Glaze Layers and Protective Coatings for Thermal Printheads

Thermal printheads have heating elements on top of a glaze layer, which is covered by protective coatings. Glaze layers and protective coatings are important factors that determine the performance of thermal printheads, such as printing quality and lifetime.

This article describes the glaze layers and protective coatings for thermal printheads.

Glaze Layer

The glaze layer is a heat storage layer placed between the ceramic substrate and the wiring in a thermal printhead. The role of the glaze layer is to regulate the dissipation of heat from the heating element to the highly heat-dissipating ceramic substrate by utilizing the heat storage properties of the glaze layer. The glaze layer helps to efficiently transfer to thermal paper and other media the heat generated when an electric current flows through the heating element, thereby contributing to printer energy savings and improved print quality.

The higher the heat storage capacity of the glaze layer, the more efficiently heat can be transferred to the media. On the other hand, the high heat storage capacity can slow down the thermal response, which can result in less clear printing at the beginning or end of a print run.

ROHM offers two types of glaze layers, partial glaze and full glaze, depending on the application.

Partial Glaze

Partial glaze is a glaze layer formed only under the heating elements. This type of glaze layer is designed to be thinner to reduce its heat storage capacity. Thermal printheads employing a partial glaze layer are suitable for high-speed printing because their fast thermal response prevents unclear printing at the beginning and end of printing. However, such thermal printheads require a high-voltage power supply because they cannot efficiently transfer heat from the heating elements to the media.

Main applications: High-speed printing at 150 mm/s or higher, stationary printers

Full Glaze

Full glaze is a glaze layer formed to cover the entire surface of the substrate. This type of glaze layer is designed to be thicker to increase its heat storage capacity. Thermal printheads employing a full glaze layer have a slower thermal response, but can be driven at lower voltages due to more efficient heat transfer from the heating elements to the media.

Main applications: Low-speed printing at around 50 mm/s, mobile type printers with built-in batteries

Protective Coatings

Protective coatings serve to protect the internal circuitry of thermal printheads. Through their materials and structures, protective coatings extend the life of thermal printheads and reduce maintenance frequency by improving wear resistance, environmental resistance, and resistance to temperature changes. They are also an important factor in achieving high-quality printing, for example by smoothing the surface in contact with the thermal paper.

The harder the protective coating is, the less abrasion there is during printing and the longer the circuit can be protected. In addition, the higher the thermal conductivity of the protective coating, the more efficiently the heat of heating elements is transferred to the media. When selecting a protective coating, other factors such as anti-sticking performance (to prevent sticking to the media) and antistatic performance must also be considered.

There are four types of protective coatings for ROHM thermal printheads: N-coat, B-coat, X-coat and W-coat. The characteristics and applications of each are summarized below.

N-coat

This is the standard specification and is the least expensive of the four coatings. Guaranteed lifetime and anti-stick performance are not excellent.

B-coat

This coating has antistatic specifications. Conductive materials are used to prevent static electricity buildup in the thermal printhead.

X-coat

Anti-sticking and anti-static specifications. Designed to prevent sticking to the medium during printing by using a special material.

W-coat

Long life specification using a hard material, and undergoes the least wear of the four types. Anti-static measures are also provided, but the anti-stick performance is inferior to that of the X-coat.

Summary

The performance of a thermal printhead varies depending on the type of glaze layer and protective coating used. Each type has its own advantages and disadvantages, and should be selected according to the application. In addition to the lineup introduced here, ROHM can also provide special specifications, so please feel free to contact us.