CERAMIC DEVICE AND JOINED BODY

Provided is a piezoelectric device which has high reliability for the joint between an external electrode and low-melting-point solder. This piezoelectric device is a fired body including a main body part and an external electrode. The external electrode includes a surface electrode that covers upper and lower surfaces of the main body part, and a side-surface electrode that covers a side surface of the main body part and makes a connection to the surface electrode. This piezoelectric device is obtained by co-firing all of constituent members. The surface electrode is configured to include platinum (Pt) or palladium (Pd). The side-surface electrode is also configured to include platinum (Pt) or palladium (Pd). In addition, the side-surface electrode contains gold (Au). The content ratio of gold in the side-surface electrode is preferably 3 to 20 weight %.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a ceramic device, and more particularly, a ceramic device that functions as a piezoelectric device. The piezoelectric device is also referred to as a piezoelectric/electrostrictive device.

Description of the Background Art

As an example of this type of piezoelectric device, WO 2012/132661 discloses a piezoelectric device800that is a fired body including a main body part810and an external electrode811as shown inFIG. 11. In the piezoelectric device800shown inFIG. 11, the main body part810is a laminated body that has piezoelectric layers820and internal electrodes821laminated alternately. The internal electrodes821include internal electrodes821A,821B. The external electrode811includes a pair of surface electrodes840A,840B partially covering an upper surface860of the main body part810, and a pair of side-surface electrodes850A,850B at least partially covering corresponding side surfaces870A,870B of the main body part810, and making connections to the corresponding internal electrodes821A821B and the surface electrodes840A,840B. Typically, the external electrode811(surface electrodes840A,840B, and side-surface electrodes850A,850B) and the internal electrodes821are configured to include platinum (Pt) or palladium (Pd) (hereinafter, referred to as “platinum or the like”). This is based on the fact that platinum or the like has the property of “being high in melting point and less likely to be oxidized (thus, able to be fired stably in an oxygen atmosphere without being oxidized).”

This type of piezoelectric device has been actively developed as an element for controlling the position of an optical lens (for example, an ultrasonic motor for an auto focus or zoom for a camera), a position control element for an element for reading and/or writing magnetic information or the like (for example, an actuator for a magnetic head in a hard disc drive), or a sensor that converts mechanical vibrations to electrical signals.

SUMMARY OF THE INVENTION

Now, the piezoelectric device800shown inFIG. 11may be, for example, mounted on substrates910A,910B with the use of solder parts920A,920B, as shown inFIG. 12. In the example shown inFIG. 12, while both ends880A,880B of a lower surface861of the piezoelectric device800are placed respectively on upper surfaces950A,950B of opposed ends940A,940B of a pair of substrates910A,910B located away from each other, the pair of side-surface electrodes850A,850B of the piezoelectric device800and the ends940A,940B of the pair of substrates910A,910B are joined and fixed with the use of the solder parts920A,920B. Joining and fixing are performed to the copper terminals930A,930B.

The melting point of solder vary greatly depending on the composition (substances constituting the solder) of the solder, and there is solder with a relatively high melting point (for example, 200 to 250° C., typically composed of tin (Sn), copper (Cu), silver (Ag), and so on), as well as solder with a relatively low melting point (for example, lower than 200° C., typically composed of tin (Sn), bismuth (Bi), silver (Ag), and so on). Hereinafter, the “solder with a melting point lower than 200° C.” is particularly referred to as “low-melting-point solder.”

As mentioned above, when the side-surface electrodes of the piezoelectric device and the substrates are joined and fixed with the use of the solder, with the use of low-melting-point solder, the joining step mentioned previously can be carried out through the use of the (melted) solder at a relatively low temperature. Therefore, the amount of heat transferred from the solder to the substrates is relatively small in this step. Accordingly, the following advantages are provided.

First, low heat-resistance materials can be used for the substrates and the parts provided on the substrates. As a result, a wider choice of materials is provided. Secondly, thermal stress can be reduced which is generated in the substrates and the parts provided on the substrates in the previously mentioned step, and the possibility of crack generation or the like in the substrates and the parts can be reduced. As a result, the electrical disconnection on the substrate due to the cracks or the like are less likely to occur. Thirdly, when an adhesive containing an epoxy resin is used for the substrates, the treatment of curing the adhesive containing the epoxy resin can also be carried out simultaneously in the step. As a result, the number of steps required for the whole work can be reduced. Fourth, when the side-surface electrodes of the piezoelectric device with the piezoelectric layers (piezoelectric material) polarized are joined with the substrates, the piezoelectric layers (piezoelectric material) are less likely to be depolarized in the step. As a result, the number of steps required for the whole work can be reduced.

On the other hand, the use of the low-melting-point solder also causes the following problem. More specifically, in general, low-melting-point solder has relatively low wettability to platinum or the like. Thus, when the side-surface electrodes of the piezoelectric device are composed of only platinum or the like, the (melted) low-melting-point solder is less likely to spread on the surfaces of the side-surface electrodes. Therefore, the joint area between the side-surface electrodes and the low-melting-point solder is reduced, thereby probably resulting in decreased reliability for the joint between the side-surface electrodes and the low-melting-point solder. Improving the reliability for the joint between the side-surface electrodes (the external electrode) and the low-melting-point solder is now desired.

An object of the present invention is to provide a ceramic device (piezoelectric device) which has high reliability for the joint between the external electrode and low-melting-point solder.

In order to achieve the object mentioned above, a feature of a ceramic device (piezoelectric device) according to the present invention is that the external electrode is configured to include (as a main material) platinum or the like (platinum (Pt) or palladium (Pd)), and the external electrode (typically, the side-surface electrodes) contains gold (Au). In this regard, the external electrode (the surface electrodes+the side-surface electrodes) and the internal electrodes can be configured from the same material (that is, platinum or the like+gold). The electrodes including gold may be only the side-surface electrodes, or only the surface electrodes. The electrodes to be jointed with low-melting-point solder are configured to include gold in addition to platinum or the like. In addition, the main body part and the external electrode can be co-fired.

It has been determined that the configuration mentioned above improves reliability for the joint between the external electrode and the low-melting-point solder, as compared with when the external electrode (typically, the side-surface electrodes) of the ceramic device (piezoelectric device) is composed of only platinum or the like (details will be described later). This is assumed to be based on the following reason.

More specifically, as can be easily understood from a phase diagram (not shown) of gold (Au) and tin (Sn), it is known that in the case of gold (Au) in contact with tin (Sn) in the low-melting-point solder, gold and tin form an alloy phase even at a relatively low temperature on the order of 200° C.

Therefore, in the case of the external electrode composed of platinum or the like with gold therein as in the configuration mentioned above, when the melted low-melting-point solder at approximately 200° C. comes into contact with the surface of the external electrode, the gold in the external electrode can be dissolved in the melted low-melting-point solder. Due to the solution of gold, the platinum or the like present around the dissolved gold is also more likely to be dissolved in the melted low-melting-point solder. As a result, a compound layer including at least tin and platinum or the like can be formed at the joint part between the external electrode and the low-melting-point solder. The formation of this compound layer is assumed to improve the reliability for the joint between the external electrode and the low-melting-point solder. In other words, the gold in the external electrode is assumed to function as “an aid that dissolves platinum or the like in the external electrode to form a compound layer,” thereby improving the reliability for the joint between the external electrode and the low-melting-point solder.

The configuration mentioned above improves reliability for the joint between the external electrode and the low-melting-point solder, as compared with when the external electrode of the ceramic device (piezoelectric device) is composed of only platinum or the like.

In the piezoelectric device mentioned above, the content ratio of gold in the external electrode is suitably 3 to 20 weight %. It has been determined that this content ratio further improves reliability for the joint between the external electrode and the low-melting-point solder, as compared with when the ratio fails to fall within the range (details will be described later).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a piezoelectric device according to the present invention will be described below with reference to the drawings.

As shown inFIG. 1andFIG. 2that is a cross-sectional view taken along the line2-2ofFIG. 1, a piezoelectric device100according to the present preferred embodiment is a fired body, which includes a main body part110of a rectangular solid-like shape, and an external electrode111provided on the main body part110to at least partially cover the surface of the main body part110.

The main body part110is a laminated body which includes multiple (6 in this example) piezoelectric layers130of a piezoelectric material, and multiple (5 in this example) layered internal electrodes131, and where the piezoelectric layers130are located as an uppermost layer and a lowermost layer, and the piezoelectric layers130and the internal electrodes131are laminated alternately. The piezoelectric layers130and the internal electrodes131are laminated parallel to each other. The size of the main body part110(fired) is, for example, 0.2 to 10.0 mm in width (x-axis direction), 0.1 to 10.0 mm in depth (y-axis direction), and 0.01 to 10.0 mm in height (z-axis direction). The thickness (z-axis direction) of each of the piezoelectric layers130(fired) is 1.0 to 100.0 μm, and the thickness (z-axis direction) of each of the internal electrodes131(fired) is 0.3 to 5.0 μm.

As shown inFIG. 2, the external electrode111includes a surface electrode140partially covering upper and lower surfaces160,161of the main body part110, and a side-surface electrode141partially covering side surfaces170A,170B of the main body110. The side-surface electrode141is electrically connected to the internal electrodes131and the surface electrode140. More specifically, (three) internal electrodes131A, a surface electrode140A, and a side-surface electrode141A (hereinafter, referred to collectively as a “first electrode group150A”) are electrically connected to each other, whereas (two) internal electrodes131B, a surface electrode140B, and a side-surface electrode141B (hereinafter, referred to collectively as a “second electrode group150B”) are electrically connected to each other.

The first and second electrode groups150A,150B are connected with the piezoelectric layers130interposed therebetween as insulators, and thus electrically insulated from each other. In other words, the (three) internal electrodes131A electrically connected to each other and (two) internal electrodes131B electrically connected to each other constitute a comb-like electrode. The surface electrode140(fired) is 0.5 to 10.0 μm in thickness, whereas the side-surface electrode141(fired) is 0.5 to 10.0 μm in thickness. It is to be noted that the layer number of internal electrodes is 5 in this example, but not particularly limited (which may be zero).

In this piezoelectric device100, the deformation amount of the piezoelectric layers130(thus, the main body part110) can be controlled by adjusting the potential difference applied between the first and second electrode groups150A,150B. Through the use of this principle, the piezoelectric device100can be used as an actuator that controls the position of an object. Examples of the object include optical lenses, magnetic heads, and optical heads. In addition, in this piezoelectric device100, the potential difference produced between the first and second electrode groups150A,150B varies depending on the deformation amount of the piezoelectric layers130(thus, the main body part110). Through the use of this principle, the piezoelectric device100can be used as various types of sensors such as an ultrasonic sensor, an acceleration sensor, an angular velocity sensor, a shock sensor, and a mass sensor.

It is suitable to adopt, as a material of the piezoelectric layers130(piezoelectric material), piezoelectric ceramic, electrostrictive ceramic, ferroelectric ceramic, or antiferroelectric ceramic. Specific materials include ceramics containing, as a single material or a mixture, lead zirconate, lead titanate, lead magnesium niobate, lead nickel niobate, lead zinc niobate, lead manganese niobate, lead antimony stannate, lead manganese tungstate, lead cobalt niobate, barium titanate, sodium bismuth titanate, potassium sodium niobate, and strontium bismuth tantalate.

The materials of the external electrode111(the surface electrode140and the side-surface electrode141) and the internal electrodes131(electrode materials) are preferably composed of metals that are solid at room temperature and excellent in electrical conductivity. Specifically, the external electrode111(the surface electrode140and the side-surface electrode141) and the internal electrodes131are composed of platinum (Pt) or palladium (Pd), or an alloy thereof. This is based on the fact that platinum or palladium has the property of “being high in melting point and less likely to be oxidized (thus, able to be fired stably in an oxygen atmosphere without being oxidized).” It is to be noted that the surface electrode140and the internal electrodes131may be composed of a single element metal such as aluminum, titanium, chromium, iron, cobalt, nickel, copper, zinc, niobium, molybdenum, ruthenium, rhodium, silver, tin, tantalum, tungsten, iridium, and lead, or an alloy thereof. However, from the perspective of co-firing with the main body part110, the surface electrode140and the internal electrodes131are preferably composed of platinum (Pt) or palladium (Pd), or an alloy thereof, as with the side-surface electrode141.

In addition, in particular, the side-surface electrode141contains gold (Au). The content ratio of gold in the side-surface electrode141will be described later. The surface electrode140and the internal electrodes131may also contain gold.

Next, a method for manufacturing the piezoelectric device mentioned above will be described briefly. Hereinafter, the state of “unfired” is indicated by the addition of the term “green” to the name of a corresponding member, or the addition of the symbol “g” to the end of a sign for a corresponding member.

In this example, first, as shown inFIG. 3, on a flat-plate base material300, a large sheet of green laminated body301is formed which includes multiple (3×7) parts corresponding to piezoelectric devices (hereinafter, referred to as “green piezoelectric device corresponding parts”)100garrayed in a matrix form at predetermined intervals. This large green laminated body301includes green laminated body parts110gcorresponding to the main body part110, and green electrode films140g,140gcorresponding to the surface electrode140, which are formed on upper and lower surfaces160g,161gof the parts110g.

Each of the green laminated body parts110gcorresponding to the main body part110is formed by alternately laminating green piezoelectric sheets130gcorresponding to the piezoelectric layers130and green electrode films131gcorresponding to the internal electrodes131. The green piezoelectric sheets130gare formed by shaping a paste including the piezoelectric material with the use of one of well-known approaches such as a doctor blade method. The green electrode films131gare formed onto the green piezoelectric sheets130gby shaping a paste including a material of the internal electrodes131with the use of one of well-known approaches such as screen printing, spray coating, and ink-jet printing. In order to further ensure the adhesion between the green piezoelectric sheets130gand the green electrode films131g, green adhesive layers may be interposed between the green piezoelectric sheets130gand the green electrode films131g.In this case, the green adhesive layers are formed onto the green piezoelectric sheets130gwith the use of one of well-known approaches such as coating.

The green electrode films140gare also formed respectively onto the upper and lower surfaces160g,161gof the green laminated body part110gby shaping a paste including a material of the surface electrode140with the use of one of well-known approaches such as screen printing, spray coating, and ink-jet printing. Specifically, for example, this paste includes a powder of an electrode material such as platinum or palladium, a binder, a dispersion medium, and a solvent. Ethyl cellulose, polyvinyl alcohol, an acrylic resin, or the like can be used as the binder, and terpineol, texanol, isopropyl alcohol, or the like can be used as the solvent. This paste may contain therein a gold powder. The powder of the electrode material (and the gold powder) are, for example, 0.1 to 2.0 μm in particle size.

Then, machining such as cutting and punching is applied along cutting-plane lines (see dashed-two dotted lines)310as shown inFIG. 3. As a result, as shown inFIG. 4, on the base material300, the multiple (3×7) green piezoelectric device corresponding parts100gcan be extracted in the same step. Hereinafter, for the convenience of explanation, attention will be focused on only one of the multiple green piezoelectric device corresponding parts100gextracted, and the explanation will be continued.

FIG. 5shows a cross section of one green piezoelectric device corresponding part100gextracted, which corresponds toFIG. 2. As shown inFIG. 5, in this example, the green piezoelectric device corresponding part100gis composed of: the green laminated body110gcorresponding to the main body part110; and the green electrode films140g,140gcorresponding to the surface electrode140, which are formed on the upper and lower surfaces160g,161gof the green laminated body110g.The green laminated body110gis a laminated body with the piezoelectric sheets130glocated as uppermost and lowermost layers, and the piezoelectric sheets130gand electrode films131g laminated alternately.

Next, as shown inFIG. 6, green electrode films141gcorresponding to the side-surface electrode141are formed on predetermined sites of side surfaces170Ag,170Bg of the green piezoelectric device corresponding part100g.The green electrode films141gare also formed by shaping a paste including a material of the side-surface electrode141with the use of one of well-known approaches such as screen printing, spray coating, and ink-jet printing. Specifically, for example, this paste includes a powder of an electrode material such as platinum or palladium, a binder, a dispersion medium, and a solvent. Ethyl cellulose, polyvinyl alcohol, an acrylic resin, or the like can be used as the binder, and terpineol, texanol, isopropyl alcohol, or the like can be used as the solvent. This paste contains therein a gold powder. The powder of the electrode material and the gold powder are, for example, 0.1 to 2.0 μm in particle size.

Then, the green piezoelectric device corresponding part100gshown inFIG. 6is subjected to firing at a predetermined temperature (for example, 900 to 1200° C.) for a predetermined period of time (for example, the maximum temperature holding time is 0.5 to 3 hours). In other words, the green laminated body110g(the green piezoelectric sheets130corresponding to the piezoelectric layers130and the green electrode films131gcorresponding to the internal electrodes131) corresponding to the main body part110, as well as the green electrode films140gcorresponding to the surface electrode140and the green electrode films141gcorresponding to the side-surface electrode141is subjected to co-firing. As a result, the piezoelectric device100(fired) shown inFIGS. 1 and 2is obtained.

It is to be noted that the large green laminated body301with the electrode films140gformed is subjected to machining in the example described above. As a result, as shown inFIG. 5, at the stage of having extracted each green piezoelectric device corresponding part100gby the machining, the corresponding part100ghas the electrode films140galready formed. In contrast, the large green laminated body301without any electrode film140gformed may be subjected to machining. In this case, after the extraction of each green piezoelectric device corresponding part100gby the machining, for each green piezoelectric device corresponding part100g,the electrode films140gcan be formed, and thereafter, the electrode films141gcan be formed. Alternatively, for each green piezoelectric device corresponding part100g,the electrode films141gmay be formed, and thereafter, the electrode films140gmay be formed.

The piezoelectric device100described above according to the present preferred embodiment is mounted on substrates410A,410B with the use of solder parts420A,420B, for example, as shown inFIGS. 7 and 8. In this example, first, both ends180A,180B of the lower surface161of the piezoelectric device100are placed respectively on upper surfaces450A,450B of opposed ends440A,440B of the pair of substrates410A,410B located away from each other (more specifically, upper surfaces of copper ends430A,430B provided on the upper surfaces450A,450B of the ends440A,440B). Next, the pair of side-surface electrodes141A,141B of the piezoelectric device100and the ends440A,440B of the pair of substrates410A,410B are joined and fixed with the use of the solder parts420A,420B. Thus, the mounting of the piezoelectric device100onto the pair of substrates410A,410B is completed.

In the piezoelectric device100mounted onto the pair of substrates410A410B, the deformation amount of the piezoelectric layers130(thus, the main body part110) is changed (see the arrows shown inFIG. 8) by varying the potential difference applied between the first and second electrode groups150A150B. As a result, the distance (interval) is changed between the pair of substrates. Through the use of this principle, the piezoelectric device can be used as an actuator that controls the position of an object such as an optical lens. Alternatively, in the piezoelectric device100thus mounted onto the pair of substrates410A,410B, the deformation amount of the piezoelectric layers130(thus, the main body part110) is changed (see the arrows shown inFIG. 8) by varying the magnitude of a force applied to the pair of substrates410A,410B in the direction of changing the distance (interval) between the substrates410A,410B, and the potential difference produced between the first and second electrode groups150A,150B varies depending on the deformation amount. Through the use of this principle, the piezoelectric device100can also be used as various types of sensors such as a mass sensor.

In the mounting of the piezoelectric device as described above, it is suitable to use, as the solder, the above-described “low-melting-point solder” (solder with a melting point of 200° C. or lower). The low-melting-point solder typically includes tin (Sn), bismuth (Bi), and silver (Ag). The use of the low-melting-point solder provides various advantages as described in the section “SUMMARY OF THE INVENTION,” due to the fact that the step of joining can be carried out with the use of (melted) solder at a relatively low temperature.

However, in general, the “low-melting-point solder” has relatively low wettability to platinum or palladium, and thus, when the side-surface electrode of the piezoelectric device is composed of only platinum or palladium, the (melted) “low-melting-point solder” is less likely to spread on the surface of the side-surface electrode. Therefore, the joint area between the side-surface electrode and the “low-melting-point solder” is reduced, thereby probably resulting in a problem of decreased reliability for the joint between the side-surface electrode and the “low-melting-point solder.”

For this problem, the inventors have made various experiments and researches, in order to improve the reliability for the joint between the side-surface electrode and the “low-melting-point solder.” As a result, the inventors have found that when the side-surface electrode is composed of platinum or palladium, the side-surface electrode containing gold therein improves the reliability for the joint between the side-surface electrode and the “low-melting-point solder,” as compared with when the side-surface electrode is composed of only platinum or palladium, and the reliability for the joint is strongly correlated with the content ratio of gold in the side-surface electrode. A test for confirming the foregoing will be described below.

In this test, a device having a form shown inFIG. 9(before reflow) andFIG. 10(after reflow) was used as a sample. In this sample, a piezoelectric plate500and an electrode plate501respectively correspond to the main body part110(piezoelectric layers130) and the side-surface electrode141of the piezoelectric device100shown inFIG. 1. The piezoelectric plate500has the thin rectangular solid-like shape with 10 mm×10 mm square and 0.3 mm thick. The electrode plate501has the thin rectangular solid-like shape with 5 mm×5 mm square and 5 μm thick. Low-melting-point solder502(before reflow) shown inFIG. 9is a solder paste formed in a region 1 mm×1 mm square and 0.1 mm thick.

This sample was prepared in the following way. First, a green sheet was formed with the use of a piezoelectric paste including a powder of a piezoelectric material. Specifically, for example, a piezoelectric paste obtained by adding a solvent, a binder, and a plasticizer to a powder of a piezoelectric material was mixed with the use of a ball mill. A mixed solvent of xylene and butanol was used as the solvent, PVB was used as the binder, and DOP was used as the plasticizer. Next, the mixed piezoelectric paste was applied onto PET films by a doctor blade method, thereby forming piezoelectric green sheets.

Next, an electrode paste including a powder of an electrode material was shaped on upper surfaces of the piezoelectric green sheets with the use of screen printing or the like, thereby forming (laminating) a compact for the electrode plate. A platinum powder with a gold powder mixed therein was used as the powder of the electrode material. The platinum powder was 0.3 to 0.7 μm in particle size (before firing), and the gold powder was 0.3 to 0.7 μm in particle size (before firing). Ethyl cellulose was used as a binder for the electrode paste, and texanol was used as a solvent for the electrode paste.

Then, the unfired laminated body was subjected to co-firing. The firing temperature was 1100° C., and the firing time was 2 hours. Then, the low-melting-point solder502before reflow was placed on the fired electrode plate501. Thus, the sample shown inFIG. 9was obtained. PF142-LT7 (NIHON HANDA Co., Ltd.: Sn-57Bi-1Ag) was used as the low-melting-point solder.

This sample shown inFIG. 9was subjected to reflow (heating) for the low-melting-point solder. Specifically, first, preliminary heating was carried out over 80 seconds at 120 to 130° C. Thereafter, the temperature was continuously increased so that the heating time at 140° C. or higher was 80 seconds or more in total. The maximum temperature during the heating was 195° C. As a result, as shown inFIG. 10, the low-melting-point solder503was melted and deformed, and the low-melting-point solder503was joined onto an upper surface of the electrode plate501(soldering completed).

In this test, the thus soldered sample was evaluated for “Wettability of Low-melting-point Solder,” “Condition of Co-fired Electrode Plate,” and “Peeling of Low-melting-point Solder.” The “Wettability of Low-melting-point Solder” was evaluated by measuring the magnitude of the contact angle of the low-melting-point solder to the upper surface of the electrode plate (the wettability is better as the contact angle is smaller). The “Condition of Co-fired Electrode Plate” was evaluated by determining whether or not the electrode plate underwent agglomeration by firing (the condition of the electrode plate is better without any agglomeration). The “Peeling of Low-melting-point Solder” was evaluated in a simplified way with the use of a tape test method. Specifically, an adhesive tape (Scotch tape (from 3M)) was attached to the upper surface of the low-melting-point solder by pushing with a finger for 10 seconds, and thereafter, the tape was peeled instantaneously in a direction perpendicular to the attachment surface. Thus, the low-melting-point solder was peeled from the electrode plate. Then, the peeled interface of the low-melting-point solder peeled was observed.

In this test, prepared were multiple samples with varying content ratios (Au content ratio, weight %) of gold (Au) in the electrode plates. Specifically, as shown in Table 1, eleven types of levels were prepared. Ten samples (N=10) were prepared for each level.

The “Au Content Ratio (weight %)” in Table 1 refers to the proportion (%) of “the total weight of Au in the side-surface electrode” to “the total weight of the side-surface electrode.” The value of the “Au Content Ratio” listed for each level in Table 1 refers to a value (average value for N=10) after the firing. In Table 1, a conventional composition (Au Content=0%) with “a side-surface electrode composed of only platinum without containing Au” was adopted only for Level 1. The Au content ratio was adjusted by the amount (ratio by weight) of the gold powder mixed in the electrode paste mentioned above.

Then, the ten samples were each subjected to the evaluations described above for each level. In Table 1, as for the “Solder Wettability,” the mark “x” refers to a contact angle larger than 90°, the mark “0” refers to a contact angle of approximately 90°, and the mark “0” refers to a contact angle smaller than 90°. As for the “Condition of Co-fired Electrode Plate,” the mark “0” refers to no agglomeration found in the electrode plate, and the mark “A” refers to agglomeration found partially in the electrode plate. As for the “Peeling of Solder,” the mark “x” refers to peeling caused between the electrode plate and the solder in one or more samples, and the mark “0” refers to no sample with peeling caused between the electrode plate and the solder. It is to be noted that at Levels 10, 11, agglomeration was caused excessively in the electrode plate during co-firing, thereby making the electrode plate fail to retain its shape, and thus resulting in failure to obtain any sample. Accordingly, the respective evaluations were not able to be carried out.

As can be understood from Table 1, it can be determined that when the Au content ratio is 3 weight % or more (see Levels 4 to 9), the low-melting-point solder is less likely to be peeled with better wettability, as compared with when the Au content ratio is less than 3 weight % (see Levels 1 to 3). In addition, when the Au content ratio is 5 weight % or more, the low-melting-point solder has particularly good wettability. When the Au content ratio is 15% or less, the co-fired electrode plate has a particularly good condition. It is to be noted that as described above (see also Table 1), when the Au content ratio exceeds 20 weight %, the shape of the electrode plate is unable to be retained due to agglomeration in the electrode plate. Accordingly, the Au content ratio is desirably 3 weight % or more and 20 weight % or less, further desirably 5 weight % or more and 20 weight % or less, particularly desirably 5 weight % or more and 15 weight % or less. The Au content ratio may be 3 weight % or more and 15 weight % or less.

As just described, the fact the electrode plate composed of platinum with gold therein improves reliability for the joint between the electrode plate and the “low-melting-point solder” as compared with that without gold therein is assumed to be based on the following reason.

That is, as can be easily understood from a phase diagram (not shown) of gold (Au) and tin (Sn), it is known that in the case of gold (Au) in contact with tin (Sn) in the low-melting-point solder, gold and tin form an alloy phase even at a relatively low temperature on the order of 200° C. Therefore, in the case of the electrode plate composed of platinum with gold therein, when the melted “low-melting-point solder” at approximately 200° C. comes into contact with the surface of the electrode plate, the gold in the electrode plate can be dissolved in the melted “low-melting-point solder.” Due to the solution of gold, the platinum present around the dissolved gold is also more likely to be dissolved in the melted “low-melting-point solder.” As a result, a compound layer including at least tin and platinum can be formed at the joint part between the electrode plate and the “low-melting-point solder.” The formation of this compound layer is assumed to improve the reliability for the joint between the electrode plate and the “low-melting-point solder.” In other words, the gold in the electrode plate is assumed to function as “an aid that dissolves platinum in the electrode plate to form a compound layer,” thereby improving the reliability for the joint between the electrode plate and the “low-melting-point solder.”

When the Au content ratio is less than 3 weight %, the solder is poor in wettability and likely to be peeled as compared with when the Au content ratio is 3 weight % or more. This is believed to be due to the fact “the aiding function of gold” cannot be sufficiently fulfilled because of the excessively low Au content ratio. In addition, when the Au content ratio exceeds 20 weight %, agglomeration is excessively caused during the firing, thereby making the electrode plate fail to retain its shape. This is believed to be due to the fact that the Au content ratio is excessively high, thereby decreasing the melting point of the whole electrode plate.

As just described, the side-surface electrode composed of platinum with gold therein improves the reliability for the joint between the side-surface electrode and the “low-meting-point solder,” as compared with when the side-surface electrode is composed of only platinum. Furthermore, when the Au content ratio of the side-surface electrode is 3 to 20 weight %, the reliability for the joint is further improved.

While an example of using platinum as a material of the electrode plate has been provided above in the experiment mentioned above, it has been separately confirmed that exactly the same result is obtained also when palladium is used in place of platinum as a material of the electrode plate. More specifically, the electrode plate composed of palladium with gold therein improves the reliability for the joint between the electrode plate and the “low-melting-point solder,” as compared with when the electrode plate is composed of only palladium. Furthermore, when the Au content ratio of the electrode plate is 3 to 20 weight %, the reliability for the joint is further improved. Furthermore, it has been separately confirmed that exactly the same result is obtained when an alloy of platinum and palladium is used in place of platinum as a material of the electrode plate.

The present invention is not limited to the preferred embodiment mentioned above but various modification examples can be adopted within the scope of the present invention. For example, in the preferred embodiment mentioned above, the side-surface electrode141composed of platinum or palladium contains gold therein, and the low-melting-point solder is joined to the side-surface electrode141. However, the surface electrode140composed of platinum or palladium may contain gold therein, and the low-melting-point solder may be joined to the surface electrode140.

In addition, while the main body part110is a laminated body that has the piezoelectric layers130and internal electrodes131laminated alternately in the preferred embodiment mentioned above, the main body part110may be a piezoelectric body composed of only a piezoelectric material (without any internal electrode). In addition, the main body part110may be a ceramic body composed of only a ceramic material other than piezoelectric materials (without any internal electrode).

It is to be noted that it has been separately confirmed that the reliability for the joint between the external electrode and the low-melting-point solder is improved even when silver (Ag) or copper (Cu) is added in place of gold (Au) in the case of the external electrode composed of platinum (Pt). This is also assumed to be based on the same mechanism as in the case of gold. More specifically, in the case of the external electrode composed of platinum with “silver or copper” therein, when the melted “low-melting-point solder” at approximately 200° C. comes into contact with the surface of the external electrode, the “silver or copper” in the external electrode is dissolved in the melted low-melting-point solder, thereby forming an alloy phase of “silver or copper” and tin. Due to the dissolution of “silver or copper,” platinum present around the “silver or copper” dissolved is also dissolved in the melted low-melting-point solder, and as a result, a compound layer including at least tin and platinum can be formed at the joint part between the external electrode and the “low-melting-point solder.” The formation of the compound layer is assumed to improve the reliability for the joint between the external electrode and the “low-melting-point solder.” In other words, the “silver or copper” in the external electrode is assumed to function as “an aid that dissolves platinum in the external electrode to form a compound layer,” thereby improving the reliability for the joint between the external electrode and the “low-melting-point solder.”