Side-Wettable Package Pads With Undercuts for Solder Inspection
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Solution Overview
Problem
Conventional semiconductor packages with exposed lead frames face challenges such as thick lead frames making it difficult to reduce package height, precise half-cut and etching processes required, burrs causing short circuits, and copper oxide formation due to exposed lead frames.
Innovation Solution
A side wettable package design with peripheral conductive pads forming undercuts on the side surface, manufactured using a single sawing and over-etching process, allowing for a thinner lead frame and eliminating the need for precise cutting and etching, while a protective electroplating layer prevents copper oxide formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the lead frame is made thick to enable twice cutting (half-cut and full-cut), then the cutting process can be completed, but the package height cannot be reduced
Solution Approach 1:
The patent divides the cutting process into two stages: first performing a half-cut to create a stepped portion, then performing a full-cut to separate packages. This segmentation allows the use of a thinner lead frame while still enabling complete separation, thereby reducing package height without compromising manufacturing feasibility
Solution Approach 2:
The half-cut is performed as a preliminary action before the full-cut. By pre-forming the stepped portion through half-cutting, the subsequent full-cut can be performed more easily on a thinner lead frame, achieving both height reduction and successful package separation
2Manufacturing precision
If the half-cut process is precisely controlled to avoid excessive sawing or accidental cut off, then cutting accuracy is improved, but the manufacturing complexity increases
Solution Approach 1:
The stepped portion formed by the half-cut serves a dual function: it is both the result of the first cutting stage and the guide structure for the second full-cut. The geometry of the stepped portion naturally guides the sawing blade during the full-cut process, making the process self-regulating and reducing the need for complex external control mechanisms
Solution Approach 2:
The horizontal surface of the stepped portion created by half-cutting provides a stable reference plane that ensures consistent positioning for the subsequent full-cut. This equipotential surface eliminates positioning variations and reduces the complexity of alignment control
3Productivity
If the lead frame is fully cut to form individual packages, then package separation is achieved, but burrs remain on the side wall causing short circuit
Solution Approach 1:
The patent converts the potentially harmful burrs generated during full-cutting into beneficial features by forming the stepped portion. The half-cut creates a geometry where the lead frame thickness varies, and the resulting edges after full-cutting are less likely to form excessive burrs. Additionally, the stepped structure provides mechanical clearance that prevents burr-related short circuits between adjacent packages
Solution Approach 2:
The invention introduces a vertical dimension variation through the stepped portion, creating different height levels on the lead frame. This dimensional change ensures that even if burrs are generated during full-cutting, they remain isolated on the upper level and cannot bridge to adjacent packages, thereby maintaining electrical reliability
4Ease of operation
If the lead frame is exposed from the side wall without protective layer, then the side wettable structure is formed, but copper oxide forms on the cross section
Solution Approach 1:
The patent applies a protective plating layer (such as nickel or palladium) on the lead frame before the cutting and exposure processes. This preliminary protective coating prevents copper oxidation during subsequent handling and storage, while still allowing the lead frame to be exposed on the side wall for solder wetting and inspection purposes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach simplifies manufacturing, reduces package thickness, eliminates burrs and copper oxide issues, and lowers costs by using a single cutting and etching step, while ensuring reliable solder connections.
Implementation Method 1
A solder layer 44a is plated on a bottom surface of the lead frame 41 by an electroplating process
Data Source
AI summary
A side wettable package includes a molding compound, a chip and multiple conductive pads exposed from a bottom surface of the molding compound. The conductive pads include peripheral conductive pads arranged near a side wall of the molding compound. Each of the peripheral conductive pads is over etched to form an undercut. When the side wettable package is connected to a circuit board via solder, the solder ascends to the undercut of the peripheral conductive pads for improving connection yield and facilitating inspection of soldering quality.