Thermal Print Head Bonding Pad Exposure and Corrosion Control
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Solution Overview
Problem
Existing thermal print heads face issues with poor wire bonding due to incomplete exposure of bonding pads and the presence of aluminum chloride, leading to potential capillary contact with the opening edge during wire bonding, which can result in poor joining and corrosion.
Innovation Solution
A thermal print head configuration with a substrate, insulating layer, wiring layer, resistor layer, and protective layer, where a raised portion is formed on the substrate, and the wiring layer has bonding pads with a specific width ratio and no aluminum chloride in the side surfaces, and an opening in the protective layer that is wider than the bonding pad, facilitating complete exposure and reducing corrosion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the opening in the protective layer is made just large enough to expose the bonding pad, then the protective layer provides good coverage and protection, but the capillary may contact the opening edge during wire bonding causing poor joining and corrosion
Solution Approach 1:
The patent divides the opening structure into multiple functional zones: a first opening portion with a first width and a second opening portion with a second width. This segmentation allows different regions of the opening to serve different purposes - one region provides adequate exposure for bonding while another region prevents capillary contact with edges, thereby resolving the contradiction between reliable wire bonding and avoiding corrosion.
2Ease of operation
If the bonding pad is completely exposed from the protective layer, then wire bonding can be performed effectively, but aluminum chloride may be present on the bonding pad surface causing corrosion
Solution Approach 1:
The patent applies local quality by creating a raised portion with specific dimensional characteristics (width in a first direction and length in a second direction) that is positioned at a specific location relative to the bonding pad. This localized structural modification ensures complete exposure of the bonding pad for effective wire bonding while the specific geometry controls aluminum chloride presence, preventing corrosion in the critical bonding area.
3Manufacturing precision
If a raised portion is formed on the substrate to control bonding pad exposure, then complete exposure is achieved, but the manufacturing process becomes more complex
Solution Approach 1:
The raised portion is formed on the substrate before the protective layer is applied. This preliminary action allows the bonding pad exposure to be pre-configured with precise dimensional control, ensuring that when the protective layer is subsequently formed, the bonding pad is completely exposed without requiring complex post-processing or adjustment steps.
Data Source
AI summary
A thermal print head includes: a substrate; an insulating layer; a wiring layer; a resistor layer; and a protective layer. The substrate has a first surface and a second surface opposite to the first surface. A raised portion protruding toward a side opposite to the second surface and extending along a first direction in a plan view is formed on the first surface. The insulating layer is disposed on the first surface. The wiring layer is disposed on the insulating layer with the resistor layer interposed therebetween. The wiring layer has a bonding pad. A constituent material of the wiring layer is aluminum or an aluminum alloy. The protective layer is disposed on the insulating layer to cover the wiring layer. An opening that exposes the bonding pad is formed in the protective layer.