Manual Soldering and Repair Challenges in the Lead … repair challenges.pdfManual Soldering and Repair Challenges in the Lead-free Soldering Era ... (Bellcore): GR-78-CORE standard

Manual Soldering and Repair Challenges in the Lead-free Soldering Era

Inge Schildermans

Alcatel Bell, Geel

RoHS Gorinchem 23/11/05 — 2 All rights reserved © 2004, Alcatel

Introduction

> RoHS directive

> Solder processes

• Reflow soldering

• Wave soldering


Content

1. Manual Soldering

> Introduction

> Lead-free solder wire evaluation

> Impact of higher melting temperature: a physics perspective

2. Component replacement (repair)

> Introduction

> Convection heating limitations

> Radiative heating: overcoming the limitations


1. Manual soldering: Introduction

Manual soldering:

Create a solder joint between component lead and board pad or PTH using flux-cored solder wire as solder supply and conduction heating using a soldering iron as heat source.



Soldering stations:



Important parameters of soldering stations:

> Power rating (W): heating power capacity of the soldering iron

> Temperature control

• Temperature controlled iron (power control circuitry)

• Bimetallic soldering pins with specific temperature set point (Metcal)

> Soldering bits or tips: different shapes and sizes



Solder

> Flux-cored solder wire

> Parameters:

• Alloy

• Type of flux

• Flux amount

• Diameter (0.25-1.5mm)

> Lead-free:

• higher melting temperature

• reduced wetting


1. Manual soldering: Lead-free solder wire evaluation

> Alloy choice: SnAg3-4%Cu0.5-0.7% is becoming the industries lead-

free alloy choice.

> Solder wiring evaluation:

• Contrary to solder pastes and fluxes there is very little standardisation around solder wire testing.

• Define solder wire requirements

• Define solder wire tests:– Reliability evaluation: corrosion, SIR, Electro-Migration

Wire performance requirements

– Control of flux amount

– Tackiness of flux remainders

– Flux spitting: contamination of PBA, esthetics, ergonomics

– Solderability test

> Flux-cored solder wires from different suppliers were evaluated.


Reliability requirements

> Evaluation of corrosiveness of fluxes

• J-STD-004 flux classification

• acceptable classes: L0, L1, (M0)

> SIR (Surface Insulation Resistance), electromigration evaluationaccording to telecommunication standard

• Telcordia (Bellcore): GR-78-CORE standard

> Test results supplied by solder material supplier



Performance requirements

Metal/flux content

• The actual weight percentage of flux contained in the wire is measured.

• Goal: quality control of wire

• Procedure– Weigh adequate amount (10g .. 50g) ofsolder wire.

– Melt wire in a beaker,

– Remove flux residues with IPA

– Dry

– Weigh left-over metal

– Determine flux and metal content


Extracted metal



Flux remainder tackiness

> Flux residues should not be tacky to avoid contaminating particles to accumulate between solder joints which may cause SIR issues

> Test:

• Reflow solder wire on a copper surface

• Cover with chalk powder

• Should be possible to wipe off the powder easily with a soft brush




Flux spitting

> Rapid heating of the flux cored wire lead flux spattering on PCB and operators hands. Should be minimal.

> Test

• Melt specific amount of solder wireunder controlled conditions

• Collect droplets of flux that are ejected

• Determine the weight of the collected flux

• Calculate percentage of ejected flux




Solderability (solder spread)

• A solder wire ring with an inner diameter of 3mm, is allowed to wet a preconditioned copper foil.

• Visual evaluation of solder spread


++

0

--



Spread factor:

++ 0 --

Performance evaluation results:



Not all commonly available solder wires are fulfilling the no-clean reliability requirements!

Be careful with those that wet well!



Reliability evaluation results:

+ROL1SW11 supplier G

+REL0SW10 supplier F

+ROL1SW9 supplier E

-ORL0SW8 supplier E

?ORL0SW7 supplier D

-ROM1SW6 supplier D

-ROM1SW5 supplier D

-ROM0SW4 supplier D

-ROM1SW3 supplier C

-ROM1SW2 supplier B

--SW1 supplier A

Telecom requirement fullfilled?

Flux Act. Class.J-STD-004

Selected wire


How to define the manual soldering process parameters?

Parameters on which the temperature of the soldering area depends:

• PBA/PCB: thickness, build-up, land/PTH-size, material (thermal conductivity, thermal capacity), component to be soldered,...

• Soldering iron: set temperature, power, thermal capacity, temperature control, tip properties (dimensions, shape, material,...),...

• Soldering method: solder supply, board preheating temperature, iron position, contact time,...

What approach should we take?

> Leave it to the (chinese) operator to find out?

> Set-up a 10+ parameter full-factorial Design-of-Experiments?

Let’s have a look at some basic thermal physics.

1. Manual soldering: Impact of higher melting temperature



PCB

iron

Basic thermal elements totake into account



Basic physical model:

• T: solder joint temperature

• TB: soldering iron temperature

• T0: base temperature of the PBA

Power steering conditions: P

c

BBB

c

Bp

rr

TTP

t

TTC

R

TT

rr

TT

t

TTC

+−

−=∂−∂

−−

+−

=∂−∂

0

00

set

BB

c

BBB

c

Bp

TTrr

TT

t

TTC

R

TT

rr

TT

t

TTC

≥→+−

−=∂−∂

−−

+−

=∂−∂

0

00

At temperature control around TBset

or

T

TB

T0



How to reach a higher temperature?

Method 1: Increase iron temperature

• Too high a temperature can damage the component or PCB

• Select a solder tip that matches the soldering area

• Take care of proper tinning of the solder tip to reduce contact resistance

( )00 TTrrR

RTT set

B

c

−++

=−The stationary case:

Method 2: Minimize resistance r+rc of the iron and the contact areaGood soldering practice



Dynamical analysis: solve the set of differential equations

−++

=⇒=

−++

+

−+

−=

−−

−+

−+

−=

−−

0

214321

02

4

1

3

0

0

02

2

1

1

0

0

)(,,,,,,,'

)('exp

'exp

'exp

'exp

TT

Prr

R

rr

C

CfAAAA

RC

tt

TT

PrrRtA

tA

TT

TT

TT

RPtA

tA

TT

TT

set

B

cc

p

Bi

p

set

B

c

set

B

B

set

B

set

B

ττ

ττ

ττ

Board temperature profile depends on a set of dimensionless ratios:

• Iron power rating P versus iron temperature setting TBset

• Thermal resistance of the iron r+rc versus PCB resistance R

• Thermal capacity of iron CB versus local PBA capacitance Cp



Normalised temperature profile (power steering only):

TBset

solderingdomain

Tmelt



Conclusion

> For save soldering one needs to limit the soldering iron temperature while still meeting the higher melting temperature requirements of lead-free soldering.

> Soldering temperature conditions on the board should be obtained in a pre-defined region of the temperature profile for reproducible results.

> This should be done by carefully selecting soldering tips, ironsand the temperature/power setting in accordance with the thermal properties of the PBA to be soldered. Thermal capacity, thermal resistance and heating power are of major importance.

> Physical modelling gives a basic tool to master the large range of parameters influencing manual soldering.


Content

1. Manual Soldering

> Introduction

> Lead-free solder wire evaluation

> Impact of higher melting temperature: a physics perspective

2. Component replacement (repair)

> Introduction

> Convection heating limitations

> Radiative heating: overcoming the limitations


2. Component replacement: introduction

For components with interconnections under the body of the component,e.g., BGA (Ball Grid Array) and LLP (LeadLess Package)removal and resoldering the components using a soldering iron is not possible. Dedicated repair machines are required

Most of the machines are based on forced convection heating. A temperature profile approaching the standard reflow profile is used to replace these components.



Principle of forced convection:


Depends on: temperature setting per heating period, number of heating periods, air velocity, nozzle shape/dimension, PCB Pre-heating,...

PBGA

0,00

20,00

40,00

60,00

80,00

100,00

120,00

140,00

160,00

180,00

200,00

220,00

240,00

260,00

0 50 100 150 200 250 300 350 400

Seconds

°C

Board Centre (max 230°)


Increased melt temperature results in increased operating temperature and smaller process window for lead-free rework.

Temperature profilingis critical:


2. Component replacement: Convection heating limitations

( )00max1

TTHR

RTT air −+=−Maximum temperature:

( )R

TTTTH

t

TTC airp

00 −−−=

∂−∂

First order thermal model:

How to increase the temperature?

Method 1: increase air temperature

• Danger of local overheating of component

• Equipment limitation

Method 2: increase the convection heat transfer coefficient H

• H ~ area x air speed• problematic for small components: small area

• air speed increase limited: risk of blowing away components.


TEBGA

0

20

40

60

80

100

120

140

160

180

200

220

240

260

0 50 100 150 200 250 300 350 400 450 500

Seconds

°C

Comp Centre (max 249°)

Comp Corner (max 261°)

Board Centre (max 231°)

Board Corner (max 236°)

2. Component replacement: Convection heating limitations

Increased temperature difference across the component

∆T = 30oC


2. Component replacement: Radiative heating: overcoming the limitations

Is there a solution to overcome the convection heating limitations?

Yes: Radiative heating

> No (physical) heat transfer limitation

> No danger of component blow-off

> Overcomes temperature limitation

IARTT

R

TTIA

t

TTC p

α

α

=−

−−=

∂−∂

0max

00

I intensity, α absorption coefficient, A surface area



Temperature difference across the component

A uniform temperature across a component by local heating requires heating the edges of the component more than its center.

(Note: principle demonstrating drawings only)



Convection heating: use of nozzles with peripheral openings only

> Reduction of air flow and heating area reduces the maximum temperature

> Only a limited heat input modulation is possible

Radiative heating: Exploit the disadvantage of IR heating in mass reflow soldering as an advantage for local heating.

> Heat input control using optical modulation. Near 100% modulation possible.

> Add reflective tape (Al) to limit heat input

> Add absorbing black tape to increase heat input


Conclusions

> Infrared radiative local heating technique is a very promising candidate to overcome the limitations set by the conventional local convection technique.

> It is at least an important complementary technique to convection heating in order to deal with the repair challenges imposed by the switch to lead-free soldering.

> To be considered also: laser repair



QuitQuit

Qin

Tlucht

QuitQuit

a

r

a/2A

2. Component replacement: modeling the repair convection process

( )

−

−

−−−

+Ω

+

−−−=− 1

*4exp

/exp1

44exp4

/exp1)()(

2

2

2

2

2

2

0max0

tt

t

r

a

HC

t

Ktr

a

a

r

HC

tTTTtT

σ

),,,( nozzleflowtTT setair Φ=

),,(max propertiesPCBnozzleTT airΦ=


www.alcatel.be/EMS

Thank you!Thank you!

Alcatel Bell Geel

Bell telephonelaan 3

B-2440 Geel

België

Tel.: +32 14 572 142

Fax: +32 14 572 294

[email protected]

Documents

Manual Soldering and Repair Challenges in the Lead … repair challenges.pdfManual Soldering and Repair Challenges in the Lead-free Soldering Era ... (Bellcore): GR-78-CORE standard