QFN Leadframe Grooves for Side-Wettable Solder Joints
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Solution Overview
Problem
The existing quad flat no leads (QFN) structures have insufficient solderable area on the lateral leads, leading to reduced solderability and board-level reliability, and environmental issues hinder plating processes, causing package defects and reduced lifetime.
Innovation Solution
Forming grooves on the leadframe surface, filling them with a metal paste to create a solderable portion, and cutting the leadframe to form a package structure that covers the leads, enhancing the solderable area and eliminating the need for plating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the QFN structure uses conventional cutting process to form copper-exposing surfaces, then the manufacturing process is simple, but the solderable area of the lateral leads is insufficient
Solution Approach 1:
The patent applies preliminary action by forming grooves on the leadframe surface and filling them with metal paste before the cutting process. This preliminary preparation ensures that when the leads are eventually cut and separated, they already possess enhanced solderable surfaces with increased lateral area, solving the insufficient solderable area problem without requiring complex post-cutting processing
Solution Approach 2:
The patent changes the physical and chemical parameters of the leadframe surface by introducing grooves and filling them with metal paste containing solderable materials. This parameter change transforms the ordinary copper surface into an enhanced solderable surface with modified topology (grooves) and composition (metal paste), thereby increasing the effective solderable area and improving soldering performance
2Reliability
If plating process is used to enhance solderability, then the solderable area is improved, but environmental issues arise and plating lines cannot be set up
Solution Approach 1:
The patent extracts the plating process from the manufacturing flow by replacing it with an alternative approach using grooves and metal paste. This extraction eliminates the need for plating lines and associated environmental concerns while achieving the same functional outcome of enhanced solderability through a different technical means that does not involve harmful plating chemicals or processes
Solution Approach 2:
The patent employs metal paste as a disposable material that can be directly applied to the grooves on the leadframe. This metal paste serves as a temporary but effective solderable coating that performs its function during soldering without requiring the complex, environmentally harmful plating infrastructure, thereby eliminating plating-related environmental issues
3Duration of action of stationary object
If the lateral leads have insufficient solderable area, then the manufacturing process is simple, but the solder fatigue and solder crack occur reducing package lifetime
Solution Approach 1:
The patent applies preliminary action by pre-forming grooves and filling them with metal paste on the leadframe before cutting and packaging. This preliminary enhancement of the lateral lead surfaces ensures that when the package is subjected to thermal cycling and mechanical stress during its service life, the increased solderable area provides greater resistance to solder fatigue and crack formation, thereby extending package lifetime
Solution Approach 2:
The patent provides beforehand cushioning against solder fatigue and cracking by increasing the solderable area through groove formation and metal paste application. This additional solderable material acts as a cushion or buffer that absorbs stress and prevents crack propagation during subsequent disposition on the circuit board, thereby preventing package defects before they occur
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
Increases the side wettable area, improves board-level reliability, and enhances connecting strength between the package and circuit board, while eliminating the need for plating processes.
Implementation Method 1
heating the metal paste to form a solderable portion in each of the grooves
Data Source
AI summary
A method of forming a package structure includes forming a plurality of grooves on a surface of a leadframe, disposing a metal paste in the grooves, heating the metal paste to form a solderable portion in each of the grooves and cutting a plurality of cutting streets of the leadframe to form the package structure. The leadframe includes a plurality of leads, and the grooves are disposed on a side of each of the leads. The solderable portion covers the side of each of the leads.