JP3254857B2 - Solder alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solder alloy.
More specifically, a solder alloy that is used when mounting electronic components on printed circuit boards and is suitable for solder joints of electronic components used in environments where fatigue is likely to occur due to repeated stress of temperature cycles It is.

[0002]

2. Description of the Related Art In general, when mounting electronic components on a printed circuit board, a solder alloy is used. In consideration of heat influence and workability on the electronic components and the printed circuit board, the melting point is low and soldering is performed. A Sn-Pb solder alloy of 60 to 63% by weight of Sn having good properties is used. As a soldering method using this solder alloy, there are a dipping method dipping in molten solder, a reflow method using a solder paste or a form solder, and the like.

[0003] Solder joints soldered by a dip method or a reflow method are used to turn on / off switches of electronic equipment.
As a result, it is exposed to a temperature cycle environment in which heating and cooling are repeated. When the solder joint is exposed to a temperature cycle environment, a thermal stress is generated due to a difference in thermal expansion coefficient between the electronic component and the printed circuit board, and finally, the solder is cracked to be broken. This is because the solder is weaker in strength than other members to be joined, so that the thermal stress in the joint is concentrated on the solder.

Heretofore, in a place where a solder joint is destroyed by thermal stress, the component lead of the joint is curved to have a structure in which thermal stress is not directly applied to the solder. Measures related to mounting design,
Measures have been taken regarding solder joints, such as increasing the amount of solder by adding solder to the joints after mounting electronic components by ironing work.

[0005] However, due to recent high-density mounting of electronic devices, the joints have been miniaturized, such as shortening of leads or electrodes of electronic parts and pads of a printed circuit board. It has become difficult to design a mounting form, form a good solder fillet, and secure a sufficient amount of solder. In addition, high-density mounting has enabled electronic devices to be highly functional and compact, thereby enabling portability, greatly expanding their use environment, and exposing solder joints to more severe environments than before. It is becoming. For this reason, in electronic devices, not only the reliability of solder joints but also the demand for improving fatigue resistance to solder has been further increased. In response to such a demand for solder, conventionally, the strength of the solder has been increased by adding a third element to a Pb-Sn alloy.

As a Sn-Pb alloy to which a third element is added, Te is added in Japanese Patent Application Laid-Open No. 1-127192 and disclosed in Japanese Patent Application Laid-Open Nos. 1-237095 and 3-3248.
In No. 7, Sb, In, Ag, Cu and the like are added. In JP-A-1-106591, In, Ga, Sb, Bi, A and the like are added.
g, Au, Al, etc. are added to improve the fatigue resistance.

[0007] In these solder alloys, by adding a third element to the Sn-Pb alloy, an intermetallic compound is precipitated at the grain boundaries of the Sn phase and the Pb phase in the structure of the solder, and this intermetallic compound is removed. By exhibiting a pinning effect that suppresses crystal grain growth and plastic deformation, the strength and creep resistance of the solder alloy are increased to improve fatigue resistance.

However, a solder having an alloy composition in which an intermetallic compound precipitates may cause an increase in melting point or an impairment of fluidity. In particular, when used in the dipping method, the intermetallic compound floating in the molten solder is a foreign substance, and there is a possibility that the workability may be deteriorated. Even if they are uniformly melted in a liquid state, there is a concern that the fluidity of the printed circuit board may decrease due to a temporary temperature drop of the solder when the printed circuit board comes into contact with the molten solder. Incidentally, a Te-added alloy has an effect on fatigue resistance, but Te has a high reactivity with oxygen, and concentrates into dross to decrease the amount of Te in the solder. Therefore, when used in the dipping method, the constituent components of the alloy are liable to change and lack stability for long-term use.

Although not intended to improve fatigue resistance, there is an Sn-Pb-based alloy in which Ni is added to improve solder properties. Tokiko 40-2588
In No. 5, Sn: 10 to 90% by weight, Pb: 10 to 90% by weight
% By weight, Cu: 0.2 to 2.0% by weight,
An alloy to which g: 0 to 0.01% by weight and Ni: 0 to 0.01% by weight has been proposed. This is because at the time of ironing work, the iron of the iron tip dissolves in the solder, impairs the spreadability of the solder and also wears the iron tip. Ag and Ni are intended to suppress Cu dissolution, and prevent Cu from impairing the spreadability of the solder.

In Japanese Patent Publication No. 51-32067, it is described that in a method for manufacturing a semiconductor device, a Sn-Pb-based solder alloy containing Ni in advance is used. This is because when connecting the electrode lead wires to the Ni plating film on the semiconductor substrate by soldering, the remaining Ni after soldering is used.
Since the thickness of the plating film affects the mechanical strength of the electrode, Ni in the solder alloy is added with Ni to suppress the dissolution of the Ni plating film in the solder during soldering. That is, the Ni-added Sn—Pb solder alloy is N
By leaving an effective thickness of Ni plating between the semiconductor substrate and the solder in the soldered portion of the i-plated film, the mechanical strength of the electrode can be increased. Here, it is described that an appropriate amount of Ni added to the Sn-Pb-based solder alloy is 0.05 to 0.2% by weight from the viewpoint of the melting point and mechanical strength of the solder.

[0011]

As described above, the fatigue fracture of the solder is caused by the concentration of the thermal stress at the joint to the solder. In other words, the solder absorbs the thermal stress at the joint, This alleviates the stress applied to the members to be joined. As a result, the solder is deformed as the stress is relaxed and eventually breaks. When the amount of strain acting on the solder due to thermal stress is relatively small, the solder in which the intermetallic compound precipitates may have an effect of improving the strength and creep resistance of the solder by the pinning effect of the intermetallic compound. However, if the amount of strain acting on the solder is large, the ability to alleviate the stress at the joint is required. This is even more so when a component generating a large amount of heat or a joint is minute. Since the role of the solder is to relax the stress at the joint, it is unavoidable that the solder will cause fatigue failure. Therefore, using a solder having excellent spreadability to delay the time to breakage leads to improvement in fatigue resistance. When a hard and brittle intermetallic compound is present in the solder structure, it inhibits the spreadability of the solder and causes a reduction in stress relaxation at the joint. Further, since the solder also has a role of holding the component, only the spreadability should not be considered. Therefore, a solder alloy having both strength for holding the component and extensibility for relaxing the stress is required.

The solder alloy of the present invention has been made in view of the drawbacks of the alloy in which an intermetallic compound is precipitated. The solder alloy does not generate an intermetallic compound in a liquid state or a solid state. An object of the present invention is to provide a solder alloy having excellent fatigue resistance that can be used in a soldering method.

[0013]

The inventors of the present invention have conducted intensive studies on means for improving the fatigue resistance of a solder alloy. As a result, an alloy obtained by adding a small amount of Ni to a Sn-Pb-based alloy has improved the fatigue resistance. The present invention has been found to be improved.

As is well known, since the solder alloy has a mixed structure of Sn phase and Pb phase, the properties of both Sn and Pb are affected. Sn is rich in malleability and high in strength in place of a low melting point metal, but Pb is a soft material having good malleability but small in strength. Therefore, by increasing the strength while maintaining the ductility of Pb without impairing the strength and ductility of Sn, a solder alloy having both strength and ductility can be produced.

At present, a third element is generally added as a means for improving the strength of a metal, and this means is also used in the present invention. Since a solder alloy that does not precipitate intermetallic compounds is desirable, Sn or P
b and those that form a solid solution without forming an intermetallic compound,
The amount of addition was based on the solid solubility limit at room temperature. There are many elements that form a solid solution with Sn, but if the solid solution limit is exceeded, most of them form intermetallic compounds. When a third element is added to Sn, the strength is improved, but the ductility is impaired. In particular, Sn becomes brittle when an intermetallic compound is generated. Even if the third element is added within the solid solubility limit, the tendency to inhibit the spreadability does not change, so that the third element is preferably not dissolved in Sn. Although there are many elements that form a solid solution with respect to Pb, there are few elements that form an intermetallic compound, and the strength can be improved by adding a third element. If the addition is within the solid solubility limit, excellent spreadability will not be impaired. Therefore, an element that does not form a solid solution with Sn but forms a solid solution only with Pb is selected. Only a few such elements are present, and Ni meets this condition.

According to the present invention, 10 to 95% by weight of Pb,
This is a solder alloy excellent in fatigue resistance, characterized by being composed of the balance of Sn added with 0.002 to 0.02% by weight of Ni in the melting limit.

[0017]

The reason why the composition of the solder alloy in the present invention is limited as described above will be described. In the present invention, Pb is 10 to 95.
The weight percentage is that the composition actually used for electronic devices in the Sn-Pb based solder alloy is Sn for the Pb main component.
-95Pb alloy, and Sn-10
This is because it is a Pb alloy.

Further, in the present invention, the amount of Ni added is 0.002.
The reason for limiting to 0.02 is that the solid solubility limit of Ni to Pb is 0.023% by weight at room temperature. The small amount of Ni added is based on this solid solubility limit, and Sn-
In the Pb-based alloy, the content of Ni, which corresponds to a solid solubility limit of 10 to 98% by weight of Pb, is 0.002 to 0.02%.
2% by weight. Therefore, in the case of the Sn-Pb eutectic solder, the Ni content is preferably 0.008 to 0.009% by weight.
If Ni is added excessively beyond the solid solubility limit of Pb, the excess Ni reacts with Sn in the solder and precipitates an intermetallic compound of Sn-Ni, which causes a decrease in the spreadability of the alloy. Become. As a result, stress relaxation at the joint is impaired,
The fatigue resistance is reduced.

In the present invention, Ni is based on the solid solubility limit of Pb, and since the solder alloy does not generate an intermetallic compound even in a liquid state, it does not cause an increase in melting point or an inhibition of fluidity. Therefore, it is possible to cope with any soldering method from the reflow method to the dip method.

When the soldering is a dip method, Sn-Pb eutectic solder is mainly used. However, Cu, which is a pad of a printed circuit board, is dissolved in the solder by the soldering operation. It is used in a state containing about 2 to 0.3% by weight. When Cu is contained in the Sn-Pb-based alloy in an amount of 0.2 to 0.3% by weight, fatigue resistance is reduced as compared with the initial solder containing no Cu. This is because excessive Cu in the solder generates Sn and an intermetallic compound. However, in the alloy to which Ni is added,
Even if Cu is contained in an amount of about 0.2 to 0.3% by weight, the effect of Ni is maintained without lowering its fatigue resistance. This is because, as can be seen from the Cu-Ni binary phase diagram, Cu
Since the -Ni system is an all-solid solution type, it melts in a liquid state or a solid state and does not generate an intermetallic compound. For this reason, it is assumed that the excess Cu in the solder is compatible with Ni and is dispersed in the solder. Here, the state of Cu coexistence when the soldering method is the dip method has been described. However, the same applies to the reflow method using a solder paste or a form solder. Occur.

The solder alloy of the present invention is miniaturized by high-density mounting of electronic equipment in recent years, and is further improved in function and size to improve the reliability of a solder joint exposed to a more severe environment. It is an alloy with fatigue resistance that is indispensable to the alloy and has excellent properties that it can be used for various soldering methods.

[0022]

EXAMPLES AND COMPARATIVE EXAMPLES Solders having the alloy compositions shown in Table 1 were prepared, and each of the following electronic components and a printed circuit board were soldered into test pieces. For the test, a temperature cycle test was performed.
The breaking rate at 00 points was determined and evaluated.

[Table 1]

The electronic parts are connector parts molded with resin and have a lead diameter of 0.6 mm and a pitch of 2.5.
4 mm. ○ The printed circuit board is a single-sided glass epoxy board with a Cu foil land diameter of 2 mm, a hole diameter of 1 mm, and a pitch of 2.54 mm. ○ Cycle conditions are -40 ° C (30 minutes) to + 125 ° C
(30 minutes).

[0024]

As described above, the solder alloy of the present invention has a Ni—Sn content even though the third element Ni for improving mechanical strength is added to the Sn—Pb-based solder alloy.
Since the intermetallic compound does not precipitate in the liquid or solid state solder, there is no problem of impairing the solderability or reducing the spreadability of the solder, and has the effect of relaxing the stress at the joint. It is. Therefore, the solder alloy of the present invention has excellent fatigue resistance, especially excellent properties against thermal cycling, and does not cause cracks at the joints and cause poor conduction even when used for a long time. Therefore, it is very suitable for a device such as an electronic device in which low and high temperatures repeatedly occur.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-52-10843 (JP, A) JP-A-3-128192 (JP, A) JP-A-4-333391 (JP, A) JP-A-3-333 204194 (JP, A) JP-A-4-305395 (JP, A) JP-A-63-154289 (JP, A) JP-A-48-20481 (JP, A) JP-A-54-61050 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B23K 35/26

Claims (1)

(57) [Claims] 1. 10 to 95% by weight of Pb, within the solid solubility limit for Pb
A fatigue-resistant solder alloy comprising a balance of Sn added with 0.002 to 0.02% by weight of Ni.