JP2013035016A - Lead-free solder and cream solder using the solder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide lead-free solder with an improved drop impact resistance.SOLUTION: The lead-free solder is configured of Bi, In, Cu, Sn, and inevitable impurities, and when the mass ratios of the Bi, the In, the Cu, and the Sn are defined to be A wt.%, B wt.%, C wt.%, and D wt.%, respectively, A, B, C, and D satisfy the following formulas (1)-(4), respectively. The formulas are: (1): 55.1≤A≤57, (2): 0.1≤B<5, (3): C=B×1/99, and (4): D≥A×42/58.

Description

  The present invention relates to a lead-free solder used for soldering a circuit board and a cream solder using the solder.

  In recent years, further improvement in connection reliability of lead-free solder materials has been demanded with the progress of miniaturization and high functionality of mobile devices and the like.

  With regard to mobile devices, due to their portability, device failure due to destruction of solder joints due to dropping has become a problem, and various measures such as circuit board design and housing design have been taken as measures against product failure due to drop impact.

  Moreover, Sn-3.0Ag-0.5Cu (wt%) comprised of tin (Sn), silver (Ag), and copper (Cu) is used as a lead-free solder generally used in the above equipment. can give. Since this lead-free solder has a high melting point of 219 ° C., when a reflow method or DIP method is used when soldering an electronic board to a circuit board, the melting point of the solder material is high due to the high melting point. A lot of energy is consumed.

  From the above, there is a strong demand for a solder material that achieves both excellent connection reliability with respect to drop impact and energy saving through solder bonding at low temperatures.

  As a lead-free solder composition having a melting point lower than that of conventional Sn-3.0Ag-0.5Cu (wt%) (below 219 ° C.), Sn is used as a base metal and bismuth (Bi) is added at 58 wt% and Ag is added at 1 wt%. A Sn—Bi eutectic alloy (melting point: 138 ° C.) has been proposed (see, for example, Patent Document 1).

  FIG. 7 is a graph showing the relationship between the Ag addition amount (wt%) of the Bi—Sn—Ag alloy described in Patent Document 1 and the elongation at break. The horizontal axis represents the Ag addition amount (wt%), and the vertical axis represents the breaking elongation (%).

  As shown in FIG. 7, the breaking elongation of the alloy to which 1% Ag is added is 40%, which is improved with respect to the breaking elongation of 20% of the Sn—Bi eutectic alloy containing no Ag. There is a description that the reliability of the system is improved.

  In addition, it has been proposed to secure the drop impact resistance required for products by reinforcing the solder joints using a resin material and relieving the stress generated at the time of dropping (see, for example, Patent Document 2). ). FIG. 8 is a view showing a conventional solder joint reinforcing structure described in Patent Document 2. As shown in FIG.

  In FIG. 8, there is a structure in which a solder joint is filled with a resin material to give a function of relieving stress against dropping. In the structure shown in FIG. 8, the substrate 200 and the device 230 are joined by the solder bumps 240 and the periphery thereof is covered with the adhesive material 220 to reinforce.

Japanese Patent No. 3347512 Japanese Patent No. 2589239

  However, in the connection between the component and the circuit board using the solder described in Patent Document 1, the drop impact resistance required in the mobile device depends on the reliability standard required for the device, but sufficient strength is obtained. It is difficult to say that the solder has a higher elongation at break.

  Further, in Patent Document 2, solder bonding and solder reinforcement are performed at once by using a solder material preliminarily formed on the component side and a resin material applied on the circuit board side for the purpose of reinforcing the flux function and the solder joint portion. A process to be performed is used, and a connection structure between a component using solder and a circuit board is formed. However, according to this structure, it may become a factor which increases processes, such as a resin application process and adhesive hardening.

  From the above, it is essential to improve the drop impact resistance of the solder material itself.

  An object of the present invention is to provide a lead-free solder having improved drop impact resistance and a cream solder using the same in consideration of the problems of the conventional solder.

In order to achieve the above object, the first present invention provides:
Composed of Bi, In, Cu, Sn, and inevitable impurities,
Assuming that the mass ratios of Bi, In, Cu, and Sn are Awt%, Bwt%, Cwt%, and Dwt%, respectively, A, B, C, and D are represented by the following formulas 1, 2, respectively. It is a lead-free solder that satisfies Equations 3 and 4.

The second aspect of the present invention is
The lead-free solder according to the first aspect of the present invention, wherein A is 55.1, B is 4.95, C is 0.05, and D is 39.9.

The third aspect of the present invention
Comprising the lead-free solder of the first or second aspect of the present invention and a flux;
It is cream solder.

  As described above, according to the present invention, it is possible to provide a lead-free solder having improved drop impact resistance and a cream solder using the lead-free solder.

Cu-In binary phase diagram Sn-Bi eutectic structure conceptual diagram Conceptual diagram of the structure in which Sn-Bi eutectic composition surrounds Sn primary crystal Conceptual diagram of the structure where In element is dissolved in Sn phase and Bi phase Diagram showing an overview of the indentation tester used in this example Diagram showing the relationship between load and indentation displacement The figure which shows the relationship between Ag addition amount of the Bi-Sn-Ag alloy of patent document 1, and breaking elongation. The cross-sectional block diagram seen from the front which shows the device with which the solder joint part after reflowing to the board | substrate of patent document 2 was reinforced

  Embodiments according to the present invention will be described below with reference to the drawings.

(Embodiment 1)
The lead-free solder according to the first embodiment of the present invention will be described.

The lead-free solder of the first embodiment corresponding to an example of the solder of the present invention is
Composed of Bi, In, Cu, Sn, and inevitable impurities,
Assuming that the mass ratios of Bi, In, Cu, and Sn are Awt%, Bwt%, Cwt%, and Dwt%, respectively, A, B, C, and D are represented by the following formulas 1, 2, respectively. It is a solder that satisfies Equations 3 and 4.
(Expression 1) 55.1 ≦ A ≦ 57
(Equation 2) 0.1 ≦ B <5
(Expression 3) C = B × 1/99
(Equation 4) D ≧ A × 42/58
In the first embodiment, this configuration can provide a lead-free solder having a low melting point and excellent drop impact resistance.

  When blended at a mass ratio of the above (Equation 1) to (Equation 4), Cu and In are mixed with a Cu—In eutectic composition ratio ( (Equation 2), (Equation 3)). And since the melting point in this Cu—In eutectic composition ratio is 148 ° C., the elongation of Sn—Bi solder, in particular, the impact strength is increased to increase the reliability of the solder joint, and the solder joint at a low temperature. Can be possible.

  Next, it was examined how to configure the mass ratio of each component of the solder according to the first embodiment.

  Bi lowers the melting point as an eutectic component, but when the Bi mass ratio is large, a brittle Bi phase is formed as the primary crystal, so that the solder becomes brittle. For this reason, when the mass ratio of Bi is Awt%, A is in the range of 55.1 ≦ A ≦ 57.

  In is blended so that the mass ratio Bwt% is 0.1 wt% or more for the purpose of improving the elongation of the solder and the wettability with respect to the non-bonded metal. However, when a large amount of In is added, the metal structure collapses due to the strain caused by the phase transformation of γ-Sn and β-Sn having different hardness in the thermal shock test, and the thermal shock resistance is lowered. Therefore, In is blended so that the mass ratio Bwt% is less than 5 wt%.

  Cu is added to give an effect of increasing the elongation of the solder. Since the melting point of Cu is as high as 1083 ° C., it is necessary to add Cu at a mass ratio Cwt% that can secure a low melting point as much as possible.

  FIG. 1 is a binary phase diagram of In and Cu. The vertical axis | shaft of FIG. 1 has shown temperature (degreeC). The lower horizontal axis represents In / (Cu + In) × 100 (at%), and the upper horizontal axis represents In / (Cu + In) × 100 (wt%). Since In and Cu have a mass ratio of 1Cu-99In (wt%) and a eutectic composition (melting point: 148 ° C), by adding Sn to Bi (melting point: 138 ° C) solder at this mass ratio. It is possible to form an alloy having a lower melting point than conventional Sn-3.0Ag-0.5Cu (wt%) solder having a melting point of 219 ° C.

  That is, since the eutectic composition of Cu—In that can ensure a low melting point even if Cu is added is 99: 1 wt%, Cwt% is set to C = B × 1/99 as in (Equation 3).

The balance is Sn mass ratio, but in order not to form brittle Bi phase primary crystals,
Sn is blended so as to satisfy the relationship. That is, the mass ratio (wt%) of Sn and Bi in the Sn—Bi eutectic composition is Sn: Bi = 42: 58. By adding Sn to this mass ratio or higher, Sn is equal to or higher than the eutectic composition Therefore, the Sn phase is formed as the primary crystal, and the formation of the brittle Bi phase can be suppressed.

  FIG. 2 is a conceptual diagram of the Sn—Bi eutectic composition. FIG. 3 is a conceptual diagram showing the Sn—Bi alloy composition when Bi is alloyed with the mass ratio of Sn and Bi ((Equation 1), (Equation 4)) of the present invention. In FIG. 2, a phase in which Sn and Bi are separated into Sn phase 402 and Bi phase 401 as a eutectic composition is formed.

  On the other hand, when Sn and Bi are blended as shown in (Equation 1) and (Equation 4) as compared with the mass ratio of Sn: Bi = 42: 58 (wt%) which is a Sn—Bi eutectic composition, Sn is blended more than the eutectic composition. Therefore, as shown in FIG. 3, a Sn primary crystal 501 is formed without forming a Bi primary crystal, and a structure surrounding the Sn—Bi eutectic composition is formed.

  Thus, the slip surface 502 can be formed by forming different solder composition interfaces. The formation of the slip surface 502 and the deformation of the Sn primary crystal occur, and the brittleness reduction due to the formation of the primary crystal Bi unique to the Sn—Bi alloy is avoided.

  FIG. 4 is a conceptual diagram of the structure in which the In element 601 is dissolved in the Sn phase 402 and the Bi phase 401. By dissolving the In element 601 in the Bi phase 401 and Sn phase 402 in the structure, the In is refined and dispersed in the alloy to reduce the embrittlement caused by the Bi solid solution, thereby increasing the elongation of the solder. Can be increased.

  In addition, the inevitable impurities of the present invention are originally present in the raw material, are inevitably mixed in the manufacturing process, and are originally unnecessary, but are in a trace amount that does not affect the characteristics, It is an allowed impurity.

  Next, examples will be described.

  In this example, the elongation amount was measured by an indentation test using solder formed with three different blending ratios satisfying (Equation 1) to (Equation 4). In addition, the elongation amount was also measured by an indentation test using a solder of Sn—Bi eutectic composition as a comparative example.

  Specifically, the solders of Examples 1 to 3 were formed by mixing Sn, Bi, In, and Cu at a mass ratio as shown in Table 1 below.

  In the test, a test piece having a width of 10 mm, a length of 40 mm, and a thickness of 1 mm is held with two sides at a width of 10 mm with respect to the length direction of the test piece, and a test piece is used using a kamaboko-shaped pushing tool with a tip of R0.5 mm The center part was pushed in and the amount of indentation displacement and the load were measured. FIG. 5 is a conceptual diagram of the indentation test. A test piece 10, a fixing tool 11 that fixes the long side of the test piece 10, and a pushing tool 12 that pushes the center of the test piece 10 from above are provided. By pushing the center part of the test piece 10 with this pushing tool 12 (arrow B), the test piece 10 extends in the longitudinal direction (arrow C).

  FIG. 6 is a diagram showing the relationship between the load and the indentation displacement obtained by this test. The amount of indentation at the extrapolation point A where the metal yields and breaks is defined as the elongation. The vertical axis in FIG. 6 represents the load (N), and the horizontal axis represents the indentation displacement (mm).

  The elongation was calculated as a ratio relative to the amount of elongation of the Sn—Bi eutectic alloy of Comparative Example 1 as a reference.

  (Table 1) shows an indentation test result using the Sn—Bi eutectic composition (Comparative Example 1) and the solder in Examples 1 to 3.

  As shown in Table 1 above, the amount of solder elongation, which is an index of impact resistance, is 55.1 wt% for Bi and 39.9, 4.95, 0.05 wt for Sn, In, and Cu, respectively. The solder material of Example 3 configured with a mass ratio of% was the largest, and the elongation was 2.4 times that of the Sn—Bi eutectic alloy of Comparative Example 1. From this result, it can be seen that the brittleness of the Sn—Bi eutectic composition is greatly improved.

  In addition, as shown in Examples 1 and 2, the alloy constituted by the mass ratio of the present invention has an improved elongation compared to the Sn—Bi eutectic solder even if the mass ratio of Cu and In is small. It turns out that the effect expresses even if it adds.

  In addition, the melting point of the solder in the present invention depends on the In mass ratio and decreases as the In mass ratio increases. The melting point of the 42Sn-58Biwt% eutectic composition is 138 ° C, but the blending of (Equation 1) to (Equation 4) in the present invention is 134 ° C at the maximum mass ratio of 4.95 wt%, and at low temperature. Solder joint is possible.

  As described above, according to the alloy according to the first embodiment, the elongation is greatly improved and the melting point is approximately the same as that of the Sn—Bi eutectic alloy. Energy saving is possible because solder joints can be performed at low temperatures.

(Embodiment 2)
Next, a second embodiment according to the present invention will be described. The cream solder according to the second embodiment uses the solder according to the first embodiment.

  Bi, Sn, In, and Cu are mixed and dissolved at a mass ratio of (Equation 1) to (Equation 4) of the first embodiment to obtain a solder ingot. The alloy formed by this blending is pulverized and kneaded with a flux to produce the cream solder of the second embodiment.

  When the mixed cream solder is used for soldering, in the process of heating the cream solder on the metal to be joined in the soldering operation, the melting time required at 170 ° C. above the melting point can be sufficiently melted. A method of use in which, after holding, rapidly cooled, solidified and joined is preferably employed. As a melting time, it is preferable to secure about 20 to 30 sec.

  The rapid cooling after melting is to prevent coarsening of the structure during solidification of the solder, and the cooling rate is preferably 5 to 15 ° C./sec, particularly about 10 ° C./sec. By this method, it becomes possible to sufficiently form and disperse a compound of Cu and In from Bi and powder in the alloy in the melting process, and to increase the bonding strength of the solder.

  In addition, as for flux for preparing cream solder, fluxes for air, nitrogen, RA (Rosin Activated), RMA (Rosin Mil Activated), etc. are used, including fluxes for RMA for air. In particular, the type of flux is not limited.

  The lead-free solder of the present invention and the cream solder using the lead-free solder exhibit the effect of improving the drop impact resistance, and can be applied not only to mobile devices but also to solder joining applications at low temperatures such as weak heat-resistant parts.

DESCRIPTION OF SYMBOLS 10 Test piece 11 Fixed tool 12 Push tool 200 Substrate 220 Adhesive material 230 Device 240 Solder bump 401 Bi phase 402 Sn phase 501 Sliding surface 502 Sn primary crystal 601 In atom

Claims (3)

Composed of Bi, In, Cu, Sn, and inevitable impurities,
Assuming that the mass ratios of Bi, In, Cu, and Sn are Awt%, Bwt%, Cwt%, and Dwt%, respectively, A, B, C, and D are represented by the following formulas 1, 2, respectively. Lead-free solder that satisfies Equations 3 and 4.

  The lead-free solder according to claim 1, wherein the A is 55.1, the B is 4.95, the C is 0.05, and the D is 39.9. A cream solder comprising the lead-free solder according to claim 1 or 2 and a flux.

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