JP5646230B2 - Lead-free bonding material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a lead-free bonding material and a method for manufacturing the same.

  A general power module is a BGA (ball grid array) in which a semiconductor element is bonded to a substrate, and a heat radiating plate for releasing heat generated from the semiconductor element is attached by a bonding material such as solder. In recent years, there has been a demand for further improvements in heat resistance and reliability for electric vehicles and hybrid vehicles that require increased output of the module itself and electronic control equipment. Improvement of the melting point of certain solder (application of high-temperature solder) has been studied.

  High-temperature solder includes Pb-5Sn solder (melting point: 310 to 314 ° C.) and the like, which is a component containing more lead than the composition. In recent years, due to environmental problems, it has been required to cope with Pb-free such as the RoHS command, and Sn—Ag—Cu eutectic alloys have been put to practical use as general solder components. On the other hand, Au-20Sn (melting point: 280 ° C.) is known for the high temperature solder, but it is hardly used because it is inferior to the Pb—Sn solder in terms of cost and mechanical properties. Since other component systems have not yet been put into practical use, the EU directive restricting specific harmful substances such as Pb contained in electronic devices and the like is also an excluded item for Pb high-temperature solder. Therefore, there is an increasing demand for the development of Pb-free high-temperature solder.

  Regarding the development of high-temperature solder alloys, for example, Japanese Patent Application Laid-Open No. 2003-260587 (Patent Document 1) describes Sn—Cu based solder. According to this method, a material in which Sn powder and Cu powder are mixed is used as a solder paste. During high-temperature soldering, Sn powder melts and contributes to soldering, and at the same time, reacts with Cu powder and has a high melting point Sn—Cu. An intermetallic phase is formed. This compound phase functions to maintain the strength of the soldered part without melting during reflow.

Further, since the power module generates heat during operation, a thermal cycle is applied to the bonding surface of the semiconductor element. Therefore, it is important to control the stress state at the interface between the semiconductor element and the bonding material, and it is designed so that the shape of the bonding portion, the thickness of the bonding layer, and the like are precisely controlled to obtain an optimum state. For this reason, it has been studied to control the viscosity by adding metal powder inside the molten solder to suppress the thickness variation of the bonding layer. For example, “Packaging of Double-sided Cooling Power Module”, 15 ^th Sympium on “Microjoining and Assembly Technology in Electronics”, P91-94 (Non-Patent Document 1).

Further, the inventors have disclosed in Japanese Patent Application Laid-Open No. 2008-178909 (Patent Document 2) an alloy composed of two types of elements A and B. Element A has a higher melting point than element B and is stable at room temperature composed of element B. A bonding material produced by supersaturating solid solution of element A in a room temperature stable phase composed of element B in an alloy having a room temperature stable phase AmBn composed of a phase and elements A and B (m and n are values inherent to the alloy system) Is used to maintain the bonding strength even at the reflow temperature of Pb-Sn eutectic solder and Sn-Ag-Cu lead-free solder by maintaining the temperature at which the supersaturated solid solution decomposes and depositing at room temperature stable phase AmBn I found out that I can do it.
JP 2003-260587 A JP 2008-178909 A “Packaging of Double-sided Cooling Power Module”, 15th Sympium on “Microjoining and Assembly Technology in Electronics”, P91-94.

However, according to the above-described invention of Patent Document 1, since the powder that is practically used is about 10 μm, the structure of the intermetallic compound phase formed at the interface between the Sn powder and the Cu powder is that of the Cu powder. Since it is formed on the surface, it is rough and variation in strength occurs, and in order to place importance on strength, a large amount of Cu powder must be used, so that there are problems such as deterioration in soldering characteristics due to Sn powder. Furthermore, since the formation of intermetallic compounds is due to the diffusion reaction between melted Sn and solid state Cu, the rate of formation of the Cu ₆ Sn ₅ intermetallic compound phase that contributes to strength maintenance is slow and it takes time to react. There is also a problem.

  Furthermore, when controlling the thickness of the bonding layer, normal BGA solder balls and general-purpose Sn-Ag-Cu solders are completely melted at the time of bonding. Therefore, the thickness and inclination after solidification deviate from the design values. There is also a possibility of impairing reliability in the cycle. Conversely, as in Non-Patent Document 1, in order to control the viscosity of the molten solder by adding metal powder to the molten solder, a mechanism for adding powder to the molten solder layer is required as a soldering device. In a small power module package, BGA solder or the like is used, but there is a problem that cannot be applied to this.

  Moreover, although the high temperature joining material superior to the technique mentioned in patent document 2 was shown, even if it uses the atomized powder which is a rapid cooling process in the SnCu alloy system which is the patent document range, the strength as a high temperature solder is stable. It has become clear that there is no state.

In order to solve the problems as described above, the inventors examined SnCu-based alloy and SnMn-based alloy in more detail, and showed in Patent Document 2 by appropriately controlling the rapid cooling rate from the molten metal state to solidification. By rapid solidification, a structure in which the SnCu alloy phase (Cu ₆ Sn ₅ phase) is dispersed in a very fine (nano order: 1 μm or less) in the Sn phase is obtained regardless of the presence or absence of the supersaturated solid solution. Thus, by finely dispersing the Cu ₆ Sn ₅ phase in the Sn phase, it does not affect the wettability and the uniform meltability required for soldering during Sn melting. Furthermore, since the finely dispersed phase has a high surface energy, the Cu ₆ Sn ₅ phase suddenly coarsens and becomes clear in order to lower the surface energy when held at the soldering temperature, and the melting point exceeds the normal soldering temperature. Cu ₆ Sn ₅ intermetallic compound phase having a bond and coarsening. As a result, it has been found that stable joint strength in a high-temperature solder region can be maintained.

On the other hand, for SnMn-based alloys as well, by appropriately controlling the rapid cooling rate from the molten metal state to solidification, the rapid solidification shown in Patent Document 2 is extremely A microstructure in which SnMn alloy particles (MnSn ₂ phase) are finely dispersed (nano order: 1 μm or less) is obtained. By finely dispersing in the Sn phase in this way, it does not affect the wettability and uniform meltability required for soldering at the time of Sn melting. Furthermore, since the finely dispersed phase has a high surface energy, the soldering temperature is not affected. In order to reduce the surface energy when held, the SnMn alloy particles are rapidly coarsened, and the SnMn alloy phase appears clearly, and the SnMn intermetallic compound phase is bonded and coarsened. As a result, it has been found that stable joint strength in a high-temperature solder region can be maintained.

The gist of the invention is that
(1) An alloy composed of Sn and Cu , wherein Cu is 10 to 38% by mass, the balance is composed of Sn and inevitable impurities, and a phase composed of an intermetallic compound composed of Sn and Cu is 1 μm in the Sn base. A lead-free bonding material characterized by being dispersed as the following fine particles.

(2) a Sn and Mn or Ranaru alloy, Mn is Ri Do the balance of Sn and unavoidable impurities 4-18 wt%, the Sn base phase consisting configured intermetallic compound of Sn and Mn The lead-free bonding material is characterized by being dispersed as fine particles having a size of 1 μm or less .
(3) The lead-free bonding material according to (1) or (2), wherein the phase composed of an intermetallic compound composed of Cu or Mn and Sn is a Cu ₆ Sn ₅ phase or a MnSn ₂ phase A lead-free bonding material characterized by

( 4 ) The molten metal having the alloy composition described in the above (1) or (2) is rapidly cooled and solidified to disperse the Cu ₆ Sn ₅ phase or the MnSn ₂ phase in Sn as a fine phase of 1 μm or less. A method for manufacturing a lead-free bonding material.
( 5 ) The method for producing a lead-free joining material according to ( 4 ) above, wherein the rapid cooling rate is 100 ° C./second or more.

  As described above, according to the present invention, by using SnCu alloy or SnMn alloy and rapid cooling, it is a lead-free solder component that can be used for high-temperature soldering in which conventional Pb-5Sn or the like has been used, and solder There is no variation in the strength of the soldered part, and there is an extremely excellent effect that can produce lead-free solder with excellent balance between strength stability and soldering characteristics and basic characteristics as high-temperature solder. Is.

Hereinafter, the present invention will be described in detail.
FIG. 1 is a binary system equilibrium state diagram of Cu and Sn, and FIG. 2 is a binary system equilibrium state diagram of Mn and Sn. In the equilibrium diagram shown in FIGS. 1 and 2, the reflow temperature is used at the position adjacent to SnCu alloy or Sn using metal Cu indicating the intermetallic compound Cu ₆ Sn ₅ having a melting point equal to or higher than the reflow temperature at the position adjacent to Sn. An SnMn alloy is manufactured using metal Mn indicating the intermetallic compound MnSn ₂ having the above melting point. When a method that is not a quenching process is used, this alloy has a two-phase composition of Sn and Cu ₆ Sn ₅ intermetallic compound or Sn and MnSn ₂ intermetallic compound in proportion to the equilibrium diagram depending on the Cu content or Mn content. Become an organization.

However, regardless of the presence or absence of the supersaturated solid solution by the rapid solidification shown in Patent Document 2 by preparing an alloy by controlling the cooling rate from the molten state to solidification by a rapid cooling process such as the atomizing method or the melt span method. The Cu ₆ Sn ₅ phase or MnSn ₂ phase does not grow, and a very fine Cu ₆ Sn ₅ phase or MnSn ₂ phase of 1 μm or less which is difficult to detect by ordinary microscopic observation is dispersed in Sn. Since this effect is strongly influenced by the cooling rate at the time of solidification, it is necessary to use a process having a high quenching rate such as an atomizing method or a melt span method and to ensure a cooling rate of 100 ° C./second or more. In the melt span method, there is no problem because a cooling rate of 100 ° C./second can be easily secured. In the atomization method, although it depends on the spray medium (gas species), spray pressure, spray temperature, and the amount of molten metal dropped (crucible nozzle diameter), a cooling rate of 100 ° C./second or more can be secured if the powder is 50 μm or less.

As a soldering material, the alloy produced by the rapid cooling process has a structure in which the intermetallic compound Cu ₆ Sn ₅ phase of the nano-order CuSn alloy and the intermetallic compound MnSn ₂ phase of the SnMn alloy are dispersed in the Sn phase. ing. When provided in this state, the Cu ₆ Sn ₅ phase and MnSn ₂ phase do not affect the solderability of the substrate with Sn, and uniform and good wettability can be achieved.

Furthermore, due to heating during soldering, Cu ₆ Sn ₅ and MnSn ₂ finely dispersed phases in Sn have a high surface energy, so the Cu ₆ Sn ₅ alloy phase and the MnSn ₂ alloy phase suddenly decrease in order to reduce the surface energy. In addition, the Cu ₆ Sn ₅ intermetallic compound phase and the MnSn ₂ intermetallic compound phase are combined and dispersed in the molten Sn. If it solidifies from this state, it becomes a mixed structure in which Sn is dispersed while Cu ₆ Sn ₅ and MnSn ₂ having a high melting point are bonded together, and the high-temperature strength can be stably maintained as a whole solder. Further, at the time of soldering, the viscosity in the molten solder state can be controlled by dispersing the solid intermetallic compound phase in the liquid phase. This is the basic idea of the present invention.

The Cu content in the SnCu alloy is optimally 10 to 38% by weight. The reason is determined by the balance between the Sn solid solution amount contributing to soldering and the Cu ₆ Sn ₅ intermetallic compound amount contributing to maintaining the strength after soldering. As a result of examining the range of the Cu content in detail, the inventors have found that when the Cu amount exceeds 38% by weight, the amount of coarse Cu ₆ Sn ₅ intermetallic compound produced is not 1 μm or less, even if the quenching method is used. The amount of Sn solid solution that contributes to soldering decreases and good soldering becomes difficult, so the upper limit was made 38% by weight. If the Cu content is less than 10%, both of the Cu ₆ Sn ₅ phases dispersed in Sn at 1 μm or less are not sufficient, and a sufficient amount of Cu ₆ Sn ₅ intermetallic compound that contributes to maintaining the strength after soldering is secured. Can not. As mentioned above, the range of Cu amount was 10 to 38%. Preferably it is 15 to 32%.

On the other hand, the Mn content in the SnMn alloy is optimally 4 to 18% by weight. The reason is determined by the balance between the Sn solid solution amount contributing to soldering and the MnSn ₂ intermetallic compound amount contributing to strength maintenance after soldering. As a result of examining the range of the Mn content in detail, the inventors have found that when the amount of Mn exceeds 18% by weight, even if the quenching method is used, the amount of MnSn ₂ intermetallic compound that is coarsened rather than 1 μm or less is greatly increased. Therefore, the amount of Sn solid solution that contributes to soldering decreases and it becomes difficult to perform good soldering, so the upper limit was made 18% by weight. If the amount of Mn is less than 4%, both of the MnSn ₂ phases dispersed in Sn at 1 μm or less are not sufficient, and a sufficient amount of MnSn ₂ intermetallic compound contributing to strength maintenance after soldering cannot be ensured. As mentioned above, the range of the amount of Mn was made 4-18%. Preferably it is 8 to 15%.

  In the SnCu system, a Sn- (10 to 38% by weight) Cu alloy is produced by a rapid cooling method such as an atomizing method, a melt span method, or an underwater spinning method, and a cooling rate of the molten metal is ensured to be 100 ° C./more. The shape at that time is not particularly limited, and may be powder, wire, bar, ribbon, plate or the like.

  As described above, the rapid cooling method includes the atomizing method and the melt span method, and the helium gas atomizing method and the melt span method are particularly effective as the rapid cooling means. However, disk atomization, argon atomization, and nitrogen gas atomization are very effective as means for mass production of solder powder, and the cooling rate depends on the particle size of the powder after atomization. In addition, for fine particles of 50 μm or less, the cooling rate of the molten metal can be secured at 100 ° C./more, which belongs to the category of the present invention. Further, for example, a wire rod is obtained by the underwater spinning method, and a rod wire is obtained by correcting or drawing with the medium spinning method.

The behavior of the present invention will be described below with a quenched foil strip.
FIG. 3 is an electron micrograph showing a cross section of a quenched foil strip of an alloy of Sn—Cu composition example by melt span method in SEM. When the cross section of the quenched foil strip of the alloy of the SnCu composition example by the melt span method was observed with this SEM, the structure was uniform as shown in the figure. However, as a result of surface milling and observation with an electron microscope having higher resolution, it was found that nano-order (1 μm or less) fine Cu ₆ Sn ₅ phases were dispersed.

That is, FIG. 4 is an electron micrograph of the result of surface milling and observation with an electron microscope having higher resolution. As shown in this figure, Cu ₆ Sn ₅ phase (part with pattern, average 300 nm phase) and Sn phase (part where milling progressed to form a hole) are mixed at 1 μm or less. .

The dispersed nano-order fine Cu ₆ Sn ₅ phase structure has a high surface energy and becomes unstable due to its large specific surface area. For this reason, there is no thermal stability to reduce the surface energy as compared to a micron-order or millimeter-order structure, and there is a tendency to be coarse. Therefore, as shown in FIG. 5, even in the case of heating at 180 ° C. for 10 minutes, which is a solid state, below the liquidus at 227 ° C., diffusion in a solid state requiring time in a normal solidified structure proceeds, and Cu ₆ Sn ₅ phase coarsening occurs. That is, FIG. 5 is an electron micrograph showing that the Cu ₆ Sn ₅ phase has a coarsened structure in the Sn phase. As can be seen from this figure, the Cu ₆ Sn ₅ phase is coarsened in the Sn phase although the whole is not melted.

As a result, even when the holding time at the time of soldering is short, a sufficient refractory intermetallic compound can be formed even by short-time heat treatment such as holding at 250 ° C. for 30 seconds and air cooling, such as a reflow heating pattern. I can do it. FIG. 6 is an electron micrograph showing a state where the Cu ₆ Sn ₅ phase is coarsened and a refractory intermetallic compound is formed in Sn because Sn has become a liquid phase. As shown in this figure, even after heating for a short time, Sn became a liquid phase, so the Cu ₆ Sn ₅ phase was coarsened at once, and a refractory intermetallic compound was formed in Sn.

Hereinafter, the present invention will be specifically described with reference to examples.
Table 1 is a comparative table of compositions describing the wettability during soldering and the strength of the soldered part when reheated at 250 ° C. after soldering for the CuSn alloy and Table 2 for the SnMn alloy, respectively. It is. About the manufacturing method, the quenching foil strip is pressing the molten metal of a target composition on a rotating copper roll with the single roll liquid quenching method, and producing the target foil strip. As the powder, nitrogen was used as a spray gas, the molten metal temperature was heated to 300 ° C. above the melting point of the target composition, and a rapidly cooled powder was obtained at a spray pressure of 0.45 MPa. This is sieved to obtain a powder of the desired particle size. Usually, as a solidified material, a comparative material of an example is obtained by crushing a solidified material having a target composition. Incidentally, it is denoted by the average particle diameter D ₅₀ of the cumulative volume of 50% for the particle diameter. The obtained powder could be embedded in a normal temperature resin, and then subjected to a cross-section and mirror finish by wet polishing to obtain an electron microscope observation sample. Before observation with an electron microscope, milling treatment was performed with Ar ions to remove the influence layer of surface polishing. As the high-resolution electron microscope, a field emission scanning electron microscope (FE-SEM) was used to observe the internal structure of the steel of the present invention.

In Tables 1 and 2, regarding the evaluation of wettability during soldering, changes in the solder application part after heating were confirmed with the solder applied on the Cu plate or Mn plate, and ◎: Excellent wettability Yes (solder spreads sufficiently), ○: good wettability (solder spreads), x: poor wettability (solder does not spread from the applied state). Moreover, about the evaluation about the viscosity at the time of 250 degreeC heating, when positioning by using the jig which can arrange | position Cu boards with a thickness of 1 mm, or Mn boards with the space | interval of a 50 micrometer solder layer, and it joined by this Then, the Cu position change after bonding or the Mn position change (solder phase thickness variation) was measured.
A: Variation was less than ± 1 μm, O: Variation was less than ± 5 μm, and X: Variation was ± 5 μm or more. In addition, the strength evaluation at the time of reheating at 250 ° C. is that the strength is excellent and the strength is not good (the shear strength measurement limit is below or equal to 0 MPa).

表１に示すように、Ｎｏ．１〜７は本発明例であり、Ｎｏ．８〜１３は比較例である。

As shown in Table 1, no. Nos. 1 to 7 are examples of the present invention. 8 to 13 are comparative examples.

Comparative Example No. 8 and no. No. 9 is an example of the present invention. 1 and no. 2 is the same composition as that of No. 2, but it is not subjected to rapid solidification, so the Cu ₆ Sn ₅ phase cannot be finely dispersed in the structure at 1 μm or less, and the coarse Cu ₆ Sn ₅ phase and further the Cu ₃ Sn phase The Sn phase that is formed and ensures wettability is not uniformly dispersed. Therefore, the wettability during soldering is poor. Comparative Example No. No. 10 has a component composition of 100% Sn, and when heated at 250 ° C., it is completely in a liquid phase, so there is a large variation in viscosity, and there is no intermetallic compound formation, so there is no strength at 250 ° C. reheating. .

Comparative Example No. 11 is a steel No. 11 of the present invention. 4 is obtained by gas atomization, but the particle size is large so that a cooling rate of 100 ° C./s cannot be secured, and the Cu ₆ Sn ₅ phase cannot be finely dispersed in the structure at 1 μm or less. A coarse Cu ₆ Sn ₅ phase and a Cu ₃ Sn phase are formed, and the Sn phase for ensuring wettability cannot be uniformly dispersed. Therefore, the wettability during soldering cannot be ensured. Comparative Example No. No. 12 has a component composition of 55% Cu and 45% Sn, and this was poor in solderability and could not be measured because it did not melt.

  Comparative Example No. No. 13 has a component composition of Cu 60% and Sn 40%. Similar to 12, the solderability was poor and the strength could not be measured because it did not melt. On the other hand, the present invention example No. Since all of Nos. 1 to 7 satisfy the component composition as a condition of the present invention, it is understood that the soldering wettability, the viscosity at 250 ° C. heating, and the strength at 250 ° C. reheating are excellent.

表２に示すように、Ｎｏ．１〜５は本発明例であり、Ｎｏ．６〜１１は比較例である。

As shown in Table 2, no. Nos. 1 to 5 are examples of the present invention. 6 to 11 are comparative examples.

Comparative Example No. 6 and no. 7 is an example of the present invention. 1 and no. Although the composition is the same as that of No. ₂ , the MnSn ₂ phase cannot be finely dispersed in the structure at 1 μm or less because the rapid solidification is not performed, and the Sn phase for ensuring wettability cannot be uniformly dispersed. Therefore, the wettability during soldering is poor. Comparative Example No. 8 has a component composition of Sn of 100%, and when heated at 250 ° C., it is completely in a liquid phase, so there is a large variation in viscosity, and there is no intermetallic compound formation, so there is no strength at 250 ° C. reheating. .

Comparative Example No. 9 is a steel No. 9 of the present invention. The powder is obtained by gas atomization with the same composition as No. ₄ , but because of the large particle size, a cooling rate of 100 ° C./s cannot be secured, the MnSn ₂ phase cannot be finely dispersed in the structure at 1 μm or less, and wettability The Sn phase that secures the thickness is not uniformly dispersed. Therefore, the wettability during soldering cannot be ensured. Comparative Example No. No. 10 has a component composition with Mn of 25% and Sn of 75%, and this was poor in solderability and could not be measured because it was not fused.

  Comparative Example No. No. 11 has a component composition of 40% Mn and 60% Sn. Similar to 10, the solderability was poor and the strength could not be measured because it did not melt. On the other hand, the present invention example No. Since 1 to 5 all satisfy the component composition as a condition of the present invention, it can be seen that solderability, viscosity at 250 ° C. heating, and strength at 250 ° C. reheating are excellent.

It is a binary system equilibrium state diagram of Cu and Sn. It is a binary system equilibrium state diagram of Mn and Sn. It is an electron micrograph which shows the quenching foil strip cross section of the alloy of the Sn-Cu composition example by the melt span method in SEM. It is the electron micrograph of the result of having performed surface milling and observing with the electron microscope which has high resolution further. Is an electron micrograph showing that Cu ₆ Sn ₅ phase is in the organization that coarsened in Sn phase. Since Sn became a liquid phase, it is an electron micrograph showing a state in which a Cu ₆ Sn ₅ phase is coarsened and a refractory intermetallic compound is formed in Sn.

Claims (5)

An alloy composed of Sn and Cu , Cu is 10 to 38% by mass, the balance is composed of Sn and inevitable impurities, and a phase composed of an intermetallic compound composed of Sn and Cu is finely 1 μm or less in the Sn base. A lead-free bonding material characterized by being dispersed as particles. A Sn and Mn or Ranaru alloy, Mn is Ri Do the balance of Sn and unavoidable impurities 4-18 wt%, 1 [mu] m or less phase consisting configured intermetallic compound of Sn and Mn in the Sn base A lead-free bonding material characterized by being dispersed in the form of fine particles . The lead-free bonding material according to claim 1 or 2, wherein the phase composed of an intermetallic compound composed of Cu or Mn and Sn is a Cu ₆ Sn ₅ phase or a MnSn ₂ phase. Lead-free bonding material. 3. For lead-free bonding, wherein the molten alloy having the alloy composition according to claim 1 or 2 is rapidly solidified to disperse the Cu ₆ Sn ₅ phase or the MnSn ₂ phase in Sn as a fine phase of 1 μm or less. Material manufacturing method. The method for producing a lead-free bonding material according to claim 4 , wherein the rapid cooling rate is 100 ° C./second or more.

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