JP7526116B2 - How to calculate the duration of solder melting - Google Patents

Description

The present invention relates to a method for calculating the melting duration of solder used to join components together.

Electronic devices contain many electronic components, and many of these components are joined to circuit boards using solder materials. Traditionally, PbSn solder was used for joining, but in recent years, Pb-free solder has come to be widely used due to its environmental impact.

Examples of Pb-free solders include Sn-based solders such as InSn, SnAgCu, and AuSn, and Au-based solders such as AuGe and AuSi, which have a higher melting point than Sn-based solders. Among these, SnAgCu solder is widely used because it is inexpensive and has a low melting point, while AuSn solder is preferably used for components that require high reliability. Patent Document 1 describes a method for calculating the connection conditions for connecting electronic components and circuit boards with such Pb-free solder with high joint strength.

Patent No. 4136641

In soldering, a metal film made of a metal material for bonding with the solder is formed on the electronic component or circuit board to be joined with the solder, and this metal film is joined to the solder. When the solder melts during soldering, the solder and the metal film react with each other, and an intermetallic compound made of the materials that make up the solder and the metal film is generated at the interface between the solder and the metal film. The generation of this intermetallic compound progresses as the solder melts over time.

In the case of many electronic components, the electronic components and circuit boards are joined using a large amount of solder material. In this way, when joining using a large amount of solder material, even if the solder melts for a long time, the proportion of the solder that becomes an intermetallic compound is small, and the composition of the majority of the solder does not change. Therefore, there is no significant effect on the duration of solder melting, and as long as the solder is heated to a temperature above its melting point, it can be melted and joined any number of times.

However, electronic components, particularly laser elements, require high positional accuracy and high joint reliability, and are therefore generally joined using thin-film solder with a thickness of about 1 to 3 μm. In the case of joining using such thin-film solder, the generation of intermetallic compounds progresses as the solder melts over time, and because the amount of solder is small (it is a thin film), the composition of most of the solder changes to that of metal compounds in the short melting time. If the composition of the solder fluctuates in this way, the melting point of the solder fluctuates, causing the solder to solidify immediately and preventing proper joining. Therefore, in order to obtain a joint with high reliability in which members are joined with solder, the duration of solder melting is an important factor, and it is important to control the duration of solder melting, that is, to appropriately control the temporal variation of the solder composition.

The present invention was made in consideration of the above problems, and aims to provide a method for calculating the melting duration of solder, particularly thin-film solder, and to design solder of appropriate thickness and composition based on the calculated solder melting duration, thereby contributing to the provision of highly reliable joints.

また、前記はんだの膜厚が１０μｍ以下の薄膜はんだである溶融持続時間算出方法とする。 The present invention provides a method for calculating the melting duration of solder formed on a surface of a metal material, the method calculating the melting duration of the solder based on a composition ratio of an intermetallic compound made of the solder and the metal material that is generated at the interface between the solder and the metal material when the solder is heated to a temperature equal to or higher than its melting point and melted, and a generation rate of the intermetallic compound.
Furthermore, the method for calculating melting duration is such that the solder is a eutectic solder whose main component is a binary alloy having a eutectic point, and the melting duration of the hypereutectic side from the eutectic point of the solder and the melting duration of the hypoeutectic side from the eutectic point of the solder are calculated, and the sum of the respective melting durations is determined to be the total melting duration of the solder.
Furthermore, when the solder is AuSn solder, the metal material is Pt, the atomic ratios of Au and Sn contained in the solder are M _Au and M _Sn , respectively, the atomic ratios of Au and Sn in the intermetallic compound are R _Sn and R _Au , respectively, the atomic consumption rate of the metal material is M _Pt , the atomic numbers of Au and Sn at the eutectic point of the solder are M _Sn029 and M _Au071 , respectively, and the Sn composition at which the solder stops melting at a predetermined temperature is C' _Sn , the melting duration S of the solder is calculated by the following formula.

In addition, the melting duration calculation method is for a thin-film solder having a film thickness of 10 μm or less.

The present invention makes it possible to calculate the duration of solder melting, and by designing solder of appropriate thickness and composition based on the calculated duration of solder melting, it is possible to contribute to the provision of highly reliable joints.

FIG. 1 is a diagram of a calculation model for calculating the melting duration of AuSn eutectic solder, and is a diagram showing a schematic cross-sectional structure of a thin-film submount for bonding a semiconductor laser element. FIG. 1 shows the Pt consumption rate estimated from experiments. FIG. 2 is a diagram showing the vicinity of the eutectic point in the AuSn phase diagram. FIG. 1 is a graph obtained by plotting liquidus lines on the hypoeutectic side of the eutectic point on the AuSn phase diagram. FIG. 1 is a diagram comparing experimental and calculated values of melting duration of AuSn solder.

The method for calculating the melting duration of solder according to the present invention will be described below with reference to the drawings. Here, the method for calculating the melting duration of solder will be described using thin-film AuSn eutectic solder as an example of the solder. However, please note that the technical scope of the present invention is not limited to these embodiments, but extends to the inventions described in the claims and their equivalents.

Figure 1 is a calculation model diagram for calculating the melting duration of AuSn eutectic solder, and is a diagram showing a schematic cross-sectional structure of a thin film submount for bonding a semiconductor laser element. The thin film submount 100 has AuSn solder 203 on the surface of an AlN substrate (ceramic substrate) 101 for bonding an electronic component such as a semiconductor laser element to the thin film submount 100. Between the AlN substrate 101 and the AuSn solder 203, from the AuSn solder 203 side, a Pt layer 202 called a barrier layer and a Ti layer 201 for bonding the Pt layer 202 to the AlN substrate (ceramic substrate) 101 are arranged. The metallized (Au) surface of an electronic component (not shown) is bonded to the upper surface of the AuSn solder 203 of the thin film submount 100.

When the AuSn solder 203 melts, if the compositional variation of the AuSn solder 203 is zero, it will theoretically continue to melt semi-permanently. However, in reality, when the AuSn solder 203 melts, an interface reaction occurs between the AuSn solder 203 and the Pt layer 202, and an intermetallic compound (IMC) consisting of Pt, Au, and Sn is generated at the interface. Here, the Ti layer 201 disposed under the Pt layer 202 does not react with the AuSn layer 203, so Ti is not included in the IMC. The IMC generated between the AuSn solder 203 and the Pt layer 202 is typically considered to have a Sn-rich composition such as PtSn ₄ Au, so that most of the Sn in the AuSn solder 203 is replaced by the IMC composition in proportion to the amount of IMC generated. In other words, a large amount of Sn in the AuSn solder 203 is consumed by the generation of the IMC. Similarly, Pt in the Pt layer 202 is also consumed by the generation of the IMC. When the IMC is generated in this way, the Sn contained in the IMC is supplied from the AuSn solder 203, so that the composition of the AuSn changes to the Au-rich side, and even if the heating temperature of the AuSn solder 203 is constant, the solder will eventually solidify and stop melting.

The present invention focuses on the generation of IMC and calculates the melting duration of AuSn solder 203, and more specifically, calculates the melting duration of solder based on the composition (number of atoms) of the intermetallic compound (IMC) formed at the interface between the solder and the metal material in the solder formed on the surface of the metal material, and the rate of IMC generation. The present invention is particularly useful when the solder is a thin film (very small in volume). Note that thin film here refers to a film thickness of 10 μm or less.

In the above model case, the melting duration of the AuSn solder 203 can be calculated by the following method. To obtain the melting duration of the AuSn solder 203, first, the composition of Au and Sn, which are the materials constituting the AuSn solder 203, is calculated. The composition of Au in the AuSn solder 203 can be expressed by Equation 1 from the density and thickness of Au and Sn. Here, d _Au and d _Sn are the densities (g/cm ³ ) of Au and Sn, respectively, and D _Au and D _Sn are the thicknesses (m) corresponding to Au and Sn in the AuSn solder 203. If it is assumed that the area of the reaction field of the AuSn solder 203 is constant, D _Au and D _Sn can be considered to be equivalent to the volume.

Next, the composition (number of atoms) of Au and Sn, which are materials constituting the AuSn solder 203, is calculated from the composition of Au obtained by Equation 1. By solving D _Au and D _Sn from Equation 1, D _Au and D _Sn become Equations 2 and 3, respectively.

Using the thicknesses D _Au and D _Sn of Au and Sn obtained by Equation 2 and Equation 3, the atomic numbers M _Au and M _Sn of Au and Sn are obtained by Equation 4 and Equation 5. Here, _Sr is the area where AuSn melts (area of reaction field), N _A is Avogadro's constant (6.02×1023 atoms/mol), and m _Au and m _Sn are the molar masses of Au and Sn (g/mol), respectively.

Next, since the compositional fluctuation of Au and Sn in the AuSn solder 203 determines the duration of melting, the consumption rate of Pt atoms in the Pt layer 202 reacting with AuSn is calculated in order to calculate the amount of compositional fluctuation. FIG. 2 is a diagram showing the Pt consumption rate estimated from an experiment. FIG. 2 shows the relationship between the heating temperature of the AlN substrate 101 and the consumption rate of the Pt layer 202 when the Pt layer 202 is formed on the AlN substrate 101 by vapor deposition. As shown in FIG. 2, the Pt consumption rate C _Pt can be approximated by a quadratic function of the heating temperature T, and when the heating temperature T of the AlN substrate 101 is 300° C.<T<380° C., it can be approximated by the quadratic function shown in Equation 6.

Note that Equation 6 represents the Pt consumption rate C _Pt when the Pt layer 202 is formed by a vapor deposition method, and the Pt consumption rate C _Pt changes if the manufacturing method of the Pt layer 202 is changed to, for example, a sputtering method, etc. In this way, the consumption rate of the metal material that forms the solder underlayer, represented by the Pt layer 202, changes depending on the manufacturing method and heating temperature, so it is possible to adjust the melting duration of the solder by appropriately adjusting the consumption rate.

From the Pt consumption rate C _Pt (nm/s) obtained by Equation 6, the Pt atom number consumption rate M _Pt (atoms/s) can be calculated using Equation 7. Here, d _Pt is the density of Pt, and m _Pt is the molar mass of Pt. The Pt consumption rate C _Pt and the Pt atom number consumption rate M _Pt are the rates at which the AuSn solder 203 and the Pt layer 202 react with each other, or in other words, the IMC generation rates.

Next, the melting duration of the AuSn solder 203 is calculated. FIG. 3 is a diagram showing the vicinity of the eutectic point of the AuSn phase diagram. When the AuSn solder 203 is heated above its melting point, a large amount of Sn is consumed in the IMC (for example, PtSn ₄ Au) consisting of Pt, Sn, and Au, so the composition of the AuSn solder 203 changes in a direction in which the Au composition increases (the Sn composition decreases). In other words, if the melting start point of the molten AuSn solder 203 is, for example, melting start point A (hypereutectic), the composition changes as the solder melts, passing through the eutectic point Ep and reaching the melting end point B (hypoeutectic). The melting duration is calculated by decomposing it into two processes: (1) from start point A to eutectic point Ep, and (2) from eutectic point Ep to melting end point B. In process (1), as the composition change of the AuSn solder 203 progresses (the Sn composition decreases), the liquidus line decreases toward the eutectic point, so the AuSn solder 203 does not solidify midway. In process (2), as the composition change progresses (the Sn composition decreases), the liquidus line increases, so the AuSn solder 203 continues to melt until the composition of the AuSn solder 203 at a specified heating temperature T exceeds the liquidus line and becomes solid-liquid coexistent or solid, and then solidifies when the liquidus line is exceeded.

First, the melting duration S1 (s) of process (1) is calculated. The Sn composition C _Sn (atomic %) of the AuSn solder 203 is expressed by Equation 8, where M' _Au and M' _Sn are the numbers of Au and Sn atoms in the AuSn melt.

The numbers of Au and Sn atoms _M'Au and _M'Sn can be calculated by the compound atomic ratio R _Sn of the IMC, R _Au , and the number of Pt consumed atoms M _Pt using Equation 9 and Equation 10 as follows. Here, R _Sn and R _Au are the atomic ratios of Au and Sn in the IMC, respectively. For example, if the IMC is assumed to be PtSn ₄ Au, R _Sn = 4 and R _Au = 1. Note that in process (1), the melting is performed up to the eutectic point, so the Sn composition C _Sn of the AuSn solder 203 is 0.29.

Then, by substituting Equation 9 and Equation 10 into Equation 8 and solving for S1, the melting duration S1 of the AuSn solder 203 in process (1) is given by Equation 11. However, here, the composition and generation rate of the IMC are assumed to be constant regardless of the elapsed melting time.

Secondly, the melting duration S2 (s) of process (2) is obtained. First, the Sn composition _C'Sn of the AuSn solder 203 that stops melting at temperature T is obtained. Figure 4 is a graph plotting the liquidus line on the hypoeutectic side (left side of Figure 3) of the eutectic point on the AuSn phase diagram. The approximation curve in Figure 4 is mathematically formulated, and _C'Sn can be expressed as a function of T by Equation 12.

Similarly to the determination of S1, the AuSn composition _C'Sn and the numbers of Au and Sn atoms _{M''Au and M''Sn during} melting of the AuSn solder 203 are expressed by Formulas 13, 14, and 15. Here, M _Sn029 and M _Au071 are the numbers of Au and Sn atoms at the eutectic point.

By substituting formulas 14 and 15 into formula 13 and solving for S2, the melting duration S1 of the AuSn solder 203 in process (2) is given by formula 16.

Finally, the total melting duration S _total (s) of the AuSn solder 203 is calculated by Equation 17.

Here, we will verify the melting duration of the solder calculated as above. Figure 5 is a plot comparing the experimental melting duration when AuSn with a composition of Au = 68 wt% (Sn = 32 wt%) and a thickness of 5 μm is melted on a Pt layer formed by vapor deposition, with the calculated values calculated using the calculation method described above. As shown in Figure 5, it can be seen that the experimental values and calculated values are generally consistent.

The method for calculating the melting duration of solder according to the present invention can be used in the following cases. For example, when a semiconductor laser element is bonded to a substrate, various bonding condition ranges can be considered, such as pulse heating (short time) and reflow heating (long time) in addition to the solder bonding temperature. In the above model case, the IMC (PtSn ₄ Au) generated between the Pt layer and the AuSn solder is related to the subsequent bonding strength, and if the IMC is generated in small amounts, the bonding strength decreases, and conversely, if the IMC is generated too much, Kirkendall voids are generated due to the difference in the diffusion speed of Au, Sn, and Pt, which also leads to a decrease in the bonding strength. In other words, it is necessary to moderately adjust the amount of IMC generated.

As revealed by the melting duration calculation method described above, the melting duration of the AuSn solder 203 is mainly composed of three factors: the composition and film thickness of the AuSn solder 203, and the Pt consumption rate. Therefore, it is possible to design a joint structure by arbitrarily adjusting the three factors so that the melting duration is the optimal time under the required joining conditions.

Specifically, the composition of the AuSn solder 203 can be adjusted by changing the evaporation rate of Au and Sn when the AuSn solder 203 is formed, the film thickness of the AuSn solder 203 can be adjusted by changing the time during film formation, and the Pt consumption rate can be adjusted by changing the film formation method and the temperature during film formation. By using the above methods, it is possible to design and manufacture a joint structure with any desired melting duration.

Although the method for calculating the melting duration of the solder of the present invention has been described above based on the embodiment, the present invention is not limited to this embodiment. For example, in the embodiment, the solder is AuSn solder 203 made of a binary alloy, but it may be a ternary eutectic solder of a SnAgCu alloy. In this case, the composition ratio of the intermetallic compound between the metal material in contact with the solder and the solder and the generation rate of the intermetallic compound can be calculated in the same way as in the embodiment to calculate the melting duration. In addition, the solder is not limited to the eutectic type, but may be a non-eutectic type such as SnSb peritectic solder. In this case, in the embodiment using eutectic solder as an example, the melting duration of the solder is divided into process (1) and process (2), the melting duration of the solder in each process is calculated, and the sum of the calculated values is the final melting duration, but the melting duration may be calculated in one process based on the phase diagram of the solder material. In addition, in the embodiment, the metal material in contact with the AuSn solder 203 is Pt layer 202, but it goes without saying that other metal materials may be used.

100 Thin film submount 101 AlN substrate 201 Ti layer 202 Pt layer 203 AuSn solder

Claims (4)

A method for calculating the melting duration of solder formed on a surface of a metal material, the method comprising the steps of: calculating the melting duration of the solder based on a composition ratio of an intermetallic compound consisting of the solder and the metal material, which is generated at an interface between the solder and the metal material when the solder is heated to a temperature equal to or higher than its melting point and melted; and a generation rate of the intermetallic compound. 2. The method for calculating the melting duration of solder according to claim 1, wherein the solder is a eutectic solder mainly composed of a binary alloy having a eutectic point, and the melting durations of the hypereutectic side from the eutectic point of the solder and the hypoeutectic side from the eutectic point of the solder are calculated, and the sum of the respective melting durations is determined as the total melting duration of the solder . 3. The method of claim 2, wherein the solder is AuSn solder, the metal material is Pt, the atomic ratios of Au and Sn contained in the solder are M _Au and M _Sn , respectively, the atomic ratios of Au and Sn contained in the intermetallic compound are R _Sn and R _Au , respectively, the atomic consumption rate of the metal material is M _Pt , the atomic numbers of Au and Sn at the eutectic point of the solder are M _Sn029 and M _Au071 , respectively, and the composition of Sn at which the solder stops melting at a predetermined temperature is C' _Sn , the melting duration S of the solder is calculated by Equation 1.

4. The method for calculating the melting duration of solder according to claim 1, wherein the solder is a thin film solder having a film thickness of 10 μm or less.

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