Lead-free solder alloys

Abstract

A lead-free solder alloy which has a relatively low melting temperature and which is suitable for soldering electronic devices consists essentially of from 5 to 9 mass % of Zn, from 2 to 15 mass % of Bi, optionally from 0.001 to 1 mass % of P or from 0.001 to 0.1 mass % of Ge, and a balance of Sn. The solder alloy has a liquidus temperature of at most 220° C.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to solder alloys which are completely free from lead and are suitable for use in soldering of electronic devices without producing thermal damage.[0003] 2. Description of the Related Art[0004] Sn--Pb alloys have long been used for soldering in the electronics industry, and they are still the most popular alloys for soldering electronic devices to printed circuit boards or other substrates.[0005] When electronic appliances such as televisions, radios, audio or video recorders, computers, pocket telephones, and copying or printing machines are to be discarded, they are typically disposed of in landfills, since they commonly include large amounts of synthetic resins (used for housings and printed circuit boards) and metals (used for frames and connecting wires) which are not suitable for incineration.[0006] In recent years, acid rain (the phenomenon in which rain becomes highly acidic due to discharge of sulfur oxide ...

AI Technical Summary

Benefits of technology

[0014] It is another object of the present invention to provide lead-free solder alloys which have good bonding strength when used for soldering.

[0016] 1) The alloys can be used at a soldering temperature below 250.degree. C. and preferably from 230.degree. C. to 240.degree. C. so as to prevent thermal damage to heat-sensitive electronic devices during soldering.

[0018] 3) The alloys have a narrow solidification temperature range between the liquidus and solidus temperatures such that the alloys are rapidly solidified after soldering in order to prevent the resulting soldered joints from cracking when subjected to vibration or an impact immediately after soldering, the temperature range being close to the eutectic temperature of an Sn--Pb alloy (183.degree. C.).

[0020] 5) The alloys can be easily subjected to plastic working to form wire so that the alloys can be used for soldering with a soldering iron.

[0021] The present inventors found that alloys consisting essentially of Zn, Bi, optionally one of P and Ge, and a balance of Sn in specific proportions can provide solder alloys having a low liquidus temperature which enables dip soldering to be performed in a temperature range at which electronic components being soldered will not undergo thermal damage. Furthermore, it was found that by appropriately adjusting the proportions of Zn and Bi, the solidus temperatures of the alloys can be close to their liquidus temperatures, thereby enabling molten solder to rapidly solidify following soldering to avoid problems such as cracking or detachment of soldered joints. In addition, the alloys have a tensile strength and ductility which enables them to be plastically formed into wire suitable for use with a soldering iron. Thus, these solder alloys can be satisfactorily used in place of conventional Sn--Pb alloys, and because these solder alloys are lead free, they prevent contamination of groundwater by lead which occurs with conventional Sn--Pb alloys.

Problems solved by technology

When electronic appliances such as televisions, radios, audio or video recorders, computers, pocket telephones, and copying or printing machines are to be discarded, they are typically disposed of in landfills, since they commonly include large amounts of synthetic resins (used for housings and printed circuit boards) and metals (used for frames and connecting wires) which are not suitable for incineration.

In recent years, acid rain (the phenomenon in which rain becomes highly acidic due to discharge of sulfur oxide into the atmosphere by extensive use of fossil fuels such as coals, gasolines, and fuel (heavy) oils) has become increasingly serious.

Acid rain causes the solders used in discarded electronic appliances present in landfills to dissolve and contaminate groundwater.

If groundwater contaminated with lead is ingested by a person for many years, the accumulation of lead in the person's body may result in lead poisoning.

Even if this composition, which has the lowest melting temperature among Sn--Ag alloys, is used as a solder alloy, the soldering temperature will be as high as from 260.degree. C. to 280.degree. C., which may cause thermal damage to heat-sensitive electronic devices during soldering, thereby deteriorating their functions or rupturing the devices.

Therefore, the soldering temperature is in the range of from 280.degree. C. to 300.degree. C., which is higher than that of an Sn-3.5Ag alloy, and thermal damage to heat-sensitive electronic devices cannot be avoided.

Although Bi and In are both effective for decreasing the melting temperatures of Sn--Ag and Sn--Sb solder alloys, the addition of Bi and / or In in a large amount is accompanied by a number of problems.

Addition of Bi in a large proportion makes the solder alloys very hard and brittle.

As a result, it is impossible or difficult to subject the solder alloys to plastic working to form wire, and when the solder alloys are used to solder electronic devices, the soldered joints may be readily detached when subjected to only a slight impact.

Addition of indium in a large proportion to solder alloys is undesirable due to its very high cost.

However, when attempting to lower the melting temperatures of Sn--Ag and Sn--Sb solder alloys by addition of Bi and / or In, it is difficult to decrease the liquidus temperature of the alloys to 200.degree. C. or below unless Bi and / or In is added in a large amount.

Furthermore, even though it is possible to provide a solder alloy having a liquidus temperature lowered to 200.degree. C. or less by addition of Bi and / or In, the solidus temperature thereof, at which solidification of the alloy is completed, may be excessively lowered, so that it takes a prolonged period of time to completely solidify the solder alloy in soldered joints formed by soldering.

As a result, if the soldered joints are subjected to any vibration or impact before they are completely solidified, they may crack.

Another problem of conventional lead-free solder alloys is that those lead-free alloys having liquidus temperatures which are low enough to be close to their solidus temperatures do not have satisfactory mechanical properties such as tensile strength and elongation, thereby forming soldered joints which have poor bonding strength or which are liable to be detached upon impact.