QFN Lead Stiffness and Protective Layer for Wirebonding
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Solution Overview
Problem
Conventional Quad Flat No leads (QFN) packages face reduced yield in the wirebonding process due to the lack of stiffness in the half etch portions of the leads, causing bouncing and misalignment during bonding wire attachment.
Innovation Solution
The semiconductor device package design includes leads with a lower surface divided into contact and non-contact areas, where the non-contact areas are partially encapsulated and covered with a protective layer, maintaining their thickness and preventing bouncing, while the contact areas remain exposed for electrical connection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the half etch portions of the leads are made thinner to save bonding wire length, then the signal transmission efficiency is improved, but the stiffness of the leads decreases causing bouncing during wirebonding
Solution Approach 1:
The lead structure is divided into different regions with different thicknesses: the half etch portion has reduced thickness for shorter bonding wires, while the full thickness portion maintains original thickness for stiffness. This local differentiation allows each region to optimize its function without compromising the other.
Solution Approach 2:
The lead is segmented into distinct functional zones (half etch portion and full thickness portion) with different structural characteristics. The half etch portion extends from the die pad toward the periphery with reduced thickness, while the full thickness portion provides structural support at the periphery, creating a segmented structure that balances electrical and mechanical requirements.
2Manufacturing precision
If the half etch portions of the leads are etched deeper to reduce bonding wire length, then the electrical connection efficiency is improved, but the yield of wirebonding process decreases due to bouncing
Solution Approach 1:
Different regions of the lead are given different thicknesses to serve different functions: the half etch portion is thinner for better electrical connection and shorter bonding wires, while the full thickness portion maintains structural integrity to prevent bouncing during wirebonding, thus improving both precision and yield.
Solution Approach 2:
The lead structure is pre-configured with the half etch portion and full thickness portion before the wirebonding process. This preliminary structural preparation ensures that the lead has the appropriate stiffness distribution beforehand, preventing bouncing during wirebonding and improving process yield.
3Ease of operation
If the leads are fully exposed for electrical connection, then the electrical connection accessibility is improved, but the leads are vulnerable to corrosion and oxidation
Solution Approach 1:
The protective layer is applied selectively to different regions: the full thickness portion is covered for corrosion protection, while the half etch portion remains exposed for electrical connection. This local differentiation allows simultaneous achievement of protection and accessibility.
Solution Approach 2:
Instead of fully covering or fully exposing the leads, a partial protective layer is applied only to the full thickness portion. This partial action provides sufficient corrosion protection where needed while maintaining electrical connection accessibility where required.
Data Source
AI summary
A Quad Flat No Leads (QFN) package includes a lead frame, a chip, an encapsulant, and a protective layer. The lead frame includes a plurality of leads. Each of the leads has a lower surface that is divided into a contact area and a non-contact area. The chip is configured on and electrically connected to the lead frame. The encapsulant encapsulates the chip and the leads and fills spaces between the leads. The contact areas and the non-contact areas of the leads are exposed by the encapsulant. The protective layer covers the non-contact areas of the leads.