Capillary Design for High Aspect Ratio Pillar Bumps
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Solution Overview
Problem
Conventional wire bond stud bumping methods result in low stud heights, leading to low bonding integrity and reliability, especially in fine pitch devices, as increasing bump heights requires larger pad sizes and pitch, which is not feasible for all applications.
Innovation Solution
A method using a capillary with a hole portion and a chamfer section to form pillar bumps by creating a free air ball, tensioning the bond wire, and applying a vacuum to withdraw the ball into the capillary, forming a tower with a higher aspect ratio, allowing for enhanced bonding capabilities while maintaining standard ball sizes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional capillary designs are used to form wire bond stud bumps, then the manufacturing process is simple, but the stud height is low resulting in poor bonding integrity and reliability
Solution Approach 1:
The capillary is divided into distinct functional sections: a cylindrical section with a specific diameter, a tapered section transitioning from the cylindrical section, and an outlet section. This segmentation allows each section to perform its specific function - the cylindrical section controls ball formation, the tapered section guides wire tensioning, and the outlet section enables pillar extrusion - thereby achieving high aspect ratio pillars with improved bonding integrity
Solution Approach 2:
The invention transitions from forming low aspect ratio studs to forming high aspect ratio pillars by changing the dimensional characteristics of the formed structure. The capillary geometry is specifically designed to extrude material in a controlled manner, creating a tall, narrow pillar structure rather than a short, wide stud, thus achieving the desired dimensional transformation
2Length of moving object
If bond wire diameter is increased to form taller bumps, then the bump height increases, but the pitch must also be increased which is not suitable for fine pitch devices
Solution Approach 1:
The capillary features a cylindrical section with a specific diameter (0.5mm to 1.0mm) that is optimized for forming the pillar base, followed by a tapered section that gradually reduces the diameter. This local variation in geometry allows the formation of tall, narrow pillars with controlled base diameter, enabling high bump heights without requiring increased pitch for fine pitch devices
3Reliability
If conventional capillary designs are used, then the device complexity is low, but the aspect ratio of the stud is less than 1 limiting bonding reliability
Solution Approach 1:
The capillary design incorporates a tapered section that dynamically transitions from the cylindrical section to the outlet section, allowing the material to be progressively shaped during the extrusion process. This dynamic geometry enables the formation of high aspect ratio pillars with aspect ratios greater than 1, significantly improving bonding reliability while maintaining a relatively simple single-piece capillary structure
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
The method achieves higher aspect ratio pillar bumps, improving bonding reliability and enabling the use of standard free air ball sizes for smaller pitch applications, thus addressing the limitations of conventional capillary designs.
Implementation Method 1
applying a vacuum to the capillary to withdraw a portion of the FAB back into the capillary
Implementation Method 2
tensioning the bond wire
Data Source
AI summary
A method of forming a pillar bump includes feeding a bond wire in a capillary. The capillary has a hole portion and a chamfer section arranged downstream of the hole portion. The hole portion has a length along a feed direction of the bond wire that is greater than a maximum diameter of the hole portion. The method further includes performing an electric flame off (EFO) on a free end of the bond wire extending from the chamfer section to form a free air ball (FAB), tensioning the bond wire and applying a vacuum to the capillary to withdraw a portion of the FAB back into the capillary to substantially fill the hole portion for forming a tower, attaching the FAB to a bonding site, and at least partially removing the capillary from the bonding site and breaking the bond wire above the tower.