Lead Frame Four-Layer Plating for Lead-Free Solder Wettability
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Solution Overview
Problem
The reduction in size and thickness of semiconductor devices and printed circuit boards leads to deformation during the reflowing process, causing defects such as poor solder fillet formation and solder joint open failures, especially when using lead-free solder pastes which have poor wettability characteristics, and existing platings either melt during reflow or are environmentally restricted.
Innovation Solution
A lead frame with a four-layer plating of Ni, Pd, Ag, and Au on the outer leads, where the Ag layer is between 0.05 μm and 0.5 μm thick, and the Pd layer between 0.005 μm and 0.2 μm, providing improved wettability with lead-free solder pastes even at reduced lead pitches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional SnPb eutectic plating is used to improve wettability with solder paste, then the wettability is improved and solder fillet formation is enhanced, but the plating melts during reflowing due to its low melting point
Solution Approach 1:
The plating is segmented into multiple layers with different functions: the Sn layer provides wettability with solder paste, while the Ni and Pd layers provide structural stability and prevent melting during reflow. This segmentation allows each layer to perform its specific function without compromising the others.
Solution Approach 2:
The invention uses a composite plating structure consisting of Sn, Ni, and Pd layers. This composite material approach combines the advantages of each material: Sn for wettability, Ni for structural stability and low-temperature resistance, and Pd for additional protection and solderability enhancement.
2Adaptability or versatility
If lead-free solder paste is used to meet environmental requirements, then environmental compliance is improved, but wettability and solder fillet formation deteriorate
Solution Approach 1:
The invention changes the parameters of the plating structure by introducing a multi-layer configuration with specific thickness ratios. The Sn layer thickness is optimized to be between 0.5-5 μm to provide sufficient wettability for lead-free solder, while the Ni and Pd layers provide the necessary structural support.
Solution Approach 2:
The Sn layer acts as an intermediary between the lead-free solder paste and the underlying Ni/Pd layers. It provides the chemical compatibility and wettability needed for lead-free solder bonding, while the Ni and Pd layers provide structural stability that the thin Sn layer alone cannot provide.
3Length of moving object
If the lead pitch is reduced to achieve smaller device size, then device miniaturization is improved, but coplanarity and solder joint reliability deteriorate
Solution Approach 1:
The multi-layer composite plating structure provides enhanced rigidity and dimensional stability, which helps maintain coplanarity even when leads are closely spaced. The combination of Ni and Pd layers with appropriate thicknesses (1-10 μm each) provides sufficient structural support to prevent lead deformation during assembly.
Solution Approach 2:
The invention specifies that the mounting surface of the outer leads should have a coplanarity of 50 μm or less, which represents a curved surface tolerance rather than a perfectly flat surface. This allows for some flexibility in the lead geometry while still ensuring reliable solder joint formation.
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
The four-layer plating ensures high connection reliability by enhancing solder fillet formation and preventing solder joint open failures, maintaining reliability even at smaller lead pitches and widths, while being environmentally friendly and cost-effective.
Implementation Method 1
the surface of the lead frame is plated to improve wettability between the outer leads 34 and the solder paste 31
Implementation Method 2
the surface of the lead frame is plated
Data Source
AI summary
A lead frame base is coated with a four-layer plating. The four-layer plating includes an underlayer plating (Ni), a palladium plating, a silver plating and a gold plating arranged in this order from bottom to top.