Metal treatment – Compositions – Fluxing
Reexamination Certificate
2001-01-31
2002-08-27
King, Roy (Department: 1742)
Metal treatment
Compositions
Fluxing
Reexamination Certificate
active
06440228
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a solder paste and more particularly to a lead-free, zinc-containing solder paste which comprises a powder of a zinc-containing, lead-free solder alloy in admixture with a soldering flux.
Sn—Pb alloys have been used in soldering since ancient times, and they are still the most popular solders for soldering electronic components to printed circuit boards or other substrates.
Sn—Pb alloys have a eutectic composition of approximately 63% Sn with a balance of Pb. This composition has a low melting temperature of 183° C., thereby making it possible to perform soldering in a temperature range of from 220° C. to 230° C., in which range there is no substantial thermal damage to heat-sensitive electronic components. The eutectic Sn—Pb alloy, called eutectic solder, has extremely good wettability and solderability, and since it does not have a difference between its liquidus and solidus temperatures (i.e., no solidification temperature range exists), solidification occurs instantaneously during soldering when the melting point is reached. As a result, even if vibrations or mechanical shocks are applied to parts to be soldered during soldering, cracking or detachment of the resulting soldered joints does not occur.
Discarded electronic appliances including televisions, radios, audio or video recorders, computers, copying or printing machines, etc. are generally disposed of in landfills, since such appliances are composed of various materials such as synthetic resins used for housings and printed circuit boards, and metals used for wires and other electric connections and frames, which are not suitable for disposal by incineration.
In recent years, the phenomenon of acid rain has become serious due to discharge of sulfur oxide into the atmosphere by extensive use of fossil fuels such as gasoline and fuel (heavy) oils. Acid rain penetrates into the ground and causes dissolution of the solders present in discarded electronic appliances buried in the landfills, thereby producing contamination of the groundwater with lead. If such contaminated groundwater is ingested by people for many years, the accumulation of lead in their bodies may result in lead poisoning (plumbism).
From this viewpoint, it has been desired in the electronics industry to use a lead-free solder alloy for soldering electronic components. Conventional lead-free solder alloys are Sn-based alloys such as Sn—Ag, Sn—Sb, Sn—Bi, and Sn—Zn alloys.
Sn—Ag alloys form a eutectic composition of Sn-3.5Ag, but the melting temperature, i.e., eutectic temperature of this composition is relatively high (221° C.). Even if this eutectic composition having the lowest melting temperature among Sn—Ag alloys is used as a solder alloy, the soldering temperature will be as high as from 260° C. to 270° C., which may cause thermal damage to heat-sensitive electronic components during soldering, thereby deteriorating or even destroying their functions.
Of Sn—Sb alloys, an Sn-5Sb alloy has the lowest melting temperature, but its melting temperature is as high as 235° C. on the solidus line and 240° C. on the liquidus line. Therefore, its soldering temperature is in the range of from 280° C. to 300° C., which is still higher than that of an Sn-3.5Ag alloy, and thermal damage to heat-sensitive electronic devices cannot be avoided.
Sn—Bi alloys have a eutectic composition of 42% Sn—Bi with a melting temperature of 139° C., which is considerably lower than that of the above-described conventional Sn—Pb eutectic solder (183° C.). Therefore, Sn—Bi alloys may be considered to be potentially usable as lead-free solders from the viewpoint of melting temperatures. However, Sn—Bi alloys are too brittle and hard to meet the mechanical properties such as tensile strength and elongation that are required for solder alloys.
Sn—Zn alloys have a eutectic composition of Sn-9% Zn with a melting temperature of 199° C. This eutectic composition is advantageous in that its melting temperature is close to that of conventional Sn—Pb eutectic solder (183° C.). Another advantage of Sn—Zn alloys is that their mechanical properties are superior to those of Sn—Pb alloys. However, Sn—Zn alloys have poor solderability.
In order to improve the solderability of Sn—Zn alloys and further enhance their mechanical properties, a number of solder alloys based on an Sn—Zn alloy and containing one or more additional elements such as Ag, Cu, Bi, In, Ni, and P have been proposed.
With these improved Sn—Zn based solder alloys containing one or more additional elements, a considerably satisfactory solderability can be achieved as long as these alloys are used in the form of wire solder for soldering with a soldering iron along with an appropriate soldering flux. However, when these Sn—Zn based solder alloys are used in the form of a solder paste, which is a mixture of a powder of such a solder alloy and a soldering flux in a viscous fluid, they do not work successfully or not exhibit satisfactory solderability. Thus, a solder paste formed from an Sn—Zn based solder alloy may cause non-wetting or dewetting during soldering whereby the areas of a substrate to be soldered are not wetted by the solder completely and have solder balls thereon. Even though the solder after soldering appears by visual observation to be sound or wet the areas completely, it may contain internal pit-like voids at the interface between the solder and the substrate, as can be seen when the solder is peeled off.
The solderability of a solder paste formed from an Sn—Zn based solder alloy can be improved by using an activated flux containing a strong activator which can effectively enhance the spreading of the molten solder alloy. However, the strong activator can react with zinc (Zn) present in the solder alloy to oxidize or corrode it in a short period of time and cause the solder to lose its metallic nature, resulting in a significant deterioration in solderability. Thus, a solder paste formed from an Sn—Zn based solder alloy or other Zn-containing solder alloy (such a solder paste being hereinafter referred to as “Zn-containing solder paste) generally suffers the problem of detrimental alterations, i.e. aging, after storage for a relatively short period.
The detrimental alterations with time (hereinafter referred to as aging) of a Zn-containing solder paste appear as a change in viscosity. Thus, a Zn-containing solder paste immediately after it is prepared has an appropriate viscosity which makes it easy to stir with a spatula or stirring rod and which is suitable for application by screen printing or feeding with a dispenser. However, after it is stored for a certain period of time on the order of one or two weeks, it has an increased viscosity due to aging and is difficult to stir.
When such an aged solder paste having an increased viscosity is applied to a printed circuit board by screen printing or with a dispenser and then heated in a reflow furnace, the solder may not entirely melt or a large quantity of oxides formed in the solder alloy may cause the formation of solder balls as the solder melts. Even with a freshly prepared Zn-containing solder paste, if reflow soldering is conducted in an oxygen-containing atmosphere such as air, the molten solder may not spread adequately and good solderability may not be obtained. Therefore, reflow soldering must be performed in an inert gas atmosphere, thereby adding to operating costs.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a Zn-containing solder paste which is less susceptible to aging and has an extended shelf life.
It is another object of the invention to provide a Zn-containing solder paste which exhibits satisfactory solderability when reflow soldering is performed in air.
It has been found that the addition of a glycidyl ether compound to a flux used to prepare a Zn-containing solder paste has an effect of stabilizing the resulting solder paste against aging and improving the solderability of the solder paste, although the mechanism for this effect has not been clearly e
Hirata Masahiko
Nagashima Takashi
Taguchi Toshihiko
Takaura Kunihito
Yoshida Hisahiko
King Roy
Senju Metal Industry Co. Ltd.
Tobias Michael
Wessman Andrew E.
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