We designed and manufactured Auto Insertion machine in Shenzhen China, and provide SMT equipments, spare parts services support. Please email: jasonwu@smthelp.com

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Optimising Reflow Oven

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• Heat transfer and equipment
• How to profile, variables to consider
• Understanding and designing the “Best profile”
• Understanding what the profile does

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Revision
Physics: Transferring thermal energy
• Conduction
• Radiation
• Convection

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Slide 4
• Conduction
– Hot plate/travelling hot plate – Thick film guys
– Hot bar – Specific components
– Soldering iron – Repair, odd form
• Induction - Another industry another day

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Leaves Convection
• Vapour Phase Reflow
[Condensation Soldering]
• Forced Air convection

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Vapour Phase Reflow
• Single chamber process
• Usually batch, can be conveyorised
– Boil Inert Liquid
– Heated Vapour Condenses on Product
(All Surfaces)
– Equilibrium process, heat transfer stops
at BP of liquid
– Not mass, shape or color sensitive
– Almost No ΔT at reflow

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Slide 7
1980s
21st C
Elegant and simple concept
Temperature rise rate/ RAMP rate???
Anaerobic?
Cost??
Mass Production???
Generally high mix/ low volume/
prototyping

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Slide 8
Convection/Forced convection
• Multi chamber (zone)
• Usually always conveyorised
– Air/nitrogen is heated and circulated
– Provides Even Heat
– Moderate Price
– Not usually, but can be, in equilibrium
• The dominant technology

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Slide 9
Courtesy: Electrovert

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Slide 10
• There is no universal best profile
• Profile is not determined by the paste
• Profile is not determined by the PCBA
• Profile is not determined by the reflow oven
• It’s a combination – and that combination is
unique to you
• Mostly its determined by the efficiency
of the oven and the workload. Paste is
secondary
– Any Recommended Profile is
therefore just a strong suggestion

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Slide 11
Z1 Z3 Z4 Z5
Z7
Z6Z2
Z1 Z2 Z3 Z4 Z5 Z6 Z7 CoolingCooling

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Slide 12
• Thermocouples are attached to components on the
PCBa
• The temperature of the components is measured as
the PCBa passes through the oven and is soldered.
Soak time
Time Above Liquidus
Peak Temp
Liquidus Temp
Soak Exit Temp
Soak Entry Temp
T
t
Heating Rates
°c/s
RAMP SOAK Reflow COOL
• There are 2 basic methods….

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• Uses oven/external measurement system
and long thermocouples
• Practical only on small ovens
• Measurements tend to be more variable
• Assembly is easily snagged and damaged
on moving conveyor parts

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• Use a data logger or Profiler
• Use predictive software with SPC
• What is the ‘best profile’?

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• Oven type and settings
• Solder paste and flux
• Board finish
• Components – technology
• PCB substrate and layout
• Throughput

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Slide 17
• Component Integrity
Max package temperatures currently 235-240C
Excess heating has unknown effect on device MTBF
Widespread use of ‘delicate’ package types.
• Reduced process window
Lead free pastes have liquidus temp 30-40C higher than Sn/
Pb

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Slide 18
Peak Temp Deg C
205
235
OK
TOO HOT
TOO COLD
30C
• Illustration for standard Sn63/Pb37 solder paste (TLiq = 183C)
• Solder paste spec specifies min peak of 205 C for good wetting
• Component maximum is 235C
WE HAVE A 30C PROCESS WINDOW TO WORK WITH !

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Slide 19
Peak Temp Deg C
227
235
OK
TOO HOT
TOO COLD
8C
• Illustration for lead free SnAgCu solder paste ( Tliq = 217C)
• Solder paste spec specifies min peak of 227 C for good wetting
• Max Peak ideally is 257C but component max is still 235C
WE NOW HAVE AN 8C PROCESS WINDOW TO WORK WITH !

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Slide 20
• Oven needs to maintain small delta T across the
board.
• Profiles need to be developed for each board
type
• Periodic profiling required to monitor and
maintain process

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Slide 21
• Follows the PCBa through the reflow
oven
• Data logger must be protected from
the heat
• Can be used on large or small ovens
• Generally more accurate and repeatable
• Must be small to pass through restricted oven tunnels
• Should be narrow to allow profiling of small PCB’s

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Method Advantage Disadvantage
Kapton Tape quick/non destructive Non permanent /
unreliable, errors
Adhesive metal foil quick/non destructive Non permanent /
unreliable, errors
High temperature
adhesive
robust/quick cure Rel. poor thermal
conductivity, errors
HMP solder
(290-305DegC)
robust/good conductivity Dedicated test PCBa req’d

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• Aim is to heat the board uniformly
• Components vary in size, mass, texture and
colour.
• PCB’s vary is size, shape, mass, component
densities
• Need to identify extremes of the profile envelope.

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Slide 24
• High mass/bigger components will heat up slowest
• Low mass/smaller components will heat up fastest
• Power components with integral heat-sinks
• Components connected to large copper ground planes
• Indirectly heated components ( BGA )
• Components nearer board edges
• Components nearer the centre / densely populated
• Components shadowed by others

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• DO make the TC leads long enough so that the profiler
follows at least 1 zone behind the PCB.
• DON’T pass the profiler through the oven first, always
behind the PCBa.
• DO profile an example of the actual board being processed.
• DON’T profile the test board again before it has returned to
ambient temperature.
• DO profile a populated board.

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• Allows the effect of heater and belt-speed set-point changes to be
predicted
• Saves time and money by eliminating the need to perform
unnecessary profile runs for set-up and fine tuning
• Reduces machine downtime by allowing process set-up to be
completed offline.
• Eases process set-up and change over to Lead Free paste
• Unique graphical approach intuitively provides guidance to the user
to optimise the process
• Quickly allows the user to evaluate the effect of paste changes on
the process.

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Conventional / New Profiles
Common Defects
Ideal Profile Design
Optimising Reflow

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Time 265
Temp 215
Time-s 0 90 140 190 230 250 295 325
Temp-C 30 90 130 175 183 200 183 120
Thruput Calculator
Tunnel Length cm 249 Thruput Bds/min 2.84 Obeys Dwell Criteria? Yes
Belt Speed cm/min 71 Profile Time (min) 3.51
Product Length cm 20
Product Spacing cm 5
Time
Temperature183C

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• Instantly produces run
charts for each process
parameter
• Also calculates
XBar,σ,Cp and Cpk
• Source data selected
from profile database

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1. Splatter, thermal shock
2. Insufficient solvent evaporation
3. Oxidation, too much flux activation
4. Insufficient flux activity
5. TAL
a) Long/Hot: IM too thick, component damage
b) Short/Cool: trapping of flux, voids
6. Too fast: thermal shock
Too slow: large grains=> weak joint
0
50
100
150
200
250
0 50 100 150 200 250 300 350 400
Time (seconds)
Temperature(oC)
1
3
2
5
4
6

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Conventional Profile DesignTemperature(°C)
0
75
150
225
300
Time (seconds)
0 125 250 375 500
Cold spo
Hot spot
MP
IR sensitive to variation in parts feature.
Soak zone helped to reduce temperature gradient

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Slide 32
Optimized reflow profile via
defect mechanisms consideration
Temperature(°C)
0
75
150
225
300
Time (seconds)
0 125 250 375 500
Profile
MP
Slow ramp-up to 195°C, gradual raise to
200°C, spike to 230 °C, rapid cool down.

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Defect Mechanisms Analysis
• Tombstoning / Skewing
– uneven wetting at both ends of chip

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Defect Mechanisms Analysis - II
• Wicking / Opens
– leads hotter than PCB
• slow ramp up rate to allow the board and
components reaching temperature equilibrium
before solder melts; more bottom side heating

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Slide 35
Defect Mechanisms Analysis - III• Solder balling
– spattering (slow ramp up rate to dry out paste
solvents or moisture gradually)
– excessive oxidation (minimize heat input prior to
reflow (slow ramp up rate, no plateau at soaking
zone) to reduce oxidation)

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Slide 36
Defect Mechanisms Analysis - IV
• Hot slump / Bridging
– viscosity drops with increasing
temperature
• slow ramp up rate to dry out paste
solvent gradually before viscosity
decreases too much

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Defect Mechanisms Analysis - V
• Solder beading
– Slumping (Viscosity drops w/ increasing temperature
– Spattering (Rapid outgassing under low standoff
components)
Beading is more often a result of poor
aperture design

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Slide 38
• Poor wetting
– excessive oxidation(minimize heat input prior to
reflow (minimize soaking zone, or use linear ramp-
up from ambient to solder melting temperature) to
reduce oxidation)

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Defect Mechanisms Analysis - VII
• Voiding
– excessive oxidation (minimize heat input prior to
reflow (minimize soaking zone, or use linear ramp-up
from ambient to solder melting temperature) to
reduce oxidation)
– flux remnant too high in viscosity (cooler reflow
profile to allow more solvents in flux remnant)

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Defect Mechanisms Analysis - VIII
• Charring - dark flux residue
• Leaching - grainy solder joint appearance
• Dewetting - uneven pad wetting
• Excessive Intermetallics - poor joint
reliability
– overheat (lower temperature, shorter time above Liquidus)

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• Temperature profiling forms a key part of
lead free processing.
• Used in both process setup and ongoing
process control
• Modern profiling equipment has extensive
tools to help setup and maintain your lead
free process.

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Slide 43
Further reading: In depth explanation of
what we’ve just seen

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Slide 44
Optimizing printing and
reflow processes can alleviate
almost 80% of defects.
Solder Paste Screen Printer
64%
Incoming Components
6%
Reflow
15
%
Component Placement
15%

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Welcome inquiry
1,Please visit : www.smthelp.com
2, Find us more: https://www.facebook.com/autoinsertion
3, Know more our team: https://cn.linkedin.com/in/smtsupplier
4, Welcome to our factory in Shenzhen China
5, Look at machine running video: https://youtu.be/LdpOUo_1vLk
4, See more in Youtube: Auto+Insertion