U.S. patent application number 10/684589 was filed with the patent office on 2004-06-17 for lead-free solder ball.
Invention is credited to Kato, Rikiya, Nomoto, Shinichi, Okada, Hiroshi.
Application Number | 20040112474 10/684589 |
Document ID | / |
Family ID | 32040837 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040112474 |
Kind Code |
A1 |
Kato, Rikiya ; et
al. |
June 17, 2004 |
Lead-free solder ball
Abstract
Lead-free solder balls having a smooth surface and no shrinkage
cavities or wrinkles are made of a lead-free solder which
comprises, by atomic percent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of
at least one element of the iron group and preferably Co,
optionally 0.04%-4% of P, and a balance of Sn.
Inventors: |
Kato, Rikiya; (Souka-shi,
JP) ; Nomoto, Shinichi; (Hanyu-shi, JP) ;
Okada, Hiroshi; (Mouka-shi, JP) |
Correspondence
Address: |
MICHAEL TOBIAS
#40
1717 K ST. NW, SUITE 613
WASHINGTON
DC
20036
US
|
Family ID: |
32040837 |
Appl. No.: |
10/684589 |
Filed: |
October 15, 2003 |
Current U.S.
Class: |
148/400 ;
420/560 |
Current CPC
Class: |
H05K 3/3478 20130101;
B23K 35/262 20130101; B23K 35/0244 20130101; B23K 2101/36 20180801;
C22C 13/00 20130101; B23K 1/0016 20130101; H05K 3/3463
20130101 |
Class at
Publication: |
148/400 ;
420/560 |
International
Class: |
C22C 013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2002 |
JP |
2002-303197 |
Claims
What is claimed is:
1. A solder ball made of a lead-free solder which comprises, by
atomic percent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of at least one
element of the iron group, 0%-4% of P, and a balance of Sn.
2. A solder ball as claimed in claim 1 wherein the content of P is
0.04%-4%.
3. A solder ball made of a lead-free solder which comprises, by
atomic percent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of Co, 0%-4% of
P, and a balance of Sn.
4. A solder ball as claimed in claim 3 wherein the content of P is
0.04%-4%.
5. A method of forming solder bumps comprising placing solder balls
as claimed in claim 1 on a substrate and then heating the substrate
to melt the solder balls and form them into solder bumps secured to
the substrate.
6. A method of forming soldering bumps comprising placing solder
balls as claimed in claim 3 on a substrate and then heating the
substrate to melt the solder balls and form them into solder bumps
secured to the substrate.
7. A substrate arrangement comprising a substrate for a BGA package
and a plurality of solder bumps formed on the substrate from solder
balls as claimed in claim 1.
8. A substrate arrangement comprising a substrate for a BGA package
and a plurality of solder bumps formed on the substrate from solder
balls as claimed in claim 3.
9. A BGA package comprising a substrate, a semiconductor chip
mounted on the substrate, and a plurality of solder bumps formed on
the substrate from solder balls as claimed in claim 1.
10. A BGA package comprising a substrate, a semiconductor chip
mounted on the substrate, and a plurality of solder bumps formed on
the substrate from solder balls as claimed in claim 3.
Description
TECHNICAL FIELD
[0001] The present invention relates to a lead-free solder ball
suitable for use in soldering of electronic components.
BACKGROUND ART
[0002] Nowadays, as electronic products are required to be
multifunctional and have a reduced size, electronic components,
which form the core of electronic products, are also required to be
multifunctional and have a reduced size.
[0003] In conventional electronic components, a semiconductor chip
is mounted on a lead frame made of a metal such as copper or Alloy
42 (42 Ni--Fe alloy), and the chip is electrically connected to the
lead frame by wire bonding with gold wires before being sealed with
ceramic or plastic. However, with such an electronic component
using a lead frame, there is a limit on the extent to which the
size of the electronic component can be reduced since the lead
frame occupies its own space on a printed circuit board. The use of
a lead frame also puts a limit on the speed of operation, since the
lead frame increases the length of electrical connections on which
the speed of operation depends.
[0004] In view of these problems of electronic components using a
lead frame, another type of electronic component called a BGA (ball
grid array) package was developed. A BGA package employs solder
bumps formed from solder balls in order to connect and secure the
package to a printed circuit board. Therefore, the length of
electrical connection is shorter than for electronic components
using lead frames, thereby making it possible to increase the speed
of operation. The space occupied by a lead frame is no longer
necessary, so a BGA package also makes it possible to save space.
Recently, more compact BGA packages called FBGA (fine ball grid
array) packages or CSP's (chip size packages) having nearly the
same size as the chips packaged therein and having a finer
electrode pitch have been produced. BGA packages, including CSP's,
are now widely used, and solder ball-mounting technology is
becoming prevalent in the mounting of electronic components.
[0005] A BGA package has a substrate on which a semiconductor chip
is mounted. The substrate has solder bumps on its back surface
which are formed from solder balls arranged in a grid-like array.
In a typical method for forming the solder bumps, a solder ball
feeder equipped with a suction plate is used. The suction plate has
holes arranged in the same grid-like array as the solder bumps to
be formed. Solder balls are positioned on a substrate by pulling a
solder ball into each hole of the feeder by suction applied through
the holes, and after positioning the feeder above a substrate,
releasing the suction to place or mount each ball onto the
substrate. The solder balls are temporarily kept in position on the
substrate by the stickiness of a soldering flux, which has
previously been applied to the surface of the substrate on which
solder bumps are to be formed. The substrate having the solder
balls mounted thereon is then heated in a reflow furnace to melt
the solder balls and form them into solder bumps secured to the
substrate. The substrate having solder bumps thus formed is
normally inspected by an optical inspection machine in order to
check if all the solder bumps required to make the desired
grid-like array are properly formed.
[0006] Solder balls which are used in the production of BGA
packages, including CSP's, have a spherical shape with a diameter
in the range of 0.05-2.0 mm. It is desired for solder balls to have
good sphericity and a smooth surface. If the sphericity of a solder
ball is not good or if its surface has significant surface
irregularities or defects such as shrinkage cavities or wrinkles,
problems may occur during placement of the solder ball on a
substrate, such as caused by failure of the feeder to pick up the
solder ball due to a loss of suction or a failure of the feeder to
release the solder ball due to biting of the ball into one of the
holes in the feeder.
[0007] It is also desired for solder balls to form solder bumps
having a smooth surface with a uniform gloss when heated in a
reflow furnace. The optical inspection of solder bumps formed on a
substrate is conducted by focusing normally on solder bumps having
a glossy surface or in some cases on solder balls having a
non-glossy surface, so the formation of solder bumps, some of which
have a glossy surface and others of which have a non-glossy
surface, makes it difficult to adjust the focus in the optical
inspection of the solder bumps and results in a failure to identify
some of the bumps.
[0008] The solder that has been used most widely in soldering is an
Sn--Pb alloy. Sn--Pb solder has been used since ancient times and
has the advantages of a low melting point and good solderability.
In addition, an Sn--Pb alloy having an Sn content of 63 wt % or 75
at %, which is a representative composition for an Sn--Pb solder,
has the excellent properties that it forms a soldered joint having
a smooth surface with good gloss.
[0009] Solder balls made of an Sn--Pb alloy have a smooth surface,
so they can be smoothly mounted onto a substrate by use of the
above-described solder ball feeder. In addition, after heating in a
reflow furnace, they form solder bumps having a smooth and glossy
surface which does not interfere with optical inspection of the
solder bumps.
[0010] Recently, the use of an Sn--Pb solder has been disfavored
due to the toxic nature of Pb. When waste electronic product such
as computers are disposed of, they are normally disassembled to
remove plastic and metallic parts for recycling. Printed circuit
boards on which electronic components are mounted are not adapted
for recycling since plastic and metallic portions are combined
therein, so the printed circuit boards are removed from waste
electronic products, shredded, and buried in the ground. When rain
which has been acidified due to air pollution contacts shredded
printed circuit boards buried in the ground, the lead (Pb) in the
Sn--Pb solder may be dissolved out and contaminate underground
water. If a human or animal continues to drink lead-containing
water for many years, there is the possibility of lead accumulating
in its body and causing lead poisoning.
[0011] Accordingly, it is now highly recommended, from an
environmental standpoint, to use a "lead-free" solder, which is
completely free from lead, in soldering of electronic
components.
[0012] Lead-free solders which are considered promising at present
are Sn--Ag solders and particularly Sn--Ag--Cu solders in view of
their ease of handling. However, the wettability of these lead-free
solders is generally lower than that of Sn--Pb solders. For
example, in a spreading test, an Sn--Ag--Cu lead-free solder shows
a spreading factor of approximately 80% that of an Sn--Pb
solder.
[0013] When solder balls are made from such an Sn--Ag--Cu lead-free
solder, the surfaces of the resulting solder balls have shrinkage
cavities and wrinkles, thereby increasing the occurrence of the
above-described problems during mounting of solder balls on a
substrate using the above-described solder ball feeder. In
addition, after the solder balls are heated in a reflow furnace to
form solder bumps, the surfaces of the resulting solder bumps tend
to be glossy for some bumps and non-glossy for other bumps. The
formation of glossy bumps interspersed with non-glossy bumps makes
optical inspection of the solder bumps difficult and increases the
rate of misidentification. These disadvantages have been an
impediment to the use of lead-free solders as a material for solder
balls in place of conventional Sn--Pb solders.
DISCLOSURE OF THE INVENTION
[0014] The present invention provides a solder ball of an
Sn--Cu--Ag-based, lead-free solder which has a smooth surface
having little or no shrinkage cavities or wrinkles.
[0015] More particularly, in one aspect, the present invention
provides a solder ball made of a lead-free solder which comprises,
by atomic percent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of at least
one element of the iron group and preferably Co, optionally
0.04%-4% of P, and a balance of Sn.
[0016] In another aspect, the present invention also provides a
substrate for a BGA package (including CSP) which have solder bumps
formed from the above-described solder balls.
[0017] The present invention also relates to a method of forming
solder bumps on a substrate comprising placing the above-described
solder balls on the substrate followed by heating the substrate to
melt the solder balls and form them into solder bumps secured to
the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is an electron photomicrograph of solder balls of an
Sn--Cu--Ag--Co lead-free solder according to the present
invention.
[0019] FIG. 2 is an electron photomicrograph of solder balls of an
Sn--Cu--Ag lead-free solder.
[0020] FIG. 3 is an electron photomicrograph of solder balls of a
conventional Sn--Pb solder.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Spherical solder balls can be produced either by remelting
solid masses of solder of a certain size followed by cooling, or by
forming droplets of a certain size from molten solder followed by
cooling. In either method, molten solder of a certain size is
cooled and changed into a solid. In the course of solidification,
some of the elements of the solder initially crystallize out in the
liquid mass, and the resulting crystals, which serve as nuclei for
crystal growth, gradually grow until the entire mass becomes solid.
This crystal growth may sometimes occur unidirectionally, i.e., in
a certain direction, thereby forming dendrites.
[0022] When molten solder is cooled so as to solidify, a
conventional Sn--Pb solder, and particularly a eutectic Sn--Pb
solder finishes solidifying in a short time. In contrast, an
Sn--Cu--Ag solder takes a longer time to finish solidifying, as
suggested by its DSC (differential scanning calorimetry) curve in
which the solidifying peak is wider than that of an Sn--Pb solder.
In addition, because of the actual cooling speed during
solidification which is not slow enough to result in equilibrium,
the alloy composition at the final solidification stage deviates
from the eutectic composition. With an Sn--Cu--Ag solder, this
deviation becomes greater than with an Sn--Pb solder, and the
degree of supercooling is also greater. As a result, during
transition from a liquid to a solid phase, solid solution tends to
form excessively, thereby causing the formation of dendrites or
coarse crystals, which when grown, lead to the formation of surface
detects such as shrinkage cavities and wrinkles and significant
surface irregularities.
[0023] During solidification of molten solder, if a large number of
nuclei which initially crystallize out are present in the molten
solder, it becomes difficult to form a solid solution excessively,
which in turn makes it difficult to form dendrites or coarse
crystals.
[0024] According to the present invention, at least one of the iron
group elements (Fe, Ni, and Co) and preferably Co is added to an
Sn--Cu--Ag solder in an amount of 0.01-2 atomic percent (at %).
Each of these elements has a melting point which is much higher
than those of Sn, Cu, and Ag. Therefore, during solidification of
molten solder, these elements initially crystallize out to form a
large number of nuclei for crystal growth. It has been found that
addition of at least one element of the iron group and particularly
addition of Co is effective for producing solder balls having a
smooth surface in which the formation of shrinkage cavities and
wrinkles is prevented, thereby making it possible to produce solder
balls which can be smoothly mounted on a substrate using a solder
ball feeder of the above-described type. The solder balls can form
solder bumps having a uniform glossy surface, which facilitates
optical inspection and minimizes the occurrence of
misidentification.
[0025] These effects are not appreciable if the total content of
the iron group elements is less than 0.01 at %. If this content is
more than 2 at %, the iron group elements become segregating on the
surface of the solder balls without dissolving into the inside
thereof, thereby deteriorating the wettability of the solder balls
when melted and adversely affecting the surface smoothness and
gloss thereof. Preferably, the iron group element is cobalt (Co).
The total content of the iron group elements is preferably in the
range of 0.02-0.5 at %.
[0026] The addition of at least one element of the iron group to an
Sn--Ag--Cu lead-free solder is disclosed in U.S. Pat. No. 6,179,935
and 6,231,691 and JP P11-216591A (1999). However, there is no
disclosure therein regarding the production or use of solder balls
for bump formation or the surface appearance of the solder. The
ball form referred to in column 12 of U.S. Pat. No. 6,179,935 is
the shape of solder powder for use in a cream solder.
[0027] The Ag and Cu contents of the alloy composition in a
lead-free solder ball according to the present invention are 3-6 at
% and 1-3 at %, respectively.
[0028] When present in an amount of at least 3 at %, Ag serves to
lower the melting point of the solder and improve the wettability
and strength thereof. However, when the Ag content increases so as
to exceed 6 at %, it adversely affects both the melting temperature
and wettability of the solder. Preferably, the Ag content is in the
range of 3-5 at %. In order to lower the melting point of the
solder, it is also preferable that the Ag content be selected such
that the atomic ratio of Ag to Sn is approximately 3:70.
[0029] When present in an amount of at least 1 at %, Cu serves to
improve the strength of the solder. The presence of a small amount
of Cu also provides the solder with improved wettability. However,
like Ag, the presence of an excessive amount of Cu, which is
greater than 4 at % for Cu, causes a rise in the melting
temperature of the solder and deteriorates its wettability.
Preferably, the Cu content is in the range of 1-3 at %. More
preferably, the Cu content is selected such that the atomic ratio
of Cu to Sn is approximately 1:70.
[0030] Although the addition of at least one iron group element and
preferably Co is effective for preventing the formation of
dendrites during solidification and producing solder balls having a
smooth surface with no shrinkage cavities or wrinkles, the addition
can possibly adversely affect the wettability of the solder balls.
In order to eliminate this possibility, phosphorus (P) may be added
in a small amount. Thus, the addition of P ensures that the solder
can exhibit good wettability even though it contains at least one
iron group element. When added, the content of P is in the range of
0.04-4 at %.
[0031] In a solder ball according to the present invention, Sn is
the remainder of the solder composition. In general, the Sn content
is in the range of 86-96 at %.
[0032] In addition, the solder composition may contain unavoidable
impurities such as Pb, Sb, and Bi in a total amount of at most 0.2
at %.
[0033] There is no limitation on the diameter of a solder ball
according to the present invention as long as the solder ball is
suitable for use in the formation of solder bumps on a substrate
for a BGA package or CSP. In general, the diameter is in the range
of from 0.05 mm to 1.0 mm.
[0034] Solder balls according to the present invention can be
produced by a method which comprises forming masses of molten
solder of the above-described composition having almost equal
volumes and solidifying the masses to form balls having almost
equal diameters.
[0035] Examples of such a ball forming method include an oil bath
method as disclosed in U.S. Pat. No. 5,653,783 and JP P07-300606A
(1995) and a direct method as disclosed in U.S. Pat. No. 5,445,666,
although other methods may be employed.
[0036] In the oil bath method, a wire of a solder having a
predetermined composition is prepared and cut into portions having
a given length. The wire portions are separately dropped into an
oil bath having a vertical temperature gradient in which the
temperature in an upper portion of the bath is higher than in a
lower portion, whereby the portions are allowed to melt in the
upper portion and then solidify while falling in the oil bath.
[0037] In the direct method, a molten solder having a predetermined
composition is prepared. The molten solder is dripped or allowed to
fall in droplets of a given size through an orifice or nozzle, and
then solidified while falling in a chamber.
[0038] In both of these methods, the resulting solder balls have a
spherical shape due to the action of the surface tension of the
molten solder.
[0039] The following examples are presented to further illustrate
the present invention. These examples are to be considered in all
respects as illustrative and not restrictive.
EXAMPLES
[0040] Solder balls having a diameter of 0.5 mm were produced from
each of various solders having the compositions shown in Table 1 by
a conventional oil bath method, and they were used to form solder
bumps on a substrate for a BGA package by mounting the solder balls
on the substrate using a solder ball feeder having holes for
grasping solder balls by suction and then heating the substrate in
a reflow furnace at a temperature sufficient to form solder
bumps.
[0041] The reliability of mounting solder balls was evaluated by
the percentage of solder balls with respect to which problems
occurred during mounting of solder balls, i.e., the percentage of
solder balls which were not grasped by the solder ball feeder by
suction or which were not released from the feeder due to biting
into the holes of the feeder.
[0042] In addition, the solder bumps formed on the substrate were
checked by an optical inspection machine developed for checking
solder bumps to determine the percent occurrence of
misidentification of solder bumps in this inspection (the
percentage of solder bumps which were not identified by the
inspection machine).
[0043] The results obtained with each solder are also shown in
Table 1 together with the surface appearance of the solder balls
when observed under a scanning electron microscope (SEM) and the
spreading factor determined in a conventional spreading test on a
copper plate.
[0044] Electron photomicrographs of solder balls of Run No. 1, No.
6, and No. 7 in Table 1 are shown in FIGS. 1, 2, and 3,
respectively.
1TABLE 1 Run Solder composition Ball % Mounting % Misidenti-
Spreading No. (atomic %) surface problems fication.sup.1 factor (%)
Remarks 1 Sn-3.8Ag-1.3Cu-0.02Co smooth 0.01 0.04 82 Invent..sup.3 2
Sn-3Ag-1.0Cu-0.02Co smooth 0.01 0.04 80 Invent. 3
Sn-5Ag-3.5Cu-0.02Co- smooth 0.01 0.04 78 Invent. 3.4P 4
Sn-3.8Ag-1.3Cu-0.2Co smooth 0.01 0.04 82 Invent. 5
Sn-3.8Ag-1.3Cu-1.0Co- smooth 0.03 0.06 82 Invent. 3.4P 6
Sn-3.8Ag-1.3Cu irregular.sup.2 0.08 1.2 82 Compar..sup.4 7 Sn-25Pb
smooth 0.01 0.03 92 Compar. (Notes) .sup.1% Misidentification of
solder bumps in optical inspection; .sup.2Significant surface
irregularities and shrinkage cavities were observed; .sup.3Example
of this invention; .sup.4Comparative example.
[0045] As can be seen from FIG. 2 and in Table 1, solder balls made
of a conventional Sn--Cu--Ag lead-free solder in Run No. 6 showed
significant surface irregularities and had shrinkage cavities on
their surface when observed under a SEM, thereby resulting in
significantly increased percentages of occurrence of mounting
problems for solder balls and misidentification in optical
inspection of solder bumps.
[0046] In contrast, as can be seen from FIG. 1 which shows solder
balls made of a lead-free solder of Run No. 1 in accordance with
the present invention in which 0.02 at % of Co was added to the
solder of Run No. 6, surface irregularities were significantly
suppressed and no shrinkage cavities were found on the surfaces of
the solder balls, and the surfaces of the solder balls were as
smooth as those of solder balls of a conventional Sn--Pb solder
(FIG. 3). The percentages of occurrence of mounting problems and
misidentification in all the solder balls according to the present
invention (Runs Nos. 1-5) were as low as those obtained with the
conventional Sn--Pb solder in Run No. 7.
* * * * *