Recently, ceramic materials such as Al2O3, Si3N4 and AlN with excellent thermal resistance and electrical insulation properties have been used more and more often as materials for an electronic components board. As a method of making the surface of a ceramic substrate electrically conductive, an electroless plating process that needs no supply of electrical current is sometimes used. Also, according to the electroless plating process, a plated coating can be formed with high size precision, which is also beneficial.
For example, a bonded substrate, obtained by bonding a ceramic substrate and a copper alloy layer having a predetermined pattern together with a soldering material such as silver solder, has been used extensively as a substrate with good heat dissipation property. As used herein, the “copper alloy layer” means either a layer made of copper alone or a layer made of an alloy including copper. Also, such a substrate will sometimes be referred to herein as a “Cu pattern bonded substrate”. In a Cu pattern bonded substrate, the Cu portion thereof is plated with Ni to increase the corrosion resistance and to get the soldering process done more easily. As electroless Ni plating processes, a Ni—P plating process and a Ni—B plating process are known. However, the Ni—P plating process has been adopted more extensively because the process ensures good corrosion resistance and high chemical resistance and yet can be done at a low cost.
In general, when copper is plated by the electroless Ni—P plating process, hypo-phosphoric acid is used as a reducing agent. However, since Cu is not active in reaction to hypo-phosphoric acid, the surface of copper needs to be activated with a catalyst such as palladium (Pd) or tin (Sn). Such a catalyst will be referred to herein as a “plating catalyst”. And a process that uses a plating catalyst will be referred to herein as a “catalyst process”. FIG. 8 shows the flow of the catalyst process.
First, a degreasing process step is performed to remove organic substances and other smudge from the surface of the Cu portion. If any smudge stayed on the surface of the Cu portion, then the surface of the Cu portion would not have uniform wettability to a plating solution, thus making the resultant plated coating uneven.
Thereafter, an oxide remaining on the surface of the Cu portion is etched away with an acid aqueous solution. In this process step, the surface of the Cu portion may be roughened if necessary.
Next, the surface of the Cu portion is activated by a method so-called “sensitizing activation”. Specifically, first, the surface of the Cu portion is supplied with an Sn (tin) catalyst (activation process step I) and then supplied with a Pd (palladium) catalyst (activation process step II). Since these activation process steps I and II are carried out with the Cu portion immersed in an aqueous solution, divalent cations (i.e., Sn2+ and Pd2+) will be supplied to the surface of the Cu portion in both of these process steps. Once Pd2+ has been supplied by the activation process step II, the reaction Sn2++Pd2+→Sn4++Pd occurs, thus giving Pd to the surface of the Cu portion (activation process step III).
Subsequently, if the target to be plated, including the Cu portion, is immersed in an electroless Ni—P plating solution, Ni—P will precipitate where Pd has been deposited, thus forming a Ni—P plated coating. The plating reactions of this process step are represented by the following two chemical formulae:NiSO4+NaH2PO2+H2O→2NaH2PO3+H2SO4+Ni+H2 NaH2PO2+½H2O→NaOH+P+H2O
According to the catalyst process, an aqueous solution is used as described above to give Pd onto the surface of the Cu portion, and therefore, it is difficult to form a Ni—P plated coating selectively on the surface of the Cu portion, which is a problem. Hereinafter, the problems with the catalyst process will be described with reference to FIG. 9.
If the Cu portions 12a and 12b on the surface of a ceramic substrate 11 shown in FIG. 9(a) are plated by the catalyst process described above, then Pd will be given to not only the surface of the Cu portions 12a and 12b but also the surface of the ceramic substrate 11 as well as shown in FIG. 9(b). That is why if the substrate is immersed in the plating solution, not just a Ni—P plated coating 52a, 52b will be formed so as to cover the surface of the Cu portions 12a and 12b but also Ni—P plating residues 52c and 52d will be deposited from the Pd particles that have been left on the surface of the ceramic substrate.
Thus, if the catalyst process is adopted, a Ni—P plating layer 52a, 52b is formed as schematically shown in FIG. 9(d). Consequently, the minimum gap between two adjacent portions of the Ni—P plating layer that has been formed so as to cover the two adjacent Cu portions 12a and 12b should have been Wp1 but actually becomes Wp2 (where Wp2<Wp1). If the minimum gap between adjacent portions of the Ni—P plating layer becomes smaller than a predetermined value in this manner, then a decrease in dielectric strength and other serious problems will arise.
On top of that, the aqueous solution to supply the Pd catalyst includes a lot of organic substances as a complexing agent or as a stabilizer, which could remain on the plating interface and could cause swelling or peeling when heated.
To overcome these problems, an electroless Ni—P plating process that does not use any Pd catalyst has been proposed.
For example, Patent Document No. 1 discloses a process in which either hydrazine or a derivative thereof is used as a reducing agent to form a high-purity Ni coating only on the surface of the Cu portions and then the Ni coating is subjected to the electroless Ni—P plating process. However, the present inventors discovered and confirmed via experiments that even if such a method was adopted, it was still difficult to form the Ni—P plated coating selectively only on the surface of the Cu portions and also discovered that organic substances remained on the plating interface. On top of that, since the additional process step of forming a Ni coating should be carried out, the manufacturing process gets complicated.
Meanwhile, although it is related to Ni—B plating, a galvanic initiation is known as a method for subjecting a metallic material with no catalyst activity to electroless Ni plating as disclosed in Patent Document No. 2. According to the galvanic initiation method, when a Cu plate with no catalyst activity needs to be plated, for example, not just a Cu plate but also an Fe plate with catalyst activity are immersed in a plating bath and brought into physical contact with each other. Then, electrons are produced by the oxidation of a reducing agent on the surface of the Fe plate and then supplied to the surface of the Cu plate, thereby precipitating Ni and Ni—P there.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-253454    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2002-180260