Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
1999-11-22
2001-07-10
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S784000, C257S781000, C438S652000
Reexamination Certificate
active
06259161
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a circuit electrode and a method of forming a circuit electrode. More particularly, the present invention relates to a circuit electrode formed on a package of an integrated circuit or on a printed circuit board of the same, as well as to a method of forming such a circuit electrode.
2. Description of the Background Art
A ball grid array (BGA) package and a chip scale package (CSP) have already been known as packages suitable for meeting a demand for an integrated circuit having an increased number of pins. A circuit electrode coated with a metal pattern, such as a Cu pattern, by means of electroless plating is used for such a package.
FIG. 6
is a cross-sectional view showing a conventional integrated circuit package which is equipped with a circuit electrode having the above-described structure. The conventional integrated circuit package has an organic substrate
10
. A pattern
12
is formed on the organic substrate
10
from metal, such as Cu. The metal pattern
12
is coated with an electroless Ni—P plating film
14
(hereinafter referred to as a “Ni—P film
14
”), and the Ni—P film
14
is further coated with an immersion gold plating film
16
(hereinafter referred to as an “Au film
16
”). On the surface of the organic substrate
10
is formed a passivation film
18
in order to prevent oxidation of the metal pattern
12
or flow of solder from the circuit electrode.
The Ni—P film
14
is an electroless plating film containing Ni as a major constituent and P in an amount of about 6 to 8 wt. %. This Ni—P film
14
is provided primarily for the purpose of preventing diffusion of Cu, which constitutes the pattern
12
. The Au film
16
is an electroless plating film formed on the Ni—P film
14
by means of the substitution plating technique and is provided primarily for the purpose of preventing oxidation of the Ni—P film
14
. In the above-described configuration, a circuit electrode is formed from the pattern
12
, the Ni—P film
14
, and the Au film
16
. The circuit electrode is connected to an electrode of a printed circuit board by way of a solder ball or bump to be formed on the Au film
16
or by a bonding wire to be bonded on the same.
An electroless plating film of Au can be formed by the substitution plating technique or the reduction plating technique. However, if the reduction plating technique is employed for forming the Au film on a Ni—P film
14
, existence of Ni ions hinders growth of the Au film. For this reason, the Au film
16
must also be grown by means of the substitution plating technique, before employing reduction gold plating, as mentioned previously.
The substitution plating technique is a method of growing an Au film by substituting Au for Ni contained in the Ni—P film
14
. Since P is not substituted by Au at this time, Au is first deposited on the P-free areas of the surface of the Ni—P film
14
during the course of plating of the Au film
16
. Accordingly, during the course of plating of the Au film
16
, the lower the P concentration in the Ni—P film
14
, the faster the rate of substituting reaction between Ni and Au.
In other words, if the P concentration in the Ni—P film
14
is too high, substitution of Au for Ni is not sufficiently carried out, and pinholes are likely to arise in the Au film
16
. In the event that pinholes arise in the Au film
16
, oxidation of the Ni—P film
14
cannot be prevented, thus wire bonding or soldering without use of flux becomes difficult. For this reason, the P concentration in the Ni—P film
14
must not be set excessively high.
In the conventional circuit electrode, the P concentration in the Ni—P film
14
assumes a value of 6 to 8 wt. %. In other words, P ions occupy about 6 to 8% portion of the surface area of the Ni—P film
14
. During the course of plating of the Au film
16
, erosion of Ni by Au proceeds in the vertical direction while Ni is substituted by Au in the areas where P ions do not exist. If the rate of reaction between Au and Ni is high, erosion of Ni by Au becomes noticeable, resulting in formation of crack-shaped eroded portions in the Ni—P film
14
.
The lower the P concentration of the Ni—P film
14
, the faster Au ions react with Ni ions. For example, if the P concentration of the Ni—P film
14
assumes a value of 3.5 wt. %, Au is deposited to a thickness of 0.5 &mgr;m when the Au film
16
is plated for 10 minutes. In a case where the P concentration assumes a value of 7.5 wt. %, Au is deposited to a thickness of 0.3 &mgr;m when the Au film
16
is plated for 10 minutes. Further, in a case where the P concentration assumes a value of 9.5 wt. %, Au is deposited to a thickness of 0.05 &mgr;m when the Au film
16
is plated for 10 minutes. Thus, the lower the P concentration of the Ni—P film
16
, the greater the amount of Ni ions that are substituted by Au ions during the plating process. As a result, crack-shaped eroded portions are likely to arise in the Ni—P film
14
.
The areas of the Ni—P film
14
where Ni ions are substituted by Au ions are higher in P content than are other areas. At this time, the greater the amount of Ni ions substituted by Au ions, the higher the P concentration. Accordingly, the lower the original P content in the Ni—P film
14
, the higher the differential in P content in the boundary region between the Au film
16
and the Ni—P film
14
after the Au film
16
has fully grown.
FIG. 7A
shows a solder ball
20
bonded to the circuit electrode shown in FIG.
6
.
FIG. 7B
shows removal of the solder ball
20
from the circuit electrode as a result of a shear test. In
FIGS. 7A and 7B
, like reference numerals are assigned to elements which are identical with or correspond to those shown in
FIG. 6
, and repetition of their explanations is omitted.
The solder ball
20
is formed from eutectic solder containing lead (Pb) and tin (Sn). During the course of the solder ball
20
being soldered to the circuit electrode, the Au film
16
covering the Ni—P film
14
becomes fused in the solder ball
20
. As a result, the solder ball
20
comes into direct contact with the Ni—P film
14
. After the solder ball
20
has come into contact with the Ni—P film
14
, Sn contained in the solder ball
20
react with Ni contained in the Ni—P film
14
to form an Ni/Sn compound
24
(Ni
3
Sn
4
).
During the course of generation of the Ni/Sn compound
24
, the diffusion rate of Ni is reduced by the presence of P ions in the P-containing areas on the surface of the Ni—P film
14
. If any areas of the surface of the Ni—P film
14
have a high P content, therefore, the Ni/Sn compound
24
is likely to assume a columnar profile as shown in
FIGS. 7A and 7B
. Further, since P ions are left in the Ni—P film
14
during the course of generation of the Ni/Sn compound
24
, a P-rich layer
22
whose P content is higher than that of the other areas is generated in the vicinity of the boundary region between the solder ball
20
and the Ni—P film
14
.
As a result of the Ni/Sn compound
24
acting as an adhesive, the solder ball
20
is soldered to the Ni—P film
14
. The P-rich layer
22
generated between the Ni/Sn compound
24
and the Ni—P film
14
acts as a contaminated layer and weakens the adhesive action of the Ni/Sn compound
24
. Formation of the P-rich layer
22
results in a decrease in bonding strength between the solder ball
22
and the Ni—P film
14
, which in turn renders the P-rich layer
22
susceptible to removal, as shown in FIG.
7
B.
As mentioned above, as erosion of the Ni—P film
14
associated with growth of the Au film
16
becomes more noticeable, the P concentration on the surface of the Ni—P film
14
increases. In other words, as the erosion of the Ni—P film
14
associated with growth of the Au film
16
becomes more noticeable, the P ions existing in the Ni—P film
14
tend to become more concentrated. As the P ions existing on the surface of the Ni—P film
14
become more concentrated at the time of bonding of the solder ball
20
, the P-rich layer
Tomita Yoshihiro
Wu Qiang
Clark Sheila V.
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
LandOfFree
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