Recessed QFN Lead Structure for Solder Inspection and Isolation
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Solution Overview
Problem
The existing QFN surface mount packaging technology faces challenges with dimensional and inspection limitations due to the design of the wettable flank lead, which can result in reliability issues and copper burrs during the singulation process, and lacks effective electrical isolation between adjacent leads.
Innovation Solution
The design includes a lead with a central segment and extensions on either side, featuring a recess for solder inspection and a wider copper area to reduce etch time and prevent copper burrs, with angular edges and a step-style shape to improve integrity and isolation between leads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a wettable flank lead design is used to enable visual solder inspection, then solder wettability is improved, but the lead dimensions become insufficient and copper burrs are generated during singulation
Solution Approach 1:
The lead is divided into distinct segments: a central segment with reduced width and first/second extensions with wider copper areas. This segmentation allows the wettable flank region to provide adequate dimensions for solder inspection while isolating the singulation regions to prevent copper burr generation. The central segment acts as a bridge between the extensions, maintaining electrical connectivity while enabling dimensional control in different regions.
Solution Approach 2:
Different regions of the lead are given different widths to serve different functions. The central segment has reduced width to minimize copper burr risk during singulation, while the first and second extensions have wider copper areas to provide sufficient dimensions for solder wettability and visual inspection. This local differentiation resolves the contradiction by optimizing each region for its specific purpose.
2Ease of manufacture
If traditional lead designs are used, then manufacturing is simpler, but electrical isolation between adjacent leads is insufficient
Solution Approach 1:
The lead design with central segment and extensions creates natural isolation zones. The reduced-width central segment acts as an electrical isolation barrier between adjacent leads while maintaining the overall lead functionality. This segmentation provides inherent electrical isolation without complicating the manufacturing process, as the segmented structure can be formed using standard photolithography and etching techniques.
3Reliability
If wettable feature leads are created using plating process, then solder wettability is improved, but the edge becomes curved and step cut dimensions are insufficient
Solution Approach 1:
The lead geometry is pre-designed with angular edges and a step-style shape before the plating process. This preliminary geometric configuration ensures that after plating creates the wettable surface, the lead maintains adequate dimensions for step cutting and angular edges for proper alignment. The pre-planned geometry compensates for any dimensional changes that may occur during plating.
Solution Approach 2:
The invention actually does the opposite - it uses angular edges rather than curved edges. The first and second extensions are designed with angular configurations that maintain sufficient dimensions for step cutting while providing adequate surface area for solder wettability. This angular design avoids the curved edge problem inherent in traditional plated wettable features.
Data Source
AI summary
A lead for a surface mount package for a semiconductor device, and the surface mount package employing the same. In one example, the lead includes a central segment with a first side and a second side, a first extension from a portion of the first side, and a second extension from a portion of the second side. The lead also includes a recess extending through a portion of the central segment, the first extension and the second extension.