Thermal Printhead Buffer Layer Dispersion Solder Contraction Stress
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Solution Overview
Problem
Thermal printheads face reliability issues due to stress caused by solder contraction, leading to detachment of electrodes and breakage of glaze layers, which deteriorates the connection between flexible cables and drive ICs.
Innovation Solution
A thermal printhead design featuring a buffer layer between the glaze layer and electrode, with the buffer layer protruding from the electrode's edge, and an upper electrode layer with enhanced solder wettability, helps disperse the contraction force, reducing stress on the glaze layer and improving connection reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solder is used to connect the flexible cable to the electrode, then electrical connection is achieved, but solder contraction during cooling exerts stress on the electrode and glaze layer causing detachment or breakage
Solution Approach 1:
The patent introduces a resin layer as an intermediary substance between the solder and the electrode/glaze layer assembly. This resin layer absorbs and distributes the contraction stress generated during solder cooling, preventing direct transmission of stress to the fragile glaze layer and electrode structures. The intermediary layer thus mediates between the rigid solder joint and the brittle ceramic substrate, resolving the stress concentration problem.
Solution Approach 2:
The resin layer is applied in advance before soldering, creating a protective cushioning layer that anticipates and prepares for the upcoming thermal stress. This pre-positioned cushioning structure is designed to accommodate the expected contraction forces, providing stress relief before the solder cooling process begins, thereby preventing detachment and breakage.
2Device complexity
If the electrode is directly connected to the glaze layer, then structural simplicity is maintained, but stress concentration occurs at the connection point leading to detachment
Solution Approach 1:
The resin layer serves as a mediator between the electrode and the glaze layer, providing a compliant interface that accommodates thermal expansion differences. This intermediary layer maintains the basic two-layer structure (electrode on glaze) while adding a thin functional layer that prevents stress concentration, thus balancing structural simplicity with connection strength.
3Ease of manufacture
If soldering is performed directly on the electrode, then manufacturing process is simplified, but thermal stress during solder cooling causes glaze layer breakage
Solution Approach 1:
The resin layer is applied to the electrode surface before the soldering process begins. This preliminary action prepares the surface to absorb thermal stress during the subsequent soldering operation. By pre-positioning the stress-absorbing layer, the manufacturing process remains simple while protecting against glaze layer breakage during cooling.
Solution Approach 2:
The resin cushioning layer is established beforehand to anticipate and mitigate the thermal stress that will occur during solder cooling. This pre-prepared protective structure allows direct soldering on the electrode without risking glaze layer damage, maintaining manufacturing simplicity while ensuring structural integrity.
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 solution effectively reduces stress on the glaze layer and electrode, enhancing the reliability of electrical connections by dispersing contraction forces and preventing detachment or breakage, thus ensuring stable connections between the substrate and external connection members.
Implementation Method 1
the buffer layer protrudes from the electrode at least at an end adjacent to the edge of the substrate... effectively reduces stress on the glaze layer and electrode, enhancing the reliability of electrical connections by dispersing contraction forces
Implementation Method 2
since the solder 98 contracts when it cools and solidifies, the contraction force of the solder 98 exerts on the electrode 94 and the glaze layer 92 to produce a stress
Implementation Method 3
an upper electrode layer which is formed on the pad, superior in solder wettability to the pad
Data Source
AI summary
A thermal printhead (A) according to the present invention includes a substrate (1) having an obverse surface on which a glaze layer (2) is formed, an electrode (4) formed on the glaze layer (2) and a clip connector (5) attached to an edge of the substrate (1) for connection to an external device and connected to the electrode (4) via solder (8). An input wiring portion (33) as a buffer layer is provided between the glaze layer (2) and the electrode (4), and the input wiring portion (33) protrudes from the electrode (4) at least at an end adjacent to the edge of the substrate (1).