Non-Circular Connector Design for Fine-Pitch Flip-Chip Interconnects
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Solution Overview
Problem
The existing technologies for fine-pitch flip-chip interconnects using circular copper bumps face issues with solder protrusion and shorts between adjacent substrate traces due to capillary action, limiting the achievable trace pitch and reliability of solder joints under thermomechanical stress.
Innovation Solution
The use of oblong-shaped bumps with a long axis aligned with the trace and a short axis equal to the trace width minimizes overhang, reducing the risk of solder shorts and allowing for smaller trace pitches, while the enlarged cross-section enhances the robustness of solder joints against stress and facilitates void-free underfilling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If circular copper bumps are used for fine-pitch flip-chip interconnect, then the bump fabrication is simplified through wafer-level process, but solder may protrude from the bump overhang and cause shorts between adjacent substrate traces
Solution Approach 1:
The patent applies asymmetry by changing the bump cross-section from circular to oblong (rectangular) shape. The oblong bump has a long axis aligned with the trace direction and a short axis equal to the trace width, eliminating the circular overhang that causes capillary action and solder protrusion. This asymmetric shape ensures the bump fits precisely on the trace without extending beyond the trace edges, preventing solder shorts while maintaining manufacturing simplicity through photoresist-defined patterning.
2Area of moving object
If circular bumps with 25 μm diameter are used, then the bump size is minimized for fine pitch, but the trace pitch must be kept at 40 μm or more to avoid solder shorts
Solution Approach 1:
The oblong bump shape with dimensions 40-50 μm (long axis) by 20-25 μm (short axis) replaces the circular 25 μm diameter bump. The short axis matches the trace width, eliminating overhang, while the long axis provides sufficient contact area. This shape change enables trace pitch reduction from 40 μm to 30 μm or less because the solder is contained within the trace boundaries without protruding beyond the bump edges.
3Area of moving object
If circular bumps are used, then the contact area is sufficient for 25 μm diameter, but the overhang creates capillary action that pulls in liquid solder and causes protrusion
Solution Approach 1:
The patent extracts and eliminates the harmful overhang portion that extends beyond the trace width. By designing the oblong bump with a short axis exactly equal to the trace width, the bump fits precisely on the trace without any lateral extension. This removal of the overhang eliminates the capillary action that draws liquid solder outward, preventing solder protrusion and shorts while maintaining adequate contact area through the oblong shape's larger surface area.
4Length of moving object
If castellated (neckdown) trace outlines are used with circular bumps, then the trace pitch can be reduced with staggered arrangement, but the trace structure becomes more complex
Solution Approach 1:
The oblong bump shape eliminates the need for castellated (neckdown) trace outlines by providing a rectangular footprint that matches the trace width. The straight-edged oblong bump aligns with the straight trace, allowing simple rectangular trace patterns without complex castellated contours. This simplifies the trace structure while enabling finer pitch through the staggered arrangement of oblong bumps, reducing trace pitch from 40 μm to 30 μm or less.
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 solution enables trace pitches to shrink to 30 μm or less, reduces the risk of solder shorts, and enhances the robustness of solder joints against thermomechanical stress, allowing for more reliable and stress-absorbing underfill material distribution.
Implementation Method 1
for the bumps with circular cross sections, as presently employed for flip-chip assembly, only a segment of the circular cross section is utilized for actual contact to the trace, while the rest is overhang over the trace outline. Applicants discovered that by capillary action, this overhang may pull in so much liquid solder that the solder may protrude from the overhang
Data Source
AI summary
A semiconductor device has semiconductor chip assembled on a substrate. The substrate has a first surface including conductive traces, which have a first length and a first width, the first width being uniform along the first length, and further a pitch to respective adjacent traces. The semiconductor chip has a second surface including contact pads; the second surface faces the first surface spaced apart by a gap. A conductive pillar contacts each contact pad; the pillar includes a core and a solder body, which connects the core to the respective trace across the gap. The pillar core has a non-circular cross section of a second width and a second length greater than the second width and greater than the first width.