QFN Package Leadframe Segmentation for Fine Pitch
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Solution Overview
Problem
Existing methods for packaging quad-flat non-leaded (QFN) integrated circuits using etched leadframes face challenges such as high production costs, design rigidity, wire bonding difficulties, and mold resin bleeding issues due to the limitations of lead pitch, which makes it difficult to produce packages with very small lead pitches.
Innovation Solution
A method involving a generic leadframe with wider lead lands that are shaped to create multiple leads with varying pitches, allowing for flexible design and improved wire bonding and molding processes, including the option to form heat sink fins for enhanced heat dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If etched leadframes are used to produce small lead pitch packages, then lead pitch can be reduced, but production cost increases significantly
Solution Approach 1:
The leadframe structure is segmented into modular components: a carrier substrate and multiple lead fingers that can be separately manufactured and then assembled. This allows the lead fingers to be stamped at larger pitches on the carrier, then separated and attached to the die pad, achieving small effective lead pitch without requiring expensive etching of the entire leadframe at fine pitch.
Solution Approach 2:
The invention transitions from a planar two-dimensional leadframe layout to a three-dimensional assembled structure. Lead fingers are attached to the die pad from the opposite side, creating a vertical assembly process that enables small lead pitch through spatial arrangement rather than through-plane etching, thereby avoiding the cost penalty of fine-pitch etching.
2Adaptability or versatility
If etched leadframes with custom design are used, then design flexibility is achieved, but manufacturing complexity and cost increase
Solution Approach 1:
The leadframe is divided into standardized carrier substrate and lead finger components. The carrier can be a generic stamped piece, while lead fingers are separate elements that can be customized in number, shape, and arrangement. This modular approach allows design flexibility through component selection and arrangement without requiring complex custom etching of the entire leadframe structure.
Solution Approach 2:
The carrier substrate serves multiple functions: it holds multiple lead fingers during manufacturing, provides structural support, and can be discarded after lead finger attachment. This universal component allows the same carrier design to be used across different package configurations, reducing manufacturing complexity while maintaining design adaptability through varying lead finger arrangements.
3Ease of operation
If tape is used to hold leadframe during processing, then handling is facilitated, but wire bonding reliability deteriorates due to lead bouncing
Solution Approach 1:
The tape holding function is extracted and removed from the final package structure. The leadframe is designed to be handled and processed without tape, using the die pad and lead finger geometry itself for stability during wire bonding. This eliminates the tape-induced lead bouncing problem while maintaining ease of handling through the inherent structural rigidity of the leadframe components.
4Manufacturing precision
If tiny leads are taped during molding, then positioning is improved, but mold resin bleeding occurs causing solderability failure
Solution Approach 1:
The tape positioning function is removed from the molding process. The leadframe geometry, particularly the die pad and lead finger arrangement, is designed to provide inherent positioning stability during molding without requiring tape. This eliminates the interface between tape and mold resin that causes resin bleeding and flash, while maintaining precise lead positioning through the rigid leadframe structure.
Data Source
AI summary
A QFN integrated circuit is mounted on a leadframe having multiple lead lands and resin material encapsulates the integrated circuit leaving the lead lands exposed. Subsequently a sawing operation divides the lead lands into multiple leads, and the leadframe and resin material are partitioned to form packages. The pitch of the resultant leads is not limited by the pitch of the lead lands of the leadframe, so the leadframe can be of the relatively cheap generic leadframe variety, in which the pitch of the lead lands is higher than the desired pitch of the leads of the completed package. The sawing operation may further include reshaping the diepad area of the leadframe to produce heat sink fins, for improved heat dissipation. The proposed process is suitable both to produce packages including only a single integrated circuit, and also to produce multi-chip modules.