Optimising reflow oven for SMT

transcript

www.smthelp.comSlide 1

Optimising Reflow Oven

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

Revision Physics: Transferring thermal energy

• Conduction • Radiation • Convection

• Conduction

– Hot plate/travelling hot plate – Thick film guys – Hot bar – Specific components – Soldering iron – Repair, odd form

• Induction - Another industry another day

Leaves Convection

• Vapour Phase Reflow [Condensation Soldering]

• Forced Air convection

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

21st C

Elegant and simple concept Temperature rise rate/ RAMP rate??? Anaerobic? Cost?? Mass Production???

Generally high mix/ low volume/prototyping

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

Courtesy: Electrovert

• 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

Z1 Z3 Z4 Z5

Z1 Z2 Z3 Z4 Z5 Z6 Z7 CoolingCooling

• 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

Heating Rates °c/s

RAMP SOAK Reflow COOL

• There are 2 basic methods….

• 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

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

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

• 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/

Peak Temp Deg C

TOO HOT

TOO COLD

• 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 !

Peak Temp Deg C

TOO HOT

TOO COLD

• 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 !

• 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

• 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

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

• 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.

• 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

• 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.

• 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.

Conventional / New Profiles

Common Defects

Ideal Profile Design

Optimising Reflow

Time 265Temp 215

Time-s 0 90 140 190 230 250 295 325Temp-C 30 90 130 175 183 200 183 120

Thruput CalculatorTunnel Length cm 249 Thruput Bds/min 2.84 Obeys Dwell Criteria? YesBelt Speed cm/min 71 Profile Time (min) 3.51Product Length cm 20Product Spacing cm 5

• Instantly produces run charts for each process parameter

• Also calculates XBar,σ,Cp and Cpk

• Source data selected from profile database

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 shockToo slow: large grains=> weak joint

0 50 100 150 200 250 300 350 400

Time (seconds)Te

Conventional Profile DesignTe

re (°

Time (seconds)

0 125 250 375 500

Cold spotHot spotMP

IR sensitive to variation in parts feature. Soak zone helped to reduce temperature gradient

Optimized reflow profile via defect mechanisms considerationTe

re (°

Time (seconds)

0 125 250 375 500

ProfileMP

Slow ramp-up to 195°C, gradual raise to 200°C, spike to 230 °C, rapid cool down.

Defect Mechanisms Analysis

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

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

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)

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

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

• 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)

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)

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)

• 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.

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

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

Solder Paste Screen Printer64%

Incoming Components6%

Reflow15%

Component Placement15%

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Optimising reflow oven for SMT

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