Lead Frame Deflash Using Selective Electrolytic Removal
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Solution Overview
Problem
The challenge in semiconductor packaging is the formation of unwanted mold flash during the molding process, which is difficult to remove efficiently, affecting the reliability of solder joints.
Innovation Solution
A semiconductor package design with a lead surface divided into encapsulated and exterior surface portions, where the exterior surface has a higher surface roughness, and a process involving electrolytic treatment to remove polymer material, followed by forming a solderable metal layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a molding process is used to apply encapsulation material to the die and lead frame, then the semiconductor die is surrounded and protected, but small amounts of polymer material leak out under pressure resulting in unwanted mold flash on the lead frame
Solution Approach 1:
The lead surface is divided into two distinct regions: an encapsulated surface portion that contacts the polymer material and an exterior surface portion that remains exposed. This segmentation allows the encapsulation material to be applied selectively to where it is needed while preventing polymer leakage onto the exterior surface, thereby resolving the contradiction between providing protection and avoiding mold flash.
Solution Approach 2:
Different surface properties are applied to different portions of the lead. The encapsulated surface portion is designed to be compatible with polymer adhesion, while the exterior surface portion maintains properties that prevent polymer adhesion. This local differentiation enables the lead to simultaneously provide die protection and avoid unwanted polymer accumulation.
2Object-generated harmful factors
If deflashing is performed to remove mold flash, then the unwanted polymer material is removed, but the process is difficult and affects the reliability of solder joints
Solution Approach 1:
The lead surface is pre-treated during manufacturing to create distinct encapsulated and exterior surface portions with different polymer adhesion properties. This preliminary action prevents mold flash formation in the first place, eliminating the need for difficult deflashing operations and protecting solder joint reliability from deflashing-related damage.
Solution Approach 2:
The invention converts the potential harm of polymer leakage into a beneficial selective adhesion property. By designing the encapsulated surface to intentionally attract and hold polymer material while the exterior surface repels it, the process transforms what would be a harmful defect (mold flash) into a functional feature (selective encapsulation).
3Strength
If the lead surface is made rough to improve polymer adhesion, then the encapsulation material adheres better, but the exterior surface needs to remain smooth for solderability
Solution Approach 1:
The lead surface is segmented into an encapsulated surface portion with high roughness for polymer adhesion and an exterior surface portion with low roughness for solderability. This segmentation allows each surface to be optimized for its specific function without compromising the other, resolving the contradiction between adhesion strength and manufacturing ease.
Solution Approach 2:
Different surface roughness qualities are applied locally to different portions of the lead. The encapsulated surface portion has high roughness specifically where polymer adhesion is needed, while the exterior surface portion maintains smoothness specifically where solder joints will be formed. This local quality differentiation resolves the contradiction between requiring rough surfaces for adhesion and smooth surfaces for soldering.
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 process effectively reduces mold flash, ensuring reliable solder joints by minimizing adhesion and improving the surface roughness uniformity, thus enhancing the packaging process efficiency.
Implementation Method 1
a portion of the lead, where exposed by a remaining portion of the polymer material, is removed by an electrolytic process. The electrolytic process includes biasing the lead to a positive potential with respect to an electrolytic solution contacting the exterior surface portion of the lead.
Data Source
AI summary
A semiconductor package includes a lead having an exterior surface portion, at an exterior of the semiconductor package, and an encapsulated surface portion contacting an encapsulation material. A solderable metal layer is on the exterior surface portion. The lead has a higher surface roughness at the encapsulated surface portion than at the exterior surface portion. Before the solderable metal layer is formed, polymer material of the encapsulation material may extend onto the exterior surface portion. A first portion of the polymer material on the exterior surface portion is removed, exposing areas of the lead. Metal from the lead, where exposed by a remaining portion of the polymer material, is removed by an electrolytic process. The lead is biased to a positive potential with respect to an electrolytic solution. Subsequently, the remaining portion of the polymer material is removed. The solderable metal layer is formed on the exterior surface portion.