Leadless Package With Exposed Interior Sidewalls
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Solution Overview
Problem
Leadless packages face challenges in solder joint reliability when mounted on a mounting base, particularly due to thermal expansion mismatches between the package and the mounting base, leading to mechanical stress and reduced lifespan.
Innovation Solution
The solution involves a leadless package design where the interior sidewalls of the lead section are partially exposed, allowing for a larger solder wetting area and volume, which increases the connection area between the package and the mounting base, enhancing mechanical and electrical reliability. This is achieved by using a soft and flexible molding foil to prevent encapsulant coverage of the interior sidewalls during molding, and subsequent removal to expose these areas for solder contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the encapsulant fully encapsulates the carrier including interior sidewalls of the lead section, then the package achieves complete encapsulation and protection, but the solder wetting area is reduced and joint reliability deteriorates
Solution Approach 1:
The encapsulant is designed to selectively encapsulate only certain surfaces of the carrier while leaving the interior sidewalls of the lead section exposed. This local differentiation allows the bottom surface and exterior sidewalls to be encapsulated for protection, while the interior sidewalls remain accessible for solder contact, thereby increasing solder wetting area and joint reliability without requiring complete exposure of all carrier surfaces
Solution Approach 2:
The encapsulation process is segmented into selective areas: the bottom surface and exterior sidewalls are encapsulated, while the interior sidewalls are deliberately left exposed. This segmentation of the encapsulation coverage allows simultaneous achievement of mechanical protection and enhanced solder joint reliability by creating distinct functional zones on the carrier surface
2Reliability
If the interior sidewalls of the lead section are exposed, then the solder wetting area increases and connection reliability improves, but the encapsulation process becomes more complex
Solution Approach 1:
A release foil is applied to the carrier before the encapsulation process, strategically positioned to cover areas that should remain exposed (interior sidewalls of the lead section). This preliminary action protects these areas from encapsulant coverage during molding, ensuring they remain accessible for solder contact while simplifying the overall manufacturing process by preventing the need for post-encapsulation exposure steps
Solution Approach 2:
A release foil is introduced as an intermediary element between the carrier and the encapsulant. This foil acts as a temporary barrier that prevents encapsulant adhesion to specific areas (interior sidewalls of the lead section) during the molding process, allowing these areas to remain exposed for solder contact while the rest of the carrier is fully encapsulated, thereby simplifying the manufacturing process
3Object-affected harmful factors
If the connection area between lead section and mounting base is increased, then the package withstands thermal stress better, but the package structure becomes more complex
Solution Approach 1:
The solder contact area is extended from the traditional two-dimensional bottom surface to include the vertical interior sidewalls of the lead section. This dimensional extension into the vertical space creates additional solder wetting surfaces, significantly increasing the total connection area between the package and mounting base, thereby improving resistance to thermal expansion mismatch stress without requiring changes to the horizontal package footprint
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 increased solder wetting area and volume significantly improve the board-level reliability of leadless packages, enhancing their ability to withstand thermal stress and mechanical loads, thereby extending the lifespan and reliability of the electronic device.
Implementation Method 1
using a soft and flexible molding foil to prevent encapsulant coverage of the interior sidewalls during molding
Implementation Method 2
a coupling structure electrically and mechanically coupling the package with the mounting base so that the coupling structure contacts at least part of the at least partially exposed interior sidewall of the lead section
Data Source
AI summary
A leadless package includes an at least partially electrically conductive carrier having a mounting section and a lead section, an electronic chip mounted on the mounting section, and an encapsulant at least partially encapsulating the electronic chip and partially encapsulating the carrier so that at least part of an interior sidewall of the lead section not forming part of an exterior sidewall of the package is exposed.