JP6719443B2 - Lead-free solder alloy, electronic circuit mounting board and electronic control unit - Google Patents

Description

The present invention relates to a lead-free solder alloy, an electronic circuit mounting board and an electronic control device having a solder joint formed using the lead-free solder alloy.

As a method of joining an electronic component to a conductor pattern formed on an electronic circuit board such as a printed wiring board or a module board, there is a solder joining method using a solder alloy. Previously, lead was used in this solder alloy. However, since the use of lead is restricted by the RoHS directive and the like from the viewpoint of environmental load, a so-called lead-free solder alloy soldering method that does not contain lead is becoming popular in recent years.
As the lead-free solder alloy, for example, Sn-Cu based, Sn-Ag-Cu based, Sn-Bi based, Sn-Zn based solder alloys and the like are well known. Among them, Sn-3Ag-0.5Cu solder alloy is often used for consumer electronic devices used in televisions, mobile phones, etc. and in-vehicle electronic devices mounted in automobiles.
The lead-free solder alloy is somewhat inferior in solderability to the lead-containing solder alloy. However, improvements in flux and soldering equipment have covered this solderability problem. Therefore, for example, even in a vehicle-mounted electronic circuit mounting board, Sn-3Ag-0.5Cu solder alloy is used in a case where there is a difference in temperature and temperature but a relatively mild environment, such as the interior of an automobile. There is no major problem even in the solder joint formed by the above method.

However, in recent years, for example, the placement in the engine compartment, the direct mounting on the engine, the placement in the electromechanically integrated unit with the motor, such as the electronic circuit mounting board used in the electronic control unit, has been studied and put into practical use. ing. These have a particularly severe temperature difference (for example, a temperature difference of −30° C. to 110° C., −40° C. to 125° C., −40° C. to 150° C.), and are in a severe environment where they are subjected to a vibration load. In such an environment where the temperature difference is extremely large, in the electronic circuit mounting board, the mounted electronic components and the board (in the present specification, when simply referred to as “board”, the board before forming the conductor pattern and the conductor pattern are formed) A plate that can be electrically connected to an electronic component, or a plate portion that does not include an electronic component in an electronic circuit mounting board on which the electronic component is mounted. Refers to "a plate portion that does not include electronic components in an electronic circuit mounting board on which electronic components are mounted"), and thermal displacement of the solder joint portion due to a difference in linear expansion coefficient with the stress is easily generated. The repeated plastic deformation due to the temperature difference easily causes cracks in the solder joint.
Furthermore, the stress repeatedly applied to the solder joint with the passage of time concentrates near the tip of the crack that has occurred, so that the crack easily propagates transversely to the deep portion of the solder joint. The cracks that have grown remarkably in this way cause disconnection of the electrical connection between the electronic component and the conductor pattern formed on the substrate. Especially in an environment where vibration is applied to the electronic circuit mounting board in addition to a severe temperature difference, the cracks and their development are more likely to occur.
Therefore, with the increasing number of electronic circuit mounting boards for vehicles and electronic control devices placed in the above-mentioned harsh environment, the demand for Sn-Ag-Cu-based solder alloys that can exert a sufficient crack growth suppression effect is expected in the future. It is expected to grow even larger.

In addition, many electronic components mounted on an on-vehicle electronic circuit board are mounted by a surface mounting method. However, in the case of an electronic component having a portion (insulator portion in the case of a connector) that is deformed by the influence of heat, such as a connector, the thermal deformation causes the electronic component itself to warp. The terminals and the electrodes (lands) may not be soldered.
Therefore, in the case of such an electronic component, a mounting method (through hole mounting method) is used in which a through hole is provided in a substrate, terminals of the electronic component are inserted into the through hole, and soldering is performed by a flow or reflow method. The terminal of the electronic component inserted into the through hole and the land are electrically connected to each other through a solder joint (fillet) formed on the substrate. Depending on the composition of the solder alloy, the land and the fillet may be different from each other. There is a possibility that a phenomenon (a lift-off phenomenon) may occur in which a gap is generated between them. The generation of the gap may cause the disconnection of the electrical connection between the electronic component and the conductor pattern formed on the substrate.

Therefore, in particular, in an electronic circuit mounting board for a vehicle on which surface-mounted electronic components and through-hole mounted electronic components are mixedly mounted, it is sufficient even under a harsh environment where the temperature difference is extremely large and vibration is applied. It is expected that the demand for solder alloys that can suppress the above-mentioned lift-off phenomenon while exhibiting the effect of suppressing crack growth will further increase in the future.

A method of improving the strength of the solder joint and the thermal fatigue characteristics accompanying it by adding an element such as Ag or Bi to the Sn-Ag-Cu solder alloy, and thereby suppressing crack growth of the solder joint. Have been disclosed (see Patent Documents 1 to 7).

JP-A-5-228685 JP, 9-326554, A JP 2000-19090 A JP-A-2000-349433 JP, 2008-28413, A International publication pamphlet WO2009/011341 JP2012-81521A

When Bi is added to the solder alloy, Bi enters the lattice of atomic arrangement of the solder alloy and substitutes for Sn to distort the lattice of atomic arrangement. As a result, the Sn matrix is strengthened and the alloy strength is improved, so that the addition of Bi is expected to improve the suppression of solder crack growth to a certain extent.

However, in the mounting of electronic components by the above-described through-hole mounting method, during the cooling process after the flow and reflow, the heat permeated into the substrate by the flow and the reflow has a higher thermal conductivity, that is, inside the through hole. It conducts to the land (Cu) on the substrate through the provided Cu. On the other hand, although the fillet provided on the substrate solidifies from the surface by cooling, the fillet at the interface with the land where the above-mentioned heat conduction has occurred becomes difficult to solidify.
As described above, when the fillet near the interface with the land is hard to solidify, shrinkage force (acting perpendicularly to the substrate) due to solidification shrinkage and thermal shrinkage of the substrate from near the surface of the fillet and inside the through hole occurs. If so, the surface of the fillet is easily separated from the land, and a gap may occur between the two.

This phenomenon is likely to occur particularly when Bi is added to the solder alloy forming the fillet. During the cooling process after the flow and reflow, Bi easily collects in the portion of the fillet that is hard to solidify, that is, near the interface with the land, and the Bi concentration increases near this point. Since Bi is an alloying element with a low melting point, the fillet becomes even harder to solidify near the interface where the concentration of Bi is high. Therefore, when the contracting force acting perpendicularly on the substrate is generated, the surface of the fillet from the land is more easily peeled off.

As described above, although it may be difficult to improve the reliability of the in-vehicle electronic circuit mounting board only by increasing the strength by adding Bi, these phenomena and their suppression are described in Patent Documents 1 to 7. Is neither disclosed nor suggested.

Further, in recent years, in lead parts and lower surface electrodes of electronic parts such as QFP (Quad Flat Package) and SOP (Small Outline Package) mounted on an in-vehicle electronic circuit board, Ni/Pd/Au plating or Ni/Au plating is not used. The number of Sn-plated products is increasing.
At the time of solder bonding, the Sn-plated electronic component easily causes mutual diffusion between Sn contained in the Sn-plated and solder-bonded portion and Cu contained in the lead portion or the lower surface electrode. This interdiffusion causes uneven growth of the intermetallic compound (Cu ₃ Sn layer) in a region near the interface between the solder joint and the lead portion or the lower surface electrode (hereinafter referred to as “interface region”). Easy to make. Since the Cu ₃ Sn layer originally has the property of being hard and brittle, and the Cu ₃ Sn layer that has grown large in an uneven shape becomes more brittle, the Cu ₃ Sn layer having an uneven shape is formed in an environment with a large difference in temperature. There is also a problem that crack propagation caused by the crack and electrical short circuit due to the crack are very likely to occur.

The present invention aims to provide a lead-free solder alloy capable of solving the above problems, specifically the following problems, and an electronic circuit mounting board and an electronic control device having a solder joint formed using the lead-free solder alloy. And
-The crack growth of the solder joint can be suppressed even in a severe environment where the temperature difference is large and vibration is applied.
-The addition of Bi to the lead-free solder alloy can improve the strength and also suppress the occurrence of lift-off phenomenon in through-hole mounting.
-Suppresses crack propagation near the interface between the lead portion of the electronic component or the bottom surface electrode and the solder joint, even when soldering is performed using an electronic component that is not Ni/Pd/Au plated or Ni/Au plated can do.

(1) The lead-free solder alloy of the present invention has Ag of 1 mass% or more and 4 mass% or less, Cu of 0.1 mass% or more and 1 mass% or less, and Sb of 3 mass% or more and 5 mass% or less, It is characterized in that it contains Bi in an amount of 0.1% by mass to 0.7% by mass, Ni in an amount of 0.01% by mass to 0.25% by mass, and the balance being Sn.

(2) In the structure described in (1) above, the lead-free solder alloy of the present invention is characterized by further containing 0.001% by mass or more and 0.25% by mass or less of Co.

(3) In the configuration according to (1) or (2) above, the content of Sb is 3.5% by mass or more and 5% by mass or less, The claim 1 or claim 2 characterized in that Lead-free solder alloy.

(4) In the configuration according to any one of (1) to (3) above, the lead-free solder alloy of the present invention has a vertical axis of heat flow (mW) and a horizontal axis of temperature based on the DSC measurement results. In the DSC curve with (° C.), the temperature (T1) at which the heat flow becomes less than −0.5, the heat flow (H1) at the temperature (T1), the temperature (T2) at which the heat flow shows a peak, and the temperature. It is characterized in that the heat flow (H2) in (T2) satisfies the following formula (A).
|T1-T2|/|H1-H2| ≤ 1.05 (A)
(In the above formula (A), the values of |T1-T2| and |H1-H2| are absolute values.)

(5) In the configuration according to any one of (1) to (4) above, the lead-free solder alloy of the present invention further comprises at least one of P, Ga and Ge in a total amount of 0.001% by mass. The feature is that the content is not less than 0.05% by mass.

(6) In the structure according to any one of (1) to (5) above, the lead-free solder alloy of the present invention further comprises at least one of Fe, Mn, Cr and Mo in a total amount of 0.001. It is characterized in that the content is not less than mass% and not more than 0.05 mass %.

(7) The electronic circuit mounting board of the present invention is characterized in that it has a solder joint formed by using the lead-free solder alloy according to any one of (1) to (6) above.

(8) The electronic control device of the present invention is characterized by having the electronic circuit mounting board described in (7) above.

INDUSTRIAL APPLICABILITY The lead-free solder alloy of the present invention, and an electronic circuit mounting board and an electronic control device having a solder joint formed by using the lead-free solder alloy have the following effects.
-The crack growth of the solder joint can be suppressed even in a severe environment where the temperature difference is large and vibration is applied.
-The addition of Bi to the lead-free solder alloy can improve the strength and also suppress the occurrence of lift-off phenomenon in through-hole mounting.
-Suppresses crack propagation near the interface between the lead portion of the electronic component or the bottom surface electrode and the solder joint, even when soldering is performed using an electronic component that is not Ni/Pd/Au plated or Ni/Au plated can do.

9 is a DSC curve in which the vertical axis represents heat flow (mW) and the horizontal axis represents temperature (° C.) based on the DSC measurement results of the lead-free solder alloy according to Example 1. 9 is a DSC curve in which the vertical axis represents heat flow (mW) and the horizontal axis represents temperature (° C.) based on the DSC measurement results of the lead-free solder alloy according to Example 2. A DSC curve in which the vertical axis represents heat flow (mW) and the horizontal axis represents temperature (°C) based on the DSC measurement results of the lead-free solder alloy according to Comparative Example 2. A DSC curve in which the vertical axis represents heat flow (mW) and the horizontal axis represents temperature (°C) based on the DSC measurement results of the lead-free solder alloy according to Comparative Example 3. 9 is a DSC curve in which the vertical axis represents heat flow (mW) and the horizontal axis represents temperature (° C.) based on the DSC measurement results of the lead-free solder alloy according to Comparative Example 4. 6 is an electron micrograph showing a cross section in which a void is generated in a fillet portion of a chip component in a test board according to a comparative example of the present invention. In order to show the "region under the electrode of the chip component" and the "region in which the fillet is formed" for observing the presence or absence of voids in the examples and comparative examples of the present invention, a general chip component mounting substrate was subjected to X-ray transmission. Photo taken from the chip component side using the device.

Hereinafter, one embodiment of the lead-free solder alloy, the electronic circuit mounting board, and the electronic control device of the present invention will be described in detail. The present invention is not limited to the embodiments below.

(1) Lead-Free Solder Alloy The lead-free solder alloy of the present embodiment can contain 1% by mass or more and 4% by mass or less of Ag. By adding Ag within this range, Ag ₃ Sn compound can be precipitated in the Sn grain boundary of the lead-free solder alloy to impart mechanical strength, and at the same time, it can be balanced with the extensibility of the lead-free solder alloy. Can be planned.
However, when the content of Ag is less than 1% by mass, precipitation of Ag ₃ Sn compound is small, and mechanical strength and thermal shock resistance of the lead-free solder alloy are deteriorated, which is not preferable. On the other hand, if the Ag content exceeds 4% by mass, the extensibility of the lead-free solder alloy may be impaired, and the thermal fatigue resistance of the solder joint formed using this may deteriorate, which is not preferable.
When the content of Ag is 2% by mass or more and 3.8% by mass or less, the balance between strength and drawability of the lead-free solder alloy can be improved. A more preferable Ag content is 2.5% by mass or more and 3.8% by mass or less.

The lead-free solder alloy of the present embodiment can contain Cu in an amount of 0.1% by mass or more and 1% by mass or less. By adding Cu in this range, the Cu ₆ Sn ₅ compound is precipitated in the Sn grain boundary, so that the thermal shock resistance of the lead-free solder alloy can be improved.
When the content of Cu is 0.1% by mass or more and 0.7% by mass or less, the viscosity of the lead-free solder alloy at the time of melting can be kept in a good state, and the generation of voids during reflow can be further suppressed. The thermal shock resistance of the solder joint to be formed can be improved. Furthermore, in this case, fine Cu ₆ Sn ₅ is dispersed in the Sn crystal grain boundary of the molten lead-free solder alloy to suppress the change in the crystal orientation of Sn and suppress the deformation of the solder joint shape (fillet shape). can do.
If the Cu content exceeds 1% by mass, the Cu ₆ Sn ₅ compound is likely to precipitate near the interface between the solder joint and the electronic component and near the interface between the solder joint and the land of the board, resulting in the joint reliability. It is not preferable because it may hinder the stretchability of the solder joint.

Here, in general, a solder joint formed using a lead-free solder alloy containing Sn, Ag and Cu has an intermetallic compound (eg Ag ₃ Sn, Cu ₆ Sn ₅ etc.) at the interface between Sn particles. Is dispersed, and even when a tensile force is applied to the solder joint, the structure can prevent the phenomenon that the Sn particles are slipped and deformed, whereby so-called mechanical properties can be exhibited. That is, the intermetallic compound plays a role of preventing the Sn particles from slipping.
Therefore, in the case of the lead-free solder alloy of the present embodiment, the content balance of Ag and Cu is set to 1% by mass or more and 4% by mass or less of Ag and 0.1% by mass or more and 1% by mass or less of Cu, and the content of Ag is When the content of Cu is equal to or more than the content of Cu, Ag ₃ Sn can be easily formed as the intermetallic compound, and the Cu content can exhibit relatively good mechanical properties. That is, even the Cu content was less than 1 wt%, satisfactory in both from the partially contributes to slip effect of the Ag 3 _Sn becoming intermetallic compound, Ag 3 _Sn and Cu It is thought that it can exhibit various mechanical properties.

The lead-free solder alloy of the present embodiment can contain 3% by mass or more and 5% by mass or less of Sb. By adding Sb in this range, the crack growth suppressing effect of the solder joint can be improved without impairing the stretchability of the Sn-Ag-Cu solder alloy. When the Sb content is 3% by mass or more and 4.5% by mass or less, and more preferably 3.5% by mass or more and 4% by mass or less, the crack growth suppressing effect can be further improved.

Here, in order to withstand the external stress of being exposed to a severe environment where the temperature difference is severe for a long time, the toughness of the lead-free solder alloy (the size of the area surrounded by the stress-strain curve) is increased and stretched. It is considered that it is effective to improve the properties and add a solid solution element to the Sn matrix to strengthen the solution. Then, Sb is the optimum element for performing solid solution strengthening of the lead-free solder alloy while ensuring sufficient toughness and stretchability.
That is, by adding Sb in the above range to a lead-free solder alloy in which Sn is the base material (in this specification, the main constituent elements of the lead-free solder alloy. A part of the crystal lattice is replaced with Sb, and the crystal lattice is distorted. Therefore, in the solder joint formed using such a lead-free solder alloy, the energy required for the transition in the crystal is increased by the Sb substitution of a part of the Sn crystal lattice, and the metal structure thereof is strengthened. .. Further, in this case, the fine SnSb, ε-Ag ₃ (Sn, Sb) compound is precipitated at the Sn grain boundary, thereby preventing the slip deformation of the Sn grain boundary and suppressing the progress of cracks generated in the solder joint. You can

In addition, compared to Sn-3Ag-0.5Cu solder alloy, the structure of the solder joint formed using the lead-free solder alloy containing Sb in the above range is exposed for a long time in a severe environment where the difference in temperature is severe. After that, it was confirmed that the Sn crystal maintained a fine state and that the structure was such that cracks were unlikely to propagate. This is because the SnSb and ε-Ag ₃ (Sn,Sb) compounds precipitated at the Sn grain boundaries are finely dispersed in the solder joints even after long-term exposure in a severe environment where the difference in temperature is severe. Therefore, it is considered that the coarsening of the Sn crystal is suppressed. That is, in the solder joint using the lead-free solder alloy to which Sb is added within the above range, the solid solution of Sb in the Sn matrix in the high temperature state is SnSb, ε-Ag ₃ (Sn, Sb) in the low temperature state. Since precipitation of compounds occurs, even when exposed to a severe environment where the temperature difference is severe for a long time, the process of solid solution strengthening at high temperature and precipitation strengthening at low temperature are repeated, resulting in excellent cold heat resistance. It is thought that impact resistance can be secured.

When the content of Sb is less than 3% by mass, the energy required for the transition in the crystal is increased by Sb substitution in a part of the Sn crystal lattice, and the metal structure can be solid-solution strengthened, but SnSb, Fine compounds such as ε-Ag ₃ (Sn, Sb) cannot be sufficiently precipitated at the Sn grain boundaries. Therefore, if a solder joint formed using such a solder alloy is exposed to a severe environment where the temperature difference is severe for a long time, the Sn crystal becomes enlarged and changes into a structure in which cracks are likely to develop. It is difficult to ensure sufficient thermal fatigue resistance for the parts.
If the content of Sb exceeds 5% by mass, the melting temperature of the lead-free solder alloy will rise, and Sb will not re-dissolve at high temperatures. Therefore, when a solder joint formed using such a lead-free solder alloy is exposed to a severe environment where the temperature difference is severe for a long time, only precipitation strengthening by SnSb, ε-Ag ₃ (Sn, Sb) compound is performed. Is done. Therefore, in this case, these intermetallic compounds become coarse with the lapse of time, and the effect of suppressing the slip deformation of the Sn grain boundary is lost. Further, in this case, the heat resistance temperature of the electronic component becomes a problem due to the increase in the melting temperature of the lead-free solder alloy, which is not preferable.

The lead-free solder alloy of the present embodiment can contain 0.1 mass% or more and 0.7 mass% or less of Bi. By adding Bi within this range, the lift-off phenomenon can be suppressed while improving the strength of the lead-free solder alloy. When the Bi content is 0.4% by mass or more and 0.7% by mass or less, the strength of the lead-free solder alloy can be further improved while suppressing the occurrence of the lift-off phenomenon.
When the Bi content is less than 0.1% by mass, the interface strength between the fillet and the land is not sufficiently improved, and when the Bi content exceeds 0.7% by mass, the lift-off phenomenon occurs at the interface between the fillet and the land. May occur, which is not preferable.

The lead-free solder alloy of this embodiment can contain 0.01 mass% or more and 0.25 mass% or less of Ni. In the case of the lead-free solder alloy of the present embodiment, by adding Ni within this range, fine (Cu, Ni) ₆ Sn ₅ is formed in the molten lead-free solder alloy, and is formed in the base material. Since they are dispersed, it is possible to suppress the development of cracks in the solder joints and further improve their thermal fatigue resistance.
Further, in the lead-free solder alloy of the present embodiment, even when an electronic component not plated with Ni/Pd/Au or Ni/Au is solder-bonded to the substrate, Ni is present in the interface region during solder bonding. Since it moves to form fine (Cu, Ni) ₆ Sn ₅ , the growth of the Cu ₃ Sn layer in the interface region can be suppressed, and the crack growth suppressing effect in the interface region can be improved.

However, if the Ni content is less than 0.01% by mass, the effect of modifying the intermetallic compound becomes insufficient, and thus it is difficult to sufficiently obtain the effect of suppressing cracks in the interface region. Further, when the Ni content exceeds 0.25 mass %, overcooling is less likely to occur as compared with the conventional Sn-3Ag-0.5Cu alloy, and the timing at which the solder alloy solidifies becomes earlier. In this case, in the fillet of the solder joint to be formed, the gas that tried to escape during the melting of the solder alloy solidifies while remaining in it, causing holes (voids) due to the gas in the fillet. It is confirmed that there is a case. The voids in the fillet will deteriorate the thermal fatigue resistance of the solder joints especially in an environment with a large temperature difference such as −40° C. to 140° C. and −40° C. to 150° C., and cracks caused by the voids. Is likely to occur.
As described above, Ni tends to generate voids in the fillet. However, in the structure of the lead-free solder alloy of the present embodiment, Ni is less than 0.1% due to the balance of the contents of Ni and other elements. Even if the content is 25% by mass or less, the generation of the void can be suppressed.

Further, when the content of Ni is 0.01% by mass or more and 0.15% by mass or less, it is possible to improve the effect of suppressing crack growth in the interface region and the thermal fatigue resistance, while suppressing the occurrence of voids.

The lead-free solder alloy of the present embodiment may contain 0.001% by mass or more and 0.25% by mass or less of Co in addition to Ni. In the case of the lead-free solder alloy of the present embodiment, by adding Co in this range, the above effect due to the addition of Ni is enhanced and fine (Cu, Co) ₆ Sn _{5 is} contained in the molten lead-free solder alloy. Are formed and dispersed in the base material. Therefore, in this case, the thermal fatigue resistance of the solder joint can be improved even under the environment where the temperature difference is large, while suppressing the creep deformation and the crack development of the solder joint.
In addition, the lead-free solder alloy of the present embodiment enhances the above-mentioned effect by adding Ni and improves the effect of Co even when soldering electronic components not plated with Ni/Pd/Au or Ni/Au. The fine particles (Cu, Co) ₆ Sn ₅ are formed by moving to the interface region during soldering. Therefore, in this case, the growth of the Cu ₃ Sn layer in the interface region can be suppressed, and the crack growth suppressing effect in the interface region can be improved.

However, if the Co content is less than 0.001% by mass, it is difficult to obtain the effect of modifying the intermetallic compound by the addition of Co, and if the Co content exceeds 0.25% by mass, the conventional As compared with the Sn-3Ag-0.5Cu alloy, supercooling is less likely to occur, and the timing at which the solder alloy solidifies becomes earlier. In this case, in the fillet of the solder joint to be formed, the gas that tried to escape during the melting of the solder alloy solidifies while remaining in it, and voids due to gas occur in the fillet. Is confirmed. Since the voids in this fillet are supposed to be cavities where the alloy should originally be, the solder joints are particularly vulnerable to temperatures of -40°C to 140°C and -40°C to 150°C. Therefore, the thermal fatigue resistance is deteriorated, and cracks due to voids are likely to occur.
As described above, Co tends to generate voids in the fillet. However, in the structure of the lead-free solder alloy of the present embodiment, Co is less than 0.1% due to the balance of the contents of Co and other elements. Even if the content is 25% by mass or less, the generation of the void can be suppressed.

When the Co content is 0.001% by mass or more and 0.15% by mass or less, it is possible to improve the crack growth suppressing effect and the thermal fatigue resistance, and at the same time suppress the occurrence of voids.

Further, the lead-free solder alloy of the present embodiment can contain at least one of P, Ga and Ge in an amount of 0.001 mass% or more and 0.05 mass% or less. Oxidation of the lead-free solder alloy can be prevented by adding at least one of P, Ga and Ge within the range of this total amount. However, if the total amount of these exceeds 0.05% by mass, the melting temperature of the lead-free solder alloy rises, and voids are likely to occur at the welded joint, which is not preferable.

Further, the lead-free solder alloy of the present embodiment can contain at least one of Fe, Mn, Cr and Mo in an amount of 0.001 mass% or more and 0.05 mass% or less. By adding at least one of Fe, Mn, Cr and Mo within the range of this total amount, the crack growth suppressing effect of the lead-free solder alloy can be improved. However, if the total amount of these exceeds 0.05% by mass, the melting temperature of the lead-free solder alloy rises, and voids are likely to occur at the welded joint, which is not preferable.

Note that the lead-free solder alloy of the present embodiment contains other components (elements) such as In, Cd, Tl, Se, Au, Ti, Si, Al, Mg, Zn, etc. within a range that does not impair the effect. Can be included. In addition, the lead-free solder alloy of the present embodiment naturally contains inevitable impurities.

Further, the lead-free solder alloy of the present embodiment preferably comprises Sn as the balance. The preferable Sn content is 88.7% by mass or more and 95.79% by mass or less.

The lead-free solder alloy according to the present embodiment has the balance of Ag, Cu, Sb, Bi and Ni contents as described above, so that Bi is added to improve strength and lift-off in the through-hole mounting method. It is possible to suppress the occurrence of the phenomenon.
Further, by suppressing the occurrence of the lift-off phenomenon, the dipping process after the reflow process can be omitted, so that cost reduction by reducing the number of processes can be expected.

Furthermore, in the lead-free solder alloy of the present embodiment, the heat flow is less than -0.5 in the DSC curve in which the vertical axis represents heat flow (mW) and the horizontal axis represents temperature (°C) based on the DSC measurement results. The temperature (T1), the heat flow (H1) at the temperature (T1), the temperature (T2) at which the heat flow has a peak, and the heat flow (H2) at the temperature (T2) satisfy the following expression (A). Is preferred.
|T1-T2|/|H1-H2|≦ 1.05 (A)
In the above formula (A), the values of |T1-T2| and |H1-H2| are absolute values.

The “temperature at which the heat flow becomes less than −0.5 (T1)” means the temperature at which the heat flow initially showed less than −0.5 in the DSC measurement result.

In the present embodiment, T1 and T2 are rounded down to the third decimal place, H1 and H2 are rounded down to the fifth decimal place, and |T1-T2|/|H1-H2| is rounded down to the third decimal place. did. In this case, the values of |T1-T2| and |H1-H2| are absolute values.

In this embodiment, the measurement conditions in the DSC measurement are as follows.
・Measuring device: Differential scanning calorimeter (Product name: DSC Q2000, manufactured by TA Instruments)
・Measuring jig: Aluminum pan (product name: T-ZERO pan, manufactured by TA Instruments), aluminum lid (product name: T-ZERO lid, manufactured by TA Instruments)
・Measurement conditions: Temperature rising rate 2°C/min
・Nitrogen flow rate: 50 mL/min
・Sample amount: 10mg

Among the lead-free solder alloys of the present embodiment, the lead-free solder alloy whose temperature (T1), temperature (T2), heat flow (H1) and heat flow (H2) satisfy the above formula (A) was used. When soldering is performed by through-hole mounting, while exhibiting the crack growth-promoting effect of the fillet formed by the addition of Bi, the formation of a low melting point alloy phase that occurs near the interface of the fillet land during solidification of the solder Since the number is small, the occurrence of lift-off phenomenon can be further suppressed in through-hole mounting.
It is more preferable that |T1-T2|/|H1-H2| be 1.00 or less. Also in this case, the values of |T1-T2| and |H1-H2| are both absolute values.

The solder joint of the present embodiment may be formed by any method as long as the solder joint can be formed, such as a flow method, mounting with a solder ball, and a reflow method using a solder paste composition. .. Among them, the reflow method using the solder paste composition is particularly preferably used.

(2) Solder Paste Composition Such a solder paste composition is prepared, for example, by kneading a powder of the lead-free solder alloy of the present embodiment and a flux to form a paste.

As such a flux, for example, a flux containing a resin, a thixotropic agent, an activator, and a solvent is used.

Examples of the resin include rosin such as tall oil rosin, gum rosin, wood rosin, hydrogenated rosin, polymerized rosin, heterogenized rosin, rosin derivative including acrylic acid modified rosin and maleic acid modified rosin; acrylic acid; , Methacrylic acid, various esters of acrylic acid, various esters of methacrylic acid, crotonic acid, itaconic acid, maleic acid, maleic anhydride, maleic acid ester, maleic anhydride ester, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide , An acrylic resin obtained by polymerizing at least one kind of monomer such as vinyl chloride and vinyl acetate; an epoxy resin; and a phenol resin. These can be used alone or in combination of two or more.
Of these, rosin-based resins, particularly hydrogenated acid-modified rosin obtained by hydrogenating an acid-modified rosin are preferably used. It is also preferable to use a hydrogenated acid-modified rosin and an acrylic resin in combination.

The acid value of the resin is preferably 10 mgKOH/g or more and 250 mgKOH/g or less. Further, the blending amount of the resin is preferably 10% by mass or more and 90% by mass or less with respect to the total amount of the flux.

Examples of the thixotropic agent include hydrogenated castor oil, fatty acid amides, and oxyfatty acids. These can be used alone or in combination of two or more. The blending amount of the thixotropic agent is preferably 3% by mass or more and 15% by mass or less based on the total amount of the flux.

As the activator, for example, an amine salt (inorganic acid salt or organic acid salt) such as a hydrogen halide salt of an organic amine, an organic acid, an organic acid salt or an organic amine salt can be blended. More specifically, diphenylguanidine hydrobromide, cyclohexylamine hydrobromide, diethylamine salt, acid salt, succinic acid, adipic acid, sebacic acid, malonic acid, dodecanedioic acid and the like can be mentioned. These can be used alone or in combination of two or more. The compounding amount of the activator is preferably 5% by mass or more and 15% by mass or less based on the total amount of the flux.

As the solvent, for example, isopropyl alcohol, ethanol, acetone, toluene, xylene, ethyl acetate, ethyl cellosolve, butyl cellosolve, glycol ether or the like can be used. These can be used alone or in combination of two or more. The blending amount of the solvent is preferably 20% by mass or more and 40% by mass or less with respect to the total amount of the flux.

An antioxidant may be added to the flux for the purpose of suppressing the oxidation of the lead-free solder alloy. Examples of the antioxidant include hindered phenol-based antioxidants, phenol-based antioxidants, bisphenol-based antioxidants, and polymer-type antioxidants. Among these, hindered phenolic antioxidants are particularly preferably used. These can be used alone or in combination of two or more. The blending amount of the antioxidant is not particularly limited, but it is generally preferably 0.5% by mass or more and 5% by mass or less with respect to the total amount of the flux.

Additives such as halogen, a matting agent, a defoaming agent and an inorganic filler may be added to the flux.
The amount of the additive compounded is preferably 10% by mass or less based on the total amount of the flux. Further, a more preferable blending amount of these is 5% by mass or less based on the total amount of the flux.

The compounding ratio of the alloy powder and the flux of the lead-free solder alloy is preferably 65:35 to 95:5 in terms of alloy powder:flux. A more preferable mixing ratio is 85:15 to 93:7, and a particularly preferable mixing ratio is 87:13 to 92:8.

The particle diameter of the alloy powder is preferably 1 μm or more and 40 μm or less, more preferably 5 μm or more and 35 μm or less, and particularly preferably 10 μm or more and 30 μm or less.

(3) Solder joint portion A solder joint portion formed using the solder paste composition of the present embodiment is formed by, for example, the following method.
-Surface mounting method: An electrode and an insulating layer having a predetermined pattern are formed at predetermined positions on the substrate, and the solder paste composition is printed according to the pattern. Then, an electronic component is mounted at a predetermined position on the board and reflowed at a temperature of, for example, 230° C. to 260° C., whereby the solder joint portion of the present embodiment is formed. The solder joint thus formed electrically joins the electrode (terminal) provided on the electronic component and the electrode formed on the substrate.
It should be noted that the substrate on which the electronic component is mounted is not limited to these as long as it is used for mounting and mounting the electronic component, such as an electronic circuit substrate such as a printed wiring board or a module substrate, a silicon wafer, a ceramic package substrate, or the like.

-Through hole mounting method An electrode and an insulating layer having a predetermined pattern are formed at a predetermined position on the substrate, a through hole is formed on the substrate according to this pattern, and Cu plating is performed inside the through hole. Give. Next, the solder paste composition is printed on the substrate so as to cover the upper portion of the through hole, and the terminals provided on the electronic component are mounted so as to be inserted into the through hole. Then, by reflowing this at a temperature of, for example, 230° C. to 260° C., the solder joint portion (fillet) of the present embodiment is formed. The solder joint portion thus formed electrically joins the terminal of the electronic component and the electrode formed on the substrate.
As a substrate on which electronic components are mounted, through holes can be provided such as an electronic circuit substrate such as a printed wiring board or a module substrate, a ceramic package substrate, and the like, which is used for mounting and mounting electronic components, as in the surface mounting method. Any material can be used without being limited to these.

The electronic circuit mounting board of the present embodiment preferably has the solder joint portion.
Since the solder joint has an alloy composition that exhibits a crack growth suppressing effect, even if a crack occurs in the solder joint, the crack growth can be suppressed. Further, even when the electrodes (terminals) of the electronic component are not plated with Ni/Pd/Au or Ni/Au, the effect of suppressing crack propagation near the interface between the solder joint and the electronic component is also exhibited. be able to. Further, this can also suppress the electrode peeling phenomenon of the electronic component.
Furthermore, since the solder joint has an alloy composition that can suppress the occurrence of the lift-off phenomenon, even when soldering is performed by a through-hole mounting method, both the crack growth suppressing effect and the lift-off phenomenon suppressing effect are obtained. Can be demonstrated.
The electronic circuit mounting board having such solder joints is placed in an environment with a large difference in temperature and temperature, and can be particularly suitably used for an on-vehicle electronic circuit mounting board that requires high reliability.

(4) Electronic Control Device Further, the electronic control device of the present embodiment is manufactured by incorporating the electronic circuit mounting board of the present embodiment.

Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples. The present invention is not limited to these examples.

Production of Flux The following components were kneaded to obtain fluxes according to Examples and Comparative Examples.
Hydrogenated acid-modified rosin (Product name: KE-604, manufactured by Arakawa Chemical Industry Co., Ltd.) 51% by mass
Hardened castor oil 6% by mass
Dodecanedioic acid 10% by mass (Product name: SL-12, manufactured by Okamura Oil Co., Ltd.)
Malonic acid 1% by mass
Diphenylguanidine hydrobromide 2% by mass
Hindered phenolic antioxidant (Product name: Irganox 245, manufactured by BASF Japan Ltd.) 1% by mass
Diethylene glycol monohexyl ether 29 mass%

Preparation of Solder Paste Composition 11% by mass of the flux was mixed with 89% by mass of powder of each lead-free solder alloy shown in Table 1 and Table 2 (powder particle size 20 μm to 38 μm), and Examples 1 to 6, 8 to 14, 17 to 29 , each of the solder paste compositions according to Reference Examples 7, 15, 16 and Comparative Examples 1 to 19 were produced.

(1) DSC Measurement For each of the lead-free solder alloys of Examples 1 and 2 and Comparative Examples 2 to 4, DSC measurement was performed under the following conditions, and based on the results, the heat flow on the vertical axis ( mW) and the horizontal axis was the temperature (°C), and a DSC curve was created. 1 to 5 respectively.
T1 and T2 were rounded down to the third decimal place, H1 and H2 were rounded down to the fifth decimal place, and |T1-T2|/|H1-H2| was rounded down to the third decimal place. In this case, the values of |T1-T2| and |H1-H2| are absolute values.

<Measurement conditions>
・Measuring device: Differential scanning calorimeter (Product name: DSC Q2000, manufactured by TA Instruments)
・Measuring jig: Aluminum pan (product name: T-ZERO pan, manufactured by TA Instruments), aluminum lid (product name: T-ZERO lid, manufactured by TA Instruments)
・Measurement conditions: Temperature rising rate 2°C/min
・Nitrogen flow rate: 50 mL/min
・Sample amount: 10mg

Table 3 shows T1, T2, H1 and H2 shown in FIGS. 1 to 5 and the results of |T1-T2|/|H1-H2| for each lead-free solder alloy.

(2) Solder crack test (-40 to 125°C)
a) 3.2 mm x 1.6 mm chip component (chip A)
The following tools were prepared.
・Chip parts of 3.2 mm x 1.6 mm size (Ni/Sn plating)
-Printed wiring board provided with a solder resist having a pattern capable of mounting the chip component of the above size and an electrode (1.6 mm x 1.2 mm) for connecting the chip component-A metal mask having the pattern and having a thickness of 150 µm Each solder paste composition was printed on the printed wiring board using the metal mask, and the chip parts were mounted on each.
Then, each of the printed wiring boards was heated using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation) to produce each electronic circuit mounting board having a solder joint. The reflow conditions at this time were as follows: preheat from 170° C. to 190° C. for 110 seconds, peak temperature at 245° C., time at 200° C. or higher for 65 seconds, time at 220° C. or higher for 45 seconds, peak temperature to 200° C. The cooling rate was 3° C. to 8° C./sec, and the oxygen concentration was set to 1500±500 ppm.

Next, using a thermal shock tester (product name: ES-76LMS, manufactured by Hitachi Appliances, Ltd.) set to -40° C. (30 minutes) to 125° C. (30 minutes), the thermal shock cycle was 1, Each of the electronic circuit mounting boards was exposed to the environment where 000, 1,500, 2,000, 2,500, 3,000 cycles were repeated, and then taken out to prepare each test board.
Then, the target portion of each test board was cut out and this was sealed using an epoxy resin (product name: Epomount (main agent and curing agent), manufactured by Refinetech Co., Ltd.). Further, using a wet polishing machine (product name: TegraPol-25, manufactured by Marumoto Struers Co., Ltd.), a state in which the central cross section of the chip component mounted on each test board can be seen is formed, and the formed solder joint portion is formed. Whether or not the cracks that occurred in the wire completely crossed the solder joint and led to fracture was observed using a scanning electron microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corporation), and the following criteria were used. Evaluated. The results are shown in Tables 4 and 5. The number of evaluation chips in each thermal shock cycle was 10.
⊚: No cracks completely crossing the solder joints up to 3,000 cycles ◯: Cracks completely crossing the solder joints between 2,501 and 3,000 cycles Δ: 2,001 to 2 , A crack completely crossing the solder joint was generated in 500 cycles. ×: A crack completely crossing the solder joint was generated in 2,000 cycles or less.

b) 2.0 x 1.2 mm chip component (chip B)
Each test board was prepared under the same conditions as the above 3.2 mm×1.6 mm chip part except that the following tools were used, and evaluated by the same method. The results are shown in Tables 4 and 5.
・2.0×1.2mm size chip parts (Ni/Sn plating)
-Printed wiring board provided with a solder resist having a pattern capable of mounting a chip component of the above size and an electrode (1.25 mm x 1.0 mm) connecting the chip component-A metal mask having the above pattern and having a thickness of 150 µm

(3) Solder crack test in Sn plating SON The following tools were prepared.
・6 mm x 5 mm x 0.8 tmm size 1.3 mm pitch SON (Small Outline Non-leaded package) parts (8 terminals, product name: STL60N3LLH5, manufactured by STMicroelectronics)
A printed wiring board provided with a solder resist having a pattern capable of mounting the SON component and an electrode (according to the manufacturer's recommended design) for connecting the SON component. A metal mask having a thickness of 150 μm and having the above pattern. Each of the solder paste compositions was printed using the above metal mask, and the SON component was mounted on each of them. After that, each electronic circuit mounting was prepared under the same conditions as the solder crack test (1) except that each electronic circuit mounting board was placed in an environment in which the thermal shock cycle was repeated 1,000, 2,000, 3,000 cycles. Each test board was produced by applying a thermal shock to the board.
Then, the target portion of each test board was cut out and this was sealed using an epoxy resin (product name: Epomount (main agent and curing agent), manufactured by Refinetech Co., Ltd.). Further, a wet polishing machine (product name: TegraPol-25, manufactured by Marumoto Struers Co., Ltd.) was used to make the central cross section of the SON component mounted on each test board visible, and cracks generated at the solder joints It was observed using a scanning electron microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corp.) whether or not the solder completely crossed the solder joint and led to breakage.
Based on this observation, regarding the solder joint portion, the solder base material (in this specification, the solder base material refers to a portion of the solder joint portion other than the interface of the electrode of the SON component and its vicinity. The same applies to the following. Table 4 Also, in Table 5, simply referred to as "base material") and cracks generated at the interface (intermetallic compound) of the solder joint and the electrode of the SON component were evaluated as follows. .. The results are shown in Tables 4 and 5. The number of evaluated SONs in each thermal shock cycle was 20, and one terminal of the gate electrode was observed for each SON, and the cross section of 20 terminals in total was confirmed.

・Cracks generated in the solder base metal ◎: No cracks completely traversing the solder base metal up to 3,000 cycles ○: Cracks completely crossing the solder base metal between 2,001 and 3,000 cycles Occurrence Δ: A crack completely traversing the solder base metal occurred between 1,001 and 2,000 cycles ×: A crack completely traversing the solder base metal occurred at 1,000 cycles or less

-Cracks that occurred at the interface between the solder joint and the electrode of the SON component ◎: No crack that completely crosses the interface up to 3,000 cycles is generated ◯: The interface is completely completed between 2,001 and 3,000 cycles A crack that completely crosses the interface occurs between 1,001 and 2,000 cycles. A crack that completely crosses the interface occurs at 1,000 cycles or less.

(4) Solder crack test (-40 to 150°C)
Automotive electronic circuit boards are placed in harsh environments where the temperature difference is extremely severe, so the solder alloy used for them is required to exhibit a good crack growth suppression effect even in such an environment. To be Therefore, in order to clarify whether or not the solder alloy according to the present example can exert the effect even under such a more severe condition, a liquid tank type thermal shock tester is used to measure from -40°C to 150°C. A solder crack test was conducted in the temperature difference. The conditions are as follows.
Liquid bath type thermal shock test equipment (product name: ETAC WINTECH LT80, Kusumoto Co., Ltd.) in which each electronic circuit mounting board after solder joint formation is set to a condition of -40°C (5 minutes) to 150°C (5 minutes) 3.2×1.6 mm chip part under the same conditions as in the solder crack test (1) except that the thermal shock cycle is repeated 1,000, 2,000, 3,000 cycles. Each test board was mounted and mounted with a 2.0×1.2 mm chip component.
Then, the target portion of each test board was cut out and this was sealed using an epoxy resin (product name: Epomount (main agent and curing agent), manufactured by Refinetech Co., Ltd.). Furthermore, a wet polishing machine (product name: TegraPol-25, Marumoto Struers Co., Ltd.,
Whether the cracks generated in the formed solder joints completely cross the solder joints and lead to fracture. It was observed using a scanning electron microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corporation), and evaluated according to the following criteria. The results are shown in Tables 4 and 5. The number of evaluation chips in each thermal shock cycle was 10.
⊚: No crack completely traversing the solder joints up to 3,000 cycles ◯: Cracks completely traversing the solder joints between 2,001 and 3,000 cycles Δ: 1,001 to 2 Cracks that completely cross the solder joints occur in 1,000 cycles. ×: Cracks that completely cross the solder joints occur in 1,000 cycles or less.

(5) Void test The following tools were prepared.
・2.0×1.2mm size chip parts (Ni/Sn plating)
A printed wiring board provided with a solder resist having a pattern capable of mounting a chip component of the above size and an electrode (1.25 mm x 1.0 mm) for connecting the chip component a metal mask having a thickness of 150 µm having the above pattern Each solder paste composition is printed on the printed wiring board using the metal mask, the chip parts are mounted on each, and a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation) is used. The printed wiring board was heated to produce each test board (electronic circuit mounting board) having solder joints. The reflow conditions were the same as in the solder crack test (1).
Next, the surface condition of each test board was observed with an X-ray transmission device (product name: SMX-160E, manufactured by Shimadzu Corporation), and in 40 lands of each test board, the area under the electrode of the chip component (Fig. The area ratio of voids (the ratio of the total area of voids; the same applies hereinafter) to the area (a) surrounded by the broken line in FIG. 7 and the area where fillets are formed (area (b) surrounded by the broken line in FIG. 7). The average value of the area ratio of voids occupying was calculated, and each was evaluated as follows. The results are shown in Tables 4 and 5.
⊚: The average area ratio of voids is 3% or less, and the effect of suppressing void generation is extremely good. ◯: The average value of void area ratio is more than 3% and 5% or less, and the effect of suppressing void generation. Is good Δ: The average value of the void area ratio is more than 5% and 8% or less, and the effect of suppressing void generation is sufficient. ×: The average value of the void area ratio exceeds 8%, and the effect of suppressing void generation is large. insufficient

(6) Lift-off test The following tools were prepared.
A printed wiring board having a structure in which 15 copper holes with a diameter of 1.6 mm and through holes with a diameter of 1.0 mm are arranged on a straight line at 2.5 mm pitch intervals. 200 μm thick metal mask having an opening pattern of 5 mm Each solder paste composition is printed on the printed wiring board using the metal mask, and a connector part (product name: S15B-EH(LF)) is provided in each through hole. (SN), manufactured by Nippon Crimp Terminal Manufacturing Co., Ltd. were inserted. Then, each of the printed wiring boards is heated by using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation), and each test board (electronic circuit mounting board) having a solder joint (fillet) is heated. It was made. The reflow conditions were the same as in the solder crack test (1).
Then, the target portion of each test board was cut out and this was sealed using an epoxy resin (product name: Epomount (main agent and curing agent), manufactured by Refinetech Co., Ltd.). Furthermore, a wet polishing machine (product name: TegraPol-25, Marumoto Struers Co., Ltd.,
(Made by Hitachi High-Technologies Corp.) and observed by using a scanning electron microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corporation). The evaluation was made according to the following criteria. The results are shown in Tables 4 and 5. The number of evaluation terminals was eight.
○: Lift-off phenomenon did not occur ×: Lift-off phenomenon occurred

As shown above, the solder joint formed by using the lead-free solder alloy according to the embodiment does not depend on the size of the chip even in a harsh environment in which the temperature difference is severe and vibration is applied. In addition, regardless of whether the electrode is Ni/Pd/Au plated or Ni/Au plated, the effect of suppressing crack propagation in the solder joint and the interface region can be exhibited. In particular, it can be seen that the solder joints of the examples have a good crack suppressing effect even in a very harsh environment in which the temperature difference between -40°C and 150°C is used by using the liquid tank type thermal shock test device.
Further, it is understood that the lead-free solder alloys according to the examples can exhibit both the crack growth suppressing effect and the lift-off phenomenon occurrence suppressing effect even in the solder joining by the through-hole mounting method. Such a lead-free solder alloy can be suitably used for an electronic circuit mounting board on which electronic components mounted by the surface mounting method and electronic components mounted by the through hole mounting method are mixedly mounted.
The electronic circuit mounting board having such a solder joint can be suitably used for an electronic circuit mounting board which is required to have high reliability such as an on-vehicle electronic circuit mounting board. Further, such an electronic circuit mounting board can be suitably used for an electronic control device that requires higher reliability.

Claims (8)

Ag 2 mass% or more and 4 mass% or less, Cu 0.1 mass% or more and 1 mass% or less, Sb 3 mass% or more and 5 mass% or less, and Bi 0.1 mass% or more and 0.7 mass% or less. % or less and, Ni hints 0.03 mass% 0.01 mass%, lead-free solder alloy balance being made of Sn. The lead-free solder alloy according to claim 1, further comprising 0.001% by mass or more and 0.25% by mass or less of Co. The lead-free solder alloy according to claim 1 or 2, wherein the content of Sb is 3.5% by mass or more and 5% by mass or less. Based on the DSC measurement result, in the DSC curve in which the vertical axis represents heat flow (mW) and the horizontal axis represents temperature (°C),
A temperature (T1) at which the heat flow becomes less than -0.5, and a heat flow (H1) at the temperature (T1),
The temperature (T2) at which the heat flow shows a peak and the heat flow (H2) at the temperature (T2) satisfy the following formula (A): 4. Lead-free solder alloy.
|T1-T2|/|H1-H2| ≤ 1.05 (A)
(In the above formula (A), the values of |T1-T2| and |H1-H2| are absolute values.) The lead-free solder alloy according to any one of claims 1 to 4, further containing at least one of P, Ga, and Ge in a total amount of 0.001% by mass or more and 0.05% by mass or less. .. Further, at least one kind of Fe, Mn, Cr, and Mo is contained in a total amount of 0.001% by mass or more and 0.05% by mass or less, and the lead-free according to any one of claims 1 to 5. Solder alloy. An electronic circuit mounting board having a solder joint portion formed by using the lead-free solder alloy according to any one of claims 1 to 6. An electronic control device comprising the electronic circuit mounting board according to claim 7.