1. Field of the Invention
This invention relates to a lead-free solder paste suitable for soldering electronic components to a printed circuit board, particularly by reflow soldering.
2. Description of the Related Art
Electronic components used in electronic equipment include components equipped with leads and surface mounted devices.
Components with leads have elongated leads installed on the body of the electronic component. When components with leads are soldered to a printed circuit board, the flow soldering method is usually employed.
In the flow soldering method, the elongated leads of a component with leads are inserted into holes formed in a printed circuit board from the top side of the board, and then treatment is performed such as applying flux to the bottom surface of the printed circuit board, preheating, contacting the bottom surface of the board with molten solder, and cooling to solder the leads of the components to lands of the printed circuit board.
Single-function components with leads, such as resistors and capacitors, have elongated leads on both sides of the body of the electronic component, so no matter how small the body of the electronic component is made, the leads still project outwards from the body, so there is a limit to how small the overall size of the component can be made. In the case of components with leads such as transistors and PGA (Pin Grid Array) components having a large number of leads, it is difficult to insert a large number of leads into the holes in a printed circuit board, and there are cases in which all of the leads cannot be inserted into the corresponding holes, and the leads end up being bent.
With surface mounted devices, single-function components such as resistors and capacitors are chip components having an electrode formed at each end of a body, while electronic components in the form of IC packages such as QFP (Quad Flat Package) and SOP (Small Outline Package) devices have a large number of plate-shaped leads installed on both sides of the body or on four side surfaces of the body.
When surface mounted devices are soldered to a printed circuit board, the reflow soldering method is usually employed.
In the reflow soldering method, a small amount of solder paste is applied to selected locations on a printed circuit board using a printing mechanism or a dispenser, electrodes or plate-shaped leads of surface mounted devices are made to coincide with the locations on the printed circuit board where solder has been applied, and the surface mounted devices are disposed on the printed circuit board. The printed circuit board is then heated in a reflow furnace and the solder paste is melted to solder the surface mounted devices to the printed circuit board.
Solder paste used in reflow soldering is a viscous material obtained by mixing a solder alloy in the form of a powder with a flux in the form of a paste.
The solder alloy powder used in a conventional solder paste is a Sn—Pb eutectic alloy. A solder paste employing this alloy has excellent solderability with a low melting point of 183° C., so when reflow soldering is carried out with such a solder paste, solder defects are rare, and soldering can be carried out at a temperature at which electronic components and printed circuit boards will not undergo thermal damage.
The reflow temperature employed in reflow soldering with solder paste is usually 40-50° C. higher than the melting point of the solder alloy in the solder paste, since in this temperature range, spreading of the melted solder is optimal and soldering defects decrease.
Accordingly, reflow soldering with a solder paste containing a eutectic Sn—Pb solder is usually carried out at approximately 230° C., which is roughly 50° C. higher than the melting point of 183° C. of the eutectic solder alloy. A temperature of 230° C. is sufficiently low not to cause thermal damage to electronic components being soldered.
In general, electronic equipment which has become old and difficult to use or has broken and become difficult to repair is disposed of by burial in landfills. If electronic equipment which is disposed of in landfills is contacted by acid rain containing a sulfur component, the acid rain dissolves lead from Sn—Pb solder in the equipment. Rain water containing the lead then permeates into the ground and mixes with underground water and contaminates the underground water.
If underground water containing lead is drunk for long periods of time, the lead accumulates within the body and may cause lead poisoning, so the use of Sn—Pb solder has come to be regulated. For this reason, the use of lead-free solder has been recommended.
Among the most common lead-free solders are alloys having Sn as a primary component, such as Sn—Ag alloys, Sn—Cu alloys, Sn—Bi alloys, and Sn—Zn alloys.
However, lead-free solders which have been thus far proposed have various problems, as described below.
A Sn—Ag solder alloy has a Sn-3.5Ag eutectic composition with a melting point of 220° C., so a Sn—Ag solder alloy employs a reflow temperature of at least 260° C., at which temperature electronic components can easily be thermally damaged. It is possible to lower the solidus temperature of a Sn—Ag solder alloy by the addition of a small amount of Bi or In, but the liquidus temperature (melting point) of the alloy can not be decreased. Furthermore, a Sn—Ag solder alloy has no surface gloss after soldering, so the value of the soldered product is decreased.
A Sn—Cu solder alloy has a Sn-0.7Cu eutectic composition with a high melting point of 227° C., so it is difficult to avoid thermal damage to electronic components when performing soldering with this alloy. Furthermore, it has relatively poor solderability. As is the case with a Sn—Ag solder alloy, addition of a small amount of Bi or In is not effective to decrease the melting point of a Sn—Cu solder alloy.
A Sn—Bi solder alloy has a Sn-57Bi eutectic composition with a low melting point of 139° C., so the reflow temperature of this alloy is lower than that for a conventional Sn—Pb eutectic solder. Thus, there is no worry whatsoever of thermal damage to electronic components during reflow soldering with this solder alloy. However, it is extremely brittle, so after soldering, peeling of soldered joints readily occurs if the joints are subjected to an impact.
A Sn—Zn solder alloy has a Sn-9Zn eutectic composition with a melting point of 199° C., so a typical reflow temperature is 240° C., at which temperature there is little possibility of thermal damage to electronic components. However, a large amount of oxides are formed at the time of reflow soldering with this solder alloy, and it also has poor solderability.
Thus, none of the conventional lead-free solder alloys described above is acceptable both with respect to its melting temperature and its physical properties.
One possible way of obtaining a lead-free solder paste having an acceptably low melting point is to mix a powder of a lead-free solder having a high melting point with a powder of a lead-free solder having a low melting point.
Japanese Published Unexamined Patent Application No. 9-295182/1997 discloses an invention in which at least two types of solder alloy powder with Sn as a principal component are mixed, and using the difference in specific gravity of the alloy powders, a composition which varies with depth is obtained. That invention is aimed at improving wettability and soldered strength. However, the examples of that application do not mention the reflow temperature.
Japanese Published Unexamined Patent Application No. 11-138292/1999 discloses a solder paste which in order to obtain a multi-element solder alloy uses an easily manufactured binary alloy powder and an elemental metal powder or two or more types of binary alloy powder. This composition contributes to a delay in melting at the time of reflow soldering and prevention of tombstoning of minute chip components. However, while there is mention in the examples of the temperature at the start of melting and the final melting temperature (181-219° C.), there is no disclosure concerning the reflow temperature.
The solder paste disclosed in No. 11-138292/1999 uses an alloy powder containing one of Bi, In, and Zn, but each of these elements causes problems in solder. As mentioned above, the addition of Bi to a solder alloy leads to brittle fracture of soldered portions. An In metal powder or an In alloy powder in a solder paste reacts with flux in the solder paste and produces degradation with the passage of time, and the solder paste soon becomes unusable. When a lead-free solder contains Zn, a large amount of oxides are produced at the time of soldering. Oxides are not only an impediment to solderability, but large voids are formed in the soldered portions and the bonding strength becomes weak.
Japanese Published Unexamined Patent Application No. 11-186712/1999 discloses a solder paste including at least two types of alloy powder having different compositions and different melting points. The solder paste of that invention enables the reflow temperature to be lowered from the conventional value of 260° C. down to 230° C. As shown in the examples of that application, it is necessary to use a Sn-58% Bi solder alloy (melting point of 139° C.). As a result, the apparent melting point of the final alloy composition, which is Sn-3Ag-4Bi (melting point of 210° C.) or Sn-6Bi-1.5Ag-0.4Cu (melting point of 195-214° C.), is lowered. However, either composition contains Bi, so brittle fracture of soldered portions can readily take place after soldering.
Japanese Patent No. 3027441 discloses a lead-free solder alloy having mechanical strength, gloss after soldering, and impact resistance superior to those of the above-described conventional lead-free solders.
The lead-free solder alloy of that patent is a Sn—Ag—Cu lead-free solder containing 3-5 mass % of Ag, 0.5-3 mass % of Cu, and a remainder of Sn as a main component. It does not undergo deterioration of bonding strength, and a powder of such an alloy does not react with flux in solder paste, so it suffers almost no degradation with the passage of time.
Thus, a Sn—Ag—Cu lead-free solder alloy has superior properties compared to lead-free solders of other compositions, but the lowest melting point thereof is 217° C., and in order to obtain satisfactory wettability of molten solder during reflow soldering with this alloy and avoid the formation of voids in soldered joints, it is necessary for the reflow temperature to be at least 260° C. However, a temperature this high is undesirable from the standpoint of preventing thermal damage to electronic components and printed circuit boards. Therefore, in order to prevent thermal damage, reflow soldering with a Sn—Ag—Cu lead-free solder has typically been carried out at a reflow temperature of at most 250° C., preferably at most 240° C., at which temperature wettability may be inadequate and the occurrence of voids may be high, since it is only about 30° C. higher than the lowest melting point of a Sn—Ag—Cu alloy.
Furthermore, with a solder paste containing a powder of a conventional Sn—Ag—Cu lead-free solder alloy, regardless of the reflow temperature, tombstoning (a phenomenon in which chip components stand up on one end during reflow soldering) is a frequent problem.