optimising-reflow

Nokia Internal Use Only

Slide 1

Improving Solder Paste Reflow

Mike FennerTechnical Manager

Indium EuropeMarch 2010


Slide 2

Today

• Heat transfer and equipment

• How to profile, variables to consider

• Understanding and designing the “Best profile”

• Understanding what the profile does


Slide 3

Heat transfer

RevisionPhysics: Transferring thermal energy

• Conduction

• Radiation

• Convection


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

Reflow Equipment


Slide 5

Reflow Equipment

Leaves Convection

• Vapour Phase Reflow[Condensation Soldering]

• Forced Air convection


Slide 6

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 T at reflow


Slide 7

Vapor Phase Reflow Oven(Batch Style)

1980s

21st C

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

Generally high mix/ low volume/prototyping


Slide 8

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 9

Convection Reflow Ovens:Dominate the Industry

Courtesy: Electrovert


Slide 10

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 11

Classic Profiling concept

Z1 Z3 Z4 Z5

Z7

Z6Z2

Z1 Z2 Z3 Z4 Z5 Z6 Z7 CoolingCooling


Slide 12

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.

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….


Slide 13

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 14

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 15

Thermal Profilers


Slide 16

Process Variables

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


Slide 17

The Lead Free Challenge

• Component IntegrityMax package temperatures currently 235-240C

Excess heating has unknown effect on device MTBFWidespread use of ‘delicate’ package types.

• Reduced process windowLead free pastes have liquidus temp 30-40C higher than Sn/Pb


Slide 18

Sn/Pb process window

Peak Temp Deg C

205

235OK

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 !


Slide 19

SAC process window

Peak Temp Deg C

227

235OK

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 !


Slide 20

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 21

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 22

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 23

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 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

Some pointers …


Slide 25

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 26

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 27

Conventional / New Profiles

Common Defects

Ideal Profile Design

Optimising Reflow


Slide 28

Do a proper DoEProprietary/Predictive Programs with SPC Ours is ReflowCoach™

T i m e 2 6 5T e m p 2 1 5

T i m e - s 0 9 0 1 4 0 1 9 0 2 3 0 2 5 0 2 9 5 3 2 5

T e m p - C 3 0 9 0 1 3 0 1 7 5 1 8 3 2 0 0 1 8 3 1 2 0

T h r u p u t C a l c u l a t o rT u n n e l L e n g t h c m 2 4 9 T h r u p u t B d s / m i n 2 . 8 4 O b e y s D w e l l C r i t e r i a ? Y e s

B e l t S p e e d c m / m i n 7 1 P r o fi l e T i m e ( m i n ) 3 . 5 1

P r o d u c t L e n g t h c m 2 0

P r o d u c t S p a c i n g c m 5

T i m e

Temperatu

re183 C


Slide 29

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 30

Potential Reflow Problems

1. Splatter, thermal shock2. Insufficient solvent

evaporation3. Oxidation, too much

flux activation4. Insufficient flux activity5. 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

0 50 100 150 200 250 300 350 400

Time (seconds)T

emp

erat

ure

(o

C)

1

3

2

5

4 6


Slide 31

Conventional Profile Design

0

50

100

150

200

250

0 100 200 300 400 500

Time (seconds)

Tem

pera

ture

(°C

)

Cold spotHot spotMP

IR sensitive to variation in parts feature.

Soak zone helped to reduce temperature gradient


Slide 32

Optimized reflow profile via defect mechanisms consideration

0

50

100

150

200

250

0 100 200 300 400 500

Time (seconds)

Tem

per

atur

e (°

C)

ProfileMP

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


Slide 33

Defect Mechanisms Analysis

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


Slide 34

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 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)


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


Slide 37

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 38

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 39

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 40

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 41

Voiding changes things


Slide 42

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 43

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


Slide 44

Finally

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

Solder Paste Screen Printer

64%

Incoming Components6%

Reflow15%

Component Placement15%


Slide 45

That’s it

• Thank you for your attention• Questions

Acknowledgements and thanks to Solderstar for their assistance in preparation of this presentationwww.solderstar.co.uk

optimising-reflow

Documents