QFN Leadframe Pad Structure for Delamination Resistance
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Solution Overview
Problem
Mechanical loads and thermal cycling during the manufacturing and assembly of Quad-Flat No-lead (QFN) package semiconductor devices cause delamination of semiconductor chips from the leadframe, leading to in-field device failure.
Innovation Solution
A modified leadframe design with half-etched pads and increased pad sizes on the top and bottom sides, creating pillar structures filled with pre-molding resin to enhance anchoring and resist delamination, while maintaining wettable flanks for soldering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional leadframe design with standard pad size is used, then manufacturing process is simple, but pads are prone to delamination under mechanical load and thermal cycling
Solution Approach 1:
The pad structure transitions from a two-dimensional flat surface to a three-dimensional pillar structure with vertical height. The etched recesses extend downward into the leadframe substrate, creating depth-wise anchoring features that prevent delamination by mechanically interlocking the pad with the substrate in the vertical dimension.
Solution Approach 2:
The pad is divided into distinct functional zones: a top surface for electrical connection, vertical sidewalls for mechanical anchoring, and etched recesses for resin interlocking. This segmentation allows each zone to perform its specific function optimally, with the recesses providing dedicated anchoring features separate from the electrical contact surface.
2Reliability
If pad size is increased to prevent delamination, then anchoring strength improves, but available area for wettable flanks and soldering is reduced
Solution Approach 1:
The anchoring function is moved from the horizontal plane to the vertical dimension through etched recesses and pillar structures. This allows the top surface area to remain dedicated to electrical connection and soldering, while the vertical walls provide the anchoring strength, effectively separating the functions of electrical contact and mechanical retention into different spatial dimensions.
Solution Approach 2:
Different regions of the pad structure are given different properties: the top surface maintains a flat, conductive quality for electrical connection, while the sidewalls and recesses are designed with anchoring qualities through etching and geometric features. This local differentiation allows optimal performance for both soldering and mechanical retention without compromising either function.
3Reliability
If pre-molding resin is used to fill etched volumes, then pillar structures form to resist delamination, but manufacturing process complexity increases
Solution Approach 1:
The etched recesses are created in the leadframe substrate before the pre-molding step. This preliminary action prepares the structure to receive and retain the molding resin, ensuring that when the resin is applied, it automatically fills the recesses and forms the anchoring pillars without requiring additional post-processing steps to create the anchoring features.
Solution Approach 2:
The pre-molding step combines two functions: encapsulating the semiconductor die and simultaneously forming the anchoring pillars by filling the pre-formed etched recesses. This merging of encapsulation and anchoring formation into a single process step reduces overall manufacturing complexity compared to adding separate anchoring features after molding.
Data Source
AI summary
Semiconductor chips to be singulated to individual semiconductor devices are arranged onto respective adjacent areas of a mounting substrate such as a pre-molded leadframe. The mounting substrate is made of a laminar, electrically conductive sculptured structure with molded electrically insulating material. Electrically conductive side formations in the adjacent areas of the mounting substrate include first and second pads at front and back surfaces, respectively, of the mounting substrate. The first contact pads at the front surface of the substrate include narrowed portions having side recesses. The second contact pads at the back surface of the substrate include widened portions having side extensions adjacent the side recesses. The electrically insulating material extends into the side recesses to provide anchoring formations of the insulating material to the electrically conductive sculptured structure of the mounting substrate.