Concave Lead End Faces for Smear Mitigation in QFN Packages

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Solution Overview

Problem

Leadless electronic packages, such as QFN and SON, face issues with metal smearing and burrs during singulation, leading to shorting and failed inspections in high-reliability environments due to the lack of clearance between leads, which complicates visual inspection and reliability.

Innovation Solution

The introduction of leads with exposed end faces featuring a narrow portion forming a concave recess between the top and bottom edges, providing additional clearance and mitigating metal smearing and burrs, while maintaining sufficient surface area for solder wetting and visual inspection.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Difficulty of detecting and measuring

If leads have exposed end faces coplanar with package sides to enable visual inspection, then inspection capability is improved, but metal smearing and burrs during singulation cause shorting between adjacent leads

Engineering Contradiction:

Improvevisual inspection capabilityVSAvoidshorting risk between leads

Core Design Contradiction:

Difficulty of detecting and measuringVSReliability

Solution Approach 1:

The lead structure incorporates a narrow portion with concave recess at the exposed end face, creating local geometric variation. This local quality change provides clearance between adjacent leads while maintaining the exposed end face for inspection, resolving the contradiction between inspection capability and shorting risk

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The invention introduces a narrow portion with concave recess that extends into the lead body, adding a dimensional feature that increases spacing between adjacent leads. This dimensional modification provides the necessary clearance to prevent shorting while preserving the exposed end face for visual inspection

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Ease of operation

If lead ends are cut during singulation to expose end faces, then inspection is enabled, but metal smearing and burrs are created causing failed inspections

Engineering Contradiction:

Improveinspection accessibilityVSAvoidlead end face quality

Core Design Contradiction:

Ease of operationVSManufacturing precision

Solution Approach 1:

The narrow portion with concave recess is pre-formed on the lead before singulation. This preliminary structural feature ensures that when the lead is cut during singulation, the metal smearing and burrs are contained within the concave recess, preventing them from compromising the inspection quality of the exposed end face

Inventive Principle:

Principle #10Preliminary action

3Reliability

If additional clearance is provided between adjacent leads, then shorting risk is reduced, but lead pitch increases reducing package density

Engineering Contradiction:

Improveshorting mitigationVSAvoidlead pitch

Core Design Contradiction:

ReliabilityVSLength of moving object

Solution Approach 1:

The narrow portion with concave recess is a localized feature at the exposed end face of each lead. This local modification provides the necessary clearance between adjacent leads without requiring an increase in overall lead pitch, thereby maintaining package density while reducing shorting risk

Inventive Principle:

Principle #3Local quality

Data Source

PatentUS20220359352A1Electronic package with concave lead end faces

Publication Date: 2022.11.10 TEXAS INSTRUMENTS INC

AI summary

An electronic package includes an electronic component including terminals, a plurality of leads, at least some of the leads being electrically coupled to the terminals within the electronic package, and a mold compound covering the electronic component and partially covering the leads. Each of the leads include an exposed bottom face coplanar with a bottom surface of the mold compound and an exposed end face coplanar with one of a plurality of side surfaces of the mold compound. For at least some of the leads, the exposed end face includes a narrow portion forming a concave recess, the narrow portion being between top and bottom edges of the exposed end face.