Metal-Oxide-Coated Leadframe for Tin Whisker Suppression
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Solution Overview
Problem
Copper leadframes in semiconductor packaging form intermetallic layers that cause compressive stress, leading to tin whisker growth and potential short circuits, which existing solutions like postbake steps or material modifications are inadequate in preventing.
Innovation Solution
A metal layer, such as nickel, silver, or gold, is applied to the leadframe, followed by a metal oxide layer, with a tin layer formed on the oxide layer, separating it from the copper leadframe to prevent whisker formation and corrosion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If tin is plated onto copper leadframe, then the leads are formed, but an intermetallic layer of Cu6Sn5 is formed causing compressive stress and tin whisker growth
Solution Approach 1:
A nickel barrier layer is introduced between the copper leadframe and the tin plating layer. This intermediary layer prevents direct contact between copper and tin, thereby preventing the formation of the intermetallic layer Cu6Sn5 that causes compressive stress and whisker growth, while still allowing the plating process to proceed
Solution Approach 2:
The leadframe structure is segmented into multiple layers: copper leadframe base, nickel barrier layer, and tin plating layer. This segmentation separates the copper and tin materials that would otherwise directly interact to form harmful intermetallic compounds
2Reliability
If a postbake step is performed after plating, then whisker growth is reduced by relaxing compressive stress, but the intermetallic layer still forms
Solution Approach 1:
The nickel barrier layer is applied before tin plating to prevent intermetallic layer formation in the first place. This preliminary protective action eliminates the root cause of whisker growth, making subsequent postbake steps unnecessary for whisker prevention
3Reliability
If leadframe composition is modified to iron-nickel alloy, then whisker formation is reduced, but compatibility with plating chemistries is lost
Solution Approach 1:
The nickel barrier layer serves as a universal intermediary that maintains compatibility with standard tin plating chemistries while preventing intermetallic layer formation. This approach preserves the original copper leadframe composition and allows continued use of established plating processes
4Reliability
If a metal layer and metal oxide layer are added to the leadframe, then corrosion resistance and whisker prevention are improved, but the structure becomes more complex
Solution Approach 1:
The leadframe structure uses a composite layered approach with copper leadframe, nickel barrier layer, and tin plating layer. This composite structure combines the advantages of each material: copper for conductivity, nickel for barrier properties and corrosion resistance, and tin for solderability and additional corrosion protection
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
Reduces the likelihood of short circuits by eliminating the stressed intermetallic layer and whisker formation, enhancing package reliability and corrosion resistance without requiring a postbake step.
Implementation Method 1
A metal oxide layer is formed on the metal layer
Implementation Method 2
A tin layer may then be formed over at least a portion of the metal oxide layer
Data Source
AI summary
A leadframe including a metal oxide layer on at least a portion of the leadframe are disclosed. More specifically, leadframes with a metal layer and a metal oxide layer formed on one or more leads before a tin finish plating layer is formed are described. The layers of metal and metal oxide between the one or more leads and the tin finish plating layer reduce the formation of tin whiskers, thus reducing the likelihood of shorting and improving the overall reliability of the package structure and device produced.