Electroplated QFN Terminal Edges Eliminate Tin Whiskers
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Solution Overview
Problem
Flat no-lead semiconductor packages, such as QFN and DFN, face issues with tin whiskering due to electroless tin plating, leading to electrical shorts and reliability problems, which conventional electroplating cannot address without electrical contact to all surfaces.
Innovation Solution
A method involving partial sawing to expose terminal sides, removably shorting all surfaces for electroplating, and subsequent singulation, allowing for electroplated metal finishes on QFN/DFN terminals and edges, eliminating tin whiskering while enabling visual solder inspection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If electroless tin plating is used for terminal finishes, then solderability is improved, but tin whiskering occurs causing electrical shorts and reliability problems
Solution Approach 1:
The patent changes the plating process parameters from electroless to electroplating, and modifies the terminal structure to include exposed edges that can be electrically connected. This allows the use of electroplating process parameters (electrical current, electrolyte composition, plating time) to deposit tin or tin alloy coatings that do not exhibit whiskering behavior, thereby maintaining solderability while eliminating the harmful tin whiskering effect
Solution Approach 2:
The patent employs composite terminal structures consisting of multiple material layers including the leadframe substrate, solder mask, and electroplated tin or tin alloy coating. This composite structure allows the electroplated layer to provide solderability while the underlying layers provide structural support and electrical connectivity, preventing tin whiskering that would occur with electroless plating alone
2Reliability
If conventional electroplating is used to eliminate tin whiskering, then reliability is improved, but electrical contact to all surfaces is required which complicates the process
Solution Approach 1:
The patent segments the terminal structure into distinct functional zones: the bottom surface terminals for electrical connection, the exposed side edges for plating access, and the top surface for soldering. This segmentation allows electroplating to be applied only to accessible surfaces (bottom and sides) rather than requiring electrical contact with all surfaces, simplifying the process while still eliminating tin whiskering through electroplating
Solution Approach 2:
The patent uses the exposed side edges of the terminals as intermediary surfaces that can be electrically connected during plating and then removed or left as part of the final structure. These intermediary edges provide the necessary electrical contact for electroplating without requiring complex wiring to all terminal surfaces, enabling the electroplating process to eliminate tin whiskering while maintaining process simplicity
3Manufacturing precision
If all terminal surfaces are exposed for electroplating, then complete coating is achieved, but additional processing steps are required
Solution Approach 1:
The patent performs preliminary actions during the encapsulation process where the molding compound is applied to expose the terminal edges while protecting the top surfaces. This preliminary exposure of the side edges during encapsulation prepares the terminals for subsequent electroplating without requiring additional separate exposure steps, achieving complete coating coverage while minimizing additional processing steps
Solution Approach 2:
The patent merges multiple functions into the encapsulation process: the molding compound simultaneously provides mechanical protection, electrical insulation, and terminal edge exposure. By combining these functions into a single encapsulation step, the patent achieves complete terminal surface exposure for electroplating without adding separate processing steps, thereby maintaining productivity while ensuring complete coating coverage
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 eliminates tin whiskering, enhances solderability, and allows for visual inspection instead of X-ray, improving yield and reliability by using an electroplating process that covers all surfaces, including edges, thereby preventing shorting issues.
Implementation Method 1
an electrolytic (electroplating) terminal finishing process instead of a conventional electroless plating process would remove Sn whiskering
Implementation Method 2
they have added terminal finishes, such as comprising Matt Sn (100% tin) which is applied to the terminals using an electroless (or auto-catalytic by only chemical means) plating process
Data Source
AI summary
A lead frame sheet of flat no-lead lead frames having a semiconductor die on a die pad, terminals, and plastic encapsulation except on a back side of the sheet to provide an exposed thermal die pad, exposed side walls, and exposed back sides of the terminals.A solder wetable metal or metal alloy plating layer is on the back side and on the exposed the walls of the terminals. The exposed thermal pad and the back side of the terminals each include a contact region which lacks the plating layer.