Transcript Document

Improving Solder Paste Reflow
Mike Fenner
Technical Manager
Indium Europe
March 2010
Slide 1
Today
• Heat transfer and equipment
• How to profile, variables to consider
• Understanding and designing the
“Best profile”
• Understanding what the profile does
Slide 2
Heat transfer
Revision
Physics: Transferring thermal energy
• Conduction
• Radiation
• Convection
Slide 3
Reflow Equipment
• Conduction
– Hot plate/travelling hot plate – Thick film guys
– Hot bar – Specific components
– Soldering iron – Repair, odd form
• Induction - Another industry another day
Slide 4
Reflow Equipment
Leaves Convection
• Vapour Phase Reflow
[Condensation Soldering]
• Forced Air convection
Slide 5
Reflow Equipment
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 DT at reflow
Slide 6
Vapor Phase Reflow Oven
(Batch Style)
1980s
Elegant and simple concept
Temperature rise rate/ RAMP rate???
Anaerobic?
21st C
Cost??
Mass Production???
Generally high mix/ low
volume/prototyping
Slide 7
Reflow Equipment
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
Slide 8
Convection Reflow Ovens:
Dominate the Industry
Courtesy: Electrovert
Slide 9
Before we go any further
• 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
Slide 10
Classic Profiling concept
Slide 11
Z1
Z2
Z3
Z4
Z5
Z6
Z1
Z2
Z3
Z4
Z5
Z6
Z7
Z7
CoolingCooling
Capturing a temperature profile
•
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.
T
Peak Temp
Liquidus Temp
Soak Exit Temp
Soak Entry Temp
Heating
Rates °c/s
•
Soak time
t
Time Above Liquidus
Slide 12
RAMP
SOAK
Reflow
COOL
There are 2 basic methods….
Fishing wire method
• 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
Slide 13
So in practice - how do you
determine best profile?
• Use a data logger or Profiler
• Use predictive software with SPC
• What is the ‘best profile’?
Slide 14
Thermal Profilers
Slide 15
Process Variables
•
•
•
•
•
•
Slide 16
Oven type and settings
Solder paste and flux
Board finish
Components – technology
PCB substrate and layout
Throughput
The Lead Free Challenge
• 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
Slide 17
Sn/Pb process window
• 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
Peak Temp Deg C
TOO HOT
235
OK
30C
205
TOO COLD
WE HAVE A 30C PROCESS WINDOW TO WORK WITH !
Slide 18
SAC process window
• 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
Peak Temp Deg C
TOO HOT
235
OK
8C
227
TOO COLD
WE NOW HAVE AN 8C PROCESS WINDOW TO WORK WITH !
Slide 19
Reduced process window
• 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
Slide 20
Pass through profiling system
method - AKA Data Logger
• 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
Slide 21
Methods of thermocouple attachment
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
Slide 22
Where to attach TC’s ?
• 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.
Slide 23
Some pointers …
•
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
Slide 24
Profiling do’s and don'ts
• 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.
Slide 25
Profile Prediction
• 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.
Slide 26
Optimising Reflow
Conventional / New Profiles
Common Defects
Ideal Profile Design
Slide 27
Do a proper DoE
Proprietary/Predictive Programs with
SPC
Ours is ReflowCoach™
Slide 28
Or use SPC tools which come
with good profilers
• Instantly produces run
charts for each
process parameter
• Also calculates
XBar,σ,Cp and Cpk
• Source data selected
from profile database
Slide 29
Potential Reflow Problems
250
3.
4.
5.
Splatter, thermal shock
Insufficient solvent
evaporation
1
Oxidation, too much flux
2
activation
Insufficient flux activity
TAL
200
Temperature (oC)
1.
2.
150
4
6
100
0
b)
Slide 30
3
50
a)
6.
5
Long/Hot: IM too thick,
component damage
Short/Cool: trapping of
flux, voids
Too fast: thermal shock
Too slow: large grains=>
weak joint
0
50
100
150
200
Time (seconds)
250
300
350
400
Conventional Profile Design
IR sensitive to variation in parts feature.
Soak zone helped to reduce temperature gradient
Temperature (°C)
250
200
Cold spot
Hot spot
MP
150
100
50
0
0
Slide 31
100
200
300
Time (seconds)
400
500
Optimized reflow profile via
defect mechanisms consideration
Slow ramp-up to 195°C, gradual raise to
200°C, spike to 230 °C, rapid cool down.
Temperature (°C)
250
200
150
Profile
MP
100
50
0
0
Slide 32
100
200
300
Time (seconds)
400
500
Defect Mechanisms Analysis
• Tombstoning / Skewing
– uneven wetting at both ends of chip
Slide 33
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
Slide 34
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)
Slide 35
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
Slide 36
Defect Mechanisms Analysis - V
• Solder beading
– Slumping (Viscosity drops w/ increasing
temperature)
– Spattering (Rapid outgassing under low standoff
components)
Slide 37
Beading is more often a result of poor
aperture design
Defect Mechanism - VI
• 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)
Slide 38
Defect Mechanisms Analysis - VII
• Voiding
– excessive oxidation (minimize heat input prior to
reflow (minimize soaking zone, or use linear rampup 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)
Slide 39
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)
Slide 40
Voiding changes things
Slide 41
Summary
• 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.
Slide 42
Further reading: In depth explanation of
what we’ve just seen
Slide 43
Finally
Component Placement
15%
Reflow
15%
Incoming Components
6%
Optimizing printing and
reflow processes can
alleviate almost 80% of
defects.
Slide 44
Solder Paste Screen Printer
64%
That’s it
• Thank you for your attention
• Questions
Acknowledgements and thanks to Solderstar for their assistance in
preparation of this presentation
www.solderstar.co.uk
Slide 45