COF Composite Bump Structure for Stronger Lead Bonding
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Solution Overview
Problem
Conventional chip-on-film (COF) packages face challenges with insufficient bonding strength between chip bumps and substrate leads, leading to potential short circuits due to reduced bump pitch and increased output ports, which compromises reliability.
Innovation Solution
The COF package incorporates composite bumps with a first raising strip, UBM layer, and bonding layer, featuring a first bonding rib that inserts into the lead, increasing compressive strength and bonding length, thereby enhancing the bonding process and reducing the required force for chip-substrate attachment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If bump pitch is reduced to increase chip output ports, then the number of output ports is increased, but bumps may be compressed to connect to adjacent bumps causing short circuit
Solution Approach 1:
The patent introduces a vertical dimension by adding a raising strip that protrudes from the chip surface. This vertical extension allows bumps to be positioned at different heights, creating a stepped structure that prevents adjacent bumps from contacting each other during compression, thus eliminating short circuit risk while maintaining reduced pitch
Solution Approach 2:
The bump structure is segmented into multiple components: the original bump, the raising strip, and optionally a protective layer. This segmentation allows the raising strip to serve as a mechanical separator that physically isolates adjacent bumps, preventing them from merging during the bonding process
2Strength
If higher bonding force is applied to ensure bonding strength, then bonding strength is improved, but bumps may be compressed causing short circuit
Solution Approach 1:
By adding vertical height through the raising strip, the patent creates a height difference between adjacent bumps. This dimensional change allows sufficient bonding force to be applied horizontally while the vertical separation prevents bump compression and short circuit formation
Solution Approach 2:
The raising strip acts as a pre-positioned mechanical cushion or stop that limits the compression distance during bonding. When bonding force is applied, the raising strip prevents excessive compression before short circuit can occur, effectively cushioning against the harmful effect
3Device complexity
If conventional bonding method is used, then process simplicity is maintained, but bonding strength between bumps and leads is insufficient
Solution Approach 1:
The raising strip is pre-formed on the chip surface before the bonding process. This preliminary structural preparation ensures that when conventional bonding is applied, the bumps are already positioned at an optimal height to achieve sufficient bonding strength without requiring complex bonding parameters or additional process steps
Data Source
AI summary
A COF package includes a substrate and a chip, composite bumps on the chip are bonded to leads on the substrate. Each of the composite bumps includes a raising strip, a UBM layer and a bonding layer. A bonding rib is formed on the bonding layer because of the raising strip and the UBM layer, and the bonding rib on each of the composite bumps can be inserted into each of the leads and surface-contact with each of the leads to increase weld length and bonding strength between the bonding layer and the leads and further reduce a force required for bonding the chip to the substrate in a flip-chip bonding process.