Lead Webbing in QFP Packages to Prevent Electrical Shorts
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Solution Overview
Problem
In integrated circuit (IC) packaging, the narrow lead pitch between adjacent leads increases the risk of electrical shorts due to trapped stray metal bits during trimming and forming, especially in IC devices with small size and high lead count, where the lead gap width becomes narrower.
Innovation Solution
The implementation of a dual row QFP package with alternating gull-wing and j-shaped leads and the use of lead webbing, which is part of the molding compound, to prevent conductive objects from getting lodged between leads, along with crenellated case edges and tapered recesses and tabs for easier access and reduced burr formation, thereby minimizing electrical shorts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the lead pitch is reduced to increase the number of leads in small IC devices, then the lead count increases, but the risk of electrical shorts between adjacent leads increases due to narrower lead gap width
Solution Approach 1:
The patent introduces lead webbing as an intermediary structure between adjacent leads. This webbing acts as a physical barrier that prevents conductive objects (stray metal bits) from bridging the narrow gap between leads, thereby eliminating the electrical short risk while maintaining the reduced lead pitch configuration.
Solution Approach 2:
The lead frame structure is segmented into multiple functional zones: lead regions, webbing regions, and recess regions. This segmentation allows the webbing to be positioned specifically in critical areas where electrical shorts are most likely to occur, providing targeted protection without affecting the overall lead density.
2Ease of manufacture
If conventional plain rectangular case outline is used, then manufacturing is simple, but access to leads for trimming and forming is difficult and burr formation increases
Solution Approach 1:
The case outline is segmented with crenellations (protrusions and recesses) that create targeted access points to different lead regions. This segmentation allows trimming and forming tools to reach specific leads more easily while maintaining the overall rectangular package structure for simple manufacturing.
Solution Approach 2:
The case outline features local variations in geometry (crenellations) at specific locations where lead access is needed, while the majority of the case maintains a simple rectangular form. This local quality modification provides enhanced accessibility exactly where required without complicating the overall manufacturing process.
3Reliability
If lead webbing is added between leads, then electrical short risk is reduced, but device complexity increases
Solution Approach 1:
The lead webbing structure is merged with the molding compound material and integrated into the encapsulation process. The webbing is formed as part of the same molding operation that creates the case body, so no additional manufacturing steps or materials are required, thus adding reliability without proportionally increasing complexity.
Solution Approach 2:
The lead webbing serves multiple functions simultaneously: it acts as a physical barrier against conductive objects, provides structural support to the lead frame, and maintains the positioning of adjacent leads. This multi-functionality reduces the need for separate protective structures, offsetting the added complexity.
Data Source
AI summary
A quad flat package integrated circuit (IC) device has alternating inner and outer leads that protrude from a package body. The inner leads are j-shaped leads and the outer leads are gull-wing shaped leads. The package body is formed such that it includes plastic lead webbings between adjacent leads, which help prevent metal particles from getting lodged between the leads and causing electrical shorts. The webbings are made of the same molding compound as the package body and are formed together with the package body.