JP2005197958A - Piezoelectric oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively prevent a joining defect of a surface-mounted type piezoelectric oscillator in two-storied structure constituted by integrally joining a mount substrate where an IC component is mounted with a ceramic container bottom part of a piezoelectric vibrator with lead-free solder by suppressing the amount of gold by which gold-plating layers of respective join electrode surface layers on ceramic-container and mount-substrate sides are diffused into the solder below a fixed ratio against the amount of lead-free solder in use. <P>SOLUTION: The piezoelectric oscillator is equipped with a piezoelectric vibrator 10 having a piezoelectric vibrating element 14 airtightly sealed in the ceramic container 12 and the mount substrate 1 where the IC component 3 is mounted, and a join electrode 11 of the piezoelectric vibrator and a join electrode 4 of the mount substrate are joined together in one body with lead-free solder 20 and also have gold-plating layers as their surface layers, the weight ratio of the lead-free solder and gold that the lead-free solder contains is ≤7.5 wt% . <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an improvement in the package structure of a surface-mount piezoelectric oscillator, and in particular, in a piezoelectric oscillator in which a piezoelectric vibrator and a mounting substrate on which an IC component is mounted are stacked in a two-story structure and electrodes are joined with lead-free solder. The present invention relates to a technique for solving problems that are likely to occur.

Due to the price reduction accompanying the widespread use of mobile communication devices such as mobile phones and the rapid progress of miniaturization, the price and size of piezoelectric oscillators such as crystal oscillators used in these communication devices are also reduced. There is an increasing demand for thinning.
In order to meet such a demand, a surface mount piezoelectric oscillator as shown in FIG. 4 has been proposed. The piezoelectric oscillator 100 includes an external electrode 104 for surface mounting on the bottom and a rectangular mounting substrate 101 having a recess 102 on the upper surface, a bonding electrode 105 formed on the upper surface of the outer frame of the mounting substrate 101, and solder 115. The piezoelectric vibrator 110 having the lower surface of the bonding electrode 111 to be bonded by bonding is bonded and integrated with a two-story structure, and the IC component 103 is mounted on the internal electrode on the inner bottom surface of the recess 102 of the mounting substrate 101. It has. The IC component 103 is a chip component of an oscillation circuit and a temperature compensation circuit.
The piezoelectric vibrator 110 has a configuration in which a piezoelectric vibration element 114 (not shown) is accommodated in a recess 113 of a ceramic container 112 having a recess on the upper surface, and the recess 113 is hermetically sealed with a metal lid 115. ing.
Recently, instead of conventional eutectic solder (tin-lead solder) containing lead, which is a harmful substance, due to social demands such as response to environmental problems, lead-free solder (tin-silver) that does not contain lead -Copper-based, tin-copper-based, tin-zinc-based, etc.) are frequently used. For this reason, lead-free solder 115 is also used for bonding between the bonding electrodes 105 and 111 of the mounting substrate 101 and the piezoelectric vibrator 110 constituting the piezoelectric oscillator as described above.
On the other hand, each of the bonding electrodes 105 and 111 connected using lead-free solder is formed on the mounting substrate 101 and the ceramic container 112, for example, a metallized underlayer such as tungsten or copper, an intermediate layer such as a nickel plating layer, a surface, etc. The gold plating layers are sequentially stacked. The thickness of the gold plating layer is about 0.5 to 0.8 μm. When forming each bonding electrode 105, 111 on the mounting substrate 101 or the ceramic container 112, the external electrode 104 at the bottom of the mounting substrate 101, the internal electrode in the ceramic container, etc. are simultaneously manufactured in the same procedure. Each electrode has the same laminated structure.
When soldering, for example, the piezoelectric vibrator and the mounting board are positioned so that the bonding electrode 111 of the piezoelectric vibrator 110 is placed on an appropriate amount of cream solder applied to the surface of the bonding electrode 105 of the mounting board 101. Then, a procedure for heating and cooling in a reflow furnace is performed.
By the way, when electrodes having a gold plating layer on the surface layer are joined with tin-lead solder, the gold plating layer dissolves and disappears in the solder, so that a crack or a joint failure occurs in the solder joint portion. It is known that the “eating phenomenon” occurs (Japanese Patent No. 3377850). This phenomenon is considered to occur when gold dissolves and a part of the solder layer is transformed into a low brittle alloy containing a lot of gold.
In Japanese Patent No. 3377850, in order to solve such a problem, the gold plating layer on the electrode surface is melted and removed, and then the solder plating layer is formed. The solder plating layer and the solder diffuse to each other so that they are firmly and reliably joined without causing erosion.

However, since this conventional example obviously increases the number of steps and becomes complicated, there is a strong possibility that the productivity is lowered and the cost is increased.
It has been confirmed that a phenomenon similar to the “solder erosion phenomenon” similarly occurs in lead-free solder.
5 (a), 5 (b) and 5 (c) are enlarged views showing a state in which bonding failure occurs when bonding electrodes are bonded with lead-free solder in the piezoelectric oscillator shown in FIG. 4, time-reflow temperature. It is a figure which shows a profile, and is a figure which shows the relationship between the gold | metal density | concentration in solder, and melting | fusing point.
First, in FIGS. 5A and 5A, after the lead-free cream solder 115 is printed on the bonding electrode 105 on the mounting substrate 101, the bonding electrode 111 is appropriately positioned on the bonding electrode 105. The piezoelectric vibrator 110 is placed with the cream solder 115 sandwiched between side switches. When heating in the reflow furnace is started in this state, the state shown in the figure is maintained at the stage where the melting point of the cream solder has not been reached as shown in (b) (1).
Next, FIGS. 5A and 5B show the melted state. When the melting point reaches the melting point as shown in FIGS. 5B and 2B by heating in the reflow furnace, the cream solder 115 is moved to the vibrator side. The gold plating layer constituting the surface layer of the bonding electrodes 105 and 111 starts to diffuse into the solder at this time, and the gold concentration on the solder 115 side increases as shown in FIG. The melting point increases due to the relative decrease in the amount of solder. As the melting point rises, the solder containing gold cannot be completely melted.
Next, FIGS. 5 (a) and 3 (3) show the state when the temperature is lowered. Since the gold-tin alloy is deposited on the interface and the wettability is lowered, and further, the solder tends to be rounded by the capillary phenomenon. Retraction is performed. And since the whole solder is not melt | dissolved completely at the time of a heating, a void is formed without flux gas being able to escape at the time of cooling. That is, when the solder is heated to the melting point and completely melted, sufficient agglomeration is performed and the flux gas is discharged. However, when the solder cannot be completely melted, the solder is not agglomerated. Since it becomes sufficient, the flux gas remains and cures as it is. Similarly, if the melting is insufficient, the solder tends to be rounded and cannot be spread on the bonding electrode surface.

That is, when joining the joining electrodes 105 and 111 by soldering, it is not necessary to form a gold plating layer on the surface of the joining electrode, but for the external electrode 104 of the mounting substrate 101, the internal electrode of the ceramic container 112, etc. In order to obtain an electrode structure suitable for chip mounting, it is necessary to form a thick gold plating layer. As described above, in the actual manufacturing process of the insulating substrate and the ceramic container, these external electrodes and internal electrodes are formed at the same time as the respective bonding electrodes 105 and 111. Therefore, the gold plating layer is formed on the surface layer of each bonding electrode 105 and 111. Will be formed. At this time, the gold plating layer on the surface of the bonding electrode has a thickness of about 0.5 to 0.8 μm. When the gold plating layer is increased in this way, the gold on the surface of both bonding electrodes diffuses into the solder. As a result, the weight concentration of gold with respect to the amount of solder becomes higher than a certain level (for example, about 11 to 12 wt%), the melting point of the solder portion in the gold-rich state rises, and the solder is hardened without melting. It became clear that peeling occurred, impact resistance and drop resistance were lowered, and reliability was lost.
In addition, when one bonding electrode does not include a gold plating layer and the surface layer of the other bonding electrode has a relatively thin gold plating layer of about 0.3 μm, even if bonding is performed using lead-free solder, It has been confirmed that there is no problem of solder bumping and void generation due to gold diffusion.
Japanese Patent No. 3377850

When the present inventor joins two joining electrodes having a relatively thick gold plating layer on the surface by lead-free soldering, a bonding failure occurs because the gold plating layers on the surfaces of both joining electrodes simultaneously diffuse into the solder. As a result of repeated experiments for solving the problem, when the gold content weight in the solder is not more than a predetermined value in the state where the gold on the both electrode side diffuses to the solder side by solder bonding, It has been found that such a bonding failure is less likely to occur, and the present invention has been made.
The present invention has been made in view of the above, and in a two-storey structure piezoelectric device in which a piezoelectric vibrator is connected and fixed using a lead-free solder on a mounting substrate on which an IC is mounted, a gold plating layer is provided on the surface layer. When the bonding electrode on the mounting board and the bonding electrode on the piezoelectric vibrator side having the same configuration are bonded with lead-free solder, even if the gold plating layer of both bonding electrodes diffuses into the solder, It is an object of the present invention to provide a technique capable of effectively preventing a drop in impact resistance and drop resistance by forming a void during cooling or peeling off as a bump.

In order to solve the above-described problem, the invention of claim 1 includes a piezoelectric vibrator in which a piezoelectric vibration element is hermetically sealed in a ceramic container, and a mounting substrate on which an IC component is mounted. And a piezoelectric oscillator in which the bonding electrodes of the mounting substrate are bonded and integrated with lead-free solder, and each of the bonding electrodes has a gold plating layer on each surface layer. The ratio of the gold plating layer constituting the surface layer of the bonding electrode is 7.5 wt% or less.
The invention of claim 2 is to keep the gold amount of the gold plating layer constant so that the ratio of the lead-free solder amount of claim 1 and the gold contained in the lead-free solder is 7.5 wt% or less. The amount of lead-free solder is increased.
The invention according to claim 3 is the lead-free solder amount according to claim 1 and the ratio of gold contained in the lead-free solder is 7.5 wt% or less, so that the lead-free solder amount is not increased. Alternatively, the amount of gold is reduced by thinning the gold plating layer on the surface of the bonding electrode while reducing the increase in the amount of lead-free solder.
The invention of claim 4 is to increase the ratio of the lead-free solder amount according to claim 1 and the gold contained in the lead-free solder to 7.5 wt% or less, without increasing the lead-free solder amount, The bonding area of the bonding electrode is reduced to reduce the amount of gold.
In the invention of claim 5, the value of 7.5 wt% or less, which is the ratio of the amount of lead-free solder according to claims 1 to 4 to the gold contained in the lead-free solder, is particularly 7.4 wt%. It is characterized by.

  In a surface mounted piezoelectric oscillator having a two-story structure in which a mounting substrate having IC components mounted on the bottom of a ceramic container of a piezoelectric vibrator is joined and integrated with lead-free solder, by increasing the amount of lead-free solder used, the ceramic container and Since the amount of gold diffusing into the solder from the gold plating layer on the surface of each bonding electrode on the mounting substrate side is suppressed to a certain ratio or less, it is possible to effectively prevent bonding failure.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.
FIG. 1 is a longitudinal sectional view of a surface mount piezoelectric oscillator according to an embodiment of the present invention.
The piezoelectric oscillator includes an external electrode 5 for surface mounting at the bottom, and a mounting substrate 1 on which an IC component 3 is mounted as an electronic component constituting an oscillation circuit, a temperature compensation circuit, and the like in a recess 2 formed on the top surface. And a structure in which the bonding electrode 4 formed on the upper surface of the outer frame of the mounting substrate 1 and the piezoelectric vibrator 10 having the bonding electrode 11 bonded by the lead-free solder 20 on the lower surface are bonded and integrated in a two-story structure. Yes.
The piezoelectric vibrator 10 has a configuration in which a piezoelectric vibration element 14 such as a crystal vibration element is accommodated in a recess 13 formed on the upper surface of the ceramic container 12 and the recess 13 is hermetically sealed with a metal lid 15. Yes.
The bonding electrode 4 on the mounting substrate 1 side and the bonding electrode 11 on the piezoelectric vibrator 10 side have a configuration in which, for example, a nickel film and a gold plating layer are sequentially laminated on a copper film, and the thickness of the gold plating layer is, for example, 0. It is set to about 5 to 0.8 μm.
In the present invention, when a piezoelectric vibrator is mounted on a mounting substrate on which an IC component is mounted and the bonding electrodes are bonded with lead-free solder, the bonding electrodes are bonded with a necessary and sufficient bonding force. Therefore, even when a thick metal plating layer is formed on each bonding electrode surface layer, the weight ratio (ratio) of gold diffusing into the lead-free solder during reflow is limited to a predetermined value or less. Voids are prevented from being formed or bulging during cooling.

FIG. 2 shows that the thickness of the gold plating layer on the surface of the bonding electrode 5 on the mounting substrate side is 0.8 μm, and the thickness of the gold plating layer on the surface of the bonding electrode 11 on the piezoelectric resonator (in this example, crystal resonator) side. When the thickness is 0.5 μm (when the total amount of the gold plating layer is constant), cream solder is printed on the bonding electrode 5 (or the bonding electrode 11) using a metal mask having a predetermined mask thickness and opening area. It is a figure which shows the change of the quality of the joining state judged from an external appearance, when changing the quantity to change variously and the relative weight ratio (wt%) of the gold | metal diffused in the solder 20 is changed.
According to this figure, the amount of cream solder applied on the bonding electrode 5 is relatively small, such as 70.8 mg, 81, and 9 mg, as in Samples 1 and 2, so the total amount of solder 20 When the gold concentration (wt%) increases to 11.3% and 9.9%, respectively, the connection state judged from the appearance is clearly bad, voids are formed, and It is observed that the connection state is getting worse.
On the other hand, when the amount of solder was further increased and the gold concentration was lowered to about 9.1 wt% as in sample 3, it was confirmed that the connection state was slightly improved. However, in this state, voids remain and there is also a feeling of vomit, which is still a connection state that cannot be put to practical use.
Sample 4 is a case where the gold concentration is 7.4 wt% by setting the solder amount to 108.1 mg. In this case, the connection state is good in appearance. For sample 4, sufficiently satisfactory results were obtained even in the reliability test.
As a result of further detailed experiments, when the concentration of gold contained in the solder is 7.5 wt% or less when the two bonding electrodes provided with the gold plating layer are bonded by lead-free solder, It was found that the bonding state was good.

FIG. 3 shows the results of a reliability test of a crystal oscillator with a gold concentration of 7.5% or less, and it was found that there was no abnormality in all tests.
According to the present invention, by adjusting the amount of lead-free solder to be used, bonding failure due to gold diffusion into the solder can be prevented, so that the expected effect can be achieved without incurring any increase or complexity in the bonding process. Can be realized.
In addition, as a method for setting the gold concentration in the solder after reflow to 7.5% or less in this way, in addition to the method of increasing the amount of solder while keeping the amount of gold diffusing in the solder constant, the amount of solder Without increasing the amount of solder (or reducing the amount of increase in solder), the gold plating layer on the surface of the bonding electrode is thinned to reduce the amount of gold diffused or increase the bonding electrode without increasing the amount of solder. A method of reducing the amount of gold diffused in the solder by reducing the size can be considered.
In the piezoelectric oscillator according to the present invention, a piezoelectric vibrator is stacked on a mounting substrate on which IC components constituting an oscillation circuit, a temperature compensation circuit, and the like are mounted, and the bonding electrodes of both are bonded with lead-free solder. The case can be generally applied.
In the actual process of manufacturing the piezoelectric oscillator of the present invention, a mounting board base material in which a large number of mounting board pieces are connected in a sheet form is prepared, and individual piezoelectric vibrators are mounted on the mounting board base material. Then, soldering is performed by reflow, and batch processing is performed to divide into pieces after the connection is completed.
The present invention can be applied to a piezoelectric oscillator of a type using any piezoelectric vibrator other than a crystal vibrator.

1 is a longitudinal sectional view of a surface mount piezoelectric oscillator according to an embodiment of the present invention. The figure which shows the change of the quality of the joining state judged from an external appearance, when changing the quantity of lead-free solder to be used variously when the thickness of both gold plating layers is set to a predetermined value. The figure which shows the result of the reliability test of the crystal oscillator whose gold concentration is 7.5% or less. Explanatory drawing of a prior art example. (A), (b) and (c) are enlarged views showing a state in which bonding failure occurs when bonding electrodes are bonded by lead-free soldering in the piezoelectric oscillator shown in FIG. 4, and a diagram showing a time-reflow temperature profile. The figure which shows the relationship between the gold | metal density | concentration in solder, and melting | fusing point.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Mounting substrate, 2 recesses, 3 IC components, 4 joining electrodes, 10 piezoelectric vibrator, 11 joining electrodes, 12 ceramic container, 13 recesses, 14 piezoelectric vibrating elements, 15 metal lid, 20 lead free solder.

Claims (5)

A piezoelectric vibrator in which a piezoelectric vibration element is hermetically sealed in a ceramic container, and a mounting board on which an IC component is mounted. The bonding electrode of the piezoelectric vibrator and the bonding electrode of the mounting board are made of lead-free solder. Bonded and integrated piezoelectric oscillator, each having a gold plating layer on each surface layer of each bonded electrode,
A piezoelectric oscillator, wherein a ratio of the lead-free solder to gold contained in the lead-free solder is 7.5 wt% or less.   In order to make the ratio of the lead-free solder amount according to claim 1 and the gold contained in the lead-free solder 7.5 wt% or less, the amount of the lead-free solder is increased while keeping the gold amount of the gold plating layer constant. A piezoelectric oscillator characterized by having been made.   In order to set the ratio of the lead-free solder amount according to claim 1 to the gold contained in the lead-free solder to 7.5 wt% or less, the lead-free solder amount is not increased or the amount of lead-free solder is increased. A piezoelectric oscillator characterized by reducing the amount of gold by reducing the gold plating layer on the surface of the bonding electrode while reducing the amount of gold.   In order to make the ratio of the lead-free solder amount according to claim 1 and gold contained in the lead-free solder 7.5 wt% or less, the junction area of the joining electrode is reduced without increasing the lead-free solder amount. And reducing the amount of gold.   A piezoelectric oscillator characterized in that a value of 7.5 wt% or less, which is a ratio of the amount of lead-free solder according to claims 1 to 4 and gold contained in the lead-free solder, is particularly 7.4 wt%.

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