Lead Frame Plating Design for Whisker Prevention
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Solution Overview
Problem
Conventional semiconductor lead frame manufacturing processes face challenges with wire bonding and soldering reliability, high manufacturing costs, and productivity issues due to Galvanic corrosion and whisker formation, particularly when using Fe—Ni alloys.
Innovation Solution
A lead frame design featuring inner leads with a first plating layer of Ag, Au, or Pd and outer leads with a second plating layer of Sn, where the lead frames are coupled and heat-treated to prevent whisker growth, enhancing mechanical strength and manufacturing efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a two-tone pre-plated frame is used with Ag on inner leads and Sn on outer leads, then wire-bonding and soldering quality are improved, but whisker formation occurs causing short-circuits
Solution Approach 1:
The patent extracts the harmful Sn plating from the inner lead portions where it causes whisker formation, while retaining Ag plating that provides good wire-bonding properties. This selective removal eliminates the source of whiskers while preserving the beneficial plating configuration for reliable connections.
Solution Approach 2:
The patent applies different plating configurations to different regions of the lead frame: Ag plating is applied to inner lead portions for optimal wire-bonding, while Sn plating is applied only to outer lead portions for soldering. This localized differentiation optimizes each region's function while avoiding the harmful interaction between Sn and inner leads that causes whiskers.
2Object-generated harmful factors
If a reflow process is performed to remove whiskers, then whisker formation is prevented, but processing time increases and productivity decreases
Solution Approach 1:
The patent performs preliminary plating configuration during the lead frame manufacturing process itself, applying Ag to inner leads and Sn only to outer leads before assembly. This preliminary action prevents whisker formation from occurring in the first place, eliminating the need for subsequent reflow processes and maintaining high productivity.
Solution Approach 2:
The patent converts the potential harm of Sn plating (whisker formation) into a benefit by strategically limiting Sn application to outer lead portions only. The Sn plating provides excellent soldering properties where needed, while its absence from inner lead portions prevents whisker-related short-circuits, turning a harmful material into a beneficial one through proper placement.
3Reliability
If Ag plating is applied on inner leads, then wire-bonding capability is improved, but the Ag plating layer becomes damaged by heat in reflow process
Solution Approach 1:
The patent extracts the Ag plating layer from the exposure path of high-temperature reflow processes by configuring it only on inner lead portions that remain inside the package body. This extraction protects the Ag layer from thermal damage while maintaining its wire-bonding function where it is most needed.
Solution Approach 2:
The patent uses spatial differentiation to protect Ag plating from heat damage. By applying Ag only to inner leads (internal dimension) and Sn to outer leads (external dimension), the Ag layer is shielded from external reflow heat processes, while the Sn layer on outer leads withstands the heat without causing whisker issues.
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 improves wire-bonding and soldering quality, reduces defects, and lowers manufacturing costs by preventing whisker formation and thermal damage, while maintaining mechanical strength and reducing processing time.
Implementation Method 1
a first plating layer formed on an inner surface of the inner leads, wherein the first plating layer contains at least one material selected from a group consisting of Ag, Au and Pd
Implementation Method 2
a second plating layer formed on an outer surface of the outer leads, wherein the second plating layer contains Sn as a main material
Implementation Method 3
the lead frame with the tin (Sn) plating layer disposed thereon is subjected to heat at a temperature higher than a re-crystallization temperature
Data Source
AI summary
Provided is a lead frame having an improved wire bonding property of inner leads and an improved soldering property of outer leads and preventing defects with high producing yield, and a method of manufacturing the lead frame. The lead frame includes a plurality of inner leads formed with predetermined intervals between them; and a plurality of outer leads extended from the inner leads in length directions of the inner leads, each of which has an end portion overlapped with the inner lead and coupled thereto and the other end connected to neighboring outer lead by a supporting portion.