Semiconductor Package Lead Flanks for Inspectable PCB Solder Joints
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Solution Overview
Problem
The challenge in inspecting the thin solder joints between semiconductor package pads and leads, typically around 10 microns thick, poses a difficulty in ensuring robust connections, particularly in quality control processes like those in the automotive sector.
Innovation Solution
The semiconductor package design includes exposed portions of leads partway along the encapsulant, allowing for electroplating of both the underside and flank of the pads, enabling thicker solder application and improved inspection capabilities, while maintaining electrical connectivity through a dual set of interconnecting tracks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the solder thickness between pad and leads is kept thin (around 10 microns) as in conventional designs, then the semiconductor package maintains a compact size, but the inspection of the solder joint becomes extremely difficult and unreliable
Solution Approach 1:
The patent introduces a vertical dimension to the solder joint by creating a stepped recess in the encapsulant that exposes the flank of the pad. This allows solder to be applied not only horizontally on the pad surface but also vertically on the exposed flank, transforming a 2D soldering problem into a 3D solution. The exposed portion of the pad extends partway through the encapsulant thickness, providing additional surface area for solder attachment that can be inspected from multiple angles.
Solution Approach 2:
The encapsulant is segmented to create a stepped recess that partially exposes the pad structure. This segmentation allows different portions of the pad to serve different functions: the upper surface for electrical connection and the exposed flank for enhanced soldering and inspection. The encapsulant is divided into regions with different depths, creating zones for solder application and inspection accessibility.
2Reliability
If the solder thickness is increased to improve connection robustness and inspection capability, then the reliability of the soldered joint improves, but the overall package size and complexity increase
Solution Approach 1:
The encapsulant structure is modified locally at specific pad locations rather than throughout the entire package. The stepped recess is created only where needed to expose the pad flank for soldering, while the rest of the encapsulant maintains its standard protective function. This localized modification allows enhanced soldering capability without requiring a complete redesign of the entire package structure.
Solution Approach 2:
The pad flank is pre-exposed during the encapsulant formation process by creating the stepped recess before final encapsulation. This preliminary action ensures that the pad surface ready for enhanced soldering is prepared in advance, allowing for thicker and more inspectable solder joints without adding complex post-processing steps. The electrical connectivity is maintained through dual interconnecting tracks that are also prepared in advance.
3Object-affected harmful factors
If the encapsulant fully covers all leads and pads for complete protection, then the protection against contaminants improves, but the ability to inspect and solder joints deteriorates
Solution Approach 1:
Instead of fully exposing the pad for soldering, the patent applies partial exposure by creating a stepped recess that reveals only the necessary portion of the pad flank. This partial action provides sufficient access for solder application and inspection while maintaining encapsulation coverage for environmental protection. The exposed portion extends partway through the encapsulant thickness, offering the right balance between protection and accessibility.
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 design enhances the reliability of soldered connections by allowing for wider solder application and easier inspection, ensuring secure coupling to PCBs and reducing manufacturing costs.
Implementation Method 1
an encapsulant which at least partly surrounds the die and the plurality of leads, the encapsulant defining a thickness
Implementation Method 2
allowing for electroplating of both the underside and flank of the pads, enabling thicker solder application
Data Source
AI summary
There is disclosed a semiconductor package for a PCB. The semiconductor package includes a die paddle, a die, a plurality of leads and an encapsulant. The die is coupled to the die paddle. The plurality of leads are electrically connected to the die. Each of the plurality of leads extend from the die at a first end and define a pad for attachment to the PCB at a second end. The encapsulant at least partly surrounds the die and the plurality of leads. The encapsulant defines a thickness. The one or more of the plurality of leads includes an exposed portion, the exposed portion being disposed partway along an extent of the lead between the first and second ends. The exposed portion is located partway along the thickness of the encapsulant.