Wettable Flank Lead Frames for Robust QFN Bonding
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Solution Overview
Problem
Conventional lead frames in electronic packaging lack structural rigidity, leading to handling flaws, damage, and poor bond adhesion due to mechanical instability, especially in applications like QFN packages where connections can be severed under stress and are difficult to inspect.
Innovation Solution
A lead frame with a webbed structure is created through partial patterning of a film, providing structural rigidity and allowing for the production of wettable flanks, which facilitates robust connections and easier handling by using a continuous surface for vacuum chucking and etching to form fillets for enhanced solderability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional lead frames are used with thin finger-like portions, then the lead frame can be manufactured with standard processes, but the lead frame lacks structural rigidity and is prone to bending, breaking, and deformation
Solution Approach 1:
The lead frame is segmented into a rigid base portion and separate finger-like lead portions. The base portion provides structural support while the leads extend from it, allowing each segment to be optimized for its specific function. This segmentation resolves the contradiction by concentrating rigidity where needed while maintaining the necessary lead geometry.
Solution Approach 2:
The invention adds a vertical dimension to the lead frame structure by extending leads upward from the base portion. This dimensional change allows the base to provide rigidity in the horizontal plane while the leads extend in the vertical direction, simultaneously achieving structural stability and proper lead configuration.
2Reliability
If the lead frame structure is made more rigid to prevent handling flaws and damage, then handling and positioning improve, but the complexity of the lead frame structure increases
Solution Approach 1:
By segmenting the lead frame into a rigid base and flexible leads, the invention achieves reliable bond adhesion through the stable base while keeping the lead structure simple and manageable. The segmentation allows the base to provide the necessary rigidity for reliable bonding without making the entire structure overly complex.
3Reliability
If conventional lead frames are used in QFN packages, then the packages can be manufactured, but the connections may be severed under rigorous stress and are difficult to inspect
Solution Approach 1:
The invention applies preliminary plating to the lead surfaces before final assembly, creating a robust, corrosion-resistant coating that strengthens the connections beforehand. This preliminary action ensures connection robustness while the plated surfaces also provide visual contrast that facilitates easier inspection of connection quality.
Data Source
AI summary
A method of producing wettable fillets in electronic packages. A matrix of unsingulated lead frames is provided, each including a plurality of lead elements and a chip pad. Chips are attached to the chip pads and terminals on the chips are electrically connected to lead portions of the lead elements. The top portion of the package is encapsulated. Masking is applied to the bottom surface of the lead elements and the chip pads, but at least one of the lead elements has a portion of its surfaced remaining exposed. The exposed lead element surface is etched to create a fillet. The fillets, lead elements and bottom surface of the chip pads are plated, and the packages then singulated, producing packages with wettable flanks.