Metal fusion bonding – Process – Applying or distributing fused filler
Reexamination Certificate
2001-07-13
2002-11-05
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Applying or distributing fused filler
C228S103000, C228S108000
Reexamination Certificate
active
06474537
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a soldering method using a lead-free alloy containing copper. More particularly, it relates to a continuous flow soldering method using such an alloy.
Electronic parts mounted on printed circuit boards are used in a wide variety of electrical products, electronic devices, and communications equipment. Electronic parts must be connected to a printed circuit board in a manner which both secures the parts to the printed circuit board and forms an electrical connection. The connection of electronic parts to printed circuit boards is usually carried out by soldering.
A typical conventional solder used for soldering of printed circuit boards is a Sn—Pb alloy, and particularly a 63Sn—Pb eutectic solder. This eutectic solder has a low melting point of 183° C., so soldering can be performed at a temperature which does not produce thermal damage of electronic parts. In addition, a 63Sn—Pb solder has good wettability. Therefore, this solder has the excellent property that it produces few soldering defects.
However, when products such as those described above containing printed circuit boards soldered with a Sn—Pb solder are discarded and disposed of by burying underground, the Sn—Pb solder may cause pollution of underground water. In recent years, the acidification of rainwater to form acid rain has been progressing due to the wide use of fossil fuels. When acid rain contacts lead-containing solder in discarded equipment buried underground, the acid rain can dissolve out the lead in the solder, and the dissolved lead may enter into underground water and pollute the water supply. Accordingly, for environmental reasons, the use of lead-free solders is being promoted for the soldering of electronic parts.
A typical lead-free solder has Sn as a primary component and further contains one or more alloying elements selected from Cu, Ag, Bi, In, Ni, Zn, P, Ge, Ga, and the like. For the soldering of electronic parts, a Sn—Cu lead-free solder containing a small amount of Cu (usually on the order of 1 mass % or less) is widely used. Cu is added to Sn in order to improve the mechanical properties of solder, and it has the secondary effect of lowering the melting temperature of the solder, which results in less thermal damage to parts being soldered.
Commonly used soldering methods include soldering with a soldering iron, reflow soldering, and flow soldering. Continuous soldering of printed circuit boards on which electronic parts are mounted is usually performed by reflow soldering or flow soldering.
In reflow soldering, a solder paste which is a mixture of solder powder and a flux is applied to portions of members to be soldered, and then the members are heated in a furnace to melt the solder powder. This method is generally used for soldering of surface mounted devices on printed circuit boards.
In flow soldering, the surface of a printed circuit board or other member to be soldered is contacted with molten solder in a solder bath. Although this method can be applied to soldering of surface mounted devices, it is more commonly used to solder electronic parts having leads on a printed circuit board, the leads of the electronic parts being inserted into through holes in the printed circuit boards and secured to the board by soldering on the surface of the board facing away the electronic parts. The type of flow soldering most commonly used with printed circuit boards is the wave soldering method, in which a wave is formed on the surface of molten solder, and the printed circuit boards are conveyed with the surface to be soldered facing downwards to contact the wave of molten solder. Another type of flow soldering is the dipping method, in which a printed circuit board or other member is dipped into a solder bath having a stationary surface. In the following description, flow soldering will refer to the wave soldering method since it is much more popular than dipping method.
When flow soldering of printed circuit boards is carried out using a Cu-containing lead-free solder, soldering defects and thermal damage to electronic parts tend to occur more frequently than with reflow soldering. Accordingly, there is a need for improvements in flow soldering methods when employing a Cu-containing lead-free solder.
SUMMARY OF THE INVENTION
The present inventors noticed that the primary reason for the high occurrence of soldering defects and thermal damage to electronic parts when flow soldering is carried out with a Cu-containing lead-free solder is an increase in the Cu-content of the molten solder bath with the passage of time.
The surface of a printed circuit board to be contacted with molten solder by flow (wave) soldering normally has areas of Cu exposed on that surface, which are typically in the form of lands or pads located in or near portions to be soldered. When the exposed Cu contacts molten solder in a solder bath, some of the Cu dissolves into the molten solder, and during continuous soldering, the amount of Cu contained in the molten solder progressively increases. If the molten solder is a Cu-containing lead-free solder, the Cu in the molten solder restrains the dissolving out of Cu from the printed circuit board to a certain extent, but even so, due to the activity of Sn (the primary component of the solder) for dissolving Cu, the Cu content of the molten solder gradually increases.
Since the desired Cu content of a Cu-containing lead-free solder is normally as low as 1 mass % or less, even a slight increase in the Cu content of the solder may have an appreciable adverse effect on soldering. Thus, the solderability of the molten solder become worse and soldering defects can more readily occur. In addition, as the Cu content of the molten solder increases, the liquidus temperature of the solder increases, and particularly when the Cu content exceeds the solubility of Cu, precipitation of Sn—Cu intermetallic compounds occurs, and in order to dissolve such compounds, it is necessary to increase the soldering temperature. However, doing so imparts thermal damage to electronic parts. Thus, even though the length of time for which individual printed circuit boards contact molten solder in a solder bath during flow soldering is only a few seconds, the adverse effects of dissolving out of Cu from printed circuit boards into molten solder on soldering operation should not be neglected.
In flow soldering using a wave solder bath, while a printed circuit board is conveyed along a given path passing over the solder bath, the lower surface of the board contacts the top of a wave of solder formed in the solder bath to perform soldering. It is important to maintain the level of molten solder in the solder bath substantially constant, since if the level becomes too low, solder does not adequately contact the lower surface of the board and unsoldered portions may result. The molten solder in the bath is gradually consumed due to adhesion to the portions to be soldered of printed circuit boards, so in a continuous soldering operation, the level of molten solder in the solder bath gradually decreases as more and more printed circuit boards undergo soldering. To prevent the level of molten solder in the solder bath from falling too low, a solder tank containing a solder bath is typically equipped with a level sensor which monitors the level of the molten solder and generates an alarm when the level falls below an allowable limit. When the alarm is generated, the solder bath is replenished by solder, normally in the form of a rod or wire, which is supplied to the solder bath either manually or automatically, to maintain the level of the solder bath in a prescribed range.
Conventionally, solder used to replenish a solder bath to maintain the level of the bath is the same as the solder which is initially used to form the molten solder in the solder bath, since it was thought that using the same solder would maintain the chemical composition of the solder bath constant. As a result, when continuous soldering of printed circuit boards or other members with exposed
Hasegawa Eietsu
Hasegawa Seiichiro
Dunn Tom
Pittman Zidia
Senju Metal Industry Co. Ltd.
Tobias Michael
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