JPS6235286B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for sealing a surface acoustic wave device in which a surface acoustic wave propagates on a substrate surface.

Surface wave devices can easily obtain complex frequency selection characteristics by devising the shapes of the transmitting and receiving electrodes that transmit and receive surface waves. It has now become possible to realize the video intermediate frequency filter of a television receiver, which was previously constructed with a single surface wave device, and it is being put into practical use.

Figure 1 shows the basic surface wave device, 1 is
A substrate made of a piezoelectric material such as LiTaO ₃ single crystal, PZT ceramic, ZnO thin film, crystal, etc., 2 and 3 are the substrate 1
Transmitting/receiving electrodes 4, 4 consisting of interdigital transducers for transmitting/receiving surface waves, which are provided so as to face the surfaces of It is an absorbent material made of alkyd resin, etc.

In a surface wave device having such a structure, when an electric signal is applied to the transmitting electrode 2, the transmitting electrode 2 excites a surface wave and transmits it to the receiving electrode 3 and the end face of the substrate 1 in the opposite direction to the electrode 3. Let it spread. Most of the surface waves that have reached the receiving electrode 3 are restored to electrical signals at the electrode 3. However, a small portion of the surface waves passes through the receiving electrode 3 and reaches the end surface of the substrate 1. This surface wave is reflected by the end face of the substrate 1 and propagates to the receiving electrode 3 again. This slight surface wave that propagated to the receiving electrode 3 is also converted into an electrical signal, but it has a certain delay time compared to the electrical signal that was converted first, so the electrical characteristics of the surface wave device deteriorate. invite On the other hand, the surface waves propagated directly from the transmitting electrode 2 to the end surface of the substrate 1 are similarly reflected at the end surface of the substrate 1 and reach the receiving electrode 3 via the transmitting electrode 2. In order to prevent reflection of surface waves at the end surface of these substrates 1, transmitting/receiving electrodes 2,
The absorbers 4, 4 are provided on the end face of the outer substrate 1 of No. 3, and absorb the surface waves before reflecting them.

When sealing this surface wave device, surface wave transmission and
Since the reception and its propagation are performed on the surface of the substrate 1 as described above, it is necessary to create a gap near the surface of the substrate 1 so as not to affect the surface waves. Conventional sealing methods include a method of incorporating into a TO package used for transistors, ICs, etc., and a method of molding with resin.

Among the above sealing methods, the TO package itself is expensive, and if a hermetic seal is applied, another cost-effective sealing method is the molding method. considerably more expensive than.

An example of a surface acoustic wave device sealed by this inexpensive molding method is shown in FIG. is a protective cap that covers the surface wave device 6 and forms a gap on the surface of the base plate 1;
Reference numeral 8 denotes a molded body made of epoxy resin or the like for molding the outer wall of the mount plate 5 and the protective cap 7.

However, in this mold-based sealing method, it is necessary to provide the protective cap 7 that forms a gap on the surface of the substrate 1, and the molding must be carried out while the protective cap 7 is held on the mount plate 5 by some means. It had the following drawbacks in terms of manufacturing process.

The present invention seals a surface acoustic wave device using a molding method in view of this point, and will be described in detail below with reference to FIG. 3 and subsequent figures.

The surface acoustic wave device used in the present invention is slightly different from that shown in FIG.
, and no absorbers 4, 4 are provided. The reason will become clear from the explanation below.

First, the transmitting/receiving electrodes 2 and 3 as described above are placed on the substrate 1.
The surface acoustic wave device 6, which is disposed opposite to the surface, is die-bonded to the mount plate 5. Next, a volatile low-melting substance 9 such as paraffin, which melts and evaporates when heated, is applied to the transmitting/receiving electrodes 2, 3 and the surface wave propagation path between the two electrodes 2, 3 (see Fig. 3). a,
b). As is clear from the figure, this application area is
The end surface of the outer substrate 1 of the transmitting/receiving electrodes 2 and 3, that is, the first
In the surface wave device shown in the figure, at least the portion where the absorbers 4, 4 were provided is not included. After that, the whole is molded into a molded body 8 which is porous and allows the volatile gas of the volatile low-melting substance 9 to be dissipated.
For example, it is molded using a phenol resin by coating or dipping (FIG. 4). The mold body 8
As mentioned above, among the end faces of the surface of the substrate 1, at least the transmitting/receiving electrodes 2 and 3 on which the absorbers 4 and 4 were provided
It is molded so as to cover the end surface of the outer substrate 1. Next, the molded surface wave device is heated in a vacuum to melt and volatilize the volatile low melting point substance 9,
Volatile gas is diffused to the outside through the porous mold body 8 to form voids on the transmitting/receiving electrodes 2 and 3 and on the surface wave propagation path, completing mold sealing of the surface acoustic wave device (FIG. 5).

When the volatile low melting point substance 9 is heated and volatilized, a portion thereof may remain in the molded body 8, but in the surface wave device, no influence due to the above-mentioned retention is observed in terms of its electrical characteristics. If necessary, after the sealing shown in FIG. 5 is completed, the entire surface acoustic wave device may be further coated with a water-repellent resin or the like.

Thus, the surface wave propagated from the transmitting electrode 2 is restored to an electrical signal at the receiving electrode 3, and a small amount of the surface wave transmitted through the receiving electrode 3 propagates to the end surface of the substrate 1. Conventionally, the surface waves propagated to the end surface of the substrate 1 are absorbed by the absorber 4, but in the present invention, the absorber 4 is not present, but the surface waves propagated to the end surface of the substrate 1 are absorbed by the absorber 4. The surface waves are absorbed by the molded body 8 covering the surface.
Further, the surface waves directly propagated to the end surface of the substrate 1 on the opposite side from the receiving electrode 3 are also absorbed by the molded body 8.

As another embodiment, a low melting point substance such as wax is used instead of the volatile low melting point substance 9, and when heated in vacuum to a temperature higher than its melting point, the low melting point substance melts and the molten liquid forms a porous mold. It penetrates into the body 8. If the heating is continued further, a part of the molten liquid will volatilize and be radiated outside the mold body 8. Thereafter, when the heating is stopped, the molten liquid of the low melting point substance stays in the mold body 8 and solidifies. Therefore, the transmitting/receiving electrodes 2, 3 and both electrodes 2, coated with a low melting point substance,
A gap is formed on the propagation path of the surface wave in step 3, and the sealing of the surface wave device is completed.

As mentioned above, volatile low melting point substances such as paraffin 9
Alternatively, heating a low melting point substance such as wax in a vacuum can increase the volatilization rate or impregnation rate of both low melting point substances and improve work efficiency. If such a material is selected, volatilization or melting of both of the low melting point substances mentioned above can be carried out at the same time as the mold body 8 is cured, which is extremely effective.

In the above explanation, the low melting point substance 9 was applied on the transmitting/receiving electrodes 2, 3 and the substrate between the two electrodes 2, 3. If weighting is applied according to
It is also possible to apply the coating material to substantially the entire electrode except for the portion where the electrode fingers do not intersect and do not substantially contribute to the transmission of surface waves, which is indicated by diagonal lines in the figure.

As is clear from the above explanation, the sealing method of the present invention is as follows:
Applying a volatile low-melting point substance or a low-melting point substance to the surface of the substrate involved in the transmission, reception, and propagation of surface waves,
After molding with a mold body that diffuses or impregnates these volatile gases or molten liquids, it is heated above the respective melting points to create voids in the areas where the above-mentioned low melting point substances were applied, eliminating the need for a protective cap that was previously required. Also, the surface acoustic wave device can be sealed without using the cap holding means. Moreover, since the molded body is structured to cover the outer substrate end face of the transmitting/receiving electrode, this molded body also acts as an absorber to absorb the surface waves reflected from the substrate end face, and a dedicated absorber is used. There is no need to set up

Furthermore, since a porous thermosetting resin is used in the molded body, its solidification can be performed at the same time as the volatilization and melting of the low melting point substance,
Moreover, the low melting point substance can be diffused and impregnated into the molded body reliably in a short period of time.
Therefore, if the surface acoustic wave device is sealed according to the present invention, the number of parts and work steps can be reduced, making it possible to significantly reduce costs.

[Brief explanation of the drawing]

Figure 1 is a front view of a conventional surface wave device;
The figure is a sectional view for explaining the conventional molding method, FIGS. 3a and 3b are sectional views and front views showing one step of the sealing method of the present invention, and FIG. FIG. 5 is a cross-sectional view showing a surface acoustic wave device sealed according to the present invention, in which 1 is a substrate, 2 and 3 are transmitting and receiving electrodes, and 6
8 represents a surface wave device, 8 represents a molded body, and 9 represents a volatile low melting point substance.

Claims (1)

[Claims] 1. When molding a substrate on which a transmitting/receiving electrode for transmitting/receiving surface waves is provided, the substrate is substantially over the transmitting/receiving electrode and the transmitting/receiving electrode except for the edge of the substrate where the surface wave reaches. First, a volatile low-melting point substance is applied on the surface wave propagation path between the receiving electrodes, and then the entire surface including the low-melting point substance and the edge of the substrate is molded with a mold body made of porous thermosetting resin. 1. A method for sealing a surface acoustic wave device, comprising curing a molded body by heating in a vacuum and dissipating volatile gas of the low melting point substance to the outside through the molded body. 2. When molding a substrate on which a transmitting/receiving electrode for transmitting and receiving surface waves is provided, the surface waves are substantially formed on the transmitting/receiving electrode and between the transmitting/receiving electrodes, except for the edge of the substrate where the surface wave reaches. First, a low melting point substance is applied onto the propagation path, and then the entire area including the low melting point substance and the edge of the substrate is molded with a mold body made of porous thermosetting resin, and then the mold is molded by heating in vacuum. A method for sealing a surface acoustic wave device, characterized in that the mold body is hardened and the low melting point substance is melted and impregnated into the mold body.