Transcript SMTAI Technical Forum Cleaning Optimization 9-22-04 - AAT

Optimizing Cleaning Energy
in Batch and Inline Spray
Systems
SMTAI Technical Forum
Steve Stach, Austin American Tech.
Mike Bixenman, Kyzen Corp.
Agenda
1. Technology Advancements
2. Research Questions
3. Theoretical Framework
4. 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
Process cleaning rate equation
Rp
Process
Cleaning
Rate
Rs
Static
Cleaning
Rate
Rd
Dynamic
Cleaning
Rate
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
D esign
Static
Im m ersion
D ish
W asher
Planarized
B atch
Inline A ir
Spray
W ash
% Rs
100%
%Rd
0%
70%
R inse
%Rd
0%
30%
% Rs
100
%
70%
30%
70%
20%
80%
D ry
%Rd
0%
30%
% Rs
100
%
50%
40%
60%
20%
80%
20%
80%
2%
98%
50%
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
= S had ow ed area
= circuit b oards
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
S o u rce
S u rfa ce
E n erg y
of
R ange
E n erg y
A v a ila b le
of
G o v ern in g
E q u a tio n
C ap illary actio n
0 -2 ” w c
(0 -1 .0 p si)
Pc = 2 γ / R
G ravity F lo w
0 -1 ” w c
(0 -0 .5 p si)
Pg = ρ g h
Im p in ge m ent
P ressure
0 -2 7 5 ” w c
(0 -1 0 p si)
P i = ½ (rv )
.
. .
2
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 p ressure d ifference at eq uilib riu m
10
1
p si
0 .1
0 .0 1
G ap /d iam eter, m ils
0
20
P lanar
C ylind er
40
60
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
tightest gap
– 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
S p ray T yp e
T yp ical
p ressu re @
2 ”,5 0 p si m an .
/P ressu re
lo ss/in
2 p si /
~50%
d ro p /in ch
In d icated u se
C o nical
0 .4 p si /
~75%
d ro p /in ch
W id est co v erag e
area, lo w est
k in etic en erg y,
flo od in g
ap p licatio n s
C o herent
1 0 p si /
~25%
d ro p /in ch
S m allest co v erag e,
h ig h est en erg y
d en sity o v er
lo n g est d istan ce
F an/D elta
W id e co v erag e,
o v erlap p in g h igh
im p in g e m en t fo r
clo se w o rk
d istan ce
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
M anifo ld
P ressure
Im p in ge m ent p si
@
C o verage
w id th @
0 .0 7 5 ”C o herent Jet
1”
2”
4”
1 .5 ”
4 .0 ”
30
40
50
60
15
17
19
20
10
12
13
15
6 .5
8
9 .5
11
0 .6
0 .6
0 .6
0 .6
0 .7
0 .7
0 .8
0 .8
3 .2
4 .4
6 .0
7 .2
1 .6
1 .8
2 .3
2 .5
0 .2
0 .3
0 .5
0 .5
1 .5
1 .7
1 .7
1 .8
3 .2 5
3 .6 0
4 .0
4 .0
p sig
p sig
p sig
p sig
F lo w :
(gp m )
0 .6 9
0 .8 2
0 .8 9
0 .9 7
F 4 0 -1 .0 F an N o zzle
30
40
50
60
p sig
p sig
p sig
p sig
0 .8 9
1 .0 6
1 .2 0
1 .3 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
Very Soluble
• Rate of solubility
– dependent on dissolution
rate
– temperature effect in
dissolving residue
– concentration of solvent
needed to dissolve residue
Temp
Soluble
Marginal Solubility
Dissolution Rate
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
by shifting 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
Follow on Research
• Phase II will prove hypothesis with practice
– will prove of nullify research hypothesis
– study design will focus on solubility rate of the cleaning solution
at static rate
– once cleaning solution rate is known, how will it be improved
applying physical energy to the board surface?
– The DOE will test the effect of energy applied to the board
surface
– Logic tells us that a known dissolution rate will allow for an
equation that will allow an engineer to calculate cleaning time
and distance
Authors
• Steve Stach - Austin American Technology
– [email protected]
• Mike Bixenman - Kyzen Corporation
– [email protected]