optimizing cleaning energy in batch and inline spray systems · optimizing cleaning energy in batch...
TRANSCRIPT
Optimizing Cleaning Energy
in Batch and
Inline Spray Systems
Steve Stach, Austin American Tech.Mike Bixenman, Kyzen Corp.
The Science of Cleaning Green
Agenda
Technology Advancements
Research Questions
Theoretical Framework
Conceptional Framework
Introduction
The benefit of a well defined cleaning process:
improves manufacturing efficiencies
increases process yields
Optimized process
cleaning agent effective on wide range of soils
integration of machine with chemistry
mechanical design delivers chemistry at the heart of the residue
control and re-use of fluids
Challenges
Converge of circuit boards and die packaging
technologies
higher performance electronic devices
Technical issues:
low standoff
fine pitch solder bump arrays
ionics trapped underneath active components
spacing between conductors may pose risk of
electromigration
Statement of Problem
Staying ahead of the ever-advancing technology
curve:
industry challenged to improve cleaning processes
increased complexity of board and geometry
new solder paste and flux formulations
improved performance at lower cost
Mechanical and chemical energy are the key
variables to meeting demands
Technology Advancements
New approaches to mechanical energy deliver:
performance at the heart of the residue rather than the tail of the delivery system
Advanced cleaning chemistry designs:
lower operating temperature
lower concentration
long bath life
bright and shinny solder joints
no sump side adds
Statement of Purpose
The purpose of this work is developing an
equation that will allow optimal spray
configuration and impingement pressure at the
board level, to improve cleaning performance.
Research Questions
What kind of equations defines surface energy
at board level?
How does this affect the fluid delivery design in a
cleaning system?
How much impingement pressure is needed at
the board level?
Which is better, higher pressure or high flow?
Study Hypothesis
H1: Understanding the dynamics of flow,
pressure, and dissolution, a physical equation to
determine impingement pressure to penetrate
under densely populated components can be
defined.
Theoretical Framework
Manufacturing is judged by:
Time and material metrics
consistently achieving product quality standards at the lowest possible cost
process optimization increases production while lowering cost
Spray in air inline cleaning systems
modeling to predict performance
solubility rate to the residue in the cleaning solution
physical energy available in the cleaning system
Conceptional Model
Rp=Rs+Rd
DynamicCleaningRate
Rs
Process CleaningRate Equation
Rp
ProcessCleaningRate
StaticCleaningRate
Rd
Process Cleaning Rate Equation
Units will vary depending on residue
Rates for cleaning flux residue:
Expressed as
thickness or mass removed/second
volume flushed/second
mass removed/second
solder balls/second
particles/second
Rates are temperature dependent
Defining an Optimized System
Spray-in-air reduce time by:
Increasing the Rd component
maximizing physical energy delivered at the surface to be
cleaned
Optimized spray-in-air
not overpowered or oversized
delivers necessary chemistry and energy to clean the
most difficult or sensitive areas, at a rate that will
meet the process time requirement using minimal
chemistry, energy and floor space consumption
The Rs+Rd Balance
Key to predicting optimized process
performance
understanding the nature of the soil and the
chemical and physical needs for removing it
Rates of Rinsing and Drying
Equation also applies in the rinsing and drying cycles
Rinsing cycle
removal of wash fluid
dissolved or suspended soils
Dryer
evaporation
displacement
Displacement of water by air preferred
100-1000 times longer to evaporate than to displace
Rs + Rd = Rp
Rd cleaning processes
inline “air spray”
planarized batch
Rs cleaning processes
dip tanks
spray under immersion
dishwasher
vapor degreasers
Cleaning process comparison
System
Design
Wash Rinse Dry
%Rs %Rd %Rs %Rd %Rs %Rd
Static
Immersion
100% 0% 100
%
0% 100
%
0%
Dish
Washer
70% 30% 70% 30% 50% 50%
Planarized
Batch
30% 70% 40% 60% 20% 80%
Inline Air
Spray
20% 80% 20% 80% 2% 98%
Important Differences in Designs
Dishwasher style
shadowing of parts
3 dimensional racking
Inline & planarized batch
higher % Rd
board surfaces tangent to
the direction of spray
less shadowing= Shadowed area = circuit boards
Types of Surface Energy
Energy available at the the cleaning surface
Energy measured in ergs for mass
1 Joule = 107 ergs = 0.239 calories = .73ft lbs.
=2.78x10-7 kW hrs
Velocities measure in grams and centimeters
force measured in dynes
force exerted by one gram accelerated by earth’s
gravity for one cm.
1 lb. / in2 = 68948 dynes / cm2
Spray in Air Systems
Cleaning requires energy to displace a fluid
across a distance to create the force sufficient to
achieve rate of cleaning
Fluid flow is created by spray impingement
pressure, gravity drainage, and capillary action
Source of
Surface
Energy
Range of
Energy
Available
Governing
Equation
Capillary action 0-2”wc
(0-1.0 psi)
Pc = 2. γ / R
Gravity Flow 0-1”wc
(0-0.5 psi)
Pg = ρ.g
.h
Impingement
Pressure
0-275”wc
(0-10 psi)
Pi = ½(rv2)
Surface Tension & Capillary Action
Think of surface tension as:
balloon of sorts surrounding the cleaning fluid
If it is thin and weak
cleaning fluid easily moves in and out of tight spaces
If the side wall is thick and strong
cleaning fluid will resist flow into tight spaces
Capillary attraction or repulsion is a resultant force of
adhesion
cohesion
surface tension
Equation 2: Interfacial pressure differential (for planes)
Δp = 2γ / R
Equation 3: Interfacial pressure differential (for tubes)
Δp = γ / R
Where γ = surface tension; R = radius meniscus
Interfacial Pressure Difference
Capillary forces can work for and against
They can facilitate the initial wetting of tight spaces
They can inhibit rinsing and drying steps by resisting displacement forces if insufficient
Graph #1 indicates it would take an air jet impingement force of greater than 1-psi to displace water trapped in spaces less than 1-mil
Interfacial pressure difference at equilibrium
10
1
psi
0.1
0.01
0 20 40 60
Gap/diameter, mils
Planar
Cylinder
Calculating Surface Energy
Cleaning fluids can have both potential and kinetic energy
Potential energy of one unit volume of fluid at rest2
(Equation 4)
Ep = p*g*h where:
p = density of fluid
g = acceleration due to gravity = 9800 cm/sec/sec
h = height of fluid (cm)
Kinetic energy of on unit volume of cleaning fluid in motion2
(Equation 5)
Ek =1/2 pv2 where:
p = density of fluid
v = velocity of jet
Solving Equations 4 & 5 for water systems
Reveals two important points
potential energy is small as compared to kinetic
energy
Ek is driven by v2
can be maximized by nozzle and pump design
basis for not purchasing equipment solely based on
horsepower
If jets or nozzles are not optimized for the pump and stand off
distance, excessive splash or spray atomization can retard
the energy delivered by reducing impingement velocity
Manifold Efficiency
Optimum
delivers energy of the pump efficiently over distance with
minimal losses
efficiency measured by dividing the impingement pressure
at maximum working distance by the manifold pressure
average spray manifold efficiency
5-10% of existing inline spray cleaning systems
high efficiency designs
>25% increased surface energy
improves cleaning by 2x to 10x
Bernoulli Equation Modifications
Total pressure of fluid in pipe is equal to static pressure plus the kinetic and potential energy
Equation 6: Pe + ½ ρv2 + ρgh = total pressure where:
Pe = internal pressure energy
ρ = density of fluid
v = velocity of fluid
g = acceleration due to gravity = 9800 cm/sec/sec
h = height of fluid (cm)
Applied to unrestrained jet striking the cleaning surface by removing the internal pressure energy factor Pe and adding the surface energy effect of capillary action
Modified Bernoulli Equation
Modified for surface cleaning energy
Equation 7: ½ ρv2 + ρgh + 2γ /R = total force at tightestgap
where
R is the contact radius of the meniscus in the gap
R will have a negative value if meniscus is convex instead of concave
the impact of negative force at any step
will not penetrate
overall cleaning rate will be zero in the gap
Spray System Nozzle Design
Nozzles create jets that carry the energy to the surface of part
Design and layout of nozzles is an important step in optimizing
cleaning process
Equation 5 give the Kinetic energy to the surface of the board
contains both mass and velocity of the jet
Conical and fan nozzles
spread the spray to cover larger areas at t he expense of reducing the
mass per unit and velocity of the jet
Coherent jets
hold together longer and deliver more energy to a smaller area requiring
more nozzles and a higher overall flow rate
Spray Type Typical
pressure @
2”,50psi man.
/Pressure
loss/in
Indicated use
Fan/Delta 2 psi /
~50%
drop/inch
Wide coverage,
overlapping high
impingement for
close work
distance
Conical 0.4 psi /
~75%
drop/inch
Widest coverage
area, lowest
kinetic energy,
flooding
applications
Coherent 10 psi /
~25%
drop/inch
Smallest coverage,
highest energy
density over
longest distance
Spray System Nozzle Design
All jets break-up and slow down over distance in
air
Coherent hold together longer
provides the maximum energy transfer per unit
area
overlapping jets can be an effective strategy for
increasing surface energy density as long as the
splash at the surface does not dampen the
impact force
Empirical Measurements
Impingement/flow data of coherent jet vs. fan
Manifold
Pressure
Flow:
(gpm)
Impingement psi
@
Coverage
width @
0.075”Coherent Jet 1” 2” 4” 1.5” 4.0”
30 psig 0.69 15 10 6.5 0.6 0.7
40 psig 0.82 17 12 8 0.6 0.7
50 psig 0.89 19 13 9.5 0.6 0.8
60 psig 0.97 20 15 11 0.6 0.8
F40-1.0 Fan Nozzle
30 psig 0.89 3.2 1.6 0.2 1.5 3.25
40 psig 1.06 4.4 1.8 0.3 1.7 3.60
50 psig 1.20 6.0 2.3 0.5 1.7 4.0
60 psig 1.30 7.2 2.5 0.5 1.8 4.0
Cleaning rates
Inline and planar racked batch cleaning can be
significantly improved by
designing coherent jets
combination of fan-jets and coherent jets
In addition
coherent jets in dryers offer considerable
improvement in effectiveness
Inline Cleaning, Jets and Timing
Timing and sequence in a cleaning process is critical
Pre-wash:
thoroughly wet the parts with wash solution chemistry
provide sufficient flow and contact time to bring the assembly to wash temperature
facilitates full static-cleaning rate Rs
Wash
part should see several high impingement scourings punctuated by brief soak periods
static rate is optimized by maintaining wash chemistry
dynamic rate is optimized by focusing on maximum physical energy at the part surfaces
Inline Cleaning, Jets and Timing
Chemical Isolation
ample impingement force in air manifold to wipe chemistry from part
wet iso uses laminar flow to remove wash chemistry
Power Rinse
series of high pressure nozzles removes and dilutes remaining ions using DI-water
Final Rinse
low flow/pressure final pure rinse of DI water
final soak to remove ionic contamination
Ionic cleanliness of a clean part = final rinse purity
Dry
high-speed air displacement
Dishwashers are Different
Different form planar type batch and inline cleaners
Most efficient when designed for maximum flow, not impingement, as potential energy and capillary forces must be relied upon for cleaning surface energy in the shadowed areas
Too high pressure atomizes the spray
reduces jet velocity
must farther distance
increases the splash interference with other jets
Optimized jets provide large cohesive droplets using low pressure jets
adhere rather than bounce or splash
more consistent and thorough cleaning in shadowed areas
Shadowing is minimized through fixturing
Solubility's Contribution
“Dissolve-it”
age old, tried and true
augmented with heat,
blasting and scrubbing
Rate of solubility
dependent on dissolution rate
temperature effect in
dissolving residue
concentration of solvent
needed to dissolve residue
Soluble
Very Soluble
Marginal Solubility
Dissolution Rate
Temp
Distinctive Residue Types
Very soluble (Top line)
finger salts
water soluble flux residue
High solubility (Middle line)
increasing temperature increases solubility (typically)
increased temperature reduces cleaning time
soft flux residue
rosin
Marginal solubility (Bottom line)
requires heat and physical energy
hard residue flux
no-clean flux
Equation 8: Rs = Dr x Tc x Cc
Dr: dissolution rate
Tc: the effect of temperature in dissolving residue
Cc: concentration of cleaning chemistry
Easily determined experimentally by
weighing residue dissolved in fixed period of time
ratio of temperature divided by rate at lower temperature
dividing the rate of dissolution in as a function of cleaning
chemistry concentration
Contamination Effect
Remember: like dissolve like
generally true for most solvent systems
not true for all cleaning solutions or applications
saponification
cleaning agent is depleted over time
not true for marginally soluble salts or weak organic acids
contamination of wash bath in these cases slows cleaning process byshifting the chemical equilibrium of the wash solution
consider the dynamics of the system
chemical loss on average
10-25% evaporative
50-90% drag out into rinse
make-up in the bath prolongs life
Equation 9: Pka=[anion][cation]/[residue]
Ionization potential (Pka)
complexities arise when residues contain multiple
constituents
some have high solubility
some have low solubility
dissolving the soluble constituents may leave
behind insoluble constituents as a physical
residue requiring significant time and/or physical
energy to remove
Conclusion
Science of optimizing spray-in-air requires
accurate model to predict performance
All cleaning systems are governed by two fundamental
principles:
the solubility rate of the residue for the cleaning solution
physical energy available in the cleaning system
maximizing the physical energy delivered to the surface
increases the dynamic cleaning rate
Understanding the static cleaning rate plus the dynamic
cleaning rate balance is key in predicting optimization
Conclusion
Surface energy
energy available at the cleaning surface to do the work
Modified Bernoulli equation
may be used to calculate surface energy
In spray-in-air system
work of cleaning requires energy to displace a fluid across a distance to create the force sufficient to achieve the rate of cleaning
low surface tension easily moves fluid in an out of tight spaces
Capillary forces work for and against since they work for wetting but inhibit rinsing
Understanding fluid potential and kinetic energy allows for nozzle and pump configuration that maximizes surface energy
Manifold efficiency can increase surface cleaning by as much as 25%
Conclusion
Design and layout of nozzles is an important step in optimization
Conical and fan nozzles spread the spray to cover larger areas
Coherent jets hold together longer giving maximum energy transfer
per unit area
Overlapping jets can be an effective strategy for increasing surface
energy density
Conclusion
Rate to chemical dissolution can be augmented with various forms
of physical assistance such as heating, impingement, and time.
Contamination loading can slow the cleaning process by shifting
chemical equilibrium
Chemical dynamics stead state when make up exceed soil load
Authors
Steve Stach - Austin American Technology
Mike Bixenman - Kyzen Corporation
Optimizing Cleaning Energy in Batch and Inline Spray Systems
SMTAI Technical Forum
Steve Stach, Austin American Technology
Mike Bixenman, Kyzen Corporation
SMTAI Technical Forum
September 26-30, 2004
Donald Stephens Convention Center
Rosemont, IL
Abstract
The cleaning industry is constantly challenged to improve
cleaning processes, staying ahead of the ever-advancing
technology curve as it applies to new fields of application.
Processes are commonly represented as mechanical and
chemical energy, temperature and time. With increasing
complexity of board and component geometry coupled to
more difficult solder paste and flux formulations, cost of
ownership concerns reduce the ability of temperature and
time to be effective process variables. As a result,
mechanical and chemical energy must make up for reduced
temperatures and shorter process times.
Typical approaches to increasing mechanical and chemical
effectiveness are counter productive for cost of ownership
concerns and achieve only diminished returns. New
approaches to mechanical energy are introduced in a way
to deliver performance at the heart of the residue, rather
than the tail of the delivery system. Coupled with new
formulations, which are optimized for lower temperatures
of operations, new levels of performance, cost modeling,
and throughput are achieved simply by answering these
following questions with a thoughtful perspective. What
spray configuration is needed at the board level? What type
of impingement pressure is needed at the board level?
Which is better, high pressure or high flow? All driving
towards a single question, is it possible to define a physical
equation to determine impingement pressure to penetrate
under densely populated components?
Introduction
The benefit of well defined, controlled precision cleaning
process improves manufacturing efficiencies and increases
product yields. An optimized cleaning process is one
where the cleaning agent effectively removes the
contaminant of the day, as well as those foreseen on your
corporate technology roadmap. In addition, in an
optimized process, the cleaning equipment and chemistries
must also be integrated. The hardware is responsible for
delivering the cleaning agent, as well as providing some
mechanism for control and re-use spent solvent or rinse
water after processing. After careful consideration, the net
result is an environmental safe cleaning system provides
quality product without contingent liabilities.
One of the most recent developments in electronics
manufacturing today is the convergence of circuit board
and die packaging technologies. The new developments
for each technology can be combined for higher
performance electronic devices. Technical issues such as
low standoff and fine pitch solder bump arrays increase the
difficulty of post-reflow defluxing. Ionics may be trapped
underneath an active circuit and the spacing between
conductors may pose a risk of electromigration. The
process engineer must develop expertise in process
optimization, process control and chemistries for the
requirements of the package design.
Statement of Problem
The cleaning industry is constantly challenged to improve
cleaning processes, staying ahead of the ever-advancing
technology curve as it applies to increasingly new fields of
application. Processes are commonly represented as
mechanical and chemical energy, temperature and time.
With increasing complexity of board and component
geometry coupled to more difficult solder paste and flux
formulations, cost of ownership concerns reduce the ability
of temperature and time to be effective process variables.
As a result, mechanical and chemical energy must make up
for reduced temperatures and shorter process times.
Typical approaches to increasing mechanical and chemical
effectiveness are counter productive for cost of ownership
concerns and achieve only diminished returns.
Statement of Purpose
New approaches to mechanical energy deliver performance
at the heart of the residue, rather than the tail of the
delivery system. Coupled with new formulations, which
are optimized for lower temperatures of operations, new
levels of performance, cost modeling and throughput are
achieved simply by answering these following questions
with a thoughtful perspective. The purpose of this work is
developing an equation that will allow optimal spray
configuration and impingement pressure at the board level,
to improve cleaning performance.
Research Questions
What kind of equations defines surface energy at
board level?
How does this affect the fluid delivery design in a
cleaning system?
How much impingement pressure is needed at the
board level?
Which is better, high pressure or high flow?
Study Hypothesis
H1: Understanding the dynamics of flow, pressure, and
dissolution, a physical equation to determine impingement
pressure to penetrate under densely populated components
can be defined.
Conceptual Model / Theoretical Framework
Time and material are the metrics that manufacturing is
judged by, to produce a quality product. The objective of
any production team is to keep the cost low while
consistently achieving product quality standards. To
achieve the lowest cost requires a high production rate and
an optimized process. The science of optimizing air spray
cleaning performance requires an accurate model to predict
performance. All cleaning systems are governed by two
fundamental principles: the solubility rate of the residue in
the cleaning solution and the physical energy available in
the cleaning system. Equation 1 describes this
relationship.
Equation 1: Process cleaning rate equation: Rp = Rs + Rd
Where;
Process cleaning rate = Rp
Static cleaning rate = Rs
Dynamic cleaning rate = Rd
In equation 1, Rp, Rs and Rd are the rates of removal of
contaminate per unit of time. Units will vary depending on
the residues being cleaned. Rates for cleaning flux residue,
rinsing wash chemistry, or drying water from a circuit
assembly can be expressed as thickness or mass
removed/second, volume flushed/second or mass
removed/second. If solder balls or other particles were in
the soil, then balls/second or particles/second would be
appropriate units. When comparing rates of different
cleaning processes, it may be appropriate to specify rates at
the same temperature since rates are temperature
dependent.
Defining “An Optimized System” - The objective of the
“spray in air”, batch, or inline cleaning systems is to reduce
time by increasing the Rd component. Maximizing the
physical energy delivered at the surface to be cleaned, in
general, increases the Rd. A word of caution: avoid
machine selection solely based on secondary indicators
such as horsepower of the pump or manifold pressure.
Shakespeare once wrote, “Full of sound and fury and
signifying nothing”.
An optimized “spray in air” is not overpowered or
oversized. An optimized cleaning system delivers the
necessary chemistry and energy to clean the most difficult
or sensitive areas, at a rate that will meet the process time
requirement using minimal chemical, energy and floor
space consumption. In this definition of an optimized
“spray in air” cleaning system, it is important to note that
the system should be designed to clean the toughest or
most electrically sensitive areas of the parts to be
manufactured. Minimum gap, largest or most complex
component usually leads the way to identifying these areas.
The Rs + Rd Balance
Understanding the balance of Rs and Rd is key in
predicting and optimizing process performance at each step
of the washing, rinsing, and drying process. Suppose for a
minute that we washed our clothes by throwing them in a
bucket of soapy water overnight. The water would dissolve
the salts and sugars thoroughly and rapidly because of the
high solubility rate (Rs). The oils and greases would
disappear slower and perhaps incompletely due to marginal
solubility and low dissolution rates. Earth’s persistent
gravity (9.8M/sec) would be the only energy available to
the static bucket cleaner to remove insoluble dirt and other
adherent soils and some of it might drop off, but most
would remain behind. Understanding the nature of the soil
and the chemical and physical needs for removing it, allow
us to select the bucket for a silk blouse with a coke stain or
the heavy-duty cycle on our washing machine for kids play
clothes.
Rates of Rinsing and Drying
Equation 1 also applies in the rinsing or drying cycles. In
the rinsing cycle, we are removing wash fluid with the
soils dissolved or suspended. In the dryer, we are
removing relatively clean rinse water by evaporation or
displacement. Displacement of water by air impingement
is preferred in automated systems, as evaporative dryers
require 100 to 1000 times more time than displacement
dryers and can leave residues or etch metallic surfaces.
Normally, Rs or Rd dominates a given cleaning process
step. Inline “air spray” cleaners and planarized batch
cleaners are examples of cleaning processes dominated by
high Rd’s. Dip tanks, “spray under immersion”,
“dishwasher style” batch “air spray” cleaners and vapor
degreasers are examples of cleaning processes dominated
by high Rs. Typical dominance of spray in air cleaner
designs are shown in Table 1:
Table 1: Comparison of Rs and Rd for typical cleaning
processes
It is important to note the differences between “dish
washer style” batch systems, and inline “air spray”
systems. These differences primarily arise due to
“shadowing” of parts and certain board surfaces in 3
dimensional racking baskets (see figure 1). The inline and
planarized batch process has a higher % Rd because all
board surfaces are delivered tangent to the direction of
spray and are therefore much less subject to shadowing.
Figure 1: Shadowing in Dishwasher Style Cleaners
Understanding Surface Energy - Surface
Types of Surface Energy
Surface energy is the energy available at the cleaning
surface to do work. Remember that Rd in equation 1 is the
rate work as contributed by the cleaner. This rate is
exclusively driven by surface energy. Energy is measured
in ergs for mass, and velocities are measured in grams and
centimeters.
For conversion convenience, 1 Joule = 107 ergs = 0.239
calories = .73 ft.lbs = 2.78X10-7kW.hrs. Force is measured
in dynes and is the force exerted by one gram accelerated
by earth’s gravity for one cm. Translated from English to
metric, 1 lb/in2 = 68948 dynes /cm2.
In air spray systems, the work of cleaning requires energy
to displace a fluid across a distance to create the force
sufficient to achieve the rate of cleaning, rinsing and
required drying. Fluid flow is created by spray
impingement pressure, gravity drainage, and capillary
action. Table 2 shows impingement pressure can be the
most significant force available to do work.
Table 2: Sources of surface energy in aqueous systems
wc = water column
Surface Tension & Capillary Action
Surface tension can be thought of as a balloon of sorts
surrounding the cleaning fluid. If it is thin and weak, the
cleaning fluid can easily move in an out of tight spaces. If
surface tension is strong, it will resist flow into tight
spaces.
Capillary attraction or repulsion is a force resultant of
adhesion, cohesion, and surface tension of cleaning fluid in
contact with the parts to be cleaned. The resultant
interfacial pressure is referred to as the capillary force.
Graph 1 shows the interfacial pressure difference for water
in a planar and cylindrical glass interface as a function of
the gap. The pressure can be calculated by equation 2 or 3.
System
Design
Wash Rinse Dry
%Rs %Rd %Rs %Rd %Rs %Rd
Static
Immersion
100% 0% 100
%
0% 100
%
0%
Dish
Washer
70% 30% 70% 30% 50% 50%
Planarized
Batch
30% 70% 40% 60% 20% 80%
Inline Air
Spray
20% 80% 20% 80% 2% 98%
Source of
Surface
Energy
Range of
Energy
Available
Governing
Equation
Capillary action 0-2”wc
(0-1.0 psi)
Pc = 2. γ / R
Gravity Flow 0-1”wc
(0-0.5 psi)
Pg = ρ.g.h
Impingement
Pressure
0-275”wc
(0-10 psi)
Pi = ½(rv2)
= Shadowed area = circuit boards
Graph 1
Equation 2: Interfacial pressure differential (for planes)1
Δp = 2γ / R
Equation 3: Interfacial pressure differential (for tubes)1
Δp = γ / R
Where γ = surface tension; R = radius meniscus
Capillary forces can work for you and against you. They
can facilitate the initial wetting of tight spaces. They also
will inhibit the rinsing and drying steps by resisting
displacement forces if insufficient. Graph #1 indicates it
would take an air jet impingement force of greater the 1-psi
to displace water trapped in spaces less than 1 mil.
Calculating Surface Energy
Cleaning fluids can have both potential and kinetic energy.
The potential energy can be expressed as shown in
equation 4.
Equation 4: Potential energy of one unit volume of fluid at
rest2 = Ep= ρ.g.h
Where;
ρ = density of fluid
g = acceleration due to gravity = 9800 cm/sec/sec
h = height of fluid (cm)
The kinetic energy contribution can be calculated as shown
in equation 5.
Equation 5: Kinetic energy of one unit volume of cleaning
fluid in motion2 = Ek = ½ ρv2
Where ρ = density of fluid
v = velocity of jet
Solving equations 4 and 5 for water systems reveals two
important points. First, the potential energy is small as
compared to the kinetic impingement. Second, the Ek is
driven by the v2 term, which can be maximized by nozzle
and pump design. This is the basis of the earlier warning
not to purchase equipment solely based on horsepower. If
the jets or nozzles are not optimized for the pump and
stand off distance, excessive splash or spray atomization
can retard the energy delivered by reducing impingement
velocity. A well-designed spray manifold will deliver
energy of the pump efficiently over distance with minimal
losses of this type.
The efficiency of a manifold can be measured by dividing
the impingement pressure at maximum working distance
by the manifold pressure. An average spray manifold
efficiency of 5-10% is typical of today’s “spray on air”
cleaning systems. New high efficiency spray manifold
designs can achieve 25% and increase surface cleaning
energy by a factor of 1/2V2. This can boost surface energy
2X to 10X in the cleaning, rinsing and drying steps.
Bernoulli Equation Modifications
Daniel Bernoulli suggested in the 1700’s that the total
pressure of a fluid in a pipe is equal to the static pressure
plus the kinetic energy and the potential energy. This
relationship is well known and published in every physics
textbook2.
Equation 6: Pe + ½ ρv2 + ρgh = total pressure = Constant
Where;
Pe = internal pressure energy
ρ = density of fluid
v = velocity of fluid
g = acceleration due to gravity = 9800 cm/sec/sec
h = height of fluid (cm)
We can apply the Bernoulli equation to an unrestrained jet
striking the cleaning surface by removing the internal
pressure energy factor Pe and adding the surface energy
effect of capillary action. This gives a modified Bernoulli
equation, which can be used to predict the cleaning surface
energy.
Equation 7: Bernoulli Equation modified for surface
cleaning energy:
½ ρv2 + ρgh + 2γ /R = total force at tightest gap
R is the contact radius of the meniscus in the gap. It will
change with gap size from surface to surface. R will have
a negative value if meniscus is convex instead of concave.
Interfacial pressure difference at equilibrium
10
1
psi
0.1
0.01
0 20 40 60
Gap/diameter, mils
Planar Cylinder
The implication of a negative force at any step is that the
washing, rinsing, or drying fluids will not penetrate and the
overall cleaning rate would be zero in that gap.
Spray System Nozzle Design
Nozzles are used in air spray systems to create jets that
carry the energy to the surface of the part to be cleaned,
rinsed, or dried. The design and layout of the nozzles
becomes important if the cleaning system is to be truly
optimized. Equation 5 gives the kinetic energy of the jet at
the surface of the board. This equation has both mass and
velocity components. Conical and fan nozzles spread the
spray to cover larger areas at the expense of reducing the
mass per unit and velocity of the jet. Coherent jets hold
together longer and delivers more energy to a smaller area
requiring more nozzles and a higher over all flow rate.
Table 3: Comparison of Fluid Jets
Spray Type Typical
pressure @
2”,50psi man.
/Pressure loss/in
Indicated
use
Fan/Delta 1-2 psi /
~50% drop/inch
Wide
coverage,
overlapping
high
impingemen
t for close
work
distance
Conical 0.2-0.4 psi /
~75% drop/inch
Widest
coverage
area, lowest
kinetic
energy,
flooding
applications
Coherent 10-20 psi /
~10%
drop/inch
Smallest
coverage,
highest
energy
density over
longest
distance
All jets will break-up and slow down over distance in air.
Coherent jets hold together longer giving the maximum
energy transfer per unit area. Overlapping jets can be an
effective strategy for increasing surface energy density as
long as the splash at the surface does not dampen the
impact force.
Empirical measurements of coherent and fan jet nozzles
are listed in table 4 and show a coherent jet delivers 5-10 X
more impingement pressure than fan nozzles at the same
manifold pressure depending on distance3.
Table 4: Impingement/flow data of coherent jet vs. fan
The cleaning rates in inline and planar racked batch
cleaners can be significantly improved if designed with
coherent jets or a combination of fan-jets and coherent jets
in the wash and rinse sections. As previously mention,
coherent jets in the dryers offers considerable improvement
in effectiveness.
Inline Cleaning, Jets and Timing
The timing and sequence of events in a cleaning process is
critical. Each section or step in the process requires careful
thought and understanding. The pre-wash should
thoroughly wet the parts with the wash solution chemistry
and provide sufficient flow and contact time to bring the
assembly to wash temperature. This facilitates the full
static-cleaning rate, Rs, in the cleaning chemistry.
Residues are softened and dissolved in the soak zone
between the pre-wash and the wash segment.
In the wash zone, the part should see several high
impingement scourings, punctuated by brief soak periods.
This optimizes the static rate by maintaining fresh cleaning
fluid and optimizes the dynamic rate by focusing the
maximum physical energy at the part surfaces.
Manifold
Pressure
Flow:
(gpm)
Impingement psi
@
Coverage
width @
0.075”Coherent Jet
1”
2”
4”
1.5” 4.0”
30 psig 0.69 15 10 6.5 0.6 0.7
40 psig 0.82 17 12 8 0.6 0.7
50 psig 0.89 19 13 9.5 0.6 0.8
60 psig 0.97 20 15 11 0.6 0.8
F40-1.0 Fan Nozzle
30 psig 0.89 3.2 1.6 0.2 1.5 3.25
40 psig 1.06 4.4 1.8 0.3 1.7 3.60
50 psig 1.20 6.0 2.3 0.5 1.7 4.0
60 psig 1.30 7.2 2.5 0.5 1.8 4.0
In the chemical isolation section there should be ample
impingement force in the first air jet manifold to strip the
wash chemistry from the assembly so that it can be
returned to the wash tank. A second jet air and/or water
manifold should thoroughly remove any remaining residue
to drain. In the power rinse section, a series of high-
pressure nozzles removes and dilutes the remaining ions
using de-ionized water. A low flow/pressure final pure
rinse of DI water and one more soak to ring out the very
last ions, and boom, there ready for a high-speed
impingement displacement dry.
Dishwashers are Different
“Dishwasher” style cleaners are different from planar type
batch and inline cleaners. Dishwasher style cleaners are
most efficient when designed for maximum flow, not
impingement, as potential energy and capillary forces must
be relied upon for cleaning surface energy in the shadowed
areas. Using too high pressure tends to atomize the spray
reducing jet velocity much faster over distance and
increases the splash interference with other jets.
Optimized jets in “dishwasher” style batch cleaners
provide large cohesive droplets from relatively large, low-
pressure coherent jets. These large droplets adhere to the
assemblies rather than bounce or splash like their high
pressure cousins and provide a more consistent and
through cleaning in shadowed areas.
Energy in “dishwasher” style cleaners can be optimized by
fixturing the assemblies in a way such that the chance of
shadowing is minimized.
Solubility’s Contribution
The traditional approach “dissolve it” is age old, tried and
true. Although it is often augmented with various forms of
physical assistance such as heating, blasting and scrubbing,
dissolving the residue remains fundamental to most
cleaning applications. The Rate of solubility, Rs, in any
given cleaning system is dependent on the dissolution rate
(Dr); the effect temperature of the residue being dissolved
(Tc); and the concentration of the residue in the solvent
(Cc).
Table 2: Effect of temperature on various residue types
Table 2 represents three distinct residue types. The top
line indicates residues like finger salts that are very soluble
at room temperature. The only challenge here is to assure
all surfaces are wetted and rinsed properly. The middle
line represents a residue, which is much more soluble in
heated wash solution. Heating here would shorten
cleaning time and improve cleaning efficiency. The third
line represents a marginally soluble residue. It requires
heat and/or physical energy to assist in the removal
process.
Equation 8: Rs = Dr x Tc x Cc
The Dr, Tc, and Cc can easily be determined
experimentally by weighing the residue dissolved in a
fixed period of time. The temperature coefficient, Tc is the
ratio of the rate at a higher temperature, divided by the rate
at a lower temperature. Similarly, concentration
coefficient, Cc can be determined by dividing rate of
dissolution in contaminated solvent by the rate of
dissolution in pure solvent.
Contamination Effect
Remember what you learned in chemistry class, “like
dissolves like”. This is generally true for most solvent
wash solutions, but not all cleaning solutions or cleaning
applications. It is true that the more rosin you dissolve in
terpene, the better it works, up to a point. It eventually
gets too thick or difficult to rinse. This adage holds when a
solvent chemically similarly to the residue is used to
remove a residue.
This would not hold true with a saponification
cleaning system or other cleaning systems where one or
more of the cleaning agents are depleted over time. This
also would not be the case if marginally soluble salts or
weak organic acids were being used in an aqueous system.
Contamination of the wash bath in these cases slows the
cleaning process by shifting the chemical equilibrium of
the wash solution. The ionization potential (Pka) and
corresponding relationship to dissolved and un-dissolved
ionic concentrations in equilibrium is shown in equation 94.
Equation 9: Pka=[anion][cation]/[residue]
The complexities of this arise when residues contain
multiple constituents. Some have high solubility, and
some may have low solubility. Dissolving the soluble
constituents may leave behind the insoluble constituents as
a physical residue requiring significant time and/or
physical energy to remove.
Conclusion
The science of optimizing air spray cleaning performance
requires an accurate model to predict performance. All
cleaning systems are governed by two fundamental
principles: the solubility rate of the residue in the cleaning
solution and the physical energy available in the cleaning
Soluble
Very Soluble
Marginal Solubility
Dissolution Rate
Temp
system. Maximizing the physical energy delivered to the
surface to be cleaned increases the dynamic cleaning rate.
An optimized cleaning system delivers the necessary
chemistry and energy to clean the most difficult or
sensitive areas, at a rate that will meet the process time
requirement using minimal chemical, energy and floor
space consumption. Understanding the static cleaning rate
plus the dynamic cleaning rate balance is key in predicting
and optimizing process performance at each step of the
washing, rinsing, and drying process.
Surface energy is the energy available at the cleaning
surface to do work. The surface energy can be calculated at
any point using a modified Bernoulli Equation. In air
spray systems, the work of cleaning requires energy to
displace a fluid across a distance to create the force
sufficient to achieve the rate of cleaning, rinsing, and
required drying. If surface tension is thin and non-
restrictive, the cleaning fluid can easily move in and out of
tight place. Capillary forces can work for and against the
process since they facilitate initial wetting but inhibit
rinsing and drying steps. Understanding the fluids potential
and kinetic energy allows for a nozzle and pump
configuration that maximizes surface energy. The
efficiency of the manifold can increase surface cleaning by
as much as 25%.
The design and layout of the nozzles is an important step
toward optimization. Conical and fan nozzles spread the
spray to cover larger areas. Coherent jets hold together
longer giving maximum energy transfer per unit area.
Overlapping jets can be an effective strategy for increasing
surface energy density.
The rate of chemical dissolution can be augmented with
various forms of physical assistance such as heating,
impingement and time. Contamination loading can slow
the cleaning process by shifting the chemical equilibrium
of the wash solution. Contaminant complexities arise when
residues contain multiple constituents.
Follow On Research
Phase II of this research proves or nullifies the research
hypothesis. The study design will focus on the solubility
rate of the cleaning solution at static conditions. Once the
cleaning solution rate is known, how will it be improved
applying physical energy to the board surface? The
designed experiment will test the effect of energy applied
to the board surface. Logic tells us that a known
dissolution rate and a known surface energy configuration
will allow for an equation that will allow an engineer to
calculate cleaning time and distance.
References
1. Ira N. Levine, Physical Chemistry, 2nd edition. McGraw
and Hill, p.347
2. Gettys, Keller, Skove. Classical and Modern Physics,
1989,McGraw and Hill; p.339
3. Steve Stach, Austin American Technology Lab
notebook, 2004; p.45
4. Lange, Handbook of Chemistry, 8th edition; p.1777
Authors
Steve Stach, President of Austin American Technology,
which is a market leader of aqueous bath and aqueous
inline cleaning equipment. E-mail address: sstach@aat-
corp.com
Mike Bixenman, Chief Technology Officer of Kyzen
Corporation, which is leading provider of engineered
cleaning materials for electronic assembly manufacturing
environments. E-mail address: [email protected]