Method of soldering electronic part

摘要

A cream solder obtained by kneading an Sn—Ag—Cu alloy together with a flux, wherein the Sn—Ag—Cu alloy includes a mixture of a first powdery alloy and a second powdery alloy, the first powdery alloy is represented by an Sn—Ag phase diagram having a solid-liquid coexistence region and has a given silver amount which is larger than that in the eutectic composition (3.5 wt. % silver), and the second powdery alloy has a silver amount which is that in the eutectic composition (3.5 wt. % silver) or which is close to that in the eutectic composition and is smaller than that in the first powdery alloy. This cream solder has excellent strength and thermal stability, and satisfactory bonding properties. It is based on an inexpensive Sn—Ag—Cu solder alloy. It is suitable for use as a high-temperature-side lead-free solder material conformable to temperature gradation bonding. Also provided is a method of soldering.

主权项

1. A method of soldering an electronic part by two steps of high- and low-temperature soldering in a reflow process employing at least two kinds of solders having different reflow temperatures, characterized in that;
high-temperature soldering as the former step is carried out by employing a solder cream obtained by kneading an Sn—Ag—Cu alloy with a flux, wherein the Sn—Ag—Cu alloy comprises a mixture of a first powder alloy for a high melting point and a second powder alloy for a low melting point, the first powder alloy comprises 10 to 30% by weight of Ag and 2 to 20% by weight of Cu with a balance consisting of Sn and unavoidable impurities, the second powder alloy comprises smaller compositions (% by weight) of Ag and Cu than the first powder alloy and having a melting point lower than that of the first powder alloy and, the mixture contains a total of not more than 35% by weight of Ag and Cu, and further, the total amount of Ag component in the alloy composition of the mixture after melting is more than 10% by weight, and whereafter low-temperature soldering as the latter step is carried out by employing a solder cream obtained by kneading the second powder alloy with a flux.