Grooved Leadframe Solder Wicking Control
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Solution Overview
Problem
Surface-mount devices face assembly difficulties due to solder wicking issues, especially with lead-free solders, which are exacerbated by shrinking package dimensions and the need for higher reflow temperatures, and there is a need for a cost-effective solution to prevent solder wicking across various semiconductor product families and assembly variations.
Innovation Solution
The implementation of leadframes with strategically positioned grooves in the lead segments to accumulate solder, slowing and stopping the wicking process by balancing surface energy and gravitational forces, using either stamped or etched grooves, which can be applied during the final assembly stage or as a precaution in new devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If lead-free solders are used to address environmental concerns, then environmental compliance is improved, but solder wicking problems worsen
Solution Approach 1:
The patent applies the 'Blessing in disguise' principle by converting the harmful solder wicking effect into a beneficial containment mechanism. Grooves are strategically positioned on the leadframe to intercept and contain the solder wicking that occurs with lead-free solders, transforming the unwanted lateral flow into a controlled accumulation at the joint location, thereby maintaining environmental compliance while eliminating the harmful wicking effect.
Solution Approach 2:
The patent uses the grooves as an intermediary structure between the solder and the leadframe. These grooves act as mediators that intercept the solder wicking process, providing a controlled path for solder accumulation and preventing uncontrolled lateral flow, thus resolving the contradiction between using lead-free solders and preventing solder wicking.
2Volume of moving object
If package dimensions are shrunk to reduce device size, then device compactness is improved, but solder attachment surface area is reduced
Solution Approach 1:
The patent applies the 'Local quality' principle by creating localized grooves at specific positions on the leadframe where solder accumulation is needed. Rather than uniformly modifying the entire leadframe surface, the grooves are strategically placed to provide localized solder containment exactly where the joint formation occurs, thereby maintaining compact package dimensions while ensuring adequate solder attachment surface area at critical locations.
3Reliability
If reflow temperature is increased to accommodate lead-free solders, then soldering process compatibility is improved, but solder wicking control becomes more difficult
Solution Approach 1:
The patent applies the 'Preliminary anti-action' principle by pre-positioning grooves on the leadframe before the soldering process. These grooves are prepared in advance to counteract the solder wicking that will occur during high-temperature reflow processing with lead-free solders. The grooves create a preliminary structural constraint that prevents uncontrolled solder flow even when elevated temperatures increase solder fluidity and wicking tendency.
4Ease of manufacture
If noble metal usage is reduced to lower leadframe cost, then manufacturing cost is reduced, but solder wicking resistance is weakened
Solution Approach 1:
The patent applies the 'Segmentation' principle by dividing the leadframe surface into distinct regions separated by grooves. This segmentation creates multiple localized zones for solder containment rather than relying on uniform noble metal coating across the entire surface. The grooves partition the solder flow path, allowing effective solder wicking control even with reduced or eliminated noble metal usage, thereby lowering manufacturing cost while maintaining soldering performance.
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 effectively reduces solder wicking, ensuring reliable device attachment and improved process yields by allowing solder to accumulate in grooves, preventing it from being pulled away from attachment points, thus enhancing device reliability and reducing the risk of poor board attachment.
Implementation Method 1
When liquid solder is wicking, it is drawn by the capillary force, based on surface tension, along the solid surface underneath.
Implementation Method 2
When liquid solder is wicking, it is drawn by the capillary force, based on surface tension, along the solid surface underneath.
Implementation Method 3
the work of lifting solder continues until the surface energy gained is balanced by the energy needed to lift the mass of solder against gravity
Data Source
AI summary
A packaged surface-mount semiconductor device has the outer, un-encapsulated lead segments structured in five adjoining portions: The first portion protrudes from the encapsulation about horizontally; the second portion forms a convex bend downwardly; the third portion is approximately straight downwardly; the fourth portion forms a concave bend upwardly; and the fifth portion is straight horizontally. Each segment has across the width a first groove in the third portion, either on the bottom surface or on the top surface. Preferably, the groove is about 2 leadframe thicknesses vertically over the bottom surface of the fifth lead portion. When stamped, the groove may have an angular outline about 5 and 50 μm deep; when etched, the groove may have an approximately semicircular outline about 50 to 125 μm deep. A second groove may be located in the second segment portion; a third groove may be located in the transition region from the third to the fourth segment portions.