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Improving Solder Paste Reflow. Mike Fenner Technical Manager Indium Europe March 2010. Today. Heat transfer and equipment How to profile, variables to consider Understanding and designing the “Best profile” Understanding what the profile does. Heat transfer. Revision

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improving solder paste reflow
Improving Solder Paste Reflow

Mike Fenner

Technical Manager

Indium Europe

March 2010

Slide 1

today
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
Heat transfer

Revision

Physics: Transferring thermal energy

  • Conduction
  • Radiation
  • Convection

Slide 3

reflow equipment
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 equipment5
Reflow Equipment

Leaves Convection

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

Slide 5

reflow equipment6
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
Vapor Phase Reflow Oven(Batch Style)

1980s

  • Elegant and simple concept
    • Temperature rise rate/ RAMP rate???
    • Anaerobic?
    • Cost??
    • Mass Production???
  • Generally high mix/ low volume/prototyping

21st C

Slide 7

reflow equipment8
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

before we go any further
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
Classic Profiling concept

Z7

Z1

Z2

Z3

Z4

Z5

Z6

Z7

Cooling

Cooling

Z1

Z2

Z3

Z4

Z5

Z6

Slide 11

capturing a temperature profile

T

Peak Temp

Liquidus Temp

Soak Exit Temp

Soak Entry Temp

Heating Rates °c/s

Soak time

t

Time Above Liquidus

RAMP

SOAK

Reflow

COOL

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.
  • There are 2 basic methods….

Slide 12

fishing wire method
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
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

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

Slide 16

the lead free challenge
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
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

30C

OK

205

TOO COLD

WE HAVE A 30C PROCESS WINDOW TO WORK WITH !

Slide 18

sac process window
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

8C

OK

227

TOO COLD

WE NOW HAVE AN 8C PROCESS WINDOW TO WORK WITH !

Slide 19

reduced process window
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
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

where to attach tc s
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
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
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
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

slide27

Optimising Reflow

Conventional / New Profiles

Common Defects

Ideal Profile Design

Slide 27

or use spc tools which come with good profilers
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
Splatter, thermal shock

Insufficient solvent evaporation

Oxidation, too much flux activation

Insufficient flux activity

TAL

Long/Hot: IM too thick, component damage

Short/Cool: trapping of flux, voids

Too fast: thermal shockToo slow: large grains=> weak joint

Potential Reflow Problems

5

3

4

1

6

2

Slide 30

slide31

Conventional Profile Design

IR sensitive to variation in parts feature.

Soak zone helped to reduce temperature gradient

Slide 31

slide32

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.

Slide 32

slide33

Defect Mechanisms Analysis

  • Tombstoning / Skewing
    • uneven wetting at both ends of chip

Slide 33

slide34

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

slide35

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

slide36

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

slide37

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

Slide 37

defect mechanism vi
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

slide39

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)

Slide 39

slide40

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

summary
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

finally

Component Placement

15%

Reflow

15%

Finally

Solder Paste Screen Printer

64%

Incoming Components

6%

Optimizing printing and reflow processes can alleviate almost 80% of defects.

Slide 44

that s it
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