Metal treatment – Compositions – Fluxing
Reexamination Certificate
2002-12-03
2004-05-18
Jenkins, Daniel (Department: 1742)
Metal treatment
Compositions
Fluxing
C148S024000
Reexamination Certificate
active
06736907
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a solder paste used for soldering of electronic equipment, and in particular it relates to a lead-free solder paste using an Sn—Zn based lead-free solder.
2. Description of the Related Art
From ancient times, an Sn—Pb based alloy has been used as solder. An Sn—Pb alloy has the advantages that it has a low melting point and it has good solderability. For example, an Sn63%-Pb37% alloy which is a eutectic composition and is the most typical solder, has a melting point of 183° C., and soldering can be carried out at a temperature on the order of 200-230° C.
This eutectic Sn—Pb solder has been generally used in the past for soldering of electronic equipment. Mounting of electronic parts on printed circuit boards has recently come to be widely carried out by surface mount technology (SMT) because it permits reductions in size, a higher density, higher performance, and lower costs.
In SMT, a solder paste (also called cream solder) which comprises solder powder uniformly mixed with a soldering flux, and particularly a rosin flux, is used. Typically, soldering is carried out by reflow soldering. In general, in reflow soldering, a solder paste is supplied to a printed circuit board by printing or dispensing, a chip-type electronic part is temporarily secured thereon utilizing the adhesion of the solder paste, the entire printed circuit board is heated in a reflow furnace to melt the solder and thus perform securing and connection of the part to the printed circuit board. Accordingly, the electronic part to be mounted is also exposed to the soldering temperature in reflow soldering. When a eutectic Sn—Pb solder is used, since the soldering temperature is relatively low as described above, thermal damage of electronic parts does not take place easily even with reflow soldering, which involves heating of the parts.
The development of electronic equipment is rapid, and large quantities thereof are discarded. Printed circuit boards which are removed from disassembled discarded electronic equipment are often shredded and then disposed of by burial underground. If rain water which has become acidified in recent years (acid rain) contacts shredded printed circuit boards which have been disposed of by burial underground, the lead constituent in the Sn—Pb solder is dissolved out as lead ions, which pollutes underground water. If humans or animals drink water containing lead ions over long periods, the lead ions accumulates in their bodies, and there is concern over lead poisoning occurring. Therefore, the use of lead-free solder which does not contain any lead has come to be recommended for soldering of the electronic equipment.
It is desired that a lead-free solder be an alloy constituted by elements that are harmless to the human body. For example, Cd which is harmful cannot be used even though it has an effect of lowering the melting point. Lead-free solders which are promising at present are Sn based alloys which have Sn as a main constituent to which one or more alloying elements such as Ag, Cu, Bi, In, Sb, and Zn are added.
Among such lead-free solders, Sn based alloys containing Ag such as Sn—Ag alloys and Sn—Ag—Cu alloys (referred to below as Sn—Ag based solders collectively) are advantageous in that they are easy to use since they have good wettability among lead-free solders. However, Sn—Ag based lead-free solders have a melting point of around 220° C., which is approximately 30-40° C. higher than that of an Sn—Pb eutectic solder, so the soldering temperature increases by that amount, and it exceeds a temperature of 250° C. Therefore, Sn—Ag based solders cannot be used for reflow soldering of some electronic parts which are heat sensitive. Thus, as far as reflow soldering is concerned, Sn—Ag based solders can not be described as general purpose solders.
Another Sn-based solder which contains Zn (referred to below as an Sn—Zn based solder) is known as a lead-free solder having a lower melting point. Zn is a metal which is indispensable to and therefore harmless to the human body, there is an ample resources therefor, and it is an inexpensive metal compared to Ag, Cu, Bi, In, and the like. Therefore, an Sn—Zn based solder is advantageous from the standpoints of safety and economy. A representative composition of an Sn—Zn based lead-free solder is Sn-9Zn. This alloy has a melting point of 199° C., which is approximately 20° C. lower than the melting point of an Sn—Ag based lead-free solder. Thus, it can be used in reflow soldering of heat sensitive electronic parts, with which an Sn—Ag based lead-free solder cannot be used. Furthermore, since the melting point of 199° C. of the Sn-9Zn alloy is near the melting point (183° C.) of an Sn—Pb eutectic alloy, it has another advantage that it can be used for reflow soldering using an existing reflow furnace designed for use with an Sn—Pb based solder paste.
However, an Sn—Zn based lead-free solder has extremely poor wettability compared to an Sn—Ag based lead-free solder, and soldering defects in the form of voids and solder balls can easily form. This is because Zn is a metal which has a high ionization tendency and which is hence very susceptible to oxidation. Thus, the Zn which is present on the solder surface is oxidized by contact with air, and an inactive oxide layer is formed on the solder surface.
In particular, in solder paste, solder is in the form of a powder having a large surface area, and the effect of surface oxidation is marked. Furthermore, a soldering flux, which is mixed with the solder powder to form the solder paste, contains reactive components such as an activator, which may also cause oxidation of the solder powder and make the surface oxidation of solder powder severer. Therefore, a solder paste of Sn—Zn based solder has extremely poor wettability of solder, and adequate solderability is not obtained therewith.
As a countermeasure, in order to improve wettability of such solder paste, it is conceivable to increase the amount of the activator component of the flux in a solder paste. However, in this case, the viscosity of the solder paste can easily increase due to reaction of the solder powder in the solder paste with the activator component in the flux, and the viscosity of the solder paste increases prematurely, and the supply of solder paste by printing or dispensing no longer proceeds smoothly.
Therefore, with an Sn—Zn based solder paste, in order to improve solder wettability, a method has been proposed in the past in which an Sn—Zn based solder powder is coated with a suitable material before being mixed with a flux so as to prevent the solder powder from reacting with flux and undergoing surface oxidation. As the coating material, a precious metal such as Au or Pd, an inorganic oxide formed from a hydrolyzable organosilicon compound or the like, or an organic material such as an imidazole or triazole compound can be used.
However, a coating for a solder powder as described above enormously increases the manufacturing costs of solder paste. In addition, depending on the type or method of coating, oxidation of the solder powder may be promoted during the coating operation, so coating is not necessarily effective for improving the wettability or solderability of an Sn—Zn based solder paste.
SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to provide a solder paste of an Sn—Zn based solder which has good solder wettability and solderability without pretreatment of a solder powder such as coating.
A solder paste used in SMT is made by mixing a solder powder and a soldering flux. A typical flux for solder paste is a rosin flux which is formed by dissolving a rosin as a main component together with additives such as an activator, a thixotropic agent, and the like in a solvent. A flux is required to have highly reliable insulating properties and corrosion resistance. In particular, the selection of an activator which has a large effect on wettability of solder largely influences the reliability of flux. If too much wettabil
Hirata Masahiko
Nagashima Takashi
Taguchi Toshihiko
Takaura Kunihito
Yoshida Hisahiko
Jenkins Daniel
Senju Metal Industry Co. Ltd.
Tobias Michael
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