Thermal Printhead Structures and Manufacturing Methods

Thermal printheads (TPHs) can be broadly classified into thick- or thin-film types based on the structure of the heating elements and the method of manufacture. The thin-film devices include products that employ ROHM’s proprietary “step-free structure” and “STPH structure.”
The most significant difference between thick- and thin-film TPHs is the thickness of the heating element, although the former are also produced by screen printing whereas the latter are made by sputtering. Additionally, TPHs can either comprise solely a ceramic substrate or a combination of a printed circuit board (PCB) and a ceramic substrate. This article discusses the differences between thick- and thin-film TPHs and also examines the varying configurations of products consisting solely of a ceramic substrate or incorporating a PCB. Thick-film TPH and thin-film TPH have their own advantages and disadvantages.

TPH Structures

The structures of thick-film TPH and thin-film TPH are explained, with their differences and features.

Heating-element structures

The structure of the heating element differs greatly between thick- and thin-film TPHs. In the case of the former, this element has a thickness of several microns while the heating element in the latter will typically have a sub-micron thickness. In addition, thick-film TPHs are produced by screen printing while thin-film TPHs are produced by sputtering.

TPH product configurations

There are two basic configurations for TPH products. The first comprises a single ceramic substrate (meaning a single-unit structure) and the second comprises a combination of a ceramic substrate and a PCB (representing a dual-unit structure).

A TPH incorporates driver integrated circuits (ICs) to control the heating elements. These ICs receive input signals that control the TPH and generate output signals that go to the heating element. Either wiring or flip-chip bonding is used to mount these ICs. A thermistor is also mounted on the device to monitor temperature increases resulting from the operation of the heating elements during use, which can potentially lead to malfunction and physical damage.

Some models also incorporate mounted connectors and a heatsink. The connectors provide an interface for signal input to the TPH and can be either flat-cable or wire-cable compatible. The heatsink allows the temperature of the unit to be more readily controlled.

Manufacturing Methods

This section describes the methods used to manufacture thick-film TPHs by screen printing and thin-film TPHs by sputtering. Sputtering is a type of deposition method that forms a film on a wafer surface.

Thick-film TPH substrate manufacturing

• 1. A glaze layer is printed on a ceramic substrate. After printing, the substrate is subjected to drying and firing processes.
• 2. Wiring is not formed solely by printing but also using photolithography and etching after printing of the wiring materials.
• 3. Heating elements, common electrodes and a protective film are fabricated by printing.
• 4. A hard protective film is deposited by sputtering.
• 5. The heating element resistance is adjusted to meet the product specifications.

Thin-film TPH substrate manufacturing

• 1. A glaze layer is printed on a ceramic substrate. After printing, the structure is subjected to drying and firing processes.
• 2. Heating elements and wiring materials are sputtered and then patterned using photolithography and etching.
• 3. After masking, a protective film is deposited by sputtering.
• 4. The heating element resistance is adjusted to meet the product specifications.

Mounting process

During fabrication, multiple driver ICs are mounted on a single ceramic substrate or on a dual substrate structure (also called a separate structure). The input/output pads of the ICs are connected to the substrate circuitry by wire bonding. In the case of flip-chip ICs, mounting and bonding are performed simultaneously. Wires linked to individual heating elements are connected one-to-one with the output pads of the driver ICs. In contrast, input-signal wires are connected serially or in parallel to multiple driver ICs. The driver ICs are subsequently coated with a protective resin, after which a connector is mounted and a heatsink is attached.

Structural Characteristics

There are advantages and disadvantages associated with thick- and thin-film TPHs and with the different product configurations.

Thick-film TPHs vs. thin-film TPHs

Product configurations