Selectively Plated Lead Frame for IC Package
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Solution Overview
Problem
Copper lead frames in integrated circuit packages are prone to oxidation, leading to failures in welded connections, and existing plating methods do not adequately prevent oxidation or ensure proper epoxy adhesion and encapsulation, resulting in defective IC packages due to wire bond breakages during thermal cycles.
Innovation Solution
A lead frame design with selectively plated silver regions for wire bonds and un plated central body portion for epoxy adhesion, featuring longitudinal grooves and slots that prevent epoxy overflow and facilitate encapsulant locking, reducing relative displacement between the lead frame and encapsulant.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If copper lead frames are plated with silver to prevent oxidation, then wire bond reliability is improved, but epoxy adhesion deteriorates due to oxidation-resistant surface
Solution Approach 1:
The lead frame is selectively plated with silver only in specific regions (ear portions, finger members, and peripheral areas) while leaving the central body portion unplated. This local differentiation allows the plated regions to provide oxidation resistance for wire bonds while the unplated central region maintains surface properties suitable for epoxy adhesion, thereby resolving the contradiction between wire bond reliability and epoxy adhesion.
2Strength
If epoxy is applied generously to ensure complete die attachment, then die attachment strength is improved, but epoxy overflow onto adjacent regions increases causing interference with packaging processes
Solution Approach 1:
The lead frame central body portion is segmented into a die attach pad region and peripheral regions by longitudinal grooves. These grooves act as physical barriers that segment the epoxy application area, allowing generous epoxy application for strong die attachment while preventing overflow onto adjacent regions where it would interfere with packaging processes.
3Productivity
If lead frame strip is cut apart after encapsulation to separate individual packages, then individual IC packages are obtained, but wire bond breakages occur during thermal cycles
Solution Approach 1:
The lead frame is preliminarily prepared with selective silver plating on peripheral regions and finger members before die mounting and wire bonding. This preliminary plating action ensures that wire bonds are attached to oxidation-resistant surfaces, pre-preventing oxidation-related failures that would occur during subsequent thermal cycles after singulation, thereby maintaining wire bond reliability through the package separation process.
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 design enhances wire bond reliability by preventing oxidation and ensuring proper epoxy adhesion, reducing the occurrence of wire bond failures and improving the overall integrity of integrated circuit packages.
Implementation Method 1
Copper lead frames are subject to oxidation, which may cause welded connections thereto, such as wire bonded connections, to be subject to failure. For this reason, copper lead frames are often plated with other metals, such as silver, which prevents oxidation.
Implementation Method 2
After placing each die on a corresponding patch of epoxy paste, the lead frame strip is moved to a heat source such as a curing oven where the epoxy is cured.
Implementation Method 3
The applied mold compound is heated until it cures to a solid state. The cured mold compound protects the encapsulated portion of each lead frame, the associated die and wire bond connections.
Data Source
AI summary
A method of making an integrated circuit (“IC”) device includes forming a lead frame in a lead frame strip. Only portions of the lead frame are plated with a conductor. A die pad is attached to an unplated portion of said lead frame.