Through Mold Via Geometry for Fine-Pitch QFN Alignment
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Solution Overview
Problem
The aspect ratio constraint in forming vias for semiconductor devices using laser direct structuring (LDS) technology limits the formation of fine-pitch Quad-Flat No-leads (QFN) packages, leading to misalignment and potential metal short-circuits due to the nearly 1:1 aspect ratio of conventional vias.
Innovation Solution
The formation of through mold vias (TMVs) with a larger upper collar and a frusto-conical bottom section, achieved in two ablation steps, allows for vias with a diameter smaller than the mold cap thickness, simplifying the molding and metallization processes without additional steps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional laser direct structuring is used to form vias with nearly 1:1 aspect ratio, then the molding process is simpler, but misalignment with leads occurs and metal short-circuits result
Solution Approach 1:
The via structure is segmented into three distinct portions: an upper cylindrical portion, an intermediate frustoconical portion, and a lower cylindrical portion. This segmentation allows each portion to serve a specific function - the upper and lower cylindrical portions provide alignment precision with leads and bonding pads, while the intermediate frustoconical portion facilitates proper metallization filling, thereby resolving the alignment issue without excessive complexity
Solution Approach 2:
Different portions of the via have different geometries tailored to their specific functions. The upper and lower portions are cylindrical for precise alignment, while the intermediate portion is frustoconical for optimal metallization. This local differentiation of geometry optimizes each region's performance while maintaining overall via functionality
2Ease of manufacture
If via diameter is increased to improve filling, then metallization is easier, but misalignment with leads occurs and shorts result
Solution Approach 1:
The via is divided into segments with different diameters - the upper and lower cylindrical portions have smaller diameters for precise alignment, while the intermediate frustoconical portion has a gradually varying diameter that facilitates metallization filling. This segmentation allows metallization ease in the intermediate portion without compromising alignment precision in the upper and lower portions
Solution Approach 2:
The via geometry parameters are changed along its length - the diameter varies from smaller at the upper and lower portions to larger in the intermediate frustoconical portion. This parameter variation optimizes both alignment precision (smaller diameter portions) and metallization filling (larger intermediate portion) simultaneously
3Volume of moving object
If fine-pitch QFN packages are manufactured with conventional vias, then package size is reduced, but aspect ratio constraints prevent proper via formation
Solution Approach 1:
The via structure is segmented to accommodate fine-pitch requirements - the upper and lower cylindrical portions provide precise alignment for fine-pitch leads, while the intermediate frustoconical portion ensures proper metallization filling even in the constrained space of miniaturized packages, thereby enabling fine-pitch QFN manufacturing with high via formation quality
Solution Approach 2:
The frustoconical intermediate portion is nested within the overall via structure, fitting between the upper and lower cylindrical portions. This nested configuration allows the via to achieve the necessary aspect ratio for proper filling while maintaining the compact dimensions required for fine-pitch packages
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 enables the production of vias with a global aspect ratio greater than 1:1, improving manufacturability and package miniaturization while avoiding misalignment issues, thus facilitating the creation of fine-pitch QFN packages.
Implementation Method 1
laser machining a through mold via in two ablation steps, to form first a collar recess and then a frusto-conical bottom via
Implementation Method 2
electrical conductivity of formations such as vias and lines or traces is facilitated via plating (e.g., electroless metallization followed by galvanic deposition)
Data Source
Figure 1
Figure 2~3
Figure 4A~4B
AI summary
A semiconductor device (10) comprises a semiconductor die (14) arranged on a substrate such as a leadframe (12A, 12B) and an encapsulation of laser direct structuring, LDS material (16; 161, 162) molded onto the semiconductor die (14). A through mold via or TMV (182) extending through the encapsulation of LDS material (16; 161, 162) comprises: an enlarged collar section (182A) that extends through a first portion (161) of the encapsulation (16) from an outer surface (1613) to an intermediate plane (1612) of the encapsulation (16), the enlarged collar section (182A) having a cross-sectional area at the intermediate plane (1612) of the encapsulation (16), and a frusto-conical section (182B) that extends through a second portion (162) of the encapsulation (16; 161, 162) from a first end having a first diameter at the intermediate plane (1612) to a second end having a second diameter away from the intermediate plane (1612) of the encapsulation (16; 161, 162). The first end of the frusto-conical section (182B) has an area smaller than the cross-sectional area of the enlarged collar section (182A) at the intermediate plane (1612) and the second diameter of the frusto-conical section (182B) is smaller than the first diameter of the frusto-conical section (182B). The through mold via (182) can thus have an aspect ratio which is not limited to 1:1.