Nickel or nickel alloy electroplating bath and plating process using the same

Disclosed is a nickel electroplating bath or a nickel alloy electroplating bath used for electroplating a conductor partially masked with an organic high-molecular resist layer, wherein the nickel electroplating bath contains a water-soluble nickel salt, and the nickel alloy electroplating bath contains both a water-soluble nickel salt and a water-soluble salt of a metal capable of being alloyed with nickel. The above electroplating bath is incorporated with an electrical conductive salt containing at least one cation selected from the group consisting of an ammonium ion, magnesium ion, calcium ion, aluminum ion, and barium ion. Further, the electrical conductive salt substantially does not contain a sodium ion and a potassium ion as cations. Such an electroplating bath is capable of electroplating a conductor partially masked with an organic high-molecular resist layer without floating of the resist layer.

BACKGROUND OF THE INVENTION 
The present invention relates to a nickel or nickel alloy electroplating 
bath used for electroplating a conductor partially masked with an organic 
high-molecular resist layer, for example, a printed circuit board 
partially masked with a solder resist layer, and to an electroplating 
process using the bath. In particular, the present invention concerns a 
nickel or nickel alloy electroplating bath capable of electroplating the 
above conductor without floating (peeling) of the above resist layer 
during electroplating. 
Nickel or nickel alloy electroplating has been used for printed circuit 
boards. Specifically, an organic high-molecular resist film (which is 
often called a solder resist film in the field of printed circuit boards) 
is partially formed on a printed circuit board, and a portion of the board 
excluding the area masked with the resist film is subjected to nickel or 
nickel alloy electroplating. 
The above electroplating has been performed using a Watts type nickel 
electroplating bath mainly containing nickel sulfate and nickel chloride. 
Such a Watts type nickel electroplating bath, however, has been poor in 
macrothrowing power, and therefore, it has been expected to develop a new 
electroplating bath enhanced in macrothrowing power. 
The nickel or nickel alloy plating baths enhanced in macrothrowing power 
have been known, for example, from Japanese Patent Publication Nos. Hei 
2-22158, Hei 2-22160, Hei 2-44911, Hei 3-19308, and Hei 3-19309. In each 
of these plating baths, the concentration of a plating metal such as 
nickel is made relatively low, and an electrical conductive salt such as a 
halide, sulfate or sulfamate of an alkali metal, alkali earth metal, or 
aluminum is incorporated at a high concentration. In particular, a sodium 
salt or potassium salt has been practically used as the above electrical 
conductive salt. 
The above-described nickel or nickel alloy plating bath enhanced in 
macrothrowing power, however, has problems. Specifically, in the case 
where a printed circuit board masked at a specific region thereof with the 
above resist layer is subjected to electroplating using the nickel or 
nickel alloy plating bath containing a sodium salt or a potassium salt in 
a large amount, there possibly occurs a floating (or peeling) phenomenon 
of the resist layer, although the macrothrowing power is undoubtedly 
enhanced. The floating of the resist layer leads to problems in that a 
copper underlayer of the printed circuit board is exposed and the copper 
layer thus exposed is corroded, and that the floating resist layer can no 
longer keep the solder resisting function resulting in occurrence of a 
solder bridging phenomenon upon soldering. These problems leads to 
critical defects of the printed circuit board. The floating of the resist 
layer has another problem. Specifically, although the resist layer is 
formed at the area where plating should not be performed, there is a 
possibility that a plating film is formed at a portion where the resist 
layer is floated (peeled). 
It is to be noted that the above-described Watts type plating bath does not 
exhibit the floating phenomenon of the resist layer; however, as described 
above, it is poor in macrothrowing power. 
In view of the foregoing, it has been required to develop a nickel or 
nickel alloy electroplating bath capable of giving a high macrothrowing 
power without floating of a resist layer. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a nickel or nickel alloy 
electroplating bath capable of electroplating a conductor partially masked 
with an organic high-molecular resist layer at a high macrothrowing power 
without floating of the resist layer, and to provide an electroplating 
process using the bath. 
The present inventors have experimentally studied to achieve the above 
object, and found that an electrical conductive salt used for a nickel or 
nickel alloy electroplating bath exerts a large effect on floating of a 
resist layer. Specifically, it has been found that the use of an 
electrical conductive salt having a sodium ion or a potassium ion as a 
cation causes the floating of the resist layer; however, the use of an 
electrical conductive salt containing an ammonium ion, magnesium ion, 
aluminum ion, calcium ion, or barium ion as a cation does not cause the 
floating of the resist layer. As a result, it has been found that a nickel 
or nickel alloy electroplating bath capable of giving a high macrothrowing 
power without floating of the resist layer can be achieved by addition of 
a high concentration of an electrical conductive salt containing 
NH.sub.4.sup.+, Mg.sup.2+, Al.sup.3+, Ca.sup.2+, and/or Ba.sup.2+. The 
present invention has been thus accomplished on the basis of the above 
knowledge. 
Accordingly, the present invention provides: 
(1) A nickel electroplating bath or a nickel alloy electroplating bath used 
for electroplating a conductor partially masked with an organic 
high-molecular resist layer, wherein the nickel electroplating bath 
contains a water-soluble nickel salt, and the nickel alloy electroplating 
bath contains both a water-soluble nickel salt and a water-soluble salt of 
a metal capable of being alloyed with nickel, characterized in that the 
nickel or nickel alloy electroplating bath is incorporated with an 
electrical conductive salt containing at least one cation selected from 
the group consisting of an ammonium ion, magnesium ion, calcium ion, 
aluminum ion, and barium ion, the electrical conductive salt substantially 
not containing a sodium ion and a potassium ion as cations; 
(2) a nickel or nickel alloy electroplating bath according to the term (1), 
wherein the content of the water-soluble nickel salt for the nickel 
electroplating bath or of the water-soluble nickel salt and the 
water-soluble salt of a metal capable of being alloyed with nickel for the 
nickel alloy electroplating bath is within a range of 5 g/l to 40 g/l on 
the basis of nickel ion conversion; 
(3) a nickel or nickel alloy electroplating bath according to the term 1 or 
2, wherein the macrothrowing power of the electroplating bath, which is 
measured using a Haring cell in a condition that a distance ratio between 
two cathode plates and an anode plate is taken as 5, is 10% or more; and 
(4) a nickel or nickel alloy electroplating process, comprising the steps 
of: dipping a conductor partially masked with an organic high-molecular 
resist layer in the electroplating bath described in any one of terms (1) 
to (3); and applying a current to the electroplating bath using the 
conductor as a cathode; whereby a portion of the conductor exposed from 
the area masked with the resist layer is formed with a nickel or nickel 
alloy plating film. 
A mechanism why the electroplating bath of the present invention prevents 
the floating of a resist layer, is not clear. However, for the related art 
electroplating bath including an electrical conductive salt containing 
Na.sup.+ or K.sup.+, it may be considered that Na.sup.+ or K.sup.+ 
permeates in the resist layer on the basis of a mechanism such as 
electrosmosis, and raises the coated resist layer, to thereby cause the 
floating of the resist layer. In this case, it may be also considered that 
a metal oxide between a base material and the resist layer is reduced due 
to generation of hydrogen or the generated hydrogen permeates under the 
resist layer and gasified, to raise the resist layer, thereby causing the 
floating of the resist layer; however, there is not observed any 
relationship between the generated amount of hydrogen upon nickel 
electroplating and the floating of the resist layer. Besides, each 
hydrated molecule of a cation (NH.sub.4.sup.+, Mg.sup.2+, Al.sup.3+, 
Ca.sup.2+, Ba.sup.2+), which does not cause the flowing of the resist 
layer, has a large size, and thereby it is hard to permeate in the resist 
layer, with a result that it becomes difficult to cause the floating of 
the resist layer. Alternatively, for an electrical conductive salt 
containing Na.sup.+ or K.sup.+, when hydrogen is generated, NaOH or KOH 
is produced at the plating interface and it makes stronger the alkalinity 
of the plating interface; however, for an electrical conductive salt 
containing a cation not causing the floating of the resist layer, the 
alkalinity of the plating interface is not so stronger as compared with 
the electrical conductive salt containing Na.sup.+ or K.sup.+. After all, 
the accurate mechanism why the electroplating bath of the present 
invention prevents the floating of the resist layer is not evident.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A nickel or nickel alloy electroplating bath of the present invention 
mainly contains a water-soluble salt of a plating metal and a 
water-soluble electrical conductive salt. 
In the case of a nickel electroplating bath, as the water-soluble salt of a 
plating metal, there is used a water-soluble nickel salt such as nickel 
sulfate, nickel chloride, nickel bromide, nickel sulfamate, or nickel 
methanesulfonate. In the case of a nickel alloy electroplating bath, as 
the water-soluble salts of plating metals, there is used, in addition to 
the above water-soluble nickel salt, a water-soluble salt of a metal 
capable of being alloyed with nickel such as cobalt or iron, for example, 
a sulfate, chloride, bromide, sulfamate, methanesulfonate of cobalt, iron 
or the like. 
The content of the above water-soluble salt(s) of the plating metal(s) is 
preferably within a range of 5 to 40 g/l, more preferably within a range 
of 7 to 40 g/l on the basis of nickel ion conversion. When it is less than 
5 g/l, the cathode current density required for electroplating is made 
small to such an extent as not to be practically used; while when it is 
more than 40 g/l, the macrothrowing power of the electroplating bath is 
reduced. 
The water-soluble electrical conductive salt contains at least one cation 
selected from the group consisting of an ammonium ion, magnesium ion, 
aluminum ion, calcium ion, and barium ion. It is to be noted that the 
water-soluble electrical conductive salt substantially does not contain a 
sodium ion and potassium ion as a cation. Further, the water-soluble 
electrical conductive salt contains at least one anion selected from the 
group consisting of anions of sulfuric acid, hydrochloric acid, 
hydrobromic acid, sulfamic acid, and methanesulfamic acid. As the 
water-soluble electric conductive salt(s), there can be used at least one 
selected from the group consisting of ammonium salts, magnesium salts, 
calcium salts, aluminum salts, and barium salts of the above acids. 
The content of the above electrical conductive salt is preferably within a 
range of 50 to 800 g/l, more preferably within a range of 150 to 500 g/l. 
When it is less than 50 g/l, the electric resistance of the plating bath 
is not reduced so much and thereby the macrothrowing power thereof cannot 
be sufficiently ensured; while it is more than 800 g/l, the additional 
effect is not significantly increased and thereby the practical usability 
is reduced. 
The plating bath of the present invention can be incorporated with a 
buffer. Specific examples of the buffers may contain boric acid and 
water-soluble organic carboxylic acids such as citric acid, tartaric acid, 
succinic acid, and acetic acid, and salts thereof. As these salts, there 
may be preferably used an ammonium salt, magnesium salt, aluminum salt, 
calcium salt, and barium salt. The concentration of the buffer is 
preferably within a range of 20 to 100 g/l. 
The plating bath of the present invention is also added with a known 
primary or secondary nickel plating brightener such as saccharin, 
butynediol or a derivative thereof. In this case, the amount of the 
primary brightener is preferably within a range of 0.1 to 5 g/l and the 
amount of the secondary brightener is preferably within a range of 0.01 to 
3 g/l. Further, there may be added a phosphorous acid, phosphinic acid, or 
a salt thereof; a hydrazine compound, dimethylamine borane, or 
trimethylamine borane in an amount of 0.01 to 10 g/l in the form of 
co-deposition. 
The plating bath of the present invention substantially does not contain a 
sodium ion and a potassium ion. The wording "substantially does not 
contain a sodium ion and a potassium ion" means that the content of the 
sodium ion and potassium ion is 5 g/l or less, particularly, 3 g/l or 
less. For example, saccharin may be added as a sodium salt in a very small 
amount, or sodium carboxylate or potassium carboxylate may be added as a 
buffer in an amount less than the above limited value. 
The pH of the plating bath of the present invention is preferably within a 
range of 3.5 to 5.4. 
The macrothrowing power of the plating bath of the present invention, which 
is measured using a Haring cell in a condition that a distance ratio 
between two cathode plates and an anode plate is taken as 5, is preferably 
within a range of 10% or more, more preferably within a range of 15% or 
more. The method of measuring the macrothrowing power using the Haring 
cell is as shown in FIG. 1 and in accordance with that described in 
Japanese Patent Publication No. Hei 2-22158. In FIG. 1, a Haring cell 10 
has a length of 240 mm, a width of 63 mm and a depth of 100 mm. 1500 ml of 
a plating solution 12 is contained therein. Numeral 14 is an anode and 
numerals 16 and 18 are cathodes to be plated. The anode 14 is provided so 
that the ratio of the distance A between the anode 14 and the first 
cathode 16 and the distance B between the anode 14 and the second cathode 
18 is 5 (=B/A) in the present invention. Electroplating is conducted in a 
predetermined time to deposit a plating film to the cathodes 16 and 18. 
The weight of the plating film deposited to the cathodes 16 and 18 is 
measured. The macrothrowing power is calculated from the following 
equation; 
##EQU1## 
wherein T: macrothrowing power 
P: distance ratio B/A (=5) 
M: weight ratio M.sub.1 /M.sub.2 in which M.sub.1 is a weight of the 
plating film deposited to the first cathode 16 and M.sub.2 is a weight of 
the plating film deposited to the second cathode 18. 
The plating bath enhanced in macrothrowing power is advantageously used for 
electroplating of, for example, a printed circuit board or the like. That 
is, as compared with a plating bath low in macrothrowing power, a 
variation in thickness of a nickel plating film formed at the same area is 
small in the same plating condition. More specifically, the minimum 
plating thickness is thicker than that of the case using the plating bath 
low in macrothrowing power so that corrosion resistance and heat 
resistance are good irrespective of using a copper underlayer, and also 
the maximum plating thickness is thinner than that of the case of using 
the plating bath low in macrothrowing power so that the number of failures 
in wire bonding is reduced when gold plating is applied on the above 
plating film. 
The plating bath of the present invention is used for plating a substance 
to be plated (conductor) partially masked with an organic high-molecular 
resist layer. In this case, the kind of the substance is not particularly 
limited, but a printed circuit board is preferably used as the substance. 
The process of forming an organic high-molecular resist layer is not 
particularly limited. For example, there may be used a process of coating 
a resist material (called a solder resist) of a solvent type, 
thermosetting type or development (alkali development) type and forming a 
resist layer by a known manner; or a process of applying a plating masking 
material such as a masking tape. 
The plating condition for electroplating the above substance using the 
plating bath of the present invention may be suitably selected. For 
example, the plating temperature is preferably within a range of 45 to 
65.degree. C., and the cathode current density is preferably within a 
range of 0.1 to 4 A/dm.sup.2. The agitation is preferably performed by 
cathode rocking at a velocity of 0.2 to 7 m/min. Liquid agitation such as 
slow air agitation or propeller agitation may be used; however, strong air 
agitation or strong liquid agitation is not desirable. In addition, there 
can be used as an anode, a nickel plate or the like which is the same as 
that used for a known nickel or nickel alloy plating. 
A known pre-treatment can be applied to a substance to be plated by using 
the nickel or nickel alloy electroplating bath of the present invention. 
From the viewpoint of the above floating of a resist layer, such a 
pre-treatment as to generate hydrogen gas by electrolysis should be 
avoided. For example, dipping degreasing not generating hydrogen gas is 
superior to cathode electrolytic degreasing. In the case of using an 
electrolytic degreaser, the floating of the resist layer can be prevented 
not by use of Na.sup.+ and K.sup.+ but by use of NH.sub.4.sup.+, 
Mg.sup.2+, Al.sup.3+, Ca.sup.2+ and/or Ba.sup.2+ as cation(s). 
After the above nickel or nickel alloy electroplating, gold strike plating, 
gold plating, palladium or palladium alloy plating, silver plating, or 
platinum plating may be performed. 
The electroplating bath of the present invention, as described above, is 
capable of electroplating a substance to be plated (conductor) partially 
masked with an organic high-molecular resist layer without floating of the 
resist layer. 
EXAMPLE 
The present invention will be more clearly understood with reference to the 
following inventive examples and comparative examples. 
Example 1 
In this example, there was used a nickel electroplating bath having the 
following composition: 
______________________________________ 
MgSO.sub.4.7H.sub.2 O 
400 g/l 
NiCl.sub.2.6H.sub.2 O 
50 g/l 
boric acid: g/l.5 
pH: 4.6 
______________________________________ 
A printed circuit board was coated at a specific area thereof with a solder 
resist film of a thermosetting type or development (alkali development) 
type to a thickness of 5 to 100 .mu.m, and then it was subjected to nickel 
electroplating using the nickel electroplating bath having the above 
composition. The electroplating condition was as follows: 
plating temperature: 55.degree. C. 
agitation: cathode rocking at velocity of 1 m/min perpendicularly to 
plating surface 
______________________________________ 
plating time: 35 min 
cathode current density: 
1 A/dm.sup.2 
anode: electrolytic nickel 
thickness of plating film: 
6-9 .mu.m 
______________________________________ 
As a result, it was confirmed that the macrothrowing power was good and the 
floating of the solder resist film was not recognized al all. 
Example 2 
The procedure in Example 1 was repeated except for the use of a 
nickel-cobalt alloy electroplating bath having the following composition: 
______________________________________ 
ammonium sulfamate: 
200 g/l 
NiCl.sub.2.6H.sub.2 O 
70 g/l 
CoSO.sub.4.7H.sub.2 O 
1 g/l 
triammonium citrate: 
50 g/l 
pH: 4.6 
______________________________________ 
As a result, it was confirmed that the thickness of the plating film was 
within a range of 6 to 10 .mu.m and the macrothrowing power was good, and 
the floating of the solder resist film was not recognized at all. 
Example 3 
The procedure in Example 1 was repeated except that MgCl.sub.2.6H.sub.2 O 
was further added to the electroplating bath used in Example 1 in an 
amount of 70 g/l. As a result, it was confirmed that the thickness of the 
plating film was within a range of 6.7 to 9 .mu.m and the macrothrowing 
power was further enhanced, and the floating of the solder resist film was 
not recognized at all. 
Comparative Example 1 
The procedure in Example 1 was repeated except that magnesium sulfate (400 
g/l) in Example 1 was replaced with sodium sulfate in an amount of 200 
g/l. As a result, it was confirmed that the thickness of the plating film 
was within a range of 6 to 9 .mu.m just as in Example 1, but the floating 
of the solder resist film was recognized. 
Comparative Example 2 
The procedure in Example 1 was repeated except for the use of a Watts type 
nickel electroplating bath having the following composition: 
______________________________________ 
NiSO.sub.4.6H.sub.2 O 
280 g/l 
NiCl.sub.2.6H.sub.2 O 
50 g/l 
boric acid: 
46 g/l 
pH: 4.6 
______________________________________ 
As a result, it was confirmed that the floating of the solder resist layer 
was not recognized, but the thickness of the plating film was within a 
range of 2 to 20 .mu.m and the macrothrowing power was poor. 
Next, the macrothrowing power of the electroplating bath in each of 
Examples 1, 2, 3 and Comparative Examples 1, 2 was measured by a manner of 
performing electroplating using the Haring cell (distance ratio of two 
cathode plates and an anode plate: 5) at a current of 2 A while performing 
air agitation for 30 min. The results are shown in Table 1. 
TABLE 1 
______________________________________ 
macrothrowing power (%) 
______________________________________ 
Example 1 25 
Example 2 23 
Example 3 35 
Comparative Example 1 
25 
Comparative Example 2 
6 
______________________________________ 
From the results shown in Table 1, it becomes apparent that the 
macrothrowing power in Comparative Example 2 using the Watts type nickel 
electroplating bath is significantly poor as compared with those in 
Examples 1, 2, 3. 
While the preferred embodiments of the present invention have been 
described using the specific terms, such description is for illustrative 
purposes only, and it is to be understood that changes and variations may 
be made without departing the spirit or scope of the following claims.