JP2009039769A - Joining sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Pb-free joining sheet used for joining electronic component, in which the oozing out of Sn upon the joining can be suppressed or reduced, and simultaneously, joining reliability can be secured. <P>SOLUTION: Disclosed is a joining sheet in which the surface of an Ag foil material is provided with a layer composed of an Sn-Ag alloy, preferably, the average thickness of the Ag foil material is 20 to 200 μm, and more preferably, the average thickness of the layer composed of an Sn-Ag alloy is 10 to 40 μm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a joining sheet used for joining electronic components.

In recent electronic equipment industries, wire bonding using Au wire is widely used as a method for electrically connecting a semiconductor element made of Si or the like to a wiring board. In order to perform wire bonding, it is necessary to bond and fix a semiconductor element to a wiring board, and various materials have been proposed as bonding materials for the fixing. Among them, a Pb-Sn alloy containing 90% or more by mass of Pb has a melting point of about 300 ° C., is superior in terms of heat resistance compared to a resin-based bonding material, and is low in cost. It is often used when bonding a large power device semiconductor element to a wiring board.
Pb—Sn alloys are also used for applications other than fixing semiconductor elements as bonding materials for structural materials of electronic components, particularly metal lids for sealing ceramic packages. This is because the Pb—Sn alloy has a high melting point at a temperature of about 200 ° C. when the electronic component is mounted on the wiring board, and therefore, it is not remelted at the time of mounting even if it is used for a joint portion in the electronic component. It is for having. In addition, Pb-Sn alloy is rich in ductility, so when used as a joining member for electronic devices that are subject to a drop impact such as portable devices, the impact energy of the drop impact is sufficiently absorbed to prevent damage such as electrical disconnection of electronic components. can do.

On the other hand, in recent years, there has been a so-called Pb-free movement to reduce and eliminate Pb in solder and brazing materials in consideration of the natural environment, and various alternative materials for Pb—Sn alloys have been proposed.
For example, in Patent Document 1, a ceramic package is sealed using a lid member having an Au-20 mass% Sn alloy or an Au-Sn plating layer. The sealed ceramic package is soldered to the circuit at about 250 ° C. using Sn—Pb eutectic solder or Sn—Ag—Cu solder in a later step. At this time, since the melting point of the Au—Sn alloy of Patent Document 1 sealed in the ceramic package in the previous process is higher than the soldering temperature for mounting the package on the circuit board in the subsequent process, the sealed part is attached to the circuit board. It has the feature that it keeps the joined state without remelting when soldering.

Further, in Patent Document 2, a brazing material sheet in which an Sn layer, which is a low melting point metal, is formed on the surface of an Ag or Cu foil material, which is a high melting point metal, which is superior in cost to the Au—Sn alloy described in Patent Document 1. Has been proposed. This brazing material sheet is sealed by utilizing the low melting point of Sn by heating through the ceramic package and the lid. Further, after sealing, Ag or Cu of the high melting point metal foil reacts with Sn, which is a low melting point metal, to form an intermetallic compound phase having a high melting point, thereby ensuring bonding reliability.
JP 2003-229504 A JP 2006-272449 A

Although the lid material disclosed in Patent Document 1 described above does not contain Pb and is advantageous in that it maintains a bonded state even in a high temperature environment of about 250 ° C. to 300 ° C., Au is very expensive. There is a problem that the product cost of the ceramic package is significantly increased. Furthermore, the intermetallic compound of Au and Sn is relatively brittle, which is disadvantageous from the viewpoint of impact resistance.
Since the brazing material sheet disclosed in Patent Document 2 also does not contain Pb, it is advantageous in that the environmental load is small. However, since the low melting point metal Sn is first melted at the time of heating in the first joining stage, Before the liquid phase Sn generates an intermetallic compound with Ag or Cu, there is a possibility that an excessive liquid phase oozes out and comes out of the joint. If this Sn oozes out, the liquid phase Sn may come into contact with the SAW filter or crystal resonator built in the package, resulting in malfunction or short circuit. Also when it oozes out of the package, a short circuit occurs between the electrodes, causing a failure of the electronic device.

  As a result of studying the problem of Sn exudation, the present inventor formed a layer made of an alloy phase of Sn—Ag instead of a single phase of Sn on the surface of the refractory foil, so that the refractory metal foil and the metal were formed. The present inventors have found that a structure composed of an intermetallic compound phase and a low-melting point Sn phase is formed to suppress and reduce Sn oozing, and at the same time secure joint reliability.

That is, the present invention is a brazing material sheet having a layer made of an Sn-Ag alloy on the surface of an Ag foil material.
The average thickness of the Ag foil material is preferably 20 to 200 μm.
Preferably, the average thickness of the layer made of the Sn—Ag alloy is 10 to 40 μm.

  According to the present invention, it is possible to bond at the melting point of Sn, and after joining one end, the Sn-Ag alloy contributes to preventing Sn from exuding when sealing a ceramic package, for example. This is an indispensable technology for joining parts.

As described above, an important feature of the present invention is to have a solder layer made of Sn—Ag alloy on the surface of the Ag foil material. In the brazing material sheet described in Patent Document 2, since the solder layer is pure Sn, Sn as the solder layer is in a completely liquid state and Sn is likely to ooze out into and out of the package. On the other hand, since the solder layer is formed of a Sn-Ag alloy, the present invention has a two-phase state of a solid and a liquid consisting of a Sn-Ag alloy and a Sn liquid phase when heat-sealed in a package. Viscosity increases. For this reason, if heat sealing is performed in the temperature range in which the two-phase state of the solid and the liquid exists, it is possible to suppress the seepage of Sn.
Further, the Ag foil contributes to the formation of Sn—Ag intermetallic compound at the interface with the Sn—Ag alloy layer and the formation of Sn—Ag intermetallic compound particles inside the solder layer. Accordingly, the viscosity of the fluid at the joint portion can be increased during heat sealing, so that it is possible to suppress the seepage of Sn.

Examples of the method for forming the layer made of the Sn—Ag alloy include a plating method. In the plating method, it is easy to form a layer made of a thick Sn—Ag alloy, and the production rate can be increased. Further, in the plating method, by adjusting the Ag concentration in the plating bath, it is possible to directly form Sn-Ag alloys having any concentration on the foil material.
As for the density | concentration range of Ag which forms the Sn-Ag alloy of this invention, 20-55 mass% Ag is desirable. If the Ag concentration is less than 20% by mass, the ratio of the solid at the time of heat sealing is small, so that the effect of suppressing the seepage of Sn may not be sufficient. On the other hand, if the Ag concentration range exceeds 55 mass%, a large amount of Sn-Ag compound is formed inside the solder layer, the amount of liquid phase that contributes to bonding is insufficient, so-called wetting is insufficient, and bonding reliability is reduced. May be problematic in terms of sex.

  Further, the average thickness of the Ag foil material is preferably 20 to 200 μm. If the thickness of the foil is 20 μm or more, it is easy to handle at the time of producing the joining sheet or soldering. On the other hand, when the thickness exceeds 200 μm, the thickness of the bonding material occupies a considerable part with respect to the electronic component that tends to be thinned, and the amount of expensive Ag used is unrealistic.

  Moreover, it is preferable that the average thickness of the layer which consists of said Sn-Ag alloy is 10-40 micrometers. When the thickness is less than 10 μm, the bonding itself is possible, but a large amount of intermetallic compounds are formed between the Sn-Ag alloy and the bonding partner material caused by Sn, resulting in bonding reliability. May be problematic in terms of sex. On the other hand, when it is thicker than 40 μm, there is a risk that the reliability of the joint portion is lowered because the thickness of the joint portion itself, which is originally inferior in reliability such as strength, is thicker than that of the matrix portion.

Assuming that the package for electronic components is sealed, after bonding a 30 μm thick Ag foil on a 100 μm thick Fe-42 mass% Ni plate to be a lid material through a Ti vapor deposition layer, the Ag foil On the surface, a layer made of an Sn—Ag alloy shown in Table 1 was formed by plating so as to have a thickness of 10 μm, and an airtight sealing lid was produced.
Further, as a comparative example, a similar hermetic sealing lid was prepared using a bonding sheet in which an Sn layer having a thickness of 10 μm was formed on the surface of an Ag foil having a thickness of 30 μm shown in Table 1 by a vacuum deposition method.

Next, in order to confirm the presence or absence of Sn exudation at the time of bonding, the airtight sealing lid is placed on a Ni foil having poor wettability with Sn as an environment in which Sn easily exudes, and 260 in a hydrogen atmosphere. Soldering was carried out while heating the airtight sealing lid at 2.23 × 10 ⁻² MPa by heating to ° C.

The evaluation of Sn exudation was carried out by photographing the airtight sealing lid joined to the Ni foil from the plane with an optical microscope (Model VH-7000C, manufactured by KEYENCE Inc.), and using the amount of Sn exuded as the exudation area by image analysis. It was measured.
Table 1 shows the measurement results of the Sn exudation area. As shown in Table 1, it was confirmed that in the joining sheet using the Sn-20.9 mass% Ag alloy of Example 1 of the present invention, the Sn seepage area was significantly reduced as compared with Comparative Example 2.
Furthermore, in the joining sheet | seat using the Sn-54.3 mass% Ag alloy of the example 2 of this invention, there is no Sn ooze-out and it turns out that the effect with respect to the oozing-out of Sn of this invention is large.

Next, a shear strength test was performed. Assuming that the ceramic package is actually sealed, electrolytic Ni / Au plating is performed on a base material of Fe-29 mass% Ni-17 mass% Co alloy showing the same low thermal expansion coefficient as ceramic, and then Similarly prepared lid for airtight sealing was placed and heated to 260 ° C. in a nitrogen atmosphere, and soldering was performed while pressurizing the airtight sealing lid at 2.10 × 10 ⁻² MPa. Here, the reason for the nitrogen atmosphere is to prevent oxidation of Sn at the time of heat bonding, and pressurization enhances the adhesion between the base material and the bonding sheet and further secures the bonding reliability.
Under a room temperature environment, the bonding strength was measured by setting on a fixing jig of a shear strength tester (model DS100 manufactured by dage) and shearing the hermetic sealing lid on the substrate. Moreover, the same test was implemented also about the joining sheet | seat which consists of Sn-95 mass% Pb alloy and the joining sheet | seat which formed Sn layer on Ag foil as a comparative example.
The shear strength test results are shown in Table 1. As shown in Table 1, the joining sheet of the present invention has higher shear strength than the joining sheet using Sn-95 mass% Pb of Comparative Example 1 conventionally used, and has practical joining strength as a joining sheet. Was confirmed.

  Next, in order to confirm the bonding state, the cross-sectional structure of the bonded portion was observed. FIG. 1 shows a scanning electron micrograph of a bonded cross section when a hermetic sealing lid using the bonding sheet of Example 2 of the present invention is heat bonded onto a substrate plated with electrolytic Ni / Au. From FIG. 1, it was confirmed that the Ag—Sn compound was formed at the Ag foil / solder layer interface, the Ni—Sn—Au compound and the Ni—Sn compound were formed on the base material side, and the bonding was sufficiently performed. .

It is a cross-sectional structure | tissue photograph by the scanning electron microscope which shows an example of this invention.

Explanation of symbols

  1. Base material, 2. 2. electrolytic Ni / Au plating layer; Ni-Sn compound, 4. 4. Au—Ni—Sn compound; Ag-Sn compound, 6. Sn, 7. Ag foil

Claims (3)

A joining sheet comprising a layer made of an Sn-Ag alloy on a surface of an Ag foil material. The bonding sheet according to claim 1, wherein an average thickness of the Ag foil material is 20 to 200 μm. The bonding sheet according to claim 1 or 2, wherein an average thickness of the layer made of the Sn-Ag alloy is 10 to 40 µm.