US20060067853A1

US20060067853A1 - Lead free solder

Info

Publication number: US20060067853A1
Application number: US11/222,808
Authority: US
Inventors: Toshihide Takahashi; Kazutaka Matsumoto; Izuru Komatsu; Masahiro Tadauchi; Yoshiyuki Fukuda
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 2004-09-24
Filing date: 2005-09-12
Publication date: 2006-03-30
Also published as: JP4115979B2; JP2006088204A

Abstract

A lead-free solder comprises tin and zinc at a weight ratio between tin and zinc ranging from about 88:12 to about 80:20 on the basis of total weight of the lead-free solder.

Description

CROSS-REFFERENCE TO RELATED APPLICATION AND INCORPORATION BY REFERRERNCE

This application claims benefit of priority based on Japanese Patent Application filed previously by the applicant, namely, Japanese Patent Application No. 2004-278254 (filing date: Sep. 24, 2004), the contents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to lead-free solder and, in particular, to lead-free solder including tin as a base material and having an excellent creep property. Moreover, the present invention relates to lead-free solder having enough tensile strength and excellent rupture elongation.
2. Description of the Related Art
At present, lead-free solder, mainly a tin-silver-copper based alloy is at a practical stage and the substitution of lead-free solder for lead solder is scheduled to be completed. However, most of these conventional lead-free solder technologies are used for bonding materials used when electronic parts are mounted on a printed wiring board. A lead-free solder technology used for lead included in the plating of electrodes or the internal bonding material of electronic parts is not at a mature stage. For this reason, there is a further required to advance the technology for completely eliminating all lead from electronic device products including lead in the electronic parts.
For example, in a semiconductor device such as a power transistor, which has high voltage and high current applied thereto to generate a large amount of heat, a heat radiating function is indispensable and hence a heat sink is used in the device. Moreover, a ceramic material, which is a body having high thermal conductivity and insulation, is used for a substrate with a power semiconductor element thereon. Tin-lead eutectic solder was conventionally used for a bonding material in a power device for bonding this ceramic substrate to a heat sink including copper as a main component. In this case, since the ceramic substrate is greatly different in the coefficient of thermal expansion from copper used for the heat sink, when the tin-lead eutectic solder is cooled from the solidification point of tin-lead eutectic solder, that is, 183° C. to room temperature, thermal stress is developed in the solder for bonding the ceramic substrate to the copper plate. However, the conventional tin-lead eutectic solder can decrease the developed thermal stress because of its characteristic that the tin-lead eutectic solder creeps easily.
However, when a tin-silver-copper based alloy is used in place of the conventional tin-lead eutectic solder, there is a problem that thermal stress developed in the solder cannot be reduced because of its resistance to creep and, consequently, causes warping of the copper substrate. For this reason, it has been desired to provide a new lead-free solder capable of solving such a problem (for example, see Japanese Patent Application Laid-Open No. 2001-246493)
On the other hand, under circumstances where lead free solder has been desired, a composition of Sn-3Ag-0.5Cu, a composition of Sn-3Ag-1Bi-31n, a composition of Sn-3Ag-1Bi, and the like have been proposed as substitute solders for the conventional solder used for mounting electronic parts on printed wiring boards. These solders show larger tensile strength but tend to be lower in rupture elongation than the tin-lead eutectic solder. Hence, there remains room for improvement to permit wide use of these solders for bonding in the device. For this reason, it has been desired to provide a new lead-free solder having excellent tensile strength and rupture elongation.
In response to the above-described technical issues, creep characteristics of an alloy including Sn-3.0 mass % Ag-0.5 mass % Cu and an alloy including Sn-8.0 mass % Zn-3.0 mass % Bi were compared with that of an alloy including Sn-37 mass % Pb (“Stress Relaxation and Life Curve of Sn—Ag—Cu based lead-free solder”, Kaga et al., Transactions of The 9-th Symposium on Microjoining and Assembly Technology in Electronics, 9(2003), page 345-350). However, this research is only basic experiments and dose not propose an alloy that is industrially applicable and has the potential for stress relaxation.

SUMAMRY OF THE INVENTION

The present invention provides a lead-free solder comprising tin and zinc at a weight ratio between tin and zinc ranging from about 88:12 to about 80:20.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic sectional side view of a power semiconductor device.
FIG. 2 shows a graphic diagram illustrating the dependency upon the Zn content for strain rate.
FIG. 3 shows a graphic diagram illustrating the dependency upon the Ag content for strain rate.
FIG. 4 shows a graphic diagram illustrating the dependency upon the Cu content for strain rate.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, the present invention will be described by the use of embodiments but it is not intended to limit the present invention to the following embodiments.

First Embodiment

After research on various kinds of metal materials, the present inventors have found that (1) tin-zinc based solder, (2) tin-silver based solder, and (3) tin-copper based solder are useful as a lead-free solder having excellent creep characteristic. Lead-free solders, in accordance with the first embodiment, have characteristics including a high stain rate even under low stress and have excellent in creep properties. Hence, the lead-free solders can relax the thermal stress developed when substrates having different thermal coefficients of expansion, for example, a ceramic substrate, and a copper plate are bonded to each other and can relax warping of the substrates. Hereafter, these solders will be described.
(1) Tin (Sn)-Zinc (Zn) Based Solder
Tin-zinc solder with an eutectic composition is lower in melting point than tin and zinc and is also lower in surface tension than tin and zinc. Moreover, the tin-zinc eutectic composition solder is lower in melting point than the other lead-free solders and is close in the melting point to tin-lead eutectic composition solder. Hence, soldering by the tin-zinc solder can be conducted at the same bonding temperature as the conventional tin-lead solder. Moreover, the tin-zinc based solder is higher in electric conductivity than the conventional tin-lead based solder and, hence, is lower in heat generation developed by the passage of current. Therefore, the tin-zinc based solder is advantageous also in terms of energy consumption and thermal measures at the time of conductive bonding.
It is preferable that the weight ratio between tin and zinc (Sn:Zn) is from about 88:12 to about 80:20. This is because when the ratio of zinc in the weight ratio between tin and zinc is higher than about 80:20, although a solidus line temperature is constant at 199° C., a liquidus line temperature increases. Hence, the solder of this composition is hard to melt sufficiently at a soldering temperature. On the other hand, when the ratio of zinc in the weight ratio between tin and zinc is lower than about 88:12, the composition of the solder is close to a eutectic composition and the difference between a solidus line temperature and a liquidus line temperature is smaller. Thus, as compared with a case where the difference between a solidus line temperature and a liquidus line temperature is large, solidification reaction progresses quickly throughout a bonding portion to increase residual stress in the bonding portion. Within the above range of weight ratio, it is more preferable that the weight ratio between tin and zinc (Sn:Zn) is from about 86:14 to about 83:17. Here, the upper limit value of Zn content is determined from a liquidus line temperature, which is measured at a temperature increasing rate of 10° C./min by the use of a differential scanning calorimeter (DSC) and is shown in Tables 1, 2.
Moreover, the coefficient of linear expansion of the tin-zinc based solder having a weight ratio between tin and zinc (Sn:Zn) ranging from about 88:12 to about 80:20 is smaller than those of simple substances of tin and zinc, and currently used lead-containing solder (Sn-37Pb) and is close to the coefficient of linear expansion of copper (1.62×10⁻⁵[1/K]). Hence, when the tin-zinc based solder of this type is used for a device and an apparatus in which portions to be soldered are made of copper, the thermal stress developed by the repeated cycles of increasing and decreasing temperatures can be reduced and, hence, the occurrence of cracks can be prevented.

Since copper is used for the lead wires of parts and substrate electrodes of semiconductor devices, this tin-zinc based solder is particularly useful as the bonding material of devices and apparatuses including power semiconductor elements. Moreover, this tin-zinc based solder is similarly useful also for bonding parts formed of metal, except copper, having a small coefficient of linear expansion, for example, iron, nickel and the like.

TABLE 1


			Liquidus	Coefficient
			Line	Of Linear
Test	Tin	Zinc	Temperature	Expansion
Sample	Mass %	mass %	° C.	1/K

1	100	0	232	2.35 × 10⁻⁵
2	91	9	199	2.16 × 10⁻⁵
3	88	12	246	2.04 × 10⁻⁵
4	80	20	285	2.24 × 10⁻⁵
5	70	30	327	2.23 × 10⁻⁵
6	60	40	335	2.59 × 10⁻⁵
7	0	100	419	3.10 × 10⁻⁵

Next, tensile tests were conducted by the use of test samples having a thickness of 4.00 mm, a width of 5.00 mm, and a gage length of 25.0 mm on the basis of JIS Z 2241(98), JIS G 0567(98) of JIS standards and the tensile characteristics of the tin-zinc based solder were studied. The obtained results are shown in Table 2. Here, strain rate is a steady strain rate when stress is 10 MPa.

TABLE 2


			Liquidus		0.2% Proof
Test			Line	Strain Rate	Stress
Sample	Tin	Zinc	Temperature		1/sec	GPa

No.	mass %	mass %	° C.	R.T.	125° C.	R.T.	125° C.

8	94	6	205	8.3 × 10⁻¹¹	1.1 × 10⁻⁶	29.0	17.3
9	91	9	199	1.7 × 10⁻⁹	8.7 × 10⁻⁶	29.5	16.7
10	88	12	246	8.9 × 10⁻⁹	5.3 × 10⁻⁵	28.0	13.3
11	60	40	330	3.6 × 10⁻¹⁰	6.0 × 10⁻⁶	35.0	16.5

A test sample 10 having a weight ratio between tin and zinc of about 88:12 was higher in strain rate when stress was 10 MPa than a test sample 9 of eutectic composition. Moreover, the test sample 10 with a weight ratio between tin and zinc of about 88:12 was the lowest in a 0.2% proof stress. It could be said from these facts that the solder having a weight ratio between tin and zinc of about 88:12 was more easily deformed and was more excellent in creep characteristic under low stress than the eutectic composition solder. In other words, in the case of relaxing thermal stress developed in a power device, the use of solders excellent with a creep characteristic is preferable.
(2) Tin (Sn)-Silver (Ag)-Copper (Cu) Based Solder
At present, tin-silver-copper based solder is the most suitable solder for practical use as a lead-free solder. However, it is not suitable to use Sn-3.0Ag-0.5Cu, which is used for a wide variety of applications as described in the Background of the Invention, as it is, as the bonding material for a power device. This is because, since Ag₃Sn of a metallic compound crystallizes out in the structure of solder or the eutectic composition of tin, silver, and copper is hard, the solder increases in mechanical strength and, hence, does not have enough ductility and, hence, does not permit stress relaxation. However, it is possible to produce solder that is soft and has an excellent creep characteristic by decreasing the weight ratio of Ag.
Specifically, it is preferable to prepare the tin-silver based binary solder. For example, it is preferable to prepare solder in such a way that a weight ratio between tin and silver (Sn Ag) is from about 99.9:0.1 to about 98.0:2.0, more preferably, about 99.7:0.3 to about 99.0:1.0. It is possible to increase the creep characteristic of the tin-silver based solder by decreasing the weight ratio of silver less than eutectic composition.
Moreover, as is the case with the tin-zinc based solder, by decreasing the weight ratio of silver, the difference between a liquidus line temperature and a solidus line temperature can be increased, which in turn can make the solidification process progress gradually and, hence, can reduce residual stress.
Further, it is possible to increase the creep characteristic by preparing solder in such a way that a weight ratio between the total weight of tin and silver and copper ((Sn+Ag):Cu) is from about 99.9:0.1 to about 99.5:0.5, in the case a weight ratio between tin and silver (Sn:Ag) is from about 99.9:0.1 to about 98.0:2.0.
(3) Tin (Sn)-Copper (Cu) Based Solder
In the eutectic composition of tin-copper based solder, a weight ratio between tin and copper (Sn:Cu) is about 99.3:0.7 and, hence, the longest part of the solder comprises tin. For this reason, when comparison is made on the creep characteristic among tin-silver based solder, tin-zinc based solder, and tin-copper based solder, each of which has a eutectic composition, the tin-copper based solder having a eutectic composition shows the most excellent creep characteristic. However, when the tin-copper based solder with eutectic composition is solidified at a cooling rate under common mounting conditions, the 11 phase (Cu₆Sn₅) of a metallic compound, which is hard and brittle, is crystallized in the solder by supercooling. To completely prevent the n phase from being crystallized, it is effective to decrease the weight ratio of copper so that the solder has a hypo-eutectic composition. Specifically, it is preferable to prepare solder at a weight ratio between tin and copper (Sn:Cu) ranging from about 99.9:0.1 to about 99.5:0.5. When solder having this composition ratio is cooled at a cooling rate conducted in a wide variety of applications, a metallic structure of a tin-copper eutectic structure and a β-Sn phase is observed and the crystallization of a metallic compound can be prevented.
(Modifications of the First Embodiment)
(4) Tin (Sn)-Zinc (Zn)-Silver (Ag) Based Solder
The tin-zinc based solder is a solder having an excellent creep characteristic as described above and is suitable for use as the internal bonding material of a power device and also in terms of the bonding temperature and the coefficient of linear expansion. Moreover, by adding silver to the tin-zinc based solder, solder wettability can be improved.
When the weight ratio between tin and zinc (Sn:Zn) ranges from about 88:12 to about 80:20, it is preferable to prepare the solder such that the weight ratio between the total of tin and zinc and silver ((Sn⁺ Zn):Ag) ranges from about 99.99:0.01 to about 95:5. This tin-zinc-copper based solder has small surface tension and excellent solder wettability when it is melted.
When the weight ratio of silver is outside the above range, the weight ratio of silver in an eutectic phase is increased to make the eutectic phase stronger or to crystallize Ag₃Sn of a metallic compound thereby to losing the conventional characteristic of an excellent creep characteristic, which is not preferable.
Therefore, as to the weight ratio of silver, the weight ratio between the total of tin and zinc and silver ((Sn+Zn):Ag) preferably ranges from about 99.99:0.01 to about 95:5, more preferably, from about 99:1 to about 97:3.
(5) Tin (Sn)-Zinc (Zn)-Copper (Cu) Based Solder
The solidus line temperature of the tin-zinc based solder is approximately 199° C. and, hence, soldering by the tin-zinc based solder can be conducted at a bonding temperature that is the same as using the tin-lead eutectic solder currently used. In addition, the addition of copper to the tin-zinc based solder, like the addition of zinc, decreases the melting point.
Specifically, it is preferable that the weight ratio between tin and zinc (Sn:Zn) ranges from about 88:12 to about 80:20 and that the weight ratio between the total of tin and zinc and copper ((Sn+Zn):Cu) ranges from about 99.9:0.1 to about 99.5:0.5. The solidus line temperature of tin-zinc based solder of this composition is approximately 194° C. When the weight ratio of copper is outside the above range, the addition of copper is conducive to the enhancement of mechanical strength, in particular, tensile strength, and, hence, is not preferable in terms of losing the creep characteristic intrinsic to the tin-zinc based solder. Therefore, as to the weight ratio of copper, the weight ratio between the total of tin and zinc and copper ((Sn+Zn):Cu) preferably ranges from about 99.9:0.1 to about 99.5:0.5, more preferably, from about 99.9:0.1 to about 99.7:0.3.

Second Embodiment

The second embodiment relates to a semiconductor including: a first body to be bonded; a second body to be bonded which is different in the coefficient of thermal expansion from about the first body; and lead-free solder that is interposed between the first body to be bonded and the second body to be bonded. The solder contains tin and zinc and the zinc content in a surface in contact with the first body to be bonded is larger than the zinc content in a surface in contact with the second body to be bonded. In other words, the second embodiment relates to solder for bonding the first body to the second body which is different in the coefficient of thermal expansion from the first body to be bonded and to lead-free solder which contains tin and zinc. The zinc content in a surface in contact with the first body is larger the a zinc content in a surface in contact with the second body to be bonded. Such lead-free solder has the function and effect of having excellent tensile strength and rupture elongation. Hereafter, referring to the Figure, the second embodiment will be described.
In a power semiconductor device as shown in FIG. 1, the second body 8 to be bonded is arranged on the first body 2 to be bonded with lead-free solder 4 and a metal circuit board 6 sandwiched there between. Moreover, mounted on the second body 8 is a power semiconductor element 20 a including a metal circuit board 10 a, a solder resistor 12 a arranged on the metal circuit board 10 a and having high-temperature solder 14 a embedded therein, a power semiconductor pellet 16 a arranged on the high-temperature solder 14 a; and a power semiconductor element 20 b having the same construction as the power semiconductor element 20 a.
The power semiconductor pellets 16 a, 16 b such as power transistors generate a large amount of heat when high voltage and high current are applied thereto. For this reason, when the power semiconductor device mounted with the power semiconductor pellets 16 a, 16 b is repeatedly activated and deactivated to repeat the cycles of increasing and decreasing temperature, a thermal stress is developed by the difference in the coefficient of linear expansion between materials to cause strain in the bonded portions of parts in the power semiconductor device. When the temperature is rapidly increased, the high- temperature solders 14 a, 14 b, comprising the bonded portions, are melted. When physical damage, such as crack and rupture, is caused by the melting, the performance of the power semiconductor device is changed. In short, thermal fatigue has a large effect. For this reason, to efficiently dissipate heat generated by the power semiconductor pellets 16 a, 16 b, a metal plate (heat sink) mainly formed of copper or copper alloy, which is referred to as the first body 2, is fixed to the bottom of the power semiconductor device. However, the second body 8, on which the power semiconductor pellets 16 a, 16 b are directly mounted, is made of aluminum, silicon oxide, or silicon nitride to ensure the strength of the whole power semiconductor device. Hence, the coefficient of linear expansion of the second body 8 is greatly different from that of the first body 2 and the difference in the coefficient of linear expansion is approximately 2 to 10 times (for example, the coefficient of linear expansion of aluminum nitride (AIN) is 4×10⁻⁶/K, whereas that of copper (Cu) is 17.7×10⁻⁶/K).
In this case, the conventionally used tin-lead (Sn—Pb) eutectic solder has an excellent ductility property in a range from room temperatures to 125° C. In other words, even if the tin-lead eutectic solder is heated and melted to bond the first body 2 to the second body 8 and then the difference in contraction between the first body 2 and the second body 8 is large in a cooling process, the solder can follow the contraction due to its deformability. Hence, the solder does not cause substantial cracking at the bonding portions and, hence, is useful for ensuring the reliability of the power semiconductor device. However, from the viewpoint of recent environmental protection concerns, solder containing lead cannot be used.
As a result of research on various kinds of metallic materials, the present inventors have found that tin (Sn)-zinc (Zn) based solder, in which the zinc content is changed in a functionally gradient manner from one bonding surface to the other bonding surface in one pair of portions to be bonded, is useful as a deformable solder. That is, the inventors have found it useful: to change the zinc content of tin-zinc based solder in a functionally gradient manner to increase deformability to the tin-zinc based solder; and to insert the tin-zinc based solder between one pair of surfaces to be bonded, in which the surfaces are different in the coefficient of linear expansion, to use the tin-zinc based solder as a bonding material. This can be achieved by arranging an alloy layer having low deformability characteristic and a composition high in zinc content on the second body 8 having a small coefficient of linear expansion of the power semiconductor device and by arranging an alloy layer having high deformability and a composition relatively low in zinc content on the first body 2 having a large coefficient of linear expansion. Specifically, assuming that the zinc content of the alloy having a low deformability characteristic and used on the second body 8 having a small coefficient of linear expansion of the power semiconductor device is X (mass %) and that the zinc content of the alloy having a high deformability characteristic and used on the first body 2 having a large coefficient of linear expansion is Y (mass %), to satisfy the following two equations at the same time is preferable in terms of ensuring reliability as a bonding material.
23≦X≦40 (1)
0≦Y≦(X−23) (2)
Here, equation (1) shows that when the second body 8 and the first body 2 are arranged on the upper side and on the lower side in the structure of the power semiconductor device, the zinc content of the uppermost layer of lead-free solder 4 in contact with the second body 8 on the upper side of the power semiconductor device is X (mass %). Moreover, equation (2) shows that the zinc content Y (mass %) of the lowermost layer of lead-free solder 4 in contact with the first body 2 on the lower side of the device varies as a dependent variable of X. When the value of X is smaller than 23, the effect of a functionally gradient material is reduced and when the value of X is larger than 40, the ductility of the lead-free solder 4 becomes extremely small, so it is preferable that x is within the range of equation (1). In addition, to keep the structure of the functionally gradient material within the range of the value of X shown by equation (1), in the case of sheet solder, it is preferable in terms of the ease of process control that the value of Y is within the range of the value of equation (2).
If the tin-zinc based alloys are worked in a low-oxygen atmosphere with an oxygen concentration of 100 ppm or less, the tin-zinc based alloys can be worked into the shape of a sheet while they are put into press-contact with each other. For this reason, not only by arranging two sheets of different compositions in layers as the solder construction but also arranging multiple layers, such as three and four layers of different compositions in a functionally gradient manner, deformability can be enhanced within a range satisfying equations (1) and (2).
For example, for sheet-shaped solder of two layers, which are constructed in such a way that their zinc contents X and Y satisfy equations (1) and (2), that is, X=30 and Y=5, the process temperature of an apparatus in a melting process can be set within a range from 240° C. to 270° C., which is within the range of process temperature control of a currently used soldering apparatus. Hence, the process temperature of the layers of sheet-shaped solder can be easily controlled.
When solder containing zinc is oxidized, a melting temperature is rapidly increased and wettability and strength are extremely decreased. Hence, it is preferable that a process of preparing solder, in particular, a process of melting and mixing solder is performed in such a way that the oxygen content of obtained solder is 100 ppm (by weight) or less while the solder is prevented from being oxidized by the use of a non-oxidizing atmosphere such as nitrogen or argon. Moreover, to decrease oxygen contained in tin and zinc raw materials, there is a method of adding phosphorus, magnesium or the like, which has a low melting point and easily reacts with oxygen, as a deoxidizer for these melted raw materials. The deoxidizer reacts with oxygen in the melted raw materials and floats up as a slug to the surface of the melted raw materials, so that the oxygen contained in tin and zinc as raw materials can be easily removed. It is preferable that the amount of deoxidizer is approximately 0.01 to 0.1 weight % of the raw materials. When raw materials deoxidized by this method are used, sheet-shaped solder with an oxygen content is decreased to 30 ppm or less can be prepared.
The prepared solder has flux added thereto, if necessary, after it is worked into the sheet-shaped solder, so that bonding can be completed. The flux is prepared by mixing various substances as required to achieve efficient chemical and physical action.
As a method of forming a bonded body by covering the second body to be bonded with solder, when metal on the surface is covered with metal resistant to oxidation, it is also possible to directly heat and melt the solder on the surface of the bonded body. On the other hand, when oxide is formed on the surface as is the case with copper or the like, the amount of oxygen on the surface of the bonded body is decreased by a reducing treatment. The reducing treatment may use alcohol vapor such as methanol vapor, ethanol vapor, or propanol vapor; acid vapor such as formic acid, acetic acid, or the like; and a reducing gas such as ammonia, hydrogen, or the like, whereby soldering can be performed. Moreover, a method of integrating a device with a heat sink includes reflow heating and VPS. By performing the reflow heating in a non-oxidizing atmosphere such as nitrogen gas, argon gas, or the like, or in a low-oxygen atmosphere having an oxygen concentration of 1000 weight ppm or less, the reliability of bonded surfaces can be improved.
(Use of Solder)
Lead-free solder in accordance with the first and second embodiments can be used as a substitute for the internal bonding material of the conventional power semiconductor device. The lead-free solder in accordance with the first and second embodiments is a material suitable for forming a junction and a film in a power semiconductor pellet and a device using the same. For example, the power semiconductor device is constructed in the form of a power module, such as power transistor module, a power IC, or the like, which uses a power bipolar transistor, a thyristor, a GTO thyristor, a power diode, a power MOS field effect transistor (power MOSFET), or the like as the power semiconductor pellet. The lead-free solder in accordance with the first and second embodiments can be applied to the bonding of not only parts of one kind of metal such as copper, silver, gold, nickel, aluminum, SUS stainless steel, or the like, but also parts of alloy materials, composite metallic materials, or the like. A metallic pre-coating may be previously applied to the parts by plating, press-contacting, or the like according to the materials of the parts to be bonded, and the composition of the pre-coating and a method of pre-coating can be suitably selected.
Further, as a working form, the thickness of a sheet-shaped lead-free bonding material is within a range from about 0.05 mm to about 0.5 mm and, to secure appropriate thermal conductivity, it is more preferable that the thickness is within a range from about 0.1 mm to about 0.3 mm. For example, it is also possible to enhance the strength of a bonded portion: by applying a nickel/gold flash plating, or a tin plating to; or by placing a metallic layer formed by applying and heating a paste mixed with tungsten particles or titanium particles, in advance, on a side in contact with the solder of the second body of the power semiconductor device. On the other hand, it is also recommended to employ a structure in which a zinc-rich layer is placed on the second body in place of the metallic layer in this manner, which is different from the composition of the solder. However, in this case, to enhance an anchoring effect, it is preferable to form an uneven surface having asperities of from about 0.5 μm to about 1.5 μm on the surface layer of a ceramic substrate as the second body to be bonded.

EXAMPLE

Hereafter, the present invention will be described in detail by the use of examples.

Examples 1 to 14

Comparative examples 1 to 5

(Preparation of Sheet-Shaped Solder)
An ingot of tin having a purity of 99.99% or higher was put into a rectangular solder melting tank. Then, the tin ingot was heated to the melting point by a heater attached to the outside of the solder melting tank. At the start of melting, nitrogen was introduced into the upper portion of the solder melting tank to bring the concentration of oxygen in a nitrogen atmosphere to 50 ppm or less. After the tin ingot was melted, the temperature of tin melt was kept at 450° C. by a feedback control.
Next, a zinc ingot, a silver ingot, and a copper ingot, each of which has a purity of 99.99% or higher, were added to the tin melt to be melted so as to make composition ratios shown in Table 3. The temperature of the melt was maintained at 450° C. by the feedback control. Parts of the melts were taken out of the solder melting tank and were cooled to room temperature to produce uniform materials of solder. The materials of solder produced in the above manner were cut into tensile test samples, each having a cross section of 4.0 mm×5.0 mm and a gage length of 25.0 mm. The dependency upon Zn, Ag, Cu concentrations for strain rate are shown in FIG. 2 to FIG. 4.
(Evaluation of Creep Characteristic)
Evaluations of rupture elongation (%), strain rate (1/sec), tensile strength (N/mm²), and creep characteristic were made on the obtained tensile test samples.
(Results)
Sn—Zn Based Solder:
A clear characteristic of the Sn—Zn based alloy of binary elements is that it does not form a metallic compound, and when the zinc content is from about 9 to about 20 mass %, as shown by the results of rupture elongation, a reduction in ductility was not observed. In the creep characteristic when a load of low stress was applied, the Sn—Zn based solder showed more remarkably excellent characteristics than Sn-9Zn eutectic solder, the peak of which was shown by the Sn—Zn based solder containing about 15 mass % zinc. As to its strength property, the samples of Sn—Zn based solder were excellent on the whole.
Sn—Ag based solder, Sn—Cu based solder:

Unlike the Sn—Zn based solder, in hyper-eutectic was formed a metallic compound for enhancing toughness as an alloy, so it was thought that hypo-eutectic was necessary to enhance creep characteristic. However, even in hypo-eutectic, it was thought that supercooling was caused at a practical level cooling rate in a casting process to crystallize partial metallic compounds, so that a Sn-rich composition is desired. From the test results, on the whole, as an eutectic composition was changed to a hypo-eutectic composition, both ductility (result of rupture elongation) and creep characteristic increased. However, as to the strength property, as a Sn ratio in a composition increased, the Sn property became more predominant and, hence, the strength property remarkably decreased. For this reason, to maintain 10 N/mm²as a standard value, it was considered that the addition of 0.1 mass % or more Sn was necessary as a minimum condition.

TABLE 3


composition	rupture	strain rate	creep	strength

Sn

Zn

Ag

Cu

elongation

5 MPa

10 MPa

strength

characteristic*⁾

prop-

No.

(mass %)

(%)

(1/sec)

(N/mm²)

5 MPa

10 MPa

erty**⁾

example	1	the rest	12	—	—	38	7.07 × 10⁻¹³	936 × 10⁻¹⁰	46	Δ	Δ	⊚
	2	″	15	—	—	38.2	1.51 × 10⁻¹¹	7.26 × 10⁻⁰⁹	46.8	⊚	◯	⊚
	3	″	16	—	—	31.6	1.68 × 10⁻¹²	2.11 × 10⁻⁰⁹	42.9	◯	Δ	⊚
	4	″	18	—	—	39.2	1.28 × 10⁻¹²	1.50 × 10⁻⁰⁹	45.4	◯	Δ	⊚
	5	″	20	—	—	42.4	1.47 × 10⁻¹²	1.79 × 10⁻⁰⁹	43.5	◯	Δ	⊚
	6	″	—	0.1	—	49.3	4.94 × 10⁻¹⁶	1.85 × 10⁻⁰⁶	15.3	X	⊚	◯
	7	″	—	0.3	—	40.4	3.70 × 10⁻¹²	3.38 × 10⁻⁰⁶	14.6	⊚	⊚	Δ
	8	″	—	0.5	—	57.5	9.74 × 10⁻¹¹	5.68 × 10⁻⁰⁵	12.8	⊚	⊚	Δ
	9	″	—	0.7	—	36	6.82 × 10⁻¹³	8.34 × 10⁻⁰⁸	19.6	⊚	⊚	◯
	10	″	—	1	—	34.5	1.41 × 10⁻¹¹	9.86 × 10⁻⁰⁷	17.3	⊚	⊚	◯
	11	″	—	2	—	24.5	5.80 × 10⁻¹⁵	2.45 × 10⁻⁰⁹	22	Δ	◯	⊚
	12	″	—	—	0.1	55.2	4.39 × 10⁻⁰⁹	6.71 × 10⁻⁰⁵	13.8	◯	⊚	Δ
	13	″	—	—	0.2	39.4	2.24 × 10⁻⁰⁸	6.58 × 10⁻⁰⁵	14.9	⊚	◯	Δ
	14	″	—	—	0.5	41.5	1.28 × 10⁻⁰⁹	1.44 × 10⁻⁰⁵	15.9	◯	Δ	◯
com-	1	″	—	—	—	64.8	2.54 × 10⁻⁰⁵	1.04 × 10⁻⁰¹	9.8	—	—	X
parative	2	″	9	—	—	39.3	9.42 × 10⁻¹⁴	2.68 × 10⁻¹⁰	44	—	—	⊚
example	3	″	40	—	—	32.8	4.37 × 10⁻¹³	3.69 × 10⁻¹⁰	62.5	Δ	X	⊚
	4	″	—	3	—	21.9	7.69 × 10⁻¹⁶	1.32 × 10⁻¹⁰	27.8	—	—	⊚
	5	″	—	—	0.7	34.6	1.18 × 10⁻¹⁰	3.47 × 10⁻⁰⁶	16.7	—	—	◯

note
*⁾creep characteristic: ⊚: more than 100 times, ◯: more than 10 times, Δ: more than 2 times, X: less than 2 times as much as each eutectic composition
**⁾strength property: ⊚: more than 20 N/m², ◯: more than 15 N/m², Δ: more than 10 N/m², X: less than 10 N/m²

Examples 15 to 29

Comparative Examples 6 to 12

(Preparation of Sheet-Shaped Solder)
Uniform materials of solder produced in the same manner as in the examples 1 to 8 except for making composition ratios shown in Tables 5 and 6 were cold-rolled into sheets each having a thickness of 0.1 mm.
(Wettability Test)
Next, to check the wettability of sheet-shaped solder, a sheet-shaped solder of 1 mm length×1 mm width×0.1 mm thick was placed on a copper plate (oxygen-free copper having its surface previously cleaned by acid) of 15 mm length×30 mm width×0.3 mm thick. Then, the solder was heated by a hot plate to 250° C. to check the wettability of the solder. At that time, experiments were conducted in the case of using flux (RA) and in the case of not using flux.
A case where the sheet-shaped solder was wetted and kept its initial area was assumed to be A, a case where the sheet-shaped solder wetted, though the area decreased, was assumed to be B, and a case where the sheet-shaped solder was not wet was assumed to be C.
A three-point bending test (load capacity: ±1 kN, displacement accuracy: 0.5 μm) was conducted to check the bending elastic characteristic of bonded portion of the sheet-shaped solder. A sheet-shaped solder of 1 mm length×1 mm width×0.1 mm thick was bonded to the center of a copper plate (oxygen-free copper having its surface previously cleaned by acid) of 15 mm length×30 mm width×0.3 mm thick. The center was made a reference point and a measurement was conducted at a jig displacement speed of 0.1 mm/sec.
A case where an interface between the copper plate and the sheet-shaped solder was in good bonding state was denoted by 0 and a case where cracks developed at the interface was denoted by ×.

Further, in consideration of actual bonding in a device, a ceramic substrate of 70 mm length×35 mm width×1 mm thick was bonded to a copper plate of 100 mm length×50 mm width×10 mm thick by a sheet-shaped solder of 65 mm length×30 mm width×0.1 mm thick, and the amount of warp developed at that time was measured at a bonding temperature of 25° C. and in the case of using flux (RA). Evaluations were conducted as follows: a case where the amount of warp was nearly equal to that of tin-lead eutectic solder was assumed to be A; a case where the amount of warp was nearly equal to or larger than that of Sn-3.0Ag-0.5Cu alloy was assumed to be C; and a case where the amount of warp was between A and C was assumed to be B. The obtained experiment results are shown in Table 4 and Table 5.

	TABLE 4


	wettability
	test

	Tin	Zinc	Silver	Copper	without	with	bending	amount
Example No.	mass %	mass %	mass %	mass %	flux	flux	test	of warp

15	88	12	0	0	C	A	◯	A
16	85	15	0	0	C	A	◯	A
17	80	20	0	0	C	A	◯	B
18	87	12	1	0	B	A	◯	A
19	85	12	3	0	B	A	◯	A
20	77	20	3	0	B	A	◯	B
21	87.9	12	0	0.1	C	A	◯	A
22	87.7	12	0	0.3	C	A	◯	A
23	79.5	20	0	0.5	C	A	◯	B
24	99.8	0	0.1	0.1	B	A	◯	B
25	99.2	0	0.5	0.3	B	A	◯	A
26	98.9	0	1.0	0.1	A	A	◯	A
27	98.5	0	1.0	0.5	B	A	◯	A
28	99.9	0	0.1	0	B	A	◯	B
29	99	0	1.0	0	A	A	◯	A

	TABLE 5


	Wettability
	test

comparative	tin	Zinc	Silver	Copper	Without	With	Bending	amount
example No.	mass %	mass %	mass %	Mass %	flux	flux	test	of warp

6	85	30	0	0	C	B	X	C
7	83	12	5	0	B	A	◯	C
8	75	20	5	0	C	A	X	C
9	87.2	12	0	0.8	C	A	◯	C
10	79.2	20	0	0.8	C	A	X	C
11	94.9	0	5	0.1	B	A	◯	C
12	95	0	5	0	B	A	◯	C

From the examples 15 to 29, it was determined that lead-free bonding materials could be provided, which could be prepared at low cost and with ease, by the use of raw materials, with a wide variety of applications, and can relax the thermal stress caused by the coefficient of thermal expansion of a substrate.

Examples 30 to 45

Comparative examples 13 to 16

[Preparation of Test Sample]
Respective alloys were prepared by heating and melting tin having a purity of 99.98% and zinc having a purity of 99.99% in a nitrogen atmosphere of an oxygen concentration of 100 ppm or less, so as to make the compositions of examples 30 to 45 and comparative examples 13 to 16 shown in Table 6. Then, the melts were cooled to room temperature and were rolled into sheets by a rolling machine to produce sheet-shaped materials of solder.
[Use of Solder]
Flux containing 12 mass % rosin, 0.1 mass % halogen in terms of chlorine, and isopropyl alcohol as a solvent was dropped at a rate of 0.01 cc/cm²over the surface of the produced solder and then was sandwiched between a ceramic substrate (size: 35 mm×70 mm×1 mm t) as the second body to be bonded and a copper plate (heat sink plate: 40 mm×75 mm×3 mm t) as the first body to be bonded. Further, the sandwiched product was heated and melted within a reflow peak temperature ranging from about 230° C. to about 270° C. for a maximum of 20 seconds to melt the solder to produce a bonded body. The degree of warp developed at this time was measured and evaluated as follows: when compared with the warp (mean warp: 100 μm) of a bonded body produced by the same method except for overlaying two sheets made of tin-lead eutectic solder and having a thickness of 100 μm, a case where the degree of warp is +30 or less was determined to be A; a case where the degree of warp was from +30% to +70% was determined to be B; and a case where the degree of warp was +70% or more or a case where cracks developed was determined to be C. The evaluation results are shown in Table 6.

Here, as to X and Y in Table 6, the zinc content of an alloy having a small deformability and used on the side of a ceramic substrate having a small coefficient of expansion of a power semiconductor device is denoted by X (mass %), and the zinc content of alloy having a large deformability and used on the side of a heat sink having a large coefficient of expansion is denoted by Y (mass %).

TABLE 6


	Alloy layer
	on the side		Alloy layer on
	of ceramic		the side of				Reflow
	substrate		heat sink		Ceramic		peak	Determination
	Zinc content	Thickness	Zinc content	Thickness	substrate	Heat sink	temperatur	of
	X (wt. %)	(μm)	Y (wt. %)	(μm)	material	material	(° C.)	warp

Example 30	23	100	0	100	silicon nitride	copper	230	A
Example 31	25	100	1	100	silicon nitride	copper	230	A
Example 32	25	100	2	100	silicon nitride	copper	230	B
Example 33	30	100	0	100	silicon nitride	copper	240	A
Example 34	30	100	5	100	silicon nitride	copper	240	A
Example 35	30	100	5	70	silicon nitride	copper	240	A
Example 36	30	130	5	70	silicon nitride	copper	240	A
Example 37	30	100	7	100	silicon nitride	copper	240	B
Example 38	30	100	5	100	aluminum nitride	copper	240	A
Example 39	30	100	5	100	aluminum nitride	copper	240	A
Example 40	30	100	5	100	aluminum nitride	nickel/gold	240	A
						plating
Example 41	30	100	5	100	silicon nitride	without	240	A
						(subjected
						to asperity
						producing
						treatment)
Example 42	30	100	5	100	silicon nitride	tungsten	240	A
Example 43	40	300	0	100	silicon nitride	copper	260	B
Example 44	40	300	10	100	silicon nitride	copper	260	B
Example 45	40	300	17	100	silicon nitride	copper	270	B
comparative	23	100	20	100	silicon nitride	copper	270	C
example 13
comparative	30	100	25	100	silicon nitride	copper	300	C
example 14
comparative	40	300	25	100	silicon nitride	copper	300	C
example 15
comparative	45	300	0	100	silicon nitride	copper	270	C
example 16

As described above, it was determined in the present examples 30 to 45 that since the bonding layer having a large deformability was formed by arranging the alloy with a small ductility on the side of the ceramic substrate having a small coefficient of thermal expansion and by arranging the alloy having a large ductility on the side of metal with a large coefficient of thermal expansion, even if the solder was lead-free, it was possible to prevent the occurrence of crack in the bonded portion of the power ceramic device.
According to the present invention, there is provided lead-free solder that has an excellent creep characteristic and effective in stress relaxation.
In this manner, needless to say, the present invention can include various examples which have not described above. Therefore, the technical scope of the present invention should be determined only by the following claims appropriately derived from the above descriptions.

Claims

1. Lead-free solder comprising tin and zinc at a weight ratio between tin and zinc ranging from about 88:12 to about 80:20.

2. The lead-free solder as claimed in claim 1, wherein the lead-free solder essentially consists of tin and zinc at a weight ratio between tin and zinc ranging from about 88:12 to about 80:20 on the basis of total weight of the lead-free solder.

3. The lead-free solder as claimed in claim 1, wherein the lead-free solder essentially consists of tin and zinc at a weight ratio between tin and zinc ranging from about 86:14 to about 83:17 on the basis of total weight of the lead-free solder.

4. Lead-free solder comprising tin and silver at a weight ratio between tin and silver ranging from about 99.9:0.1 to about 98.0:2.0.

5. The lead-free solder as claimed in claim 4, wherein the lead-free solder essentially consists of tin and silver at a weight ratio between tin and silver ranging from about 99.9:0.1 to about 98.0:2.0 on the basis of total weight of the lead-free solder.

6. Lead-free solder comprising tin and silver at a weight ratio between tin and copper ranging from about 99.7:0.3 to about 99.0:1.0.

7. The lead-free solder as claimed in claim 6, wherein the lead-free solder essentially consists of tin and silver at a weight ratio between tin and silver ranging from about 99.7:0.3 to about 99.9:0.1 on the basis of total weight of the lead-free solder.

8. Lead-free solder comprising tin and copper at a weight ratio between tin and copper ranging from about 99.9:0.1 to about 99.5:0.5.

9. The lead-free solder as claimed in claim 8, wherein the lead-free solder essentially consists of tin and copper at a weight ratio between tin and copper ranging from about 99.9:0.1 to about 99.5:0.5 on the basis of total weight of the lead-free solder.

10. A semiconductor device comprising:

a first body to be bonded to a lead-free solder;

a second body to be bonded to a lead-free solder that has a different coefficient of thermal expansion than the first body; and

a lead-free solder interposed between the first body and the second body; the solder comprises tin and zinc, the zinc content of a surface in contact with the first body being larger than the zinc content of a surface in contact with the second.

11. The semiconductor device as claimed in claim 10, wherein, the second body has a larger coefficient of thermal expansion than the first body, and wherein when the zinc content of a surface on a side of the first body is X (mass %) and the zinc content of a surface on a side of the second body is Y (mass %), X and Y satisfy the conditions,

23≦X≦40, and
0≦Y≦X−23.