U.S. patent application number 10/170408 was filed with the patent office on 2003-01-02 for lead-free solder balls and method for the production thereof.
Invention is credited to Kato, Rikiya, Nomoto, Shinichi, Okada, Hiroshi.
Application Number | 20030003011 10/170408 |
Document ID | / |
Family ID | 19021482 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030003011 |
Kind Code |
A1 |
Kato, Rikiya ; et
al. |
January 2, 2003 |
Lead-free solder balls and method for the production thereof
Abstract
Lead-free solder balls having a good surface appearance with no
appreciable surface defects such as seams and shrinkage cavities
comprises an alloy having a composition consisting essentially of
about 4.0% to about 6.0% by weight of Ag, about 1.0% to about 2.0%
by weight of Cu, and a balance of Sn, and they have a diameter of
from 0.05 mm to 1.0 mm. The solder balls can be produced by forming
a molten alloy having the above-described composition into
solidified balls having a diameter of from 0.05 mm to 1.0 mm using
the surface tension of the molten alloy.
Inventors: |
Kato, Rikiya; (Souka-shi,
JP) ; Nomoto, Shinichi; (Hanyu-shi, JP) ;
Okada, Hiroshi; (Souka-shi, JP) |
Correspondence
Address: |
MICHAEL TOBIAS
#40
1717 K ST. NW, SUITE 613
WASHINGTON
DC
20036
US
|
Family ID: |
19021482 |
Appl. No.: |
10/170408 |
Filed: |
June 14, 2002 |
Current U.S.
Class: |
420/560 ;
228/246; 228/259 |
Current CPC
Class: |
B22F 2009/0864 20130101;
B23K 35/262 20130101; C22C 1/04 20130101; C22C 13/00 20130101; B22F
9/08 20130101; B23K 35/0244 20130101; H05K 3/3436 20130101; H05K
3/3463 20130101; B22F 2009/0804 20130101 |
Class at
Publication: |
420/560 ;
228/259; 228/246 |
International
Class: |
B23K 035/12; C22C
013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2001 |
JP |
2001-181173 |
Claims
What is claimed is:
1. Lead-free solder balls for forming solder bumps on a substrate,
the solder balls comprising an alloy having a composition
consisting essentially of about 4.0% to about 6.0% by weight of Ag,
about 1.0% to about 2.0% by weight of Cu, and a balance of Sn, the
solder balls having a diameter of from about 0.05 mm to about 1.0
mm.
2. The lead-free solder balls of claim 1 wherein the alloy has a
composition consisting essentially of about 5.7% by weight of Ag,
about 1.3% by weight of Cu, and a balance of Sn.
3. The lead-free solder balls of claim 1 wherein the solder balls
have a good surface appearance which is substantially free of
surface defects which may interfere with the use of the solder
balls.
4. A method for producing lead-free solder balls comprising forming
a molten alloy having a composition which consists essentially of
about 4.0% to about 6.0% by weight of Ag, about 1.0% to about 2.0%
by weight of Cu, and a balance of Sn into solidified balls having a
diameter of from about 0.05 mm to about 1.0 mm using the surface
tension of the molten alloy.
5. The method of claim 4 including dropping pieces of a solder
alloy into an hot oil bath to allow the pieces to melt and allowing
the molten pieces to descend within the bath to form them into
solidified balls.
6. The method of claim 5 wherein the hot oil bath has a vertical
temperature gradient with a temperature in an upper portion thereof
which is high enough to melt the solder alloy and a temperature in
a lower portion thereof which is low enough to solidify the molten
alloy.
7. The method of claim 4 wherein forming the molten alloy into
solidified balls comprises dripping the molten alloy through an
orifice or nozzle.
Description
FIELD OF THE INVENTION
[0001] This invention relates to fine solder balls, and
particularly to fine lead-free solder balls and a method for their
production.
BACKGROUND ART
[0002] Multi-functional electronic components such as SOP's (small
outline packages) and QFP's (quad flat packages) have leads on
opposite long sides or surrounding four sides of their relatively
flat bodies. However, with an increase in integration in a single
semiconductor chip, the number of leads capable of being arranged
in such a component was sometimes insufficient to allow the chip to
exert all of its possible functions. Thus, electronic components
having a much larger number of leads than attainable with SOP's and
QFP's were desired. An increase in the number of leads was realized
by a technique in which leads are arranged in a grid array on the
back surface of a substrate on which a semiconductor chip is
mounted to form an electronic component, i.e., on the surface of
the substrate facing away the semiconductor chip. Examples of such
electronic components having a large number of leads on the back
surface of their substrates are BGA's (ball grid array packages)
and CSP's (chip size packages).
[0003] Normally a BGA or CSP (hereinafter referred to merely as a
BGA for simplicity) is mounted on a printed (wiring) board by use
of solder balls, which are fine balls of a solder alloy. In
practice, it is impossible to dispense and position solder balls at
the time of mounting a BGA on a printed board. Therefore, solder
balls are normally secured on the back surface of the substrate of
a BGA by making them adhere to the electrode pads or lands formed
on that surface of the BGA substrate to form solder bumps in a grid
array. A BGA having the thus formed solder bumps is soldered to a
printed board typically by applying a solder paste to the printed
board, bringing the solder bumps of the BGA into contact with the
surface of the printed board on which the solder paste is applied,
and heating the resulting assembly in a reflow furnace so as to
melt the solder paste and the solder bumps for soldering.
[0004] Solder balls which have conventionally been used to form
bumps on BGA's are made of an Sn--Pb alloy which has an
approximately eutectic composition of about 63% by weight of Sn and
a balance of Pb and which has a melting point of 183.degree. C.
Sn--Pb alloys are excellent as a solder since they have low melting
temperatures and good wetting power in a molten state, so they
minimize the occurrence of soldering defects.
[0005] Recently, however, the use of Sn--Pb solders has been
disfavored due to the toxic nature of Pb. When waste electronic
devices are disposed of, they are often disassembled to remove
plastic and metallic parts for recycling. Printed boards on which
electronic components are mounted are not adapted to recycling
since plastic and metallic portions are combined complicatedly
therein, so they are removed from waste electronic devices,
shredded, and buried in the ground. When rain which has been
acidified (i.e., acid rain) contacts the shredded printed 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 a concern of lead accumulating in its body to cause lead
poisoning.
[0006] Accordingly, it has been recommended in the art to use a
"lead-free" solder, which is completely free from lead, in
soldering of electronic components.
[0007] Most lead-free solders are made of Sn-based alloys such as
Sn--Ag, Sn--Cu, Sn--Bi, and Sn--Zn alloys which may optionally
contain one or more additional elements such as those selected from
Ag, Cu, Zn, In, Ni, Cr, Fe, Ge, and Ga. Thus, there are a wide
variety of lead-free solders which have their respective advantages
and disadvantages, and an appropriate lead-free solder is selected
depending on the application.
[0008] For lead-free solder balls which are used to form solder
bumps on BGA's, an Sn--Ag--Cu alloy is suitable in terms of
properties such as solderability, bonding strength, and thermal
fatigue resistance. In particular, an Sn--3Ag--0.5Cu alloy
(containing 3% Ag, 0.5% Cu, and a balance of Sn on a weight basis)
having a liquidus temperature of about 220.degree. C. is mostly
used.
[0009] However, solder balls made of an Sn--3Ag--0.5Cu alloy have a
problem that they often have surface defects such as shrinkage
cavities and seams, which are formed while the solder balls are
solidified during their production. Shrinkage cavities are pores
opening onto the surface of a solder ball and extending deep inside
the balls, while seams are streaky surface irregularities like
wrinkles (protrusions and indentations) found numerously on the
surface of a solder ball.
[0010] With solder balls having shrinkage cavities, the pore-like
shrinkage cavities are closed by molten solder when the solder
balls are heated to melt for the first time, i.e., so as to form
solder bumps on the back surface of a BGA substrate, thereby
leaving air confined within the closed shrinkage cavities. Thus,
the resulting solder bumps have closed air cells therein, and they
may form soldered joints having voids during soldering of the BGA
to a printed board, thereby causing the joints to have a decreased
bonding strength.
[0011] Solder balls having seams cause a problem when they are
delivered to the back surface of a BGA substrate and located on
each land formed on the back surface. In the production of BGA's,
the delivery of solder balls is normally performed using a ball
feeder which includes a suction plate to grasp solder balls, but
seams on the surface of solder balls may prevent the balls from
being grasped by the suction plate as described more fully
below.
[0012] The suction plate of a ball feeder has through holes with a
diameter slightly smaller than that of solder balls to be grasped
thereby. These holes are situated in exactly the same grid array as
that of the lands formed on the back surface of a BGA substrate on
which solder bumps are to be formed. The interior of the suction
plate is evacuated to generate a suction force sufficient to grasp
solder balls in all the holes. The suction plate which grasps
solder balls in all the holes is then moved over a BGA substrate
placed upside down (with its back surface facing upward). The BGA
substrate has been treated by applying an adhesive soldering flux
to the grid array spots (electrodes or lands) of its back surface
on which solder bumps are to be formed. After the suction plate is
positioned so that its holes coincide with the grid array spots of
the BGA substrate, the suction plate is moved toward the BGA
substrate until the solder balls grasped by the suction plate come
into contact with the flux on the BGA substrate. The solder balls
are then released by an appropriate technique such as injection of
air through the holes of the suction plate or application of an
impact to the suction plate, thereby causing the solder balls to
adhere to the flux. Thereafter, the BGA substrate having solder
balls placed thereon is heated in a reflow furnace or similar
heating device to melt the soldering flux and the solder balls and
form solder bumps in a grid array on the substrate.
[0013] If solder balls having seams are used in the above-described
process, it cannot be guaranteed that they are grasped by all the
holes of the suction plate, since air can pass through the gaps
formed between the balls and holes due to the irregular surfaces of
the balls having seams, and thus the suction force generated by the
suction plate is attenuated. As a result, some holes of the suction
plate may be vacant by a failure to grasp or maintain a solder ball
in the holes, and thus the ball feeder may fail to deliver solder
balls to the spots of the BGA substrate corresponding to the vacant
holes, resulting in the production of a BGA having no solder bumps
in some spots after heating is performed to melt the solder balls.
If a BGA is missing a soldering bump even in only one spot in its
grid array, it will not be able to perform its function
successfully. For this reason, it is critical that solder balls be
free from seams.
[0014] Solder balls having seams cause another problem during the
inspection of solder balls performed in the above-described process
to confirm that a suction plate of a ball feeder has a solder ball
in each hole of the plate before solder balls are delivered on a
BGA substrate, or subsequently to confirm that the BGA substrate
has a solder ball on each spot of its grid array before the
substrate is heated for soldering. Such inspection is normally
carried out by image processing using a photo detector which
detects the light reflected by a solder ball as an indication of
the presence of a ball. Seams on a solder ball may diffuse the
reflected light to such an extent that the reflected light reaching
the photo detector is insufficient for detection of the ball. As a
result, although a solder ball is actually present in a hole of the
suction plate or on a spot of the BGA substrate, the photo detector
may be unable to detect the presence of the ball and mistakenly
determine the hole or spot to be vacant.
SUMMARY OF THE INVENTION
[0015] The present invention provides lead-free solder balls having
a good surface appearance which is substantially free of surface
defects such as shrinkage cavities and seams which may interfere
with the use and in particular, delivery or inspection of the
solder balls, as described above.
[0016] The present invention also provides a method for producing
such solder balls.
[0017] The present inventors found that an Sn-Ag-Cu alloy having a
composition in a specific range makes it possible to produce solder
balls having a good surface appearance. In other words, it was
found that the surface appearance of solder balls of an Sn--Ag--Cu
alloy differs significantly depending on the alloy composition,
since a conventional Sn--3Ag--0.5Cu alloy forms solder balls having
shrinkage cavities and seams as described above.
[0018] The present invention provides lead-free solder balls having
a good surface appearance and comprising an alloy having a
composition consisting essentially of about 4.0% to about 6.0% by
weight of Ag, about 1.0% to about 2.0% by weight of Cu, and a
balance of Sn, the solder balls having a diameter of from about
0.05 mm to about 1.0 mm.
[0019] Preferably, the alloy has a composition consisting
essentially of about 5.7% by weight of Ag, about 1.3% by weight of
Cu, and a balance of Sn.
[0020] The present invention also provides a method for producing
lead-free solder balls comprising forming a molten alloy having a
composition which consists essentially of about 4.0% to about 6.0%
by weight of Ag, about 1.0% to about 2.0% by weight of Cu, and a
balance of Sn into solidified balls having a diameter of from about
0.05 mm to about 1.0 mm. The solidified balls are formed by the
action of the surface tension of the molten alloy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is an electron micrograph showing lead-free solder
balls of an Sn--5.7Ag--1.3Cu alloy according to the present
invention; and
[0022] FIG. 2 is an electron micrograph showing conventional
lead-free solder balls of an Sn--3Ag--1.0Cu alloy.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Lead-free solder balls according to the present invention
have a diameter of from about 0.05 mm to about 1.0 mm since such a
diameter is generally used to form solder bumps on BGA substrates
and CSP substrates. The solder balls also may be used for flip chip
bonding of a semiconductor chip to a substrate, if they have a
diameter which is small enough for such bonding.
[0024] The solder balls have an alloy composition consisting
essentially of about 4.0% to about 6.0% by weight of Ag, about 1.0%
to about 2.0% by weight of Cu, and a balance of Sn. They have a
good surface appearance which is free from appreciable shrinkage
cavities and seams which may interfere with the function or
performance of the solder balls.
[0025] Such a composition gives an alloy having a liquidus
temperature around 260.degree. C., which is higher than that of an
Sn--3Ag--0.5Cu alloy but is still suitable for soldering BGA's and
CSP's to printed boards as long as the electronic components
mounted on the BGA's or CSP's are heat-resistant and can withstand
such a temperature. If the electronic components are sensitive to
such a high temperature, the BGA's or CSP's having solder bumps
formed from solder balls according to the present invention may be
soldered to printed boards by heating to a lower temperature
sufficient to melt the solder paste applied to the printed boards.
In this case, the solder bumps on the BGA's or CSP's do not melt
and they are soldered to the wiring boards only with the solder
paste.
[0026] Preferably the Ag content is from about 5.0% to about 6.0%
by weight and the Cu content is from about 1.0% to about 1.6% by
weight. Most preferably the alloy composition consists essentially
of about 5.7% by weight of Ag, about 1.3% by weight of Cu, and a
balance of Sn in order to provide solder balls with the most
improved surface appearance.
[0027] The solder balls according to the present invention can be
produced by a method which comprises preparing a molten alloy
having the above-described composition and forming the molten alloy
into balls by any technique capable of forming balls having a
uniform diameter using the action of the surface tension of the
molten alloy.
[0028] Examples of such a ball forming technique include an oil
method as disclosed in U.S. Pat. No. 5,653,783 and JP-A 7-300606
and a direct method as disclosed in U.S. Pat. No. 5,445,666,
although other techniques may be employed.
[0029] In the oil method, a wire of a solder alloy having a
predetermined composition is prepared and cut into sections having
a given length. The wire sections are individually dropped into an
oil bath having a vertical temperature gradient in which the
temperature in an upper portion is higher than in a lower portion,
whereby the sections are allowed to melt in the upper portion and
then solidify while they are falling in the oil bath. Thus, the
upper portion has a temperature high enough to melt the solder
alloy and the lower portion has a temperature low enough to
solidify the molten solder alloy.
[0030] In the direct method, a molten solder alloy 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 in a gas stream flowing against the
gravity or on a rotating plate, for example.
[0031] In both of these methods, the resulting solder balls have a
spherical shape due to the action of the surface tension of the
molten alloy itself. If a conventional lead-free solder alloy of
Sn--3Ag--0.5Cu is used in either of these methods, or in any other
method in which balls are formed by the action of the surface
tension of a molten alloy, it will result in the formation of
solder balls having the above-described shrinkage cavities and
seams which are formed during solidification of the balls.
[0032] In contrast, according to the present invention, the use of
an Sn--Ag--Cu solder alloy having a different composition
containing about 4.0-6.0% Ag and about 1.0-2.0% Cu makes it
possible to prevent the formation of shrinkage cavities and seams
during solidification and to form solder balls having a good
surface appearance, as demonstrated in the example given below.
When these solder balls are used to form solder bumps on BGA's, it
is possible to avoid a decrease in bonding strength due to the
formation of voids which is attributable to shrinkage cavities in
the solder balls, so the resulting solder bumps have good
reliability. Since the solder balls have a good gloss with no
appreciable seams, they can be delivered and positioned
successfully onto BGA substrates by a suction plate of a ball
feeder to ensure that there are no vacant holes in the suction
plate which have not grasped or retained a solder ball. Such a good
surface appearance of the solder balls also enables them to be
inspected correctly by image processing without misidentification
due to diffusion of the light reflected by the balls.
[0033] 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.
[0034] In the examples, an oil method is used to produce solder
balls of lead-free solder alloys. The oil method is performed in a
silicone oil bath in a vertical cylindrical vessel having a funnel
structure with a stopcock at its bottom. The vessel has a coil
heater wound on its exterior wall in an upper portion of the
vessel. The level of the silicone bath is slightly above the coil
heater. Thus, the silicone bath is heated by the coil heater in
such a manner that the upper or heated portion of the bath has a
temperature range which is higher than the melting point of the
solder alloy used or which is high enough to melt the solder alloy,
and an unheated lower portion has a temperature which is lower than
the melting point or which is low enough to solidify the molten
alloy. Preferably the temperature in the lower portion is slightly
above ambient temperature at least for the lowermost portion of the
oil bath. If necessary, the lower portion of the vessel may be
cooled entirely or partly by a suitable cooling means such as a
water jacket.
[0035] A fine wire of a solder alloy is cut into sections having a
given length which is selected so as to provide a ball of a
predetermined diameter upon melting. The wire sections of the
solder alloy are dropped one by one onto the silicone oil bath to
allow them to descend within the oil bath by gravity. While falling
in the oil bath slowly due to the viscosity of the silicone oil,
the wire sections initially melt in the upper heated portion of
higher temperatures and transform into a spherical shape by the
action of the surface tension of the molten alloy. The resulting
molten solder particles continue to fall toward the lower portion
while keeping their spherical shape, and they solidify in the lower
portion in which the temperatures are below the melting point of
the solder alloy. The solidified spherical solder particles, i.e.,
solder balls accumulate at the bottom of the silicone oil bath. The
solder balls are recovered from the vessel by opening the stopcock,
and any silicone oil remaining thereon is removed by washing with a
detergent solution followed by rinsing with water.
EXAMPLE 1
[0036] A solder alloy having a composition of 5.7% by weight of Ag,
1.3% by weight of Cu, and a balance of Sn and having a melting
temperature of 256.degree. C. was subjected to wire drawing to
obtain a wire having a diameter of 0.2 mm. The alloy wire was cut
into sections having a length of 2 mm.
[0037] One hundred (100) of the wire sections of the solder alloy
were dropped one by one onto the above-described silicone oil bath
which was heated to 280.degree. C. in an upper portion thereof. The
temperature of a lower portion of the oil bath was 25-26.degree. C.
although no cooling means was employed. Solder balls having a
diameter of 0.5 mm were thus produced.
[0038] When these solder balls were observed under an electron
microscope, they had a good surface gloss, and no shrinkage
cavities or seams were found on their surfaces, as shown in FIG.
1.
COMPARATIVE EXAMPLE 1
[0039] Following the procedure described in Example 1, one hundred
solder balls were produced from a solder alloy having a composition
of 3.0 by weight of Ag, 1.0% by weight of Cu, and a balance of Sn
and having a melting temperature of 234.degree. C.
[0040] When these solder balls were observed under an electron
microscope, seams were found on the surfaces of most of the balls
as shown in FIG. 2. In addition, many of them had shrinkage
cavities along with seams.
[0041] Although the present invention has been described with
respect to preferred embodiments, they are mere illustrative and
not intended to limit the present invention. It should be
understood by those skilled in the art that various modifications
of the embodiments described above can be made without departing
from the scope of the present invention as set forth in the
claims.
* * * * *