JPH0651407B2 - Thermal print head - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is incorporated into a printer mechanism such as a word processor, a facsimile or an electronic typewriter, and has a thermal print head having a control IC chip directly mounted on a head substrate. Regarding the improvement of.

(Prior Art) The principle of the thermal print head is that an electric signal read from a subject is converted into a voltage, the voltage is applied to an electric resistance material to cause Joule heat generation, and the Joule heat causes thermal recording paper or a thermal ribbon. The main idea is to print and print by inducing.

To briefly describe the structure of such a thermal print head, a heating resistor and a lead wire derived from the heating antibody are adhered and formed on a head substrate such as alumina ceramics,
An IC chip is further mounted directly on the head substrate, the IC chip and the lead wires and external input wires are electrically connected by wire bonding, and an electric signal from a subject is controlled by the IC chip to generate a heating resistor. The above-mentioned printing and printing are performed by selective Joule heat generation. The IC chip is generally covered with a protective layer made of a silicone resin in order to prevent corrosion of the electrode material such as aluminum due to moisture and water.

(Problems to be solved by the invention) By the way, in the above-mentioned thermal print head, the reason for using the silicone resin as the protective layer is that the resin temperature that can withstand the heat (about 100 ° C.) from the heating resistor and before curing is high. It is low so that it does not damage other members. However, since such a silicone resin has a small elastic modulus (soft elasticity), an extremely thin bonding wire with a wire diameter of 20 to 30 μm is easily cut by the external force transmitted through the resin layer, and the thermal print head However, it has a fatal defect that the normal protective function cannot be fulfilled, and the protective layer itself is often damaged to impair the original protective function. Therefore, conventionally, a metal cover member is further mounted on the protective layer in order to cover the mechanical weakness of the protective layer, which has been a great obstacle to downsizing or cost reduction of the head.

(Object of the Invention) The present invention has been made to solve the above-mentioned drawbacks. By selecting a hard resin containing a small amount of impurity ions as the material of the protective layer, the normal function of the print head and the original property of the protective layer are obtained. It is intended to maintain the function of and to contribute to downsizing and cost reduction.

(Means for Solving the Problems) The structure of the present invention for achieving the above object will be described with reference to the accompanying drawings. FIG. 1 is a partial cutout showing an embodiment of the thermal print head of the present invention. A plan view, FIG. 2 is an enlarged view of a II portion of FIG. 1, FIG. 3 is an enlarged vertical sectional view of a III-III line of FIG. 2, and FIG. 4 is an enlarged sectional view of a IV-IV line of FIG. is there. That is, in the thermal print head of the present invention, bonding wires are provided on the lead wires 12 ... And the external input wires 3 ... on the head substrate 1 having the heating resistors 11 and the lead wires 12 led out from the heating resistors 11. An IC chip 2 which is connected through 21 ... and selectively heats the heating resistor 11.
In a thermal print head including the above-mentioned bonding wires, the IC chip 2 including the bonding wires 21 is made of an epoxy resin containing 10 ppm or less of cations and anions and 10 ⁸ N / cm ² or more of elastic modulus. It is characterized by being covered with a protective layer 4.

Epoxy resin has excellent chemical resistance and water resistance, and has a large elastic modulus.
Although it is a hard resin, it has an elastic modulus of 10 in the present invention.
⁸ N / cm ² or more is required. When it is less than this, when the protective layer 4 is formed, the concave bonding wire is cut by the external force or the surface of the protective layer 4 is damaged. Epoxy resin also contains cations (eg Na ⁺ ) during its purification process.
And anions (e.g., Cl ^-) impurity ions easily mixed such, these impurity ions to become a cause of corrosion of the electrode material, 10 ppm content of these impurities ions in the present invention
The following (preferably 5 ppm or less) epoxy resin is used. Furthermore, there is a difference in the coefficient of thermal expansion between epoxy resin and alumina ceramics (by the way, the former is about 3 × 10 ^-5 / de
g, on the other hand, the latter is 6 to 7 × 10 ⁻⁶ / deg), and if the epoxy resin is used as it is as the protective layer 4 on the head substrate 1 made of alumina ceramics, the protective layer 4 will be scratched or peeled. Therefore, it is desirable to add a filler such as alumina (Al ₂ O ₃ ) or silica (SiO ₂ ) to the epoxy resin raw material so that the coefficient of thermal expansion thereof is close to that of alumina ceramics. However, if the addition amount of this filler is too large, the viscosity before curing becomes high and the coating work becomes difficult, so the addition amount is adjusted so that the viscosity is 80 poise to 400 poise suitable for such work. Therefore, the coefficient of thermal expansion after curing is 1 to 2 × 10 ⁻⁵ / deg.

(Operation) In the above thermal print head, the IC chip 2
Also, since the protective layer 4 coated to protect the bonding wires 21 is made of an epoxy resin that is hard and has a low content of impure ions, even if an external force is applied to the protective layer 4, the external force is the same as the bonding wire. 21 ..., etc., and troubles such as cutting are not caused, and electrode materials due to the chemical action of impure ions such as cations and anions,
That is, the lead wires 12, the bonding wires 21, the external input wires 3, etc. are not corroded. Since the epoxy resin has excellent chemical resistance and water resistance, the IC chip 2 and the bonding wire are combined with the above-mentioned hard characteristics.
The protection function of 21 ... is fully exerted. Further, the epoxy resin has a high thermal softening temperature (about 180 ° C.) and can sufficiently withstand the heat transfer temperature (about 100 ° C.) from the heating resistor 11.

(Example) Next, an example will be described.

1 to 4 show a so-called line-type thermal print head used in a printer mechanism such as a word processor or a facsimile, in which a heating resistor 11 and a large number of lead wires are provided on an elongated head substrate 1 as described above. 12
Are adhered and formed, and a large number of IC chips 2 are mounted along this. Further, an external input line 3 for transmitting an electric signal from a subject is introduced to the head substrate 1, and the external input line 3 and the lead wire 12 and the IC chip 2 are electrically connected by a bonding wire 21. It is connected. A large number of IC chips 2 in a parallel state including the bonding wires 21 are covered by the protective layer 4 made of the epoxy resin described above. Reference numeral 5 in the figure is an insulating layer made of a polyimide resin for isolating the lead wires 11, ..., The input wires 3 ,. Also, 110
Is a heating resistor material deposited on the head substrate 1. The voltage applied between the terminals of the lead wires 12 causes the heating resistor material 110 in the small gap portion to generate Joule heat, and is printed as the heating resistor 11. It functions to print.

It should be noted that the present invention is not limited to the above-described embodiment, and it is obvious that the same effect can be expected by adopting the above-mentioned configuration even in a so-called serial type thermal print head applied to an electronic typewriter or the like. Needless to say, other modifications are possible without departing from the present invention.

(Effects of the Invention) As described above, in the thermal print head of the present invention, since the IC chip including the bonding wire is covered with the protective layer of the hard epoxy resin, the bonding wire is not cut by the external pressure, In addition, the protective layer itself is not damaged, and therefore it is not necessary to intentionally provide a mechanical protective cover made of metal on the protective layer, which greatly contributes to downsizing or cost reduction. In addition, since the amount of the above-mentioned impure ions in the epoxy resin is set to 10 ppm or less, the chemical action due to such impure ions does not occur, and the corrosion caused by the above does not occur. Moreover, the epoxy resin is a resin excellent in chemical resistance and water resistance, and in combination with the above-mentioned damage, the protective function is fully exerted. Furthermore, the heat resistance of the epoxy resin is very suitable for maintaining the performance of this type of thermal print head or for preparing it. In addition, add a filler to the epoxy resin to make its thermal expansion coefficient approximate to that of the head substrate. It is also possible to prevent the generation of cracks and cracks due to the difference in thermal expansion.

The thermal print head of the present invention, which has many advantages as described above, is extremely useful.

[Brief description of drawings]

FIG. 1 is a partially cutaway plan view showing an embodiment of the thermal print head of the present invention, and FIG. 2 is an enlarged view of a II line portion in FIG.
3 is an enlarged vertical sectional view taken along line III-III in FIG. 2, and FIG. 4 is an enlarged vertical sectional view taken along line IV-IV in FIG. (Description of symbols) 1 ... Head substrate, 11 ... Heating resistor, 12 ... Lead wire, 2 ... IC chip, 21 ... Bonding wire, 3
…… External input line, 4 …… Protective layer.

Claims (1)

[Claims] 1. A head substrate having a heating resistor and a lead wire derived from the heating resistor is connected to the lead wire and an external input line via a bonding wire to selectively heat the heating resistor. In a thermal print head having an IC chip mounted thereon, a protective layer of an epoxy resin, wherein the amount of cations and anions contained in the IC chip including the bonding wire is 10 ppm or less and the elastic modulus is 10 ⁸ N / cm ² or more. A thermal print head characterized by being covered with.