Flip-Chip Substrate Pad Overlap Design for Void-Free Underfill
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Solution Overview
Problem
In flip-chip mounting substrates, the formation of voids at the bonding positions of central Au bumps and pads leads to reduced mounting reliability due to incomplete sealing by underfill resin, causing stress from thermal expansion differences and potential damage during heat application processes.
Innovation Solution
The design of the flip-chip mounting substrate includes a solder resist with an opening portion where the edge overlaps or is in contact with the connecting pad, allowing for a shallower depth and ensuring the underfill resin can flow smoothly and fill the bonding area, preventing void formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the opening portion of the solder resist is designed with a frame shape that does not overlap with the connecting pad, then the pad is fully exposed for bonding, but the underfill resin cannot flow into the bonding position, causing void formation and reduced mounting reliability
Solution Approach 1:
The solder resist opening portion is pre-designed to overlap with the connecting pad, creating a shallow depth structure before the bonding and underfill processes. This preliminary structural preparation enables the underfill resin to naturally flow into the bonding position without requiring additional process steps, thereby preventing void formation and ensuring reliable mounting.
Solution Approach 2:
The depth parameter of the solder resist opening portion is changed from a deep frame shape to a shallow overlapping structure. This parameter modification allows the underfill resin to access the bonding position effectively, resolving the contradiction between pad exposure and resin filling while improving mounting reliability.
2Productivity
If central Au bumps are arranged at the central position to increase terminal density, then the total number of terminals increases, but voids form at the bonding position due to incomplete underfill resin sealing
Solution Approach 1:
The solder resist opening portion is designed with different local characteristics: it overlaps with the connecting pad at the central bonding position to create a shallow structure that facilitates underfill resin flow, while maintaining appropriate exposure for bonding. This local quality differentiation ensures both high terminal density and bonding reliability.
3Area of stationary object
If the solder resist opening portion has a deep frame shape, then the pad is fully exposed for bonding, but the underfill resin flows away from the bonding position, creating voids and reducing structural integrity
Solution Approach 1:
Instead of designing the solder resist opening to extend beyond the pad (traditional approach), the opening is designed to overlap with the pad, inverting the conventional geometry. This inversion creates a shallow structure that prevents underfill resin from flowing away, thereby maintaining structural integrity while ensuring adequate pad exposure for bonding.
Data Source
AI summary
A solder resist and a central pad to which a central Au bump provided on a semiconductor chip is flip-chip bonded are formed on a substrate main body. In a flip-chip mounting substrate where an underfill resin is provided after the semiconductor chip is mounted, a central opening portion for exposing the central pad is formed in the solder resist, and also, an edge portion forming the central opening portion of the solder resist is partially overlapped with the outer peripheral portion of the central pad.