U.S. patent application number 09/907862 was filed with the patent office on 2003-01-23 for gas dilution method and apparatus.
Invention is credited to Pennington, Michael A..
Application Number | 20030015220 09/907862 |
Document ID | / |
Family ID | 25424768 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015220 |
Kind Code |
A1 |
Pennington, Michael A. |
January 23, 2003 |
Gas dilution method and apparatus
Abstract
A method and apparatus for preparing dilute HF for fluxless
soldering, to provide very dilute anhydrous hydrogen fluoride to
effect surface modification of the parts to be soldered. There are
six main parts of the apparatus: a vacuum sub-system; a source gas
sub-system; a dilution gas sub-system; a source gas/dilution
make-up sub-system; a diluted gas storage sub-system; and a control
system.
Inventors: |
Pennington, Michael A.;
(Cary, NC) |
Correspondence
Address: |
Lynn E. Barber
P.O. Box 16528
Fort Worth
TX
76162
US
|
Family ID: |
25424768 |
Appl. No.: |
09/907862 |
Filed: |
July 18, 2001 |
Current U.S.
Class: |
134/2 |
Current CPC
Class: |
B23K 35/383 20130101;
B23K 35/38 20130101; B23K 35/3605 20130101 |
Class at
Publication: |
134/2 |
International
Class: |
C23G 001/00 |
Claims
What is claimed is:
1. A method for fluxless pretreatment of a substrate comprising
exposing parts to be soldered to a gas phase pretreatment of
diluted anhydrous hydrogen fluoride in a dry carrier gas, said
hydrogen fluoride being diluted sufficiently to avoid metallic
etching during the pretreatment.
2. The method of claim 1, further comprising adjusting the dilution
of the anhydrous hydrogen fluoride based on the substrate being
pretreated and on ambient environmental conditions.
3. The method of claim 1, wherein the concentration of the diluted
anhydrous hydrogen fluoride is from 30 ppm to 100,000 ppm and
preferably in the range 300 ppm to 30,000 ppm.
4. The method of claim 1, wherein the dry carrier gas is an inert
gas.
5. The method of claim 4, wherein the dry carrier gas is selected
from the group consisting of dry air, dry nitrogen gas, and dry
argon.
6. The method of claim 1, wherein the carrier gas has a dew point
below -40.degree. C.
7. The method of claim 1, wherein the pressure during pretreatment
is at or near atmospheric pressure.
8. An apparatus for the delivery and dilution of hydrogen fluoride
to a processing tool, comprising: (a) a vacuum sub-system for
removing residual gases from the apparatus; (b) a source gas
sub-system for supplying anhydrous hydrogen fluoride; (c) a
dilution gas sub-system for delivering an inert dilution gas; (d) a
source gas/dilution make-up sub-system for delivering the diluted
anhydrous hydrogen fluoride gas mixture to a first storage tank and
to a second storage tank to form a controlled diluted anhydrous
hydrogen fluoride gas mixture, and for maintaining the controlled
gas mixture in the second storage tank; (e) a diluted gas storage
sub-system for containing and delivering the diluted anhydrous
hydrogen fluoride gas mixture to the process tool to be soldered or
otherwise treated; and (f) a control system for controlling
pressure within the apparatus, flow of gases, and vacuum evacuation
of the apparatus.
9. The apparatus of claim 8, wherein the diluted gas storage
sub-system comprises a flow control valve to deliver highly diluted
hydrogen fluoride to the process tool.
10. The apparatus of claim 8, wherein the vacuum sub-system
comprises a mechanically operated vacuum pump.
11. The apparatus of claim 8, wherein the inert dilution gas is
selected from the group consisting of dry air, dry nitrogen, and
dry argon.
12. The apparatus of claim 8, wherein the vacuum sub-system further
comprises a vacuum source and a supply of gas for purging the
apparatus.
13. The apparatus of claim 12, wherein the gas for purging the
apparatus is selected from the group consisting of dry air, dry
nitrogen, and dry argon.
14. The apparatus of claim 8, wherein the concentration of the
diluted anhydrous hydrogen fluoride gas is from 30 ppm to 100,000
ppm and preferably in the range 300 ppm to 30,000 ppm.
15. The apparatus of claim 8, wherein the carrier gas has a dew
point below -40.degree. C.
16. The apparatus of claim 8, wherein the entire apparatus is
provided with a heating means so that the apparatus may operate in
a temperature range from ambient to 120 degrees C. in order to aid
in maintaining a dry condition within the apparatus.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This invention is disclosed in Disclosure Document No.
460022, filed Jul. 28, 1999, with the U.S. Patent Office.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to methods for fluxless soldering,
and in particular, relates to gas dilution method and apparatus
allowing fluxless soldering with very dilute hydrogen fluoride.
[0004] 2. Description of the Related Art
[0005] Fabrication and assembly of electronic circuits requires
that certain components of the circuit, such as integrated circuits
(chips) or other components such as resistors, capacitors, etc., be
in electrical contact with each other. They are for this reason
mounted on substrates such as printed wiring boards so that the
chip and substrate make electrical contact and are held together
with the substrate supporting the component.
[0006] Soldering processes for connecting the components and the
substrate generally involve pre-cleaning and deoxidation of surface
oxides, generally using a liquid flux material, such as a weak
organic acid applied prior to soldering and activated during the
soldering process (e.g., at 230.degree. C.) in a wave solder bath
to prepare the surface to be soldered by removing contaminants
including metal oxides from the solder surface. After the
pre-cleaning, the next step of solder reflow and/or reflow joining
can be performed after all oxides are removed from the solder
surface because the oxides prevent the wetting of the two surfaces
to be joined by the soldering reflow. When solder is heated it
reflows and joins the surfaces which are in contact with solder.
Finally, post-soldering cleaning removes the flux residue. This
step is particularly difficult due to the small size of typical
electronic components, and the difficulty of getting the cleaning
agents to reach the minute areas between components.
[0007] Numerous investigators have attempted to eliminate the flux
requirement for soldering. Fluxless soldering, or soldering without
use of a liquid flux, replaces the first step with an alternate
treatment, and can eliminate the third cleaning step. One example
of fluxless soldering methods is the process of Dishon and Bobbio
which utilizes plasma excitation using innocuous fluorinated gases,
such as SF.sub.6 or CF.sub.4, to remove surface oxides from solder
surfaces (U.S. Pat. No. 4,921,157). It is believed that these and
other fluorinated gases of non-reactive substances were used as a
source of plasma dissociated fluorine atoms, so that these atoms
would react with and fluorinate the tin oxide (SnO.sub.2) surface
layer on the solder. Once this reaction occurs, the solder surface
tension is sufficiently lowered so that joining is possible. In
this method, the joining or reflow may be done any time within two
weeks of the fluorination. The disclosure of this patent and all
other patents and publications referred to herein is incorporated
herein by reference.
[0008] Other more recent examples of fluxless soldering include
wave soldering (U.S. Pat. No. 5,044,542 of Deambrosio), palladium
enhanced fluxless soldering (U.S. Pat. No. 5,048,744 of Chang et
al.), use of various inert gases (U.S. Pat. No. 5,139,193 of Todd),
use of reducing agents such as lithium, calcium, strontium and
cesium (U.S. Pat. No. 5,139,704 of Holland et al.), use of a laser
beam (U.S. Pat. No. 5,164,566 of Spletter et al.), use of COHF
(U.S. Pat. No. 5,071,486 of Chasteen), use of a heated reducing or
non-reactive gas (U.S. Pat. No. 5,205,461 of Bickford et al.), and
use of carbon-fluoride compositions (U.S. Pat. No. 5,380,557 of
Spiro).
[0009] Most relevant to this invention is U.S. Pat. No. 5,609,290
of Bobbio et al. where it was found that use of HF, having very
strong internal bonds and not dissociating easily to yield free
fluorine, is an efficient method of fluxless soldering and provides
a surface layer on the solder which allows fluxless reflow. In this
work, anhydrous HF is reflowed directly over the parts to be
soldered, without dilution (e.g., about 100% HF). Prior to the
invention herein, it was thought that a high concentration of HF
was required for enabling fluxless soldering after HF
treatment.
[0010] There are problems, however, with using concentrated HF,
including problems with disposal of this concentrated toxic gas,
and potential problems due to the need to prevent leakage of the
effluent to the environment. Also, at high HF concentrations, if
there is moisture present, undesired etching of silicon oxide
(SiO.sub.2) or other metal oxides occurs. These oxides may be
present on the parts to be soldered. Although water vapor can be
minimized in an open environment by making sure the gases and
treatment environment are dry plus the use of mild heating in the
treating environment, it cannot be completely eliminated, and
therefore oxide etching is a usual problem with the use of a high
HF concentration as is known in the art.
[0011] It is therefore an object of the invention to provide a
method and apparatus for fluxless soldering which uses anhydrous
hydrogen fluoride (HF) at a very high dilution for surface
modification of the parts to be soldered.
[0012] It is a further object of the invention to provide a method
and apparatus that achieves the dilution of anhydrous HF in a safe
and controlled manner.
[0013] Other objects and advantages will be more fully apparent
from the following disclosure and appended claims.
SUMMARY OF THE INVENTION
[0014] The invention herein is a method and apparatus for preparing
dilute HF for fluxless soldering, comprising providing very dilute
anhydrous hydrogen fluoride to effect surface modification of the
parts to be soldered. There are six main parts of the apparatus: a
vacuum sub-system; a source gas sub-system; a dilution gas
sub-system; a source gas/dilution make-up sub-system; a diluted gas
storage sub-system; and a control system.
[0015] Other objects and features of the inventions will be more
fully apparent from the following disclosure and appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic diagram of the apparatus of the
invention.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
THEREOF
[0017] The method of the present invention provides a process for
fluxless soldering which uses anhydrous hydrogen fluoride (HF) at a
very high dilution for surface modification of parts that are to be
soldered, so that the parts are solderable without the use of flux.
The apparatus and method of the invention allow the anhydrous HF
gas bottle to be isolated from the system except during short
periods when the system is recharging its storage tank.
[0018] The method for fluxless soldering of the invention comprises
exposing the parts to be soldered to a gas phase pretreatment of
diluted anhydrous hydrogen fluoride in a dry carrier gas. The
preferred concentration of anhydrous hydrogen fluoride is from
about 30 ppm to about 100,000 ppm, with a preferred range of about
300 ppm to 30,000 ppm (0.03% to 3% HF). The amount of dilution of
the HF that is optimum depends upon the avoidance of etching (or
otherwise damaging) any metallic oxides that may be present
consistent with efficacious fluxless treatment of the solder.
Because the parts to be treated bring different amounts of water
into the treatment region, the optimum amount of dilution therefore
depends on the type of substrate or components being treated with
the HF. Thus, the treatment of parts containing relatively large
quantities of entrapped moisture within their structures (for
example, porous material like FR4 fiberglass circuit board
substrate material) shows a tendency to etch metallic oxides at
lower HF gas concentrations than in the case of less porous
materials, such as silicon substrates. The etching effect is not
linear and for a given set of samples operation below a certain
concentration leads to virtually no etching while operation above
that level produces significant damage. Neither the fact that
samples of practical interest (e.g. functional circuit boards)
bring a damaging amount of water into the treatment region nor the
fact that dilution of the hydrogen fluoride could effectively
eliminate the damage were anticipated in prior work (Bobbio et al.,
U.S. Pat. No. 5,609,290).
[0019] The amount of dilution that may be used also depends on the
ambient environmental conditions, such as relative humidity. In one
instance, utilizing the method of the invention herein to supply
diluted HF to a conveyor-driven atmospheric pressure treatment oven
with dry dilution gas where glass epoxy circuit boards were also
present, there was no metallic oxide etching at anhydrous HF
concentrations up to 30,000 ppm. In a case where only test samples
of silicon oxide on silicon wafers were present and dry dilution
gas was used there was no etching at levels up to 100,000 ppm.
Beyond this level, metallic oxide etching has always been observed.
The metallic oxide etch rate determines the absolute maximum
practical level of HF as well as the maximum within the suggested
range of operation. The minimum levels are determined by the
observation that the absolute minimum for which the treatment has
led to fluxless reflow was 30 ppm on solder samples with no circuit
boards present and 300 ppm for solder coated on glass epoxy
boards.
[0020] The dry carrier gas is an inert gas, preferably dry air, dry
nitrogen gas, dry argon or other inert gas preferably at a dew
point below -40.degree. C. The pressure during pretreatment is
preferably at or near atmospheric pressure.
[0021] It is very important that the preparation of the diluted HF
of the invention be controlled carefully and automatically, so that
the anhydrous HF gas source for the fluxless treatment apparatus
may be safely operated in a manufacturing environment. The method
of building the apparatus and the method of operation of the
apparatus to dilute the concentrated gas and deliver the diluted
mixture to the fluxless processing tool are designed with this in
mind.
[0022] Absolute pressure transducers as are known in the art are
used to measure and control the diluted anhydrous HF delivery
pressure and the anhydrous HF gas concentration. Adjusting the
partial pressure of the anhydrous HF and dilution gas allows
precise concentration control. One unique feature of this invention
is that the highly concentrated HF is only connected to the process
system for short periods of time. This feature provides a measure
of safety due to the fact that there is only a limited source of
potentially hazardous gas. In addition, using a flexible control,
the concentration of the process gas can be controlled precisely
between 0 to 100% concentrations.
[0023] System components. FIG. 1 shows the HF gas delivery
apparatus of the invention. There are six main parts of the
apparatus: (A) the vacuum sub-system; (B) the source gas
sub-system; (C) the dilution gas sub-system; (D) the source
gas/dilution makeup sub-system; (E) the diluted gas storage
sub-system; and (F) the control sub-system. The apparatus of the
invention provides diluted HF which can then be delivered to a
process tool, such as a soldering system (not shown) as is known in
the art.
[0024] The vacuum sub-system (A) removes residual gases from both
storage tanks (described in more detail below) and the gas lines.
This insures a repeatable starting point for establishing the
desired dilutions of anhydrous HF. A mechanical vacuum pump 10 or
other source of vacuum as is known in the art, exhausted by an
exhaust connection 12, is connected via a pump inlet connection 14
to a first tank, make-up storage tank 16. An electropneumatic
isolation valve 18 on the pump inlet connection 14 is normally in
the closed position. A supply 20 of gas for purging, as described
below, is connected to the mechanical vacuum pump 10 with
connection 22. The vacuum subsystem also includes gas flow check
valve 24, pneumatic cavity purge gas supply isolation valve 26 and
control valve 28 on connection 22, the use of which is discussed
below.
[0025] The source gas sub-system (B) delivers concentrated
anhydrous HF to the first and second storage tanks, storage tanks
16 and 30 respectively, from a source gas supply 32. A source gas
manual isolation valve 34 enables the flow of HF to be stopped
manually. Source gas supply valve 36 is normally closed, and is
only opened when concentrated HF gas is needed.
[0026] The dilution gas sub-system (C) delivers the dilution gas
for the anhydrous HF, as discussed below, and is composed of a
dilution gas supply 38, dilution gas supply flow check valve 40,
and a normally closed dilution gas electropneumatic supply valve
42.
[0027] The source gas/dilution make-up sub-system (D) delivers the
diluted anhydrous HF (a controlled mixture of gases from the source
gas supply 32 and from the dilution gas supply 38) to the make-up
storage tank 16 and provides the means to maintain this dilute gas
mixture. This sub-system also includes a make-up storage tank
electropneumatic isolation valve 44, which is normally closed, and
an absolute pressure/vacuum transducer 46.
[0028] The diluted gas storage sub-system (E) is used to contain
and deliver the diluted anhydrous HF gas mixture to the process
tool containing parts to be soldered or otherwise treated, and
includes a second storage tank, diluted gas storage tank 30, a
storage tank electropneumatic isolation valve 48 that is normally
closed, a diluted gas flow control valve 50, and a storage tank
absolute pressure transducer 52, for control and measurement of the
diluted gas pressure.
[0029] The control system (F) is composed of a dedicated controller
(a personal computer, for example, IBM Aptiva Personal Computer,
IBM Corporation, New York City; a programmable logic controller,
for example, model SYSMAC C20K, Omicron Electronics Inc.,
Schaumburg, Ill.; a dedicated logic controller, for example, a
Phase 4 Control System of IEI, Inc., Cary, N.C.; and a custom-built
dedicated sequence controller designed to execute the system as
detailed (e.g., a dedicated central processing unit (CPU) such as
an Intel Single Board Computer equipped with a read only memory
(ROM) program to execute the steps in sequential order, or other
devices, as are known in the art for flow control. The control
system monitors the two pressure transducers 46,52, operates all
electropneumatic valves, and operates the flow control devices.
[0030] Dilution and delivery of the diluted HF are accomplished
through operation by the control system of the pneumatic valves and
flow control valves in a predetermined sequence. The preferred
sequence of operation of the gas dilution system of the invention
follows. Prior to initiation of use of the apparatus, all
electropneumatic and manual valves are closed. The source gas
supply 32, contains concentrated anhydrous HF gas; the dilution gas
supply 38 contains, for example, dry air, dry nitrogen, dry argon,
or other inert gas; and the cavity purge gas supply 20 contains,
for example, dry air, dry nitrogen, dry argon, or other inert
gas.
[0031] In FIG. 1, the wider lines connecting the sub-systems are
preferably larger diameter pipes to facilitate high conductance
during the evacuation of the two tanks 16,30 (discussed below), and
valves 18, 44 are sized to match the line diameter as is known in
the art (for example, if a 1/2-inch line is used, 1/2-inch or
larger diameter valves should be used). The remainder of the lines
shown can be sized according the desired flow. Thus, for up to 5
CFM (cubic feet per minute), 1/4-inch diameter lines would be
suitable, or sized for particular selected valves 24, 26, 28, 34,
36, 40, 42, 48, and 50. The exhaust connection 12 and the cavity
purge connection 22 can be sized to match the selected vacuum pump
(7) connections (typically 1/4-inch to 3/8-inch for cavity purge
connection 22 and 1 to 2 1/2 inches for the pump exhaust connection
12. All gas and vacuum lines are fabricated from materials that are
compatible with the gas chemistry. Thus, for using HF as a source
gas, 316L is suitable. For the dilution gas, copper or 316L
stainless steel is suitable. Additional lines as are known in the
art, but are not shown separately in the figure would be part of
the control sub-system connection to the remainder of the invention
components, and are electrical wires or pneumatic lines to operate
the automatic valves 18, 26, 28, 36, 42, 44, 48, and 50. The
control sub-system selected and the type of valves selected would
determine the nature of the lines between the control sub-system
and the various automatic valves as is known in the art.
[0032] In the above description apparatus of the invention, several
valves are mentioned as being preferably electropneumatic valves.
Alternatively, electrical or pneumatic or electropneumatic valves,
or a combination thereof may be used. The critical valve
requirements are that the automatic valves can be remotely operated
by a control subsystem and the valve construction and material of
construction is compatible with the selected chemistry and
pressures used in the apparatus. Examples of suitable valves are as
follows: (a) valves 26, 36, 42, and 48: model SS-92M4-S4 (1/4-inch
diameter normally closed pneumatic toggle valve of stainless steel,
Swagelok Co., Solon Ohio); (b) valves 18 and 44: model SS-45S8-33C
(1/2-inch diameter normally closed pneumatic ball valve, made from
stainless steel, Whitey Co., Highland Heights, Ohio); (c) valves 28
and 50: model 1179A Mass-Flo Controller (1/4-inch diameter normally
closed electronically controller variable flow valve, MKS
Instruments, Andover, Mass.); (d) valves 24 and 40: model SS-6C-1
(1/4-inch diameter 1 PSI flow check valve, made from stainless
steel, Swagelok Co.); and (e) valve 34: model SS-4BKT-V51 (1/4-inch
manually operated isolation valve, made from stainless steel,
Swagelok Co.). One of ordinary skill in the art would know how to
substitute other suitable types and manufacturers for the
above-examples of valves to assemble the apparatus of the
invention.
[0033] The transducers used in the invention may be any suitable
for the function as specified herein. Examples of transducers that
may be used in the invention are: (a) transducer 46: model MKS
Baratron Types 623A/624A/625A, absolute pressure transducer, 0-1000
Torr range, constructed of stainless steel, MKS Instruments,
Andover, Mass.); and (b) transducer 52: model Pressure Transmitter
Model 625-50 or 625-30 absolute pressure transducer, 0-30 psig or
0-50 psig range, constructed of stainless steel, Dwyer Instruments
Inc., Michigan City, Ind.).
[0034] Although the above description of the apparatus of the
invention utilizes only two tanks, the apparatus could be
configured to supply several systems with diluted gas (at the same
concentration or at different concentrations) by replicating the
diluted gas storage sub-system E in a multi-delivery configuration,
each replenished by a single arrangement of sub-systems A-D and F
Initiation. To initiate apparatus operation, the operator opens the
source gas manually operated shut-off source gas manual isolation
valve 34. The mechanical vacuum pump exhaust connection 12 is
connected to a suitable industrial exhaust system, as is known in
the art.
[0035] Evacuation. The control sub-system is then used to open the
mechanical pump electropneumatic isolation valve 18 and the make-up
storage tank electropneumatic isolation valve 44, which connects
the diluted gas storage tank 30 and make-up storage tank 16 to the
mechanical vacuum pump inlet connection 14. Pressure transducers 46
and 52 are monitored by the control system. When a predetermined
pressure set point is reached at the transducer 46 (e.g., 0.001
Torr to 10.0 Torr or 1.93.times.10.sup.5 PSIA to 0.193 PSIA (pounds
per square inch absolute), the control system closes mechanical
vacuum pump electropneumatic isolation valve 18. The purpose of
evacuation is to remove gas present within tanks 16 and 30 prior to
introduction of source gas and dilution gas. To accomplish this a
low initial pressure is desired. The lowest value usable is a
function of the vacuum pump 10 that is used. Typical vacuum pumps
used for this apparatus achieve base pressures of approximately
0.001 Torr and thus the lowest expected value would be 0.001 Torr.
The upper value of the range is determined by (a) the time required
to achieve the desired starting lower pressure limit, which is
variable and depends on the pumping speed of the vacuum pump 10,
the size of the connecting lines and valves 18, 44, and the initial
contents of tanks 16, 30; and (b) the desired accuracy of the
diluted gas HF concentration. For example, if the final diluted gas
storage tank 30 pressure is desired to be 24.7 PSIA and the
evacuation set point of the make-up storage tank 16 is 0.001 Torr,
the error in the final mix concentration would be approximately
.+-.0.0081% of the expected concentration. For effective fluxless
pretreatment using the diluted anhydrous HF, experimental results
indicate that the concentration can vary as much as .+-.1.00% of
the desired concentration. This implies that the upper limit on the
evacuation set point can be 10 Torr.
[0036] Adding source gas to tanks. The control system opens the
source gas supply valve 36, which connects the source gas supply 32
to the make-up storage tank 16 and the diluted gas storage tank 30.
When the control system senses that pressure transducers 46 and 52
obtain a predetermined value (e.g., 2 Torr), the control system
closes the source gas supply valve 36. The range of the transducer
46 determines the acceptable pressure range. Thus, with the
preferred transducer, this range would be between 0 to 1000 Torr (0
to 19.3 PSIA). At this point, the make-up storage tank 16 and the
diluted gas storage tank 30 contain a partial pressure (e.g., 2
Torr) of the concentrated source gas.
[0037] Adding dilution gas. Next, the control system is used to
open the dilution gas electropneumatic supply valve 42 connecting
the dilution gas supply 38 to the make-up storage tank 16 and the
diluted gas storage tank 30. The dilution gas supply flow check
valve 40 prevents gas back-flow into the dilution gas supply source
38. When the control system senses that pressure transducer 52 has
reached a predetermined value, for example, 10 PSIG (pounds per
square inch gauge, which is equal to PSIA minus atmospheric
pressure, so that 10 PSIG is equal to 24.7 PSIA at sea level), the
control system closes the dilution gas electropneumatic supply
valve 42, isolating the dilution gas supply 38. The acceptable
range of pressures at transducer 52 is between 0 PSIG to 50 PSIG.
At this time, the diluted process gas mixture is in the make-up
storage tank 16 and the diluted gas storage tank 30. For example,
the values of 2 Torr of source gas and 10 PSIG of dilution gas
would yield a diluted process gas concentration of 0.156% in the
diluted gas storage tank 30. The replenished diluted gas should be
at the concentration as determined only by the partial pressures of
the P.sub.source/P.sub.dilution ratio, regardless of the size of
the tanks.
[0038] Delivery of diluted gas-from diluted gas storage tank. The
control system then closes the storage tank electropneumatic
isolation valve 44. At this point, the diluted process gas is now
ready for delivery. The control system accomplishes delivery by
opening the storage tank electropneumatic isolation valve 48 and
controlling the diluted gas flow control valve 50, and the diluted
gas is delivered to the treatment system. The gas flows out to the
treatment system due to a pressure differential between tank 30 and
the pressure within the connected treatment system. The amount of
flow is determined by the flow control valve 50.
[0039] Evacuating the make-up storage tank. In the next step, the
control system connects the mechanical vacuum pump inlet connection
14 of the mechanical vacuum pump 10 to the make-up storage tank 16
by opening the electropneumatic isolation valve 18. When the
control system detects that the storage tank pressure transducer 46
is below a predetermined value (e.g., 10 milliTorr), the control
system isolates the make-up storage tank 16 from the mechanical
vacuum pump inlet connection 14 by closing the mechanical vacuum
pump electropneumatic isolation valve 18. During delivery of the
diluted process gas via the storage tank electropneumatic isolation
valve 48 and the diluted gas flow control valve 50, the control
system monitors the storage tank pressure transducer 46.
[0040] Refilling of storage tanks. When the pressure within the
diluted gas storage tank 30 drops below a predetermined value
(e.g., 8.5 PSIG) as measured by the storage tank pressure
transducer 46, the control system fills the make-up storage tank 16
with the source gas by opening the source gas supply valve 36. When
the pressure within the make-up storage tank 16 reaches the
predetermined value (e.g., 2 Torr) as measured by the make-up
storage tank pressure transducer 52, the control system closes the
source gas supply valve 36. Following this, the control system
opens the dilution gas supply valve 42 and the make-up tank storage
isolation valve 44. This introduces the dilution gas that is
contained in the dilution gas supply 38 into both the make-up
storage tank 16 and the diluted gas storage tank 30. The control
system monitors the storage tank pressure transducer 52 while both
tanks 16 and 30 are being refilled with the diluted process gas.
When the control system detects that the storage tank pressure
transducer 52 has reached a predetermined value (e.g., 10 PSIG),
the control system closes the dilution gas supply valve 42 and the
make-up storage tank isolation valve 44.
[0041] Repetition. The evacuation and refilling steps are repeated
to maintain the diluted gas storage tank 30 pressure between
predetermined points (e.g., 8.5 to 10 PSIG) at a fixed source gas
concentration as determined by the control system programmable
pressure set-points, as monitored by pressure transducers 46 and
52.
[0042] Cavity purge. During the operation of the gas dilution
system, the mechanical vacuum pump 10 and the mechanical vacuum
pump exhaust connection 12 are exposed to repeated cycles of
potentially corrosive and/or hazardous gases from the source gas.
To facilitate removal of any residual hazardous/corrosive gases and
to extend the working lifetime of the mechanical vacuum pump 10 and
the mechanical vacuum pump exhaust connection 12, a cavity purge is
provided. The gas contained within the cavity purge gas supply 20
should be a gas chosen to provide a non-reactive environment for
the selected source gas, such as dry nitrogen, dry air, dry argon
or other inert gases. During the operation of the invention as
described above, the control system opens the cavity purge gas
supply isolation valve 26, and cavity purge gas flows through the
cavity purge flow check valve 24 and through the cavity purge
control valve 28 into the mechanical vacuum pump 10 through the
cavity purge connection 22 and out the mechanical vacuum pump
exhaust connection 12. This provides a continuous flow of a safe
gas medium in the mechanical vacuum pump 10 and the mechanical
vacuum pump exhaust connection 12, effectively sweeping any
residual hazardous/corrosive source gas out of the mechanical
vacuum pump 10 and the mechanical vacuum pump exhaust connection
12.
[0043] In addition to delivering the diluted HF formed by the
method of the invention with the apparatus of the invention to a
processing tool comprising solder on a surface, the method and
apparatus of the invention may be used for treatment of solder
powder which is used in solder paste. Conventional solder paste is
a composite of solder powder and a vehicle which contains solder
flux where the vehicle acts as a suspending medium for the powder.
Solder paste may be applied to components and circuit boards in a
number of ways including screen printing. During reflow, the flux
dissolves surface oxides on the powder to allow the powder
particles (now liquid) to flow smoothly together and to the parts
to be joined. As with other conventional soldering processes, the
flux leaves a residue which should be cleaned after joining. Using
HF formed by the method of the invention also allows fluxless
soldering with solder paste in which the solder powder has been
pretreated using the methods described herein.
[0044] Preferably the entire apparatus is provided with a heating
means and fan as is known in the art so that the apparatus may
operate in a temperature range from ambient to 120 degrees C. in
order to aid in maintaining a dry condition within the
apparatus.
[0045] While the invention has been described with reference to
specific embodiments, it will be appreciated that numerous
variations, modifications, and embodiments are possible, and
accordingly, all such variations, modifications, and embodiments
are to be regarded as being within the spirit and scope of the
invention.
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