Flexible Lead Chip Resistor Thermal Stress Relief
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Solution Overview
Problem
Surface mounted chip resistors with flat resistive elements, particularly those without leads, experience failures due to mechanical and thermal stresses when soldered to a Printed Circuit Board (PCB) due to mismatched coefficients of thermal expansion between the chip's ceramic substrate and the PCB, leading to potential solder joint failures under varying temperatures and mechanical loads.
Innovation Solution
The transformation of lead-less chip resistors into devices with flexible leads by attaching lead-frames comprising multiple sections of electrically conductive metal, which are bent and attached to the termination pads, providing mechanical and thermal strain relief through a combination of solder joints and adhesive attachment, allowing for robust assembly and reduced stress on the solder joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If lead-less chip resistors are used to reduce size and manufacturing cost, then device dimensions and cost are improved, but solder joint reliability deteriorates under mechanical and thermal stresses
Solution Approach 1:
The lead frame is divided into multiple sections including a body portion and leg portions that can be bent at different rates. This segmentation allows different parts of the lead frame to accommodate thermal expansion differently, with the leg portions absorbing stress through bending while the body portion remains relatively stable, thereby protecting the solder joints from stress concentration
Solution Approach 2:
The lead frame acts as an intermediary component between the rigid chip resistor and the PCB. It provides a flexible connection that mediates the stress transfer, absorbing mechanical and thermal stresses through its bendable structure rather than transmitting them directly to the solder joints, thus protecting the connection integrity
2Strength
If rigid chip structure is used to maintain structural integrity, then structural strength is improved, but adaptability to thermal and mechanical stress changes deteriorates
Solution Approach 1:
The lead frame is designed with dynamic characteristics, allowing it to change shape in response to external stresses. The leg portions can bend and deform elastically when subjected to thermal expansion or mechanical forces, providing adaptability while the rigid chip body maintains its structural integrity for electrical functionality
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 solution enhances the reliability and robustness of the assembly by allowing the flexible leads to accommodate thermal and mechanical strains, preventing solder joint failures and ensuring high reliability and durability of the chip resistors during temperature changes and mechanical loads.
Implementation Method 1
Each of the flexible leads is formed from a plurality of lead sections for facilitating bending of each of the flexible leads around one of the ends of the chip resistor
Implementation Method 2
attaching the first flexible lead to a first termination pad of the leadless chip resistor, attaching the second flexible lead to a second termination pad of the leadless chip resistor
Data Source
AI summary
A chip resistor having first and second opposite ends includes a rigid insulated substrate having a top surface and an opposite bottom surface, a first electrically conductive termination pad and a second electrically conductive termination pad, both termination pads on the top surface of the rigid insulated substrate, a layer of resistive material between the first and second electrically conductive termination pads, and a first and a second flexible lead, each made of an electrically conductive metal with a solder enhancing coating. The first flexible lead attached and electrically connected to the first electrically conductive termination pad and the second flexible lead attached and electrically connected to the second electrically conductive termination pad. Each of the flexible leads has a plurality of lead sections facilitating bending around the end of the chip resistor.