Self-aligning conductive clips via solder surface tension
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Solution Overview
Problem
In the packaging of integrated circuits, misalignment of conductive clips during the solder reflow process can occur due to differential heating rates, leading to reliability issues, especially in high-current applications where multiple bonding wires are impractical.
Innovation Solution
A leadframe structure with tub-like recesses is used to mount conductive clips, featuring solder pockets that increase the wettable surface area on one side, causing the molten solder to pull the components into their designed alignment during the solder reflow step.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If pre-formed clips are placed on the die/leadframe assembly using robotic pick-and-place machines, then the assembly process can be automated, but misalignment of clips during placement and swimming during solder reflow can occur leading to reliability issues
Solution Approach 1:
The clip structure performs self-alignment during the solder reflow process through its asymmetric geometry. The wider portion of the U-shaped or L-shaped clip naturally orients itself perpendicular to the leadframe surface as the molten solder wets and solidifies around it, eliminating the need for precise manual or robotic placement alignment.
Solution Approach 2:
The clip is designed with asymmetric geometry where one end has a wider portion and the other has a narrower portion. This asymmetry creates a preferred orientation during solder reflow, with the wider portion naturally positioning itself in a specific orientation relative to the leadframe, ensuring consistent alignment without requiring precise placement.
2Power
If multiple bonding wires are used to connect die to leadframe, then higher current loads can be accommodated, but the complexity and impracticability of the assembly increases
Solution Approach 1:
Multiple electrical connections are consolidated into a single pre-formed clip structure. The clip makes multiple contact points with the die simultaneously, providing a combined electrical connection that handles high current loads while requiring only one placement operation instead of multiple individual wire bonds.
Solution Approach 2:
The complex process of individually bonding multiple wires to handle high current is extracted and replaced with a single pre-formed clip component. This clip is designed to provide multiple electrical contact points in one piece, simplifying the assembly process while maintaining the required current handling capability.
3Ease of manufacture
If solder paste is used as temporary adhesive to hold the clip, then the clip is retained during handling, but the clip can swim or move around on liquefied solder during reflow process
Solution Approach 1:
The natural tendency of clips to swim or move on molten solder during reflow is converted into a beneficial self-alignment mechanism. The asymmetric clip geometry ensures that as the solder liquefies and the clip moves, it naturally orients itself in the correct perpendicular position, transforming a potential defect into a self-correcting alignment feature.
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 solution ensures accurate self-alignment and secure electrical and mechanical connections by utilizing the surface tension of molten solder to draw components into their correct positions, preventing misalignment and enhancing the reliability of high-current applications.
Implementation Method 1
the tendency of liquid or molten solder to minimize its surface area causing the parts to draw to and toward their final as-designed alignment
Implementation Method 2
the solder paste is thermally liquefied and thereafter solidified to secure the die and leadframe together
Data Source
AI summary
A lead frame having a die thereon connects a conductive area on the die to a lead frame contact using a conductive clip that includes a structural portion that is received with a recess-like “tub” formed in the lead frame contact. The end of the clip received in the tub is held in place during subsequent handling by a solder paste deposit until the clip and leadframe undergo solder reflow to effect a reliable electrical connection. The effective surface area between one side of the clip and the other side of the clip within the tub is different so that the surface tension of the liquefied solder formed during the solder reflow step will “draw” the clip into a preferred alignment against a “stop” surface.