Optimizing Particle Removal in Ultrasonic
Baths
D. John
Hunt, Applications Research Engineer and Bill Thorpe, Associate
Support Engineer

Introduction

Part cleanliness becomes increasingly important as
mechanical, optical, and electronic parts shrink in size and formerly
inconsequential particulates grow in comparison.

To remove even sub-micron particles, semiconductor,
micro-electromechanical systems (MEMS), optical and hard disk drive (HDD)
components, micro inertial sensors, laser gyroscopes, compact discs (CD’s) and
optical coating applications all employ liquid ultrasonic cleaning baths for
both intermediate and final cleaning steps.

In contrast, traditional industrial cleaning
applications often use liquid sprays to remove machining debris, oils, and large
particulates from engine blocks, transmission parts, and other cast and machined
parts of many types. However, intermediate industrial applications; e.g. natural
gas solenoid valves, injector nozzles, compressors, air bag sensors, and many
others are now using ultrasonic cleaning baths as well. In these items,
particles may not be yield limiting as they are in the high technology arena,
but they can be just as important to long-term function, and can even be
potentially catastrophic.

Understanding, and optimizing, ultrasonic cleaning
bath parameters can result in major improvements in part cleanliness, cleaning
efficiency, and ultimately part reliability and lifetime.

Ultrasonic
Cleaning Parameters

Ultrasonic baths formerly employed chlorofluorocarbon
(CFC) cleaning fluids and other volatile organic compounds (VOC’s). However, the
Montreal Protocol has restricted the use of these fluids to avoid potential
ozone depletion, consequently today’s ultrasonic baths use predominantly
water-based solutions. Thus, the hot DI water ultrasonic bath has become one of
the most popular cleaning processes in use today, for both high technology and industrial applications.

An excellent CleanTech 1999 paper, entitled “Advanced Ultrasonic Surface Cleaning”,
by Ahmed Busnaina[1], presented the
variations in surface cleaning efficiency of sub-micron sized particles on
semiconductor wafers using the aqueous ultrasonic bath.

There are many parameters affecting the cleaning
ability of an aqueous ultrasonic bath, including; contamination type, quantity,
and application manner; water temperature, auxiliary cleaning agent and
concentration; ultrasonic power, frequency, and sweep rate; filter particle
size, recirculation rate, and bath flow dynamics.

Mr. Busnaina’s paper examines particle removal
efficiency by an aqueous ultrasonic cleaning bath, versus temperature,
ultrasonic power, ultrasonic frequency and cleaning duration. The contamination
particles used in those tests were typically single sized, sub-micron, plastic
calibration particles; and the particle counts were performed on the cleaned
part, after completion of the aqueous
cleaning and subsequent drying.

In contrast, this paper is focussed on the
industrial, rather than the high technology, realm of industry. Natural silicate
particles, spanning a large size range, were used as test contaminants in place
of single sized plastic spheres; and real time particle counts were measured in the wash bath, not on the dried
part!

Traditionally, particle counters do not fare well in
the wash bath, due to the cleaning
agents and their concomitant bubbles, which appear as false particles. However,
by taking special precautions, the graphs in this paper show the actual particle
removal process, taking place directly in the wash bath! One graph includes the
characteristic signature of bubble interference in comparison.

Part
Cleanliness Tools

Many techniques are traditionally used to determine
part cleanliness; water break test, contact angle, UV photoelectric emission,
optically stimulated electron emission (OSEE), scanning electron microscopy
(SEM), secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy
(AES), electron spectroscopy for chemical analysis (ESCA), evaporative rate
analysis (MESERAN), nonvolatile residue analysis (NVR), surface scatter particle
counting, thermogravimetric analysis, total organic carbon analysis (TOC), phase
imaging, gas chromatrography (GC), mass spectrophotometry (MS), and Fourier
transform infra-red spectroscopy (FTIR).

Most of these tests measure chemical contamination,
while the few that do measure particulates cannot be used in a liquid
medium.

Particle Counting Advantages and Concerns

The application of optical single particle counters
to operating ultrasonic baths provides a convenient means of monitoring the
cleaning process by particle size and in real time. However, several questions
have been raised regarding particle counting’s efficacy and the application of
its possibly unique advantages.

Does the
particle counter function properly while sampling from an operating ultrasonic
bath? Does the ultrasound energy disrupt its particle counting ability?

Can an optical
particle counter yield stable data when used in a detergent bath?

Can particle
removal efficiency be determined using a particle counter?

Can an
“optimal” process be established to clean all parts, even if confined to a
single part type and contaminant?

Finally, if a
single “optimal” cleaning pass is achievable, how can it be maintained?

These questions were investigated in a series of
tests using silicate contaminants, typical of many environmental and industrial
soils, and utilizing an on-line particle counter sampling from within an
operating ultrasonic bath.

Test
Configuration

A simple recirculating bath system was constructed using a 150 ml glass
beaker, 0.3 micron absolute filter, a small heated ultrasonic bath, and a
battery operated portable particle counter with its own internal pump.

The beaker contained 100 ml of aqueous cleaning fluid, typically 93-100%
DI water, which was filtered and recirculated at a high 100% per minute rate,
i.e. one bath refresh each minute. This recirculation speed provided a rapid
particle capture rate for better enumeration of total particle load. The system
is shown in figure 1, below.

Figure 1 – Particle Counter, Filter, and Ultrasonic Bath Test Setup

Test Procedure

The general test procedure began by verifying the
initial beaker and system clean up to an acceptable background particle level
for particles larger than 2 microns. Particle counts less than 10 particles/ml
were considered acceptable. Counts of 5-6 particles per ml were readily achieved
and, with sufficient patience, even levels of 0-1 p/ml were observed. Refer to
Graphs 1 & 2 for examples of
beaker clean up. Please note, all graphs
show cumulative particle counts, for the sizes noted, on a per milliliter
basis.

A cleaned test item was then suspended in the bath and the minute by
minute particle count of the wash water was recorded. Once the “cleaned” part
achieved a low particle value, often arbitrarily set to 10-20 p/ml, then a
second contaminated test item replaced the original and the “dirty” part
particle response was recorded.

The cleaning agent type and/or its concentration was varied and the test
repeated on identically contaminated items.

Test
Contaminant

The parts were contaminated using a water solution of known concentration
of AC Fine Test Dust (ACFTD), which was then baked, or air-dried, onto the test
item. ACFTD is an ASTM (American Society for Testing and Materials) recognized
material, composed of a known distribution of mostly silicate particulates.

This now obsolete material is currently being replaced with a series of
ISO certified, and markedly more expensive, test dusts of similar composition,
but with better documented particle distributions.

Test Parameters

Tests were performed for variations in item material, shape,
contamination level, cleaning agent type and concentration. All tests were
performed at a nominal bath temperature of 50 °C,
filtered recirculation rate of 100 ml/min (1 bath change per minute), and
continuous ultrasonic agitation (45 W average/135 W peak power at 38.5+2 kHz
swept), unless otherwise stated.

Test Items

A number of common test items were also employed to present a variety of
materials, surface finishes, and shapes. These items proved to be amazingly
varied in their ability to harbor and dislodge particulates.

Results

Aluminum Sheet Metal

Originally, simple strips of 2024-aluminum alloy,
1.5”wide x 2.4” long x 1/16” were tested in a heated DI water ultrasonic bath.
These test coupons were cleaned and tested to confirm their ability to achieve
low background particle count levels. The particle response of a typical cleaned
coupon is shown in Graph 3. This graph shows the pulsed
ultrasonic particle response initially of the beaker itself, and then the
Aluminum test coupon. Ultrasonic energy was experimentally applied every 10
minutes in this test, but was applied continuously for all other tests shown in
the remaining graphs.

The beaker quickly achieved very low background
counts, but despite leaving the coupon in the bath for over an hour, the
particle counts never returned to the low baseline values of the initial bath.
Examination of the cleaned and dried coupons revealed a copper-oxide colored
stain. Since this alloy of aluminum contains about 5% copper, we suspect the
copper was being exposed by the effects of the heated DI water. We observed the
same effect on commercial aluminum foil, which was exposed only to the vapor
from the DI bath.

After this failure to achieve a low particle count
baseline using an aluminum coupon, stainless steel disks were tested to avoid
generation of particles by possible chemical corrosion.

Stainless Steel

Stainless steel rod, grade #303, measuring 1 ½” in
diameter was cut into approximate ¼” thick pieces, then machined flat, and
washed prior to particle count testing. No staining of any kind was observed
after the hot DI ultrasonic cleaning. However, high particle counts were again
observed for the duration of the bath cleaning, with no indication that the
parts would ever clean up. Observe Graph 4 for the particle cleanup
rate of an uncontaminated Stainless Steel coupon. It too appears to reach a
constant particle level until the test is terminated.

From the failures of both these metals to clean up
satisfactorily, we postulated the cause might be due to the surface texture of
the metal. Both metal coupons displayed the typical porous surface finish of
unpolished metal.  To test this
theory, we tested glass microscope slides, which had mostly smooth glassy
portions, but also a rough etched section.

Glass Slides

Glass microscope slides, measuring 1” x 3”, with a ¾”
frosted end on one side, were tested next and demonstrated excellent clean up,
provided the frosted end was not immersed. See Graph 5 for these test results.

Based on their superior particle cleanup ability,
smooth glass slides were then used for subsequent contamination testing.
However, the particle removal between slides of different contamination levels
proved inconsistent. Refer to Graph 6, which shows less particle removal from
contaminated slides than for “clean” slides in a detergent (Citranox)
solution.

Visual inspection of the inconsistent slides revealed
residual stains where the particle solution had been baked onto the surface.
Apparently, variations in how the contaminant dried onto the surface had
produced differing adhesion strengths and thus removal rates.

The bath particle concentration was tested by directly injecting
100 to 500 ml
of ACFTD particle solution, i.e. 50-250 mg
of actual dust directly into the bath. Within the range of experimental error,
this produced the expected linear results, see Graph 7.

Two sets of 5 slides each were similarly
contaminated. One set was rapidly dried at 200F and then run in the bath, while
the other set was dried overnight at room temperature before testing. As
expected, the quickly dried contamination dislodged fewer particles than the
slowly dried slides.

Graph 8 shows markedly fewer
particles are liberated from the baked on slides as compared to the free bath
injection (Graph 7). Also, generally more particles were
released from the air-dried slides of equal contamination (not shown), but their
release appeared more erratic and drawn-out.

Cleaning Agents Remove Residual Baked On
Contamination

Uniformly contaminated glass test slides were used as
test coupons to evaluate the effects of several commercial cleaning aids when
added to the DI water of the ultrasonic bath.

The cleaning agent manufacturer recommends to first
try a low concentration solution, followed by an anticipated over-concentration,
then an intermediate concentration that might prove nominal. However, we chose
to steadily increase, or decrease, the concentration of the cleaning agent in
our experimental application for the sake of test convenience.

Cleaning agents are formulated for different
contamination conditions, e.g. acid or base solution and plastic, metal, or
glass parts. Selecting the correct cleaning agent can be part science and part
art. Manufacturers often assist in selecting the correct product for each
particular application.

Citranox[2]

Citranox cleaning
agent was tested for its particle removing ability at different concentrations
using ACFTD contaminated glass microscope slides. Graph 9
shows the particle removal results
of 3%, 5%, and 7% solutions of Citranox in DI water upon three identically
contaminated glass slide test coupons. Other results for concentrations of 0.5 %
and 1.75% (not shown) reveal a nominal concentration of about 3%; see table
below.

%
Citranox

0.5%

1.75%

3%

5%

7%

Particle Count
Peak
(particles/ml)

53

97

165

120

95

Detergent 8

After the fashion of the Citranox testing above,
Detergent 8 was similarly tested at 1% and 3% concentrations and those results
appear in Graph 10. Both concentrations yield
essentially identical particle removal peaks of about 100 p/ml. However, so much
difficulty with false mixing counts was encountered that no additional
concentrations were tested.

The low initial particle removal peaks, trouble in
achieving low background counts, rapid recovery of slide counts to base line
values, and highly visible residue on the slides after being washed and dried,
portend against using Detergent 8 for this application.

Triton
X-100

The Triton X-100 results generally follow the above Citranox example, as
shown in Graph 11. The following table lists
the peak particle count removal values versus Triton X-100 concentration.

%
Citranox

1%

3%

5%

7%

Particle Count
Peak
(particles/ml)

99

125

73

101

Again, the 3% concentration appears to be optimal for
removing particles. The chart appears to show a possible inflection point for 7%
concentration, but Graph 11 reveals three distinct
peaks for the 3% concentration, whereas there is only one such peak for the 7%
solution. Thus, many more particles were removed at 3% concentration, though
they did not occur all in the first peak.

Note – Triton X-100 left an oily residue on the
slides and required great amounts of rinsing with hot water to clean it
thoroughly from the particle counter and tubing/beaker system.

Summary

Surface
Texture

Simple smooth glass microscope
slides provided a convenient vehicle for testing basic contamination removal
techniques and verifying theory. However, real-world parts do not typically
resemble smooth glass and are frequently complex in structure and material
finish. These factors can greatly complicate optimal cleaning determination, as
was observed with the simple aluminum and stainless coupon results shown
earlier.

Particle Counts During Sonication

The particle count shedding behavior (particulate
cleaning) of both test coupons and common machined and molded pieces (not shown
here) were successfully observed in
situ, within the bath, during ultrasonic cleaning.

Measuring particle counts directly in ultrasonic
baths is possible, even a bath containing a sudsy cleaner, as evidenced by the
near zero particle counts in a clean sonicating bath and the proper measurement
of varying amounts of known contaminants

Clean Up Time

Particle
clean up rate varied according to the manner in which the part was contaminated.
Low temperature evaporation of the contamination conveying fluid from the part
resulted in lower particle surface adhesion, while high temperature evaporation
produced stubborn bonding of particulates to the test part.

Different materials, of different shapes and surface
finishes, may require a surprisingly varied range of cleaning conditions to
achieve acceptable cleanliness.
Under some conditions, an ultrasonic bath cleaning process can “stall
out”, providing no further removal of the remaining particulates despite
indefinite extensions of cleaning time.

Universal vs. Optimal Cleaning Process

It may be possible, but probably not economical, to
establish a single universal process
to clean each and every item completely, every time. However, it may require
such a large investment in equipment, time, and cleaning agents, that it would
be prohibitively expensive and would result in inefficient and wasteful
over-cleaning of most items.

However, an “optimal” cleaning “recipe” may be able
to be formulated for each part type
by running several parts through an exhaustive matrix of conditions and measure
their resultant cleanliness by traditional methods.  Again, this challenging task would need
to be repeated for each part type.

Alternative Optimal Cleaning

An alternate technique may be to monitor particulate
removal in real time, for each part type, thus “optimizing” particle removal for
each batch of components being cleaned, regardless of part size, shape, and
contamination level.

Each “recipe” would accommodate a specific part type,
material, shape, and finish, while the degree of contamination would be
monitored on-line to determine the nominal cleaning duration.

By whatever means an “optimized” process is
determined, it may need subsequent and frequent verification and/or alteration,
especially if the part contamination loading varies, or even if the surface
texture or finish of the part is altered.

Conclusions

The in-situ particle counting of an
ultrasonic bath, during the cleaning
process – even with cleaning agents - can provide a direct means of measuring
when parts have been cleaned of particulates to the desired level. Rote reliance
on previously “optimized” process conditions need not be blindly followed, but
can be periodically (or even continuously) verified.

However, on-line particle counting results should be
correlated with another standardized cleanliness test on the finished part, to
ensure the cleaning bath did not “stall out” by reaching its maximum cleaning
ability before all the particles were removed.

If varying cleaning results are encountered, despite
following an “optimized” process, on-line particle monitoring can provide a tool
to reveal the cause, and then be used to establish and maintain a truly
“optimized” cleaning process.

Continuous in-process particle monitoring may
alleviate the rigorous cleaning conditions and shorten the duration developed
for “worst case” contamination levels, which are then imposed on each and every
part. Thus on-line particle monitoring can be employed to conserve cleaning
resources, while increasing cleaning throughput, without jeopardizing part
cleanliness.

Graph 1: Beaker
Cleanup – in DI Water

Graph 2: Beaker Cleanup – Without &
With Ultrasonics -
in DI Water

Graph 3: Aluminum -
Cycled Sonication – in DI Water

Graph 4: Stainless Steel
-
Cleanup Plateaus – DI Water

Graph 5: Clean
Glass Slides – Smooth vs. Frosted -
DI Water

Graph 6: Clean & Contaminated Glass
Slides vs. Citranox Cleaner

Graph 7:
Contamination Directly Into Beaker Solution – 100 to 500 ul

Graph 8: Slides with Contamination Baked
On – 100 to 500 ul

Graph 9:
Contaminated Slides vs. Citranox @ 3%, 5%, 7%

Graph 10: Contaminated Slides vs.
Detergent 8 @ 1 & 3%

Graph 11:
Contaminated Slides vs. Triton X-100 @1, 3, 5, & 7%

[1]
Advanced Ultrasonic Surface Cleaning,
Ahmed Busnaina, Ph.D. , Clean Tech 99
Conference, Microcontamination Research Laboratory, Clarkson University

[2] Citranox and Detergent 8 are registered trademarks of
Alcon Company, Newark, New
Jersey

Triton X-100
was
formerly a trademark of the DuPont Company, but is now widely
produced.