Lead-free solder

Abstract: A lead-free solder consisting of 1 to 4 wt% of Ag, either one or both of Bi and In of their respective amounts meeting the conditions that the value of expression (1), A, is equal to or greater than 5.00 and that the value of expression (2), B, is equal to or less than 6.90, and Sn for the balance: A = [Ag wt%] + 1.23 [Bi wt%] + 0.52 [In wt%] B = [Ag wt%] + 1.19 [Bi wt%] + 0.50 [In wt%] This solder has a tensile strength and an elongation as high as those of conventional Pb-Sn solder without containing neither lead nor cadmium which can cause environmental contamination.

Description: Field of the Invention This invention relates to a lead-free solder. Background of the Invention Pb-Sn alloy of eutectic or near-eutectic composition is well known as a representative solder, but lead is harmful. Zn-Cd alloy, which has a greater strength than Pb-Sn eutectic solder, is also known, but vaporized cadmium has an undesirable effect on workers. Zinc and tin-based solders which contain neither harmful lead nor cadmium have been proposed, but have mechanical properties that are not as good as conventional Pb-Sn solder. 3.5Ag-Sn solder, for example, has satisfactory elongation but low tensile strength. EP-A-0568952 discloses a ternary alloy, of Sn-Ag-In, for use as a solder. Summary of the Invention A lead-free solder according to the present invention consists of x wt% Ag; y wt% Bi; z wt% In; and Sn for the balance; wherein x = 1 to 4 y = 0.81 to 5.0 z = 1.9 to 12 x + 1.23y + 0.52z ≥ 5.00 x + 1.19y + 0.50z ≤ 6.90 Solder of the present invention has a tensile strength equal to or greater than 4.0 kgf/mm 2 and an elongation equal to or greater than 30%. These mechanical properties are as good as those of conventional Pb-Sn solder. Description of the Invention The presence of silver in a composition of the present invention has the effect of improving the heat-resistance, lowering the fusing temperature, and increasing the strength, spreadability and brightness of the solder. The silver content appropriate for this purpose is equal to or greater than 1 wt%. The upper limit of the silver content is 4 wt%, taking into account the expensiveness of silver. The Bi and In contents are within the ranges such that the combination of their contents fall in the hatched area in Figures 1 to 4 for 1 wt%, 2 wt%, 3 wt% and 4 wt% Ag, respectively, which can be defined by the conditions that the value of expression (1), A, is equal to or greater than 5.00 and that the value of expression (2), B, is equal to or less than 6.90, where expressions (1) and (2) represent the upper and lower boundary lines of the hatched area in Figure 3. If the value of equation (1) is smaller than 5.00, the tensile strength of the solder is lower than 4.0 kgf/mm 2 . If the value of equation (2) is greater than 6.90, the elongation of the solder is lower than 30%. If the Bi content is smaller than 0.81 wt%, the tensile strength of the solder is lower than 4.0 kgf/mm 2 . If greater than 5.0 wt%, the elongation of the solder is lower than 30%. If the In content is smaller than 1.9 wt%, the tensile strength of the solder is lower than 4,0 kgf/mm 2 . If greater than 12 wt%, the elongation of the solder is lower than 30%. However, it is preferable to set the upper limit of the In content to 4.0 wt% because In is expensive. Examples Sn, Ag, Bi, In and Pb were weighed out to prepare compositions listed in Table 1, each composition being 10 kg in total. These compositions were melted at 300 °C in an electric furnace in the air using a graphite crucible. After all metals had melted, the molten metals were thoroughly stirred to prevent the segregation caused by the gravity and cast into solder pieces of 10 mm in thickness using a mold of 150 mm x 60 mm with the height of 150 mm in inner dimensions. Test samples were cut from the bottom part of the solder pieces according to the JIS 4 standards with a machine, and their tensile strength and elongation were measured by the test method defined by the JIS Z2241 standards. The results are shown in Table 1. The strength and the elongation of Pb-Sn eutectic solder and 3.5Ag-Sn solder are also shown in Table 1 for comparison. Chemical Composition(wt%) Tensile Strength (kgf/mm 2 ) elongation (%) A g B i I n S n E x a m p l e 1 1 6 Bal. 4.1 35.3 1 1 7 Bal. 4.3 33.7 1 1 8 Bal. 4.5 32.0 1 1 9 Bal. 4.7 30.4 1 2 3 Bal. 4.0 36.4 1 2 4 Bal. 4.2 34.8 1 2 5 Bal. 4.4 33.1 1 2 6 Bal. 4.6 31.5 1 3 2 Bal. 4.3 34.2 1 3 3 Bal. 4.5 32.6 1 3 4 Bal. 4.7 30.9 1 4 2 Bal. 4.7 30.3 2 1 4 Bal. 4.1 35.5 2 1 5 Bal. 4.3 33.9 2 1 6 Bal. 4.5 32.2 2 1 7 Bal. 4.7 30.6 2 2 2 Bal. 4.2 35.0 2 2 3 Bal. 4.4 33.3 2 2 4 Bal. 4.6 31.7 2 2 5 Bal. 4.8 30.0 2 3 2 Bal. 4.6 31.1 3 1 2 Bal. 4.1 35.7 3 1 3 Bal. 4.3 34.1 3 1 4 Bal. 4.5 32.4 3 1 5 Bal. 4.7 30.8 3 2 2 Bal. 4.6 31.9 3 2 3 Bal. 4.7 30.2 4 1 2 Bal. 4.5 32.6 4 1 3 Bal. 4.7 31.0 C o m p a r a t i v e E x a m p l e 1 0 7 Bal. 3.9 37.5 1 0 12 Bal. 4.9 29.4 1 1 5 Bal. 3.9 37.0 1 1 10 Bal. 4.9 28.8 1 2 2 Bal. 3.8 38.1 1 3 0 Bal. 3.9 37.5 1 3 5 Bal. 4.9 29.3 1 4 3 Bal. 4.9 28.7 2 0 5 Bal. 3.9 37.7 2 0 10 Bal. 4.8 29.5 2 1 3 Bal. 3.9 37.1 2 1 8 Bal. 4.9 29.0 2 2 1 Bal. 3.9 36.6 2 2 6 Bal. 5.0 28.4 2 3 3 Bal. 4.8 29.5 2 4 1 Bal. 4.9 28.9 3 0 3 Bal. 3.8 37.9 3 0 8 Bal. 4.8 29.7 3 1 1 Bal. 3.9 37.3 3 1 6 Bal. 4.9 29.2 3 2 4 Bal. 4.9 28.6 3 3 1 Bal. 4.8 29.6 4 0 1 Bal. 3.8 38.1 4 0 6 Bal. 4.8 29.9 4 1 4 Bal. 4.9 29.3 4 2 2 Bal. 4.9 28.8 Pb-63Sn 3.8 30.0 S n - 3. 5 A g 2.0 70.0 It is known from Table 1 that the solder of the present invention has a tensile strength equal to or greater than 4.0 kgf/mm 2 and an elongation equal to or greater than 30% which are satisfactory mechanical properties to attain the object of the present invention. Thus, by the present invention as described above, a solder with mechanical properties as good as those of conventional Pb-Sn solder is provided.

Claims: A lead-free solder consisting of x wt% Ag; y wt% Bi; z wt% In; and Sn for the balance; wherein x = 1 to 4 y = 0.81 to 5.0 z = 1.9 to 12 x + 1.23y + 0.52z ≥ 5.00 x + 1.19y + 0.50z ≤ 6.90

Inventors: MATSUNAGA JUNICHI, NINOMIYA RYUJI, MATSUNAGA, JUNICHI, NINOMIYA, RYUJI
Applicants: MITSUI MINING & SMELTING CO, MITSUI MINING & SMELTING CO., LTD
Application year: 1995
Main classification: B23K35
IPC codes: B23K35, C22C13