JP2012200789A - Au-Sn ALLOY SOLDER - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide lead-free Au-Sn alloy solder for a high temperature being sufficiently used in joining requiring very high reliability, having an operation temperature that is <400°C, preferably, ≤370°C, being excellent in wettability, and obtaining high joining strength.SOLUTION: The Au-Sn alloy solder contains ≥18.5 mass% and ≤23.5 mass% of Sn, and contains at least one of ≥0.001 mass% and ≤0.5 mass% of P or ≥0.03 mass% and ≤1.5 mass% of Ge, and the balance Au.

Description

  The present invention relates to a solder, and more particularly to a high temperature Au—Sn alloy solder.

  In recent years, regulations on chemical substances harmful to the environment have become stricter, and this regulation is no exception for solder materials used for the purpose of joining electronic components and the like to substrates. Lead has been used as a main component in solder materials for a long time, but it has already been a regulated substance under the Rohs Directive. For this reason, the development of solder that does not contain lead (Pb) (lead-free solder, lead-free solder) has been actively conducted.

  Solders used when bonding electronic components to a substrate are roughly classified into high temperature (about 260 ° C. to 400 ° C.) and medium / low temperature (about 140 ° C. to 230 ° C.) depending on the limit temperature of use. As for the solder for medium and low temperature, it is mainly composed of Sn, and is lead-free. For example, lead-free solder containing Sn as a main component, Ag of 1.0 to 4.0% by mass, Cu of 2.0% by mass or less, Ni of 0.5% by mass or less, and P of 0.2% by mass or less. An alloy composition is described (for example, refer to Patent Document 1), containing 0.5 to 3.5% by mass of Ag, 0.5 to 2.0% by mass of Cu, and the balance consisting of Sn. (See, for example, Patent Document 2).

  On the other hand, various organizations have also developed lead-free solder materials for high temperatures. For example, a Bi—Ag brazing material containing 30 to 80% by mass of Bi and having a melting temperature of 350 to 500 ° C. is disclosed (for example, see Patent Document 3). Further, a solder alloy is disclosed in which a binary eutectic alloy is added to a Bi-containing eutectic alloy and an additional element is further added (see, for example, Patent Document 4). Although it is a system solder, it has been shown that the liquidus temperature can be adjusted and variation can be reduced.

  As an expensive lead-free solder material for high temperature, Au—Sn alloy, Au—Ge alloy or the like has already been used in MEMS (Micro Electro Mechanical Systems) or the like. For example, it contains 5 to 15% by mass of Au, 0.1 to 10% by mass of Bi, 0.1 to 10% by mass of In and 0.1 to 10% by mass of Sb, and the rest Describes a mixture of a Sn—Au alloy solder powder having a component composition composed of Sn and inevitable impurities and a flux (see, for example, Patent Document 4). Patent Document 5 contains 15 to 25% by mass of Sn, more than 100 to 500 ppm of oxygen, the remainder composed of Au and inevitable impurities, and 30% of the powder having a particle size of 10 to 35 μm. Au-Sn for solder paste with a small wetting spread and having an average particle size in the range of 10 to 35 μm as measured by a laser scattering / analysis method in which a powder having a particle size of 40 μm or less occupies 90% or more of the whole An alloy powder is described (for example, see Patent Document 5).

Japanese Patent Laid-Open No. 1999-077366 JP-A-8-215880 JP 2002-160089 A JP 2008-161913 JP 2003-260588 A

  High-temperature lead-free solder materials have been developed by various organizations, but no low-cost and versatile solder materials have yet been found. In other words, since materials having relatively low heat resistance such as thermoplastic resins and thermosetting resins are generally used for electronic parts and substrates, the working temperature must be less than 400 ° C., preferably 370 ° C. or less. There is. However, for example, in the Bi-Ag brazing material disclosed in Patent Document 3, since the liquidus temperature is as high as 400 to 700 ° C, the working temperature at the time of joining is estimated to be 400 to 700 ° C or more, and the joining is performed. It will exceed the heat resistance temperature of electronic components and boards.

  In the case of an expensive Au—Sn solder, although it is put into practical use, it is used for soldering at a location requiring particularly reliability, such as MEMS, and therefore further reliability is required. In Patent Document 4, Bi, In, and Sb are added from the point of reducing surface tension to improve wettability. In this case, since the form of the solder is a paste, a reduction effect by flux can be expected. Therefore, it is considered that the solder surface tension can be reduced and the wettability can be improved. However, when only a solder alloy, such as a solder sheet or a wire, that is, when there is no reducing agent such as flux, In is easier to oxidize than Au and Sn, and Bi and Sb are easier to oxidize than Au. Therefore, regardless of the addition amount of these elements, such as 1% or less, the solder surface is oxidized thermodynamically and wettability is reduced.

  Naturally, when the amount of oxygen in the solder is intentionally increased as in Patent Document 5, if there is no reducing agent and the solder alloy is used alone, the wettability is extremely reduced, and in some cases, electronic components or the like cannot be joined. I can imagine it easily.

The present invention can be used satisfactorily for bonding such as MEMS, and the working temperature is less than 400 ° C., desirably 370 ° C. or less, and has excellent wettability and high bonding strength. It is an object to provide a lead-free Au—Sn alloy solder for high temperature.
Sufficient wettability here means that the solder alloy melts without causing voids (voids) during the joining operation and spreads to an appropriate area on the joining surface. A contact area is ensured, and an object to be bonded can be firmly fixed to achieve highly reliable bonding. In addition, high reliability means that, as a result of strong bonding, it is possible to withstand a temperature change in a use environment and maintain a long-life and stable bonding.

The Au—Sn alloy solder of the present invention has an Au—Sn eutectic composition, specifically, a Sn content of 18.5 mass% or more and 23.5 mass% or less, and contains one or more of P or Ge. It is an Au—Sn alloy solder that is contained in a predetermined amount and the balance is made of Au, so that it is excellent in wettability and can be bonded with excellent bonding strength and reliability.
That is, the Au—Sn alloy solder of the present invention contains Sn in the range of 18.5% by mass to 23.5% by mass, 0.001% by mass to 0.5% by mass of P, or 0.03% by mass to 1%. It is an Au—Sn alloy solder containing at least one of Ge of 5 mass% or less and the balance being made of Au.

  According to the present invention, it is possible to provide a high-temperature lead-free solder having a strength necessary for joining an electronic component and a substrate. That is, by adding a metal element of P or Ge in the vicinity of the Au—Sn eutectic composition so as to have a predetermined content, solder having excellent wettability, resulting in high joint strength and high reliability. An Au—Sn alloy solder that enables bonding can be provided.

FIG. 1 is a diagram showing a cross-sectional structure of a wettability test piece.

  In order to obtain a highly reliable bond with excellent bondability and excellent bonding strength, it is important to have appropriate wettability with the substrate when the solder is melted. If sufficient wettability can be ensured when the solder is melted, a sufficient bonding area can be ensured, the bonding strength is increased, and highly reliable solder bonding with high durability can be obtained. The appropriate wettability is a standard that spreads to 110% or more of the joint area when the solder is melted. Further, in order to obtain a strong and reliable joint, it is necessary to prevent voids from being generated inside the molten solder. It is necessary to suppress the void generation rate to 5% or less.

The composition of the Au—Sn alloy solder of the present invention contains 18.5 mass% or more and 23.5 mass% or less of Sn, contains at least one of P or Ge, and contains 0.001 when P is contained. It is contained in an amount of not less than 0.5% by mass and not more than 0.5% by mass. When Ge is contained, it is not less than 0.03% by mass and not more than 1.5% by mass, and the balance is Au.
Based on the vicinity of the Au-Sn eutectic composition, by adding a predetermined amount of P or Ge, the wettability is remarkably improved, and the reliability of the joint is improved more than before by the Au-Sn binary alloy. Is possible.
That is, P and Ge are highly reducible, and have the property of reducing and removing oxide films of electronic parts and the like when the Au—Sn binary solder alloy surface is oxidized.

  First, the effect of P will be described. P is particularly highly reducing and has a great effect of removing the oxide film. Furthermore, the addition of P has the effect of reducing the generation of voids during bonding. That is, since P is highly reducible and easily oxidizes itself, oxidation proceeds preferentially over the solder component during bonding. As a result, it is possible to prevent the solder mother phase from being oxidized and to ensure wettability. As a result, good bonding is possible, and void formation is less likely to occur.

P having strong reducibility exhibits the effect of improving wettability even when added in a small amount. On the other hand, if added in a certain amount or more, the effect of improving wettability does not change, and excessive addition may cause P oxide to be generated on the solder surface, or P to form a brittle phase and become brittle. Therefore, P is preferably added in a trace amount.
Specifically, to obtain the reduction effect of P, addition of 0.001% by mass or more is necessary, and the upper limit of the addition amount of P is 0.500% by mass or less. When P exceeds this upper limit, the oxide covers the solder surface, and conversely, wettability may be reduced. Furthermore, when P is added in a large amount, brittle P oxide is segregated, and reliability is lowered. In particular, when processing sheets and wires, it has been confirmed that they are likely to cause cracks, disconnections, or defects.

  Next, Ge will be described. Ge, like P, has a reducing effect. However, the effect can be easily estimated from a thermodynamic point of view and is inferior to P. However, since Ge can reduce Sn and Cu, depending on the bonding conditions, a great improvement in wettability can be achieved by adding a small amount of Ge. Further, Ge has excellent characteristics not found in P. That is, since Ge forms a eutectic alloy with Au, workability and reliability are improved, and in addition, the melting point can be adjusted by the amount of addition. The optimum addition amount of Ge is 0.03% by mass or more and 1.5% by mass or less. If it is 0.03 mass% or less, the amount of addition is too small, and the excellent Ge effect cannot be exhibited. On the other hand, if it exceeds 1.5% by mass, Sn and a low melting point phase (the solid phase temperature of Ge—Sn binary alloy: lower than 231 ° C.) are formed, and it becomes impossible to maintain the electronic components and the like during reflow. Problems arise.

First, Au, Sn, P, and Ge having a purity of 99.9% by mass or more were prepared as raw materials. Large flakes and bulk-shaped raw materials were cut and pulverized, etc. so as to be uniform with no variation in composition depending on the sampling location in the alloy after melting, and were reduced to a size of 3 mm or less. Next, a predetermined amount of these raw materials was weighed into a graphite crucible for a high-frequency melting furnace.
The crucible containing the raw material was placed in a high-frequency melting furnace, and nitrogen was flowed at a flow rate of 0.7 L / min or more per kg of the raw material in order to suppress oxidation. In this state, the melting furnace was turned on to heat and melt the raw material. When the metal began to melt, it was stirred well with a mixing rod and mixed uniformly so as not to cause local compositional variations. After confirming sufficient melting, the high frequency power supply was turned off, the crucible was quickly removed, and the molten metal in the crucible was poured into the solder mother alloy mold. As the mold, a mold having the same shape (plate shape having a thickness of 5 mm) generally used in the production of the solder alloy was used.

  In this way, the solder mother alloy of Sample 1 was produced. Solder mother alloys of Samples 2 to 13 were produced in the same manner as Sample 1 except that the mixing ratio of the raw materials was changed. The compositions of the solder mother alloys of Samples 1 to 13 were analyzed using an ICP emission spectroscopic analyzer (S-8100 manufactured by SHIMAZU). The analysis results are shown in Table 1 below.

  Next, each solder mother alloy of the samples 1 to 13 was processed into a sheet shape by a rolling mill. Moreover, about each Au-Sn type solder alloy processed into the sheet form, the wettability evaluation, the void rate evaluation, and the reliability evaluation by the heat cycle test were performed. Note that the evaluation of solder wettability or bondability does not depend on the solder shape, and may be evaluated by the shape of a wire, ball, paste, or the like, but was evaluated by the shape of a sheet. The obtained results are shown in Table 2 below.

<Processing into sheet shape>
Each solder mother alloy (plate-shaped ingot having a thickness of 5 mm) of Samples 1 to 13 shown in Table 1 was rolled to a thickness of 0.1 mm using a rolling mill. At that time, the sheet was rolled while adjusting the feed speed of the ingot, and then cut into a width of 25 mm by slitting. The sample thus formed into a sheet shape was punched into a 10 mm square shape using a mold installed in a press machine, and used as an evaluation sample. In general, when Au—Sn solder is used, the solder thickness is often about 0.020 to 0.050 mm. However, here, when evaluating wet spread, wettability spreads. The thickness of the solder was intentionally increased so that it was easily reflected in the area.

<Evaluation of wettability (wet spreadability)>
First, a wettability tester (device name: atmosphere control type wettability tester) was started, a double cover was applied to the heater part to be heated, and nitrogen was flowed from four locations around the heater part (nitrogen flow rate: each 12 L / min). Then, the heater set temperature was set to 340 ° C. and heated.
After the heater temperature set at 340 ° C. has stabilized, the Ni plating film (2) (film thickness: 2.0 μm) as shown in FIG. 1, and the Au plating film (3) (film thickness: 1.0 μm) as the uppermost layer The Cu substrate (1) (thickness: 0.3 mm) subjected to () was set on the heater part and then heated for 25 seconds. Next, the sample solder (4) was placed on the Cu substrate (1) and heated for 25 seconds. After the heating was completed, the Cu substrate (1) was picked up from the heater part, and once installed in a place where the nitrogen atmosphere next to it was maintained, it was cooled. After sufficiently cooling, it was taken out into the atmosphere and a joint portion was confirmed. The area before melting was defined as 100%, and the area after melting and cooling was measured using the area measuring function of an optical microscope (VHX-900 manufactured by Keyence Corporation). The results are shown in Table 2.

<Evaluation of void fraction>
In order to confirm the bondability, the void ratio of the Cu substrate to which the solder obtained in the same manner as in the wettability evaluation was bonded was measured using an X-ray transmission device (TOSMICRON-6125 manufactured by Toshiba Corporation). The sample solder and Cu substrate bonding surface were transmitted through X-rays perpendicularly from the upper part of the solder, the captured image data was processed, and the void ratio was calculated using the following calculation formula (1). The results are shown in Table 2.

<Heat cycle test>
A heat cycle test was conducted to evaluate the reliability of solder joints. In addition, this test was done using Cu board | substrate with which the solder obtained similarly to the said wettability evaluation was joined. First, with respect to the Cu board | substrate with which solder was joined, -40 degreeC cooling and 150 degreeC heating were made into 1 cycle, and this was repeated predetermined cycle. Thereafter, the Cu substrate to which the solder was bonded was embedded in the resin, the cross section was polished, and the bonded surface was observed by SEM (S-4800, manufactured by Hitachi, Ltd.). The case where the joint surface was peeled or cracked in the solder was indicated as “×”, and the case where there was no such defect and the same joint surface as in the initial state was maintained as “◯”. The results are shown in Table 2.

As can be seen from Table 2, the solder mother alloys of Samples 1 to 8 that satisfy the requirements of the present invention show good characteristics in each evaluation item. That is, the wettability to the Cu substrate on which the Au layer is formed is very good. Particularly, the sample 5 added with a large amount of P and the sample 8 added with both P and Ge are very quickly wetted and spread. The solder spreading area increased by 35%. Good results were also obtained in the heat cycle test, which is a test relating to reliability, and no defects appeared after 500 times in the heat cycle test.
On the other hand, the solder mother alloys of Comparative Samples 9 to 13 that did not satisfy the requirements of the present invention had undesirable results in at least any of the characteristics. That is, Samples 9 to 13 had a higher void ratio than Samples 1 to 8, and all were 4% or more. Further, Samples 9, 10, 12, and 13 were defective in 300 times in the heat cycle test.

1 Cu substrate 2 Ni plating film 3 Au plating film 4 Solder

Claims (1)

  Sn is contained in an amount of 18.5% by mass to 23.5% by mass, and at least one of P of 0.001% by mass to 0.5% by mass and Ge of 0.03% by mass to 1.5% by mass. An Au—Sn alloy solder comprising a seed and the balance being made of Au.

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