U.S. patent application number 10/236061 was filed with the patent office on 2004-08-05 for apparatus and process for manufacturing solder balls.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Chun, Jung-Hoon, Foulke, Richard F., Rocha, Juan C., Saka, Nannaji.
Application Number | 20040149084 10/236061 |
Document ID | / |
Family ID | 31977607 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149084 |
Kind Code |
A1 |
Chun, Jung-Hoon ; et
al. |
August 5, 2004 |
Apparatus and process for manufacturing solder balls
Abstract
An apparatus and method of forming fluxless solder balls
includes forming solder balls from a supply of solder. A coating is
formed on the solder balls for limiting naturally occurring oxide
growth on the solder balls before significant natural oxide growth
on the solder balls has occurred. The coating allows the solder
balls to be soldered without using flux.
Inventors: |
Chun, Jung-Hoon; (Sudbury,
MA) ; Foulke, Richard F.; (Stoneham, MA) ;
Rocha, Juan C.; (Sunnyvale, CA) ; Saka, Nannaji;
(Cambridge, MA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
31977607 |
Appl. No.: |
10/236061 |
Filed: |
September 5, 2002 |
Current U.S.
Class: |
75/332 |
Current CPC
Class: |
B22F 2999/00 20130101;
H05K 3/282 20130101; Y02P 70/50 20151101; H05K 3/3436 20130101;
B22F 1/16 20220101; B22F 2998/10 20130101; B23K 35/0244 20130101;
B23K 35/3605 20130101; H05K 2203/095 20130101; H05K 2203/041
20130101; B22F 2998/10 20130101; B22F 9/08 20130101; B22F 1/16
20220101; B22F 2999/00 20130101; B22F 1/16 20220101; B22F 2202/13
20130101; B22F 2998/10 20130101; B22F 1/16 20220101; B22F 9/08
20130101; B22F 2999/00 20130101; B22F 1/16 20220101; B22F 2202/13
20130101 |
Class at
Publication: |
075/332 |
International
Class: |
B22F 009/08 |
Claims
What is claimed is:
1. A method of forming fluxless solder balls comprising: forming
solder balls from a supply of solder; and forming a coating on the
solder balls for limiting naturally occurring oxide growth on the
solder balls before significant natural oxide growth on the solder
balls has occurred, the coating allowing the solder balls to be
soldered without using flux.
2. The method of claim 1 further comprising forming the solder
balls from molten solder.
3. The method of claim 2 further comprising forming the solder
balls by a droplet spray process wherein the molten solder is
caused to fall as droplets which solidify while falling to form the
solder balls.
4. The method of claim 3 further comprising forming the coating on
the solder balls while the solder balls are falling.
5. The method of claim 4 in which forming the coating on the solder
balls comprises treating the solder balls with plasma products.
6. The method of claim 5 further comprising including fluorine
within the plasma products.
7. The method of claim 6 further comprising forming an oxyfluoride
layer on the solder balls.
8. The method of claim 5 further comprising forming the solder
balls at a solder ball forming station.
9. The method of claim 8 further comprising coating the solder
balls at a coating station.
10. The method of claim 9 in which the coating station comprises a
chamber containing the plasma products therein.
11. The method of claim 10 further comprising supplying the chamber
with the plasma products from a plasma generator.
12. The method of claim 11 further comprising supplying the plasma
generator with a gas containing fluorine.
13. A method of forming fluxless solder balls comprising: forming
solder balls by a droplet spray process wherein molten solder is
caused to fall as droplets which solidify while falling to form the
solder balls; and forming a coating on the solder balls before
significant natural oxide growth on the solder balls has occurred
by treating the solder balls with plasma products while the solder
balls are falling, the coating limiting naturally occurring oxide
growth on the solder balls and allowing the solder balls to be
soldered without using flux.
14. The method of claim 13 further comprising including fluorine
within the plasma products.
15. The method of claim 14 further comprising forming an
oxyfluoride layer on the solder balls.
16. The method of claim 13 further comprising forming the solder
balls at a solder ball forming station.
17. The method of claim 16 further comprising coating the solder
balls at a coating station.
18. The method of claim 17 in which the coating station comprises a
chamber containing the plasma products therein.
19. The method of claim 18 further comprising supplying the chamber
with the plasma products from a plasma generator.
20. The method of claim 19 further comprising supplying the plasma
generator with a gas containing fluorine.
21. A method of forming fluxless solder forms comprising: forming
solder forms from a supply of solder; and forming a layer on the
solder forms for limiting naturally occurring oxide growth on the
solder forms, the layer allowing the solder forms to be soldered
without using flux.
22. An apparatus for forming fluxless solder balls comprising: a
solder ball forming station for forming solder balls from a supply
of solder; and a coating station for forming a coating on the
solder balls for limiting naturally occurring oxide growth on the
solder balls before significant natural oxide growth on the solder
balls has occurred, the coating allowing the solder balls to be
soldered without using flux.
23. The apparatus of claim 22 in which the solder ball forming
station forms the solder balls from molten solder.
24. The apparatus of claims 23 in which the solder ball forming
station forms falling droplets of molten solder which solidify
while falling to form the solder balls.
25. The apparatus of claim 24 in which the coating station is
positioned below the ball forming station for forming the coating
on the solder balls while the solder balls are falling.
26. The apparatus of claim 25 in which the coating station treats
the solder balls with plasma products.
27. The apparatus of claim 26 in which the plasma products include
fluorine for forming an oxyfluoride layer on the solder balls.
28. The apparatus of claim 26 in which the coating station
comprises a chamber containing the plasma products therein.
29. The apparatus of claim 28 further comprising a plasma generator
for producing plasma and supplying the chamber with the plasma
products.
30. The apparatus of claim 29 further comprising a gas source for
supplying the plasma generator with a gas containing fluorine.
31. An apparatus for forming fluxless solder balls comprising: a
solder ball forming station for forming falling droplets of molten
solder which solidify while falling to form solder balls; and a
coating station positioned below the solder ball forming station
for forming a coating on the solder balls by treating the solder
balls with plasma products while the solder balls are falling and
before significant natural oxide growth on the solder balls has
occurred, the coating limiting naturally occurring oxide growth on
the solder balls and allowing the solder balls to be soldered
without using flux.
32. The apparatus of claim 31 in which the coating station treats
the solder balls with plasma products.
33. The apparatus of claim 32 in which the plasma products include
fluorine for forming an oxyfluoride layer on the solder balls.
34. The apparatus of claim 32 in which the coating station
comprises a chamber containing the plasma products therein.
35. The apparatus of claim 34 further comprising a plasma generator
for producing plasma and supplying the chamber with the plasma
products.
36. The apparatus of claim 35 further comprising a gas source for
supplying the plasma generator with a gas containing fluorine.
37. A fluxless solder ball for a ball grid array comprising a
generally round ball of solder, the ball of solder having a thin
brittle oxyfluoride layer which limits naturally occurring oxide
growth on the surface of the solder ball, during soldering, the
oxyfluoride layer is capable of fracturing to allow the solder to
flow, thereby permitting soldering without using flux.
Description
BACKGROUND
[0001] The use of small solder balls positioned in ball grid arrays
for making electrical interconnections in electronic chip packages
is becoming increasingly popular. A typical ball grid array may
contain over 1000 solder balls between 12-30 mils in diameter and
50 mils apart. Such a ball grid array allows a large number of
electrical connections to be made in a small area. During
manufacturing of an electronic chip package which employs solder
balls for electrical interconnections, the solder balls are placed
in the desired ball grid array configuration upon the chip package
at the appropriate location, and then later bonded thereto as well
as to any mating surfaces by reflowing the solder balls in a reflow
oven. Prior to reflow, flux is applied for chemically removing
surface oxides from the solder balls and appropriate surfaces so
that the solder balls can be properly bonded thereto. The flux also
maintains a protective layer over the cleaned surfaces during
soldering and removes reaction products. After soldering, highly
corrosive flux residues remain behind which are later removed with
solvents in a cleaning process.
SUMMARY
[0002] The present invention provides fluxless solder forms which
may be placed in ball grid arrays on chip package substrates. The
solder forms are formed from a supply of solder. A layer or coating
is formed on the solder forms for limiting naturally occurring
oxide growth on the surface of the solder forms. The layer or
coating allows the solder forms to be soldered without using
flux.
[0003] In preferred embodiments, the solder forms are solder balls
or spheres which are formed from molten solder at a solder ball
forming station by a droplet spray process. In the droplet spray
process, the molten solder is caused to fall as droplets which
solidify while falling to form the solder balls. The layer or
coating is formed on the solder balls at a coating station while
the solder balls are falling and before significant natural oxide
growth on the solder balls has occurred. The layer or coating is
formed by treating the solder balls with plasma products including
fluorine which forms an oxyfluoride layer on the solder balls. The
coating station includes a chamber containing the plasma products
therein. The solder ball forming station is positioned at the upper
end of the chamber. The chamber is supplied with the plasma
products from a plasma generator which generates plasma from a gas
containing fluorine.
[0004] The fluxless solder forms or solder balls provided by the
present invention may eliminate the step of applying flux before
soldering and the step of removing flux residues after soldering.
This not only reduces manufacturing time but also reduces the
inventory of materials and equipment required to be on hand since
the flux and the cleaning solvents associated with the eliminated
steps as well as corresponding equipment are no longer needed. The
elimination of such steps, materials and equipment reduces the
manufacturing costs of soldering or reflowing ball grid arrays.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0006] FIG. 1 is a schematic drawing of an embodiment of the
present invention solder ball apparatus.
[0007] FIG. 2 is a schematic drawing depicting the conversion of an
oxide layer to an oxyfluoride layer on a newly formed solder ball
when treated with fluorine radicals F.sup.+.
[0008] FIGS. 3A and 3B are schematic drawings depicting the
reaction of fluorine (F) atoms with tin (Sn) atoms to form tin
oxyfluorides.
[0009] FIG. 4 is a graph depicting the atomic concentration of
oxygen at particular depths for both newly produced solder balls
and fluxless solder balls having an oxyfluoride layer.
[0010] FIG. 5 is a graph depicting the atomic concentration of lead
(Pb), tin (Sn), fluorine (F), oxygen (O) and carbon (C) relative to
depth within 63 Sn/37 Pb fluxless solder balls having an
oxyfluoride layer.
[0011] FIG. 6 is a schematic drawing of the solder droplet
generator depicted in FIG. 1.
[0012] FIG. 7 is a schematic drawing of the plasma generator
depicted in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0013] Referring to FIG. 1, solder ball apparatus 10 is an
apparatus which produces treated solder balls 16 that do not
require flux when soldering or reflowing. Apparatus 10 includes a
solder ball forming station having a solder droplet generator 12
that produces uniform droplets 47 of molten solder which solidify
while falling into solder balls 15. The solder droplet generator 12
is positioned within the interior 18a of a gas tight chamber 18 at
the upper portion of the chamber 18. Chamber 18 serves as a coating
or treating station in which the solder balls 15 are coated or
treated. A plasma generator 14 for generating plasma 13 is coupled
to the interior 18a of chamber 18 by a conduit 28 and supplies
chamber 18 with plasma products 13a for coating or treating the
falling solder balls 15. The plasma products 13a contain atomic
fluorine radicals F.sup.+ that react with the solder balls 15 to
form treated or coated solder balls 16 which have an oxyfluoride
layer or coating 19 (FIG. 2) thereon. A container 20 is positioned
at the bottom of the chamber 18 for collecting the coated or
treated solder balls 16. A fitting 24 at the bottom of chamber 18
is connected to a vacuum pump for evacuating the system.
[0014] In use, the interior 18a of chamber 18 and the interior 30a
of vessel 30 of plasma generator 14 are evacuated to remove gases
from the system through fitting 24. After evacuation, nitrogen gas
(N.sub.2) is introduced therein and then removed to facilitate the
removal of oxygen. The system may be evacuated to a pressure of
0.15 Torr and then filled with N.sub.2 gas to a pressure of 13 kPa
before being removed. The introduction and subsequent evacuation of
N.sub.2 gas may be repeated to ensure that most of the oxygen in
the chamber 18 and plasma generator 14 is removed. Once the chamber
18 and plasma generator 14 are sufficiently evacuated, the plasma
generator 14 is turned on and supplied with sulfur hexafluoride gas
(SF.sub.6) from a gas source via inlet 26. The SF.sub.6 gas enters
plasma generator 14 at a controlled rate for obtaining a desired
pressure (for example, 1.5 Torr). The plasma generator 14
dissociates the SF.sub.6 gas to produce a plasma 13 containing
highly reactive atomic fluorine radicals F.sup.+. The following
equation describes electron-impact induced molecular dissociation
of SF.sub.6 gas:
e+SF.sub.6.fwdarw.SF.sub.6-x+XF+e x.ltoreq.6. (Eq. 1)
[0015] Preferably, pure SF.sub.6 gas is supplied to plasma
generator 14 to obtain the maximum concentration of atomic fluorine
radicals F.sup.+ within the generated plasma 13 for treating solder
balls 15. N.sub.2 gas may be mixed with the SF.sub.6 gas to
accelerate the decomposition of the SF.sub.6 gas, however, this
reduces the concentration of the atomic fluorine F.sup.+ within the
plasma 13. Plasma products 13a including fluorine radicals F.sup.+
flow from the inner vessel 30 of plasma generator 14 into the
interior 18a of chamber 18 via conduit 28 which is coupled
therebetween.
[0016] The heaters 42 (FIG. 6) of solder droplet generator 12 are
turned on to melt solder 38 contained within the crucible 40 of
solder droplet generator 12. The solder droplet generator 12 is
then operated to form uniform droplets of molten solder 47 which
fall downwardly within the interior 18a of chamber 18 and solidify
into solder balls 15 while falling. As the solder balls 15 fall
within chamber 18, the solder balls 15 fall or pass through the
plasma products 13a contained therein. The fluorine radicals
F.sup.+ surround and contact the surfaces of each falling solder
ball 15. Oxides formed on the surface of the solder balls 15 in a
layer 17 (FIG. 2) are treated by the fluorine radicals F.sup.+. The
fluorine radicals F.sup.+ react with the oxides in layer 17 causing
layer 17 to undergo an oxide conversion process which transforms
the layer 17 into an oxyfluoride layer or coating 19. In this
manner, solder balls 15 become treated or coated solder balls 16.
The treated solder balls 16 are collected in container 20 which may
contain a quantity of oil 22 such as silicone oil for cooling the
solder balls 16. The treatment time of solder balls 15 may be as
little as 225 milliseconds. However, the height of chamber 18 may
be sized to provide longer treatment times.
[0017] In solder balls 15 having a typical eutectic 63 Sn/37 Pb
tin/lead solder composition, the oxide conversion process may be
generally described as a conversion from Sn/Pb oxide to Sn/Pb
oxyfluoride as follows:
SnPbO.sub.x+yF.sup.+.fwdarw.SnPbO.sub.xF.sub.y. (Eq. 2)
[0018] More specifically, tin oxides (SnO and SnO.sub.2) are
usually the main surface oxide components on untreated 63 Sn/37 Pb
solder balls 15. FIGS. 3A and 3B depict the conversion of tin
oxides on the surface of solder balls 15 into oxyfluorides by the
bonding of fluorine atoms (F) with the tin atoms (Sn). The
conversion of the tin oxides may be described as follows:
SnO.sub.x+yF.fwdarw.SnO.sub.xF.sub.y (Eq. 3)
[0019] In addition to the existence of oxyfluorides in the
oxyfluoride layer 19 of treated solder balls 16, there may also be
some tin-fluoride compounds (for example, SnF.sub.2 and
Sn.sub.2F.sub.6).
[0020] As seen in the graph of FIG. 4, the plasma product treatment
significantly reduces the atomic concentration of oxygen on the
surface of a treated solder ball 16 as well as the penetration of
the oxygen into the treated solder ball 16 in comparison to solder
balls that are not treated. For example, the atomic concentration
of oxygen on the surface of a treated solder ball 16 is about 34%
while the atomic concentration of oxygen on the surface of a newly
produced untreated solder ball 15 is about 41%. In addition, the
penetration of oxygen in a treated solder ball 16 is about 60 .ANG.
deep while the penetration of oxygen in a newly produced untreated
solder ball 15 is about 90-100 .ANG. deep. The graph of FIG. 5
depicts the relative atomic concentrations of lead (Pb), tin (Sn),
fluorine (F), oxygen (O), and carbon (C) relative to solder ball
depth for plasma product treated 63 Sn/37 Pb solder balls 16.
[0021] Once treated, the oxyfluoride layer 19 (FIG. 2) on the
treated solder balls 16 allows the treated solder balls 16 to be
reflowed or soldered in a ball grid array without the use of flux
for cleaning the solder balls 16. For purposes of description,
soldering includes reflowing. The thin oxyfluoride layer 19 formed
on the treated solder balls 16 has a structure that is sufficiently
brittle to fracture or break up into small pieces when the treated
solder balls 16 melt, allowing clean solder to flow, thereby
permitting reflow as well as proper joining. Consequently, the use
of the treated solder balls 16 can eliminate the steps of applying
flux prior to soldering and then cleaning flux residues afterwards.
The oxyfluoride layer 19 also limits subsequent oxide growth 17 on
the treated solder balls 16 so that the treated solder balls 16 can
be stored for a period of time in air (for example, about six days)
before use. After about seven days, sufficient oxide growth may
form on the treated solder balls 16 to prevent fluxless soldering.
When performing a reflow process with treated solder balls 16, a
rapid reflow which occurs in about five seconds or less provides
the best results because there is little time for sufficient oxides
to grow to hamper the reflow process. If desired, the treated
solder balls 16 can also be used with various fluxes such as water
soluble flux, no-clean flux, and rosin-based flux.
[0022] A more detailed description of solder ball apparatus 10 now
follows. Referring to FIG. 6, solder droplet generator 12 includes
a housing 35 in which the lower portion forms a crucible 40 where
solder 38 is melted and contained. Heater 42 extends around
crucible 40 for heating and melting the solder 38 contained within
crucible 40. A thermocouple 33 monitors the temperature of the
molten solder 38 for maintaining the proper temperature. For a 63
Sn/37 Pb solder composition, the molten solder 38 may be maintained
at about 235.degree. C. A piezoelectric actuator 32 is clamped
against the upper disk 36a of a vibration transmitting member 36 at
the upper portion of housing 35 by a clamping plate 37 and bolts
39. The upper disk 36a is positioned over an opening 33 at the
upper portion of housing 35 with the piezoelectric actuator 32
clamped against the top surface of the upper disk 36a. Vibration
transmitting member 36 transmits vibrations produced by
piezoelectric actuator 32 to the molten solder 38. Vibration
transmitting member 36 includes a shaft 36b extending downwardly
from upper disk 36a which is connected to a lower disk 36c. The
lower disk 36c is extended into the lower portion of crucible 40
within the molten solder 38. Vibrations produced by piezoelectric
actuator 32 are transferred downwardly through upper disk 36a and
shaft 36b to the lower disk 36c of vibration transmitting member 36
for perturbing the molten solder 38.
[0023] Pressurized gas, for example, an inert gas such as nitrogen,
argon or helium, is employed to pressurize the interior of housing
35 via inlet 34. The pressurized gas is employed for forcing molten
solder 38 from crucible 40 through the orifice 44 located at the
bottom of crucible 40. A pressure differential of only about 35 kPa
(5 lb./in..sup.2) is required to force a falling jet of molten
solder 45 from crucible 40 through orifice 44, however, a pressure
differential of between about 135-700 kPa (20-100 lb./in..sup.2) is
preferred. By vibrating piezoelectric actuator 32 at a periodic
oscillation having a wave length greater than the circumference of
the jet diameter, the falling jet 45 of molten solder 38 breaks
into a train of solder droplets 47 while falling. The droplets 47
pass through an opening 46a in a charging plate 46 located below
the crucible 40. The charging plate is provided with a voltage
which charges the falling droplets 47 by electrostatic induction to
prevent merging of the droplets 47 during flight.
[0024] The diameter of orifice 44, the pressure differential within
crucible 40 and the vibration frequency of piezoelectric actuator
32, varies depending upon the size of the solder balls 16 to be
made. For example, an orifice 44 diameter of 406 .mu.m, a pressure
differential of 34.4 kPa and a vibration frequency of 1430 Hz may
be used to produce solder balls 15 that are 760 .mu.m in diameter;
an orifice 44 diameter of 254 .mu.m, a pressure differential of
44.8 kPa, and a vibration frequency of 2582 Hz may be used to
produce solder balls 15 that are 500 .mu.m in diameter; and an
orifice 44 diameter of 178 .mu.m, a pressure differential of 68.9
kPa, and a vibration frequency of 4370 Hz may be used to produce
solder balls 15 that are 300 .mu.m in diameter. In order to obtain
a particular solder ball 15 diameter, shrinkage of the solder while
cooling is also taken into account. The different orifice 44
diameters and pressure differentials within crucible 40 provides
different initial velocities of the jet 45 of molten solder. For
example, an orifice 44 diameter of 406 .mu.m and a pressure
differential of 34.4 kPa provides an initial jet velocity of 2.8
m/s; an orifice 44 diameter of 254 .mu.m and a pressure
differential of 44.8 kPa provides an initial jet velocity of 3.2
m/s; and an orifice 44 diameter of 178 .mu.m and a pressure
differential of 68.9 kPa provides an initial jet velocity of 4.1
m/s. For producing solder balls that are small in diameter, the
higher initial jet velocities in combination with the small ball
diameters allows over a million solder balls to be produced in just
a five-minute period of time.
[0025] During the operation of solder droplet generator 12,
variations in the target diameter of the solder balls 15 may be
controlled by a closed loop control system where the size of the
falling solder droplets 47 is measured by a CCD camera and the
vibration frequency of the piezoelectric actuator 32 adjusted in
response to the measurements. Typically, the solder droplets 47 are
measured by digital image analysis where the images from the CCD
camera are transformed into a pixel array and then the pixel values
are transformed into length units. The CCD camera is calibrated to
account for any optical distortion and image amplification. Such a
control system can produce solder spheres with a size variation
smaller than .+-.2.5% of the target size.
[0026] Although solder droplet generator 12 has been shown and
described for use in solder ball apparatus 10, other suitable
molten droplet generators may be employed, for example, the devices
described in U.S. Pat. Nos. 5,266,098 and 5,431,315, the contents
of which are incorporated herein by reference in their
entirety.
[0027] Referring to FIG. 7, plasma generator 14 is a microwave
plasma generator which dissociates the SF.sub.6 gas with
microwaves. Plasma generator 14 may be formed from a microwave oven
14a within which a Pyrex.RTM. inner vessel 30 is mounted. The power
of plasma generator 14 is controlled by controls 54. An iron oxide
polymer and aluminum mesh is used to prevent microwaves from
radiating to the outside environment from plasma generator 14. Two
flow meters 48a and 48b with respective inlets 26a and 26b are
coupled in communication with inlet 26. The flow of N.sub.2 gas and
SF.sub.6 gas into inner vessel 30 is controlled by respective flow
meters 48a and 48b. A pirani type pressure gauge 52 measures the
pressure within inner vessel 30. Inlet 26 and conduit 28 include
elongate tubes 31 extending within the interior 30a of inner vessel
30 for delivering the gases to and removing the plasma products 13a
containing fluorine radicals F.sup.+ from the center of the
microwave oven 14a. Pressure gauge 52 also includes an elongated
tube 31 for measuring the pressure at this central region. The
length of conduit 28 is kept to a minimum and is coupled to chamber
18 at a position for quickly delivering plasma products 13a to the
falling solder balls 15 in order to minimize the recombination of
the fluorine radicals F.sup.+ with neutral species before
contacting the solder balls 15.
[0028] A stable plasma 13 may be generated by plasma generator 14
at a power of 1000 watts, a frequency of 2.45 GHz and an SF.sub.6
gas pressure of 0.15 to 5 Torr. Typically, microwave power above
600 watts provides maximum dissociation of SF.sub.6 gas. The higher
SF.sub.6 gas pressures are preferred to provide a higher
concentration of atomic fluorine F.sup.+. A high concentration of
atomic fluorine is desirable to ensure sufficient treatment of
solder balls 15 because the treatment time of the falling solder
balls 15 is very short. The pressure of the plasma 13 within the
system may be controlled by the SF.sub.6 gas flow rate. For
example, a plasma 13 pressure of 0.8 Torr may be obtained by a
SF.sub.6 gas flow rate of 100 SCCM (standard cubic centimeters per
minute), a plasma 13 pressure of 1 Torr may be obtained by a
SF.sub.6 gas flow rate of 476 SCCM, a plasma 13 pressure of 1.5
Torr may be obtained by a SF.sub.6 gas flow rate of 985 SCCM, and a
plasma 13 pressure of 3 Torr may be obtained by a SF.sub.6 gas flow
rate of 1302 SCCM.
[0029] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
[0030] For example, although solder ball apparatus 10 is depicted
to position solder ball generator 12 in a chamber 18 that receives
plasma products 13a containing fluorine radicals F.sup.+ from
plasma generator 14 via conduit 28, alternatively, plasma generator
14 may be modified to house solder ball generator 12 within vessel
30 so that both the plasma 13 and the solder balls are produced in
vessel 30. In such a case, solder ball generator 12 must be
shielded from the microwaves. In addition, although plasma
generator 14 is described as a microwave plasma generator,
alternatively, plasma generator 14 may generate plasma by other
suitable methods, such as by radio frequency. Furthermore, although
conduit 28 preferably is mounted to the top of chamber 18 for
delivering plasma products 13a into chamber 18, alternatively,
conduit 28 may be mounted at other suitable locations, with the
vacuum fitting 24 also being positioned in an appropriate position
relative to conduit 28 to maintain an adequate plasma concentration
in the chamber 18. Although the present invention has been
described for treating solder balls having a composition of 63
Sn/37Pb, it is understood that solder balls of other compositions
may be treated, such as 10 Sn/90 Pb solder balls. Also, chamber 18
may be employed for treating solder balls that have been previously
formed. The solder balls may be loaded into a feed device which
drops the solder balls through the plasma product 13a filled
chamber 18. The solder balls may be stored in an inert environment
before and/or after treatment. The container 20 for collecting
treated solder balls may be replaced by a conveyance system.
Finally, solder forms not necessarily spherical in shape may also
be formed and treated with plasma products 13a.
* * * * *