process. This parameter change allows the underfill to be in the optimal state for dispensing and positioning, rather than being applied as a pre-cured or pre-formed material that would not flow into gaps during placement. The underfill remains in a state that allows it to fill voids as the IC die is pressed onto the substrate.
4Force
If clam-shell pressing equipment is used to apply compressive force, then IC die can be pressed to substrate, but lateral force causes misaligned joints
Solution Approach 1:
The patent segments the pressing function into two separate stages: a preliminary placement stage where the IC die is gently positioned onto the substrate with minimal force to achieve initial alignment, and a subsequent bonding stage where full compressive force is applied to form the metallic joints. This segmentation allows alignment to be achieved during placement without the lateral forces that would occur during full compression, thereby maintaining joint alignment precision while still achieving the necessary bonding force.
Solution Approach 2:
The IC die is preliminarily positioned and aligned on the substrate during the placement operation before the full compressive force is applied for bonding. This preliminary positioning ensures that the IC die is correctly aligned with the bonding conductors before any significant force is applied that could cause lateral movement. The alignment is established in advance, during the low-force placement phase, and then maintained during the high-force bonding phase.
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 significantly reduces underfill voiding and misalignment, enhancing the reliability and throughput of the microelectronic assembly process by ensuring complete filling and precise joint formation without the need for additional curing steps.
Implementation Method 1
placing the plurality of IC die on the workpiece surface... at a vacuum level corresponding to a pressure
Implementation Method 2
Heat applied during the pressing step provides heat for the solder interconnect reflow and underfill cure to occur simultaneously
Implementation Method 3
a temperature during the pressing is sufficient to cure the curable dielectric film to form underfill
Data Source
AI summary
A method for forming electronic assemblies includes providing a plurality of IC die each having IC bonding conductors and a workpiece having workpiece bonding conductors. A curable dielectric film is applied to the IC bonding conductors or the workpiece surface. The plurality of IC die are placed on the workpiece surface so that the plurality of IC bonding conductors are aligned to and face the plurality of workpiece bonding conductors to provide a first bonding. The placing is performed at a vacuum level corresponding to a pressure <1 kPa, and at a temperature sufficient to provide tackiness to the curable dielectric film. The plurality of IC die are then pressed to provide a second bonding. A temperature during pressing cures the curable dielectric film to provide an underfill and forms metallic joints between the plurality of IC bonding conductors and the plurality of workpiece bonding conductors.