Lead Free Wave Soldering

7/27/2019 Lead Free Wave Soldering

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LEAD FREE WAVE SOLDERING


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Lead in Soldering

Diluent for tin which - Reduces surface tension

Reduces the melting temperature

Reduces the cost

Low environmental impact mining

Low energy smelting

Known toxic element

Processing controls Closed loop applications desirable


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Price & Availability of

Alternatives Current availability

Current price Capability to support extra market

OK Tin, copper, zinc, antimony, silver, bismuth (minor

component)

Not capable Indium


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Relative costs of selected

alloys

$0.00 $1.00 $2.00 $3.00 $4.00 $5.00 $6.00 $7.00

Sn95.5Ag3.8Cu.7

Sn99.3Cu.7

Sn63Pb37


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Alloy Decision Tree

Tin Lead

179 183C

Tin Zinc (Bi)

~190C

Tin Rich

209 - 227C

Tin Bismuth

137C

Limited interest

- low stress

applications

Flux, stability,

reliability, issues,

nitrogen reflow


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Low Melting Temperature

OptionTin Bismuth - Sn43Bi

Eutectic melting temperature 137C

Limited wetting performance

difficult to get good flux activity at lower reflow

temperatures

Good joint strength & reliability limited upper temperature

very sensitive to Pb contamination


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Higher Melting Temperature

OptionsTin Rich Eutectics

Copper eutectic low cost diluent

Silver eutectic

high cost diluent

Tin/Silver/Copper eutectic

lowest melting temperature eutectic


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Melting temperature vs melting range

190

200

210

220

230

MeltingTem

perature,C

Apparent Pasty Range, C

0 10 15 20

Sn

SnCu0.7

SnAg3.5

SnAg3.8Cu0.7

SnAg2.5Bi1Cu0.5

SnZn8Bi3

SnAg3.5Bi3

SnAg3Bi2.5In3

SnAg3Bi2.5In6

SnPb37


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DSC curve SnAg3.8Cu0.7

Single melting

temperature

eutectic


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Tin-Silver-Copper Liquidus

22

5

235

24

5

255

E

225

Liq

225

Sn

Atom-fraction, Cu

Atom-fractio

n,

Ag

E (217 oC, 3.7 at-% Ag, 1.3 at-% Cu) Thermodynamically

calculated Equilibrium eutectic at

217C SnAg3.4Cu0.7 but

microstructure is eutectic

+ Sn dendrites

Fully eutectic

microstructure needs

higher Ag and Cu:

SnAg4.7Cu1.7


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Alloy Patent Considerations

The scope and validity of patents can only be

established when the owner defends them. Loctite wants to avoid potential conflicts for

customers

negotiated licenses that allows our customers to use

our alloys and sell their products worldwide


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Lead free Alloy Patents in

Japan and USA

0 2 4 6 8 10

1

2

3

4

5

6%Cu

%Ag

Eutectic

Both patents licensedto Loctite ISU in

USA

Senju in

Japan

Apparent eutectic atreflow cooling rates


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The Significance of Alloy

Composition Control of the initial composition

wave solder bath solder powder

solder wire

Composition of the solderin process & in thefinal joint

sources of contamination

effects of contamination


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Loctite Multicore Alloy

Specifications, %

Metal 96SC 97SC TypicalAg 3.6 4.0 2.8 3.2 -

Cu 0.6 0.8 0.4 0.6 -

Pb < 0.1


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Pb-free Alloy Wetting

Behaviour Higher melting temperature

expect to need higher soldering temperatures higher soldering temperature increases flux activity

Wetting Balance Tests

quantify the wetting rate behaviour

Spread Tests

quantify the equilibrium wetting behaviour

S tti d


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Summary wetting speed vs

temp.

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

200 210 220 230 240 250 260 270 280

Test temp eratu re, C

tim

eto

2/3m

axw

etting

forc

e,s

60/40

SnCu bas e alloys

SnAg & SnAgCu

bas e alloys


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Can Wetting Rate be

Enhanced? Flux activity is the main parameter

Alloying changes Sb, Bi have been proposed as wetting rate

enhancers

Observations show the same effect of minor andimpurity elements as known for Sn/Pb alloys


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65

70

75

80

85

90

95

LS2 LS3 HS1 HS2 HS3 LS1 with

SnPb36Ag2

%

Flux Type and Spread on Cu


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Effect of Pb-free Alloy Surface

Tension Tin-rich alloys have a higher surface tension

than Sn/Pb Pb-free alloy spread and capillary filling should

be reduced

effect is sensitive to substrate surface finish

can be demonstrated by measuring contact angles

low values indicate good spread


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Alloy Contact Angle vs

SubstrateSolder paste contact angles all in the range 20 - 25for the same flux system

REFLOWED ALLOY PELLET (Sn +)SUBSTRATE0.5Cu 3.5Ag 3.8Ag0.7Cu 3.5Ag0.5Sb 3.8Ag0.7Cu0.5Sb 37Pb

Cu 42 43 43 41 43 12

Ag 19 26 24 30 33 13Sn37Pb 19 19 22 20 22 5

Sn0.7Cu 15 11 18 11 10 17

Au overNi

9 6 10 14 5 4


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SnAgCu Alloy Properties

Lower density than Sn62/63: 7.5g/cm2

Stable dispersion of intermetallics Ag3Sn,Cu6Sn5 in tin matrix

Creep strength x4 higher than SN62, Sn63 Better high temperature strength than Sn62,

Sn63 (higher melting point)

Reliability: conservative view says performanceis equal to Sn62/63


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Summary on Alloy Selection

Alloy selection should not be a barrier to Pb-

free builds The benchmark material is Sn/Ag/Cu eutectic

introduce this as the first level change from Sn/Pb

look for technical and economic improvements once

the process is stable

normal process improvement strategy


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Wave Soldering Alloy

Cost of the alloy

favours of SnCu0.7

Process benefits

favours addition of silver

improved wetting

reduced temperature

reduced leaching from Ag finishes

BUT increased machine erosion can be prevented

Balance these factors. Look at Total Cost of Ownership


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Wave soldering alloy control

Contamination from PCB, components and

machine Drossing increase?

Nitrogen inerting - also improves wetting

Increased intermetallic - gritty joints?

Increased bridging?

Low melting phases fillet lifting


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Example Temperature Profile

0 1 3

Time [min]

0

50

100

150

200

250

Temp

erature[C]

Bottom

Top

Forsten, Steen, & Wilding, Soldering & Surface Mount Technology

Bottom

Top


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Drossing with Pb-free alloys

Wave soldering temperature higher -

creates greater risk of drossing May drive to use of nitrogen

experience shows this is not essential

Effect of impurities generally greater than forSn/Pb

Some impurities experimented with as grain

refiners behave as dross promoters as wouldbe expected with Sn/Pb solders

Addition of P can reduce drossing rates


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Dissolution of Cu during Pb-free wave

solderingSolder has

reduced Cutrack thickness

C Di l ti i t


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Cu Dissolution into

SnAg3.8Cu0.7

243 254 265

Temperature C

200

400

600

800

1000

1200

1400

1600

Dissolu

tionratenm/s

Static

Dynamic


Cu wires

immersed

in solder

S A 3 8C 0 7 B th


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SnAg3.8Cu0.7 Bath

Contamination by Cu

0,79

0,83

0,88

0,94

1,01

0,79

0,85

1,02

1 2 3 4 5 6 7

[Months since implementation]

0,6

0,7

0,8

0,9

1

1,1

1,2

Cu%-winalloy

Period 1

Period 2

Cu build up came

from the PCB

>1% Cu caused

intermetallic

particles causing

bridging defects

SnAg3.6 added to

the bath to dilute theCu @ 3 & 7 months


SnAg3 8C 0 7 Bath Ag


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SnAg3.8Cu0.7 Bath Ag

Content

1 2 3 4 5 6 7 8 9


3

3,5

4

4,5

5

[Ag%-win

alloy]

Forsten, Steen, & Wilding, Soldering &

Surface Mount Technology

Ag contentstable as

expected

Ag finishesnot

dissolved

rapidly intothe solder

SnAg3 8Cu0 7 Bath


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SnAg3.8Cu0.7 Bath

Contamination by Ni

1 2 3 4 5 6 7 8 9


0,01

0,02

0,03

0,04

0,05

[Ni%-

winalloy]

Nickel accumulation is

a result of dissolution

of nozzle and potmaterials.

Attack was observed

after 2-3 months ofoperation.

Accumulation has

been stopped by fresh

solder additions to

compensate loss by

drossing and as joints.



Eroded area on nozzle flow


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Eroded area on nozzle flow

guide



Evidence of pitting on18/8 stainless steel

nozzle after 2-3 months

production

Resolved by selecting

more resistant materials

Pb contamination of Solder


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Pb contamination of Solder

Bath Most processes will bring Pb to the bath for

some time into the future Major effect is the generation of fillet lifting

defects on double-sided boards

cosmetic

no measurable effect on reliability

has to be avoided because it is not easily

distinguished from other defects that are a reliability

hazard

SnAg3 8Cu0 7 Bath


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SnAg3.8Cu0.7 Bath

Contamination by Pb

1 2 3 4 5 6 7 8 9


0

0,1

0,2

0,3

0,4

0,5

0,6

[Pb%-winalloy]

50% of all PCBs

during theobservation period

were SnPb HASL

plated and somecomponents had

SnPb finishes.

50% of boardswere lead-free

HASL (SnCu0.7)

or immersion Goldplated.Forsten, Steen, & Wilding, Soldering &


SnAg3 8Cu0 7 Pb


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SnAg3.8Cu0.7 Pb

Contamination

0.2% Pb

4.0% Pb



Pb-rich phase,

179C melting

temperature


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Example of Fillet Lifting

Fillet Lift Fillet Lift


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X-Ray image of Fillet Lifting

Courtesy - Bob Willis, Electronic Presentation Services


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Fillet Lifting

In general fillet lifting does not affect the

strength or reliability of the soldered joint Broken lands, however, will cause functional

failure


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Fillet lifting

Occurs when Sn/Pb plated through hole components are

Used in conjunction with PB free wave soldering.

The Pb containing solder is pushed to the top of the PCB

during soldering.

Creating a lower melting point area on topside fillet.

This then lifts due to difference in solidus temp and

volume contraction.

Solder Fillet - PCB Fillet


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Solder Fillet PCB Fillet

LiftingOccurs when

solder tin-rich andcontains bismuth

soldering to Sn/Pb

coated PCBsand/orcomponentleads/terminations

both of the abovetogether

Overall view of

lifted off region

Cu land

solde

rjoint

cont

ractio

n

DIL

leg

h

eatflow

FR4 epoxy

= low melting point

region

Other Pb-free wave soldering


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Other Pb free wave soldering

defects Pad lifting

Inadequate Topside hole filling

Skips and Bridges

Microballing

Pb-free wave soldering Pad


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Pb free wave soldering Pad

Lifting Fillet forms a good joint but the pad is torn

away from the PCB real reliability issue

greater strength of the Pb-free alloy

contraction on cooling not absorbed by creep in alloy do not confuse with fillet lifting

Increase the area of the pad

increases strength of the bond to the PCB

Some general conclusions from Loctite


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Some general conclusions from Loctite

studies

More stringent requirements for preheatingdue to more critical thermal dynamics overcome by enhanced preheating, with top and

bottom convection heating.

The oxide layer developing on the top of theSnAgCu alloy is more durable than withSnPb. Deal with this by developing new nozzle structures and improved

flow characteristics,

more active and sustained flux chemistry

phosphorus doping of the solder alloy

More general conclusions from Loctite


44/65


studies

Pot and nozzle materials designed for SnPbwere found to dissolve into SnAgCu resistant stainless steels were found to be

beneficial in these critical areas.

A steady increase in copper content in the

solder bath dealt with by periodically diluting the bath using

SnAg as the top-up alloy.

Drossing rate, lead build up and defect rates,after process optimisation, did not differ fromthose of earlier SnPb process



45/65


studies

Avoid alloys with a melting range on throughhole boards

including melting range created by contaminants

not critical for single side boards

Higher temperatures may require betterboard support thinner, lower Tg boards show increased warp

In some ways, implementing a wavesoldering process is harder than a reflowprocess!

Effects of lead free solder on equipment


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Effects of lead free solder on equipment

The following picture shows an area of 20*9mm on the

original wave nozzle front flow guide, where the protective

oxide layer on the stainless steel material has broken and

wetting has taken place.

The special stainless steel has dissolved to a depth of

0.8mm after only 2 months of use.

E l f l d f ld ti ith


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Example of lead free solder reacting with

stainless steel wave former

Lead free wave soldering


48/65

Lead free wave soldering

Tools

at present any tools dropped into a lead containingpot float. Making retrieval easy.

With lead free most tools will sink. Adding to

possible contamination and causing potential

damage to machine.

Conclusions


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Conclusions

Most independent industry evaluations agree

the best alternative alloy is. Sn/Ag/Cu Solder looks and feels different.

Joint inspection criteria will have to be revised

for voiding and visual appearance.


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VOC Free

VOLATILE ORGANIC


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COMPOUNDS

VOC defined as

any carbon containing compound found in theatmosphere excluding CO and CO2

VOCs arise from

transport oil & gas productionrefining

chemicals manufacture agriculture e.t.c.

- electronics industry not a major contributor

VOCs Environmental Impact


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VOCs - Environmental Impact

VOC + Sunlight + NOx = O3

This leads to the formation of photochemical

smog at ground level.

Environmental and legislative pressures for

reduction in VOC releases from all processes.

VOCs The Legislation


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VOCs - The Legislation

United Nations Economic Commission for

Europe (UNECE): Protocol to reduce emissions by 30% from a 1988baseline by 1999.

Draft European Directive aims to cut emissions by afurther 2/3 by 2007.

European Directive on Integrated Pollution

Prevention and Control



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USA legislation varies from State to State

Federal Legislation usually less strict

tightest emission levels in California

exemption for products


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VOCs - Summary

VOC emissions lead to production of ground

level ozone

Increasingly subject to legislative control

Alcohol based fluxes in wave soldering is

significant source within electronics industry

VOC Free vs Standard Fluxes


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VOC Free vs. Standard Fluxes

Higher activity due to greater acidic dissociation

Not hazardous for transportation Reduced impact on the environment

Easier disposal

Non-flammable

Thinners not required

Activation Systems


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Activation Systems

Carboxylic acids dissociate:

RCOOH

RCOO

-

+ H

+

Water is more polar than propan-2-ol and thus

favours dissociation

This enhances fluxing potential

Activation System


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Activation System

Time/seconds

Force/mN

Aqueous Flux

Propan-2-ol Flux

1.30s

0.56s

0.97mN

0.54mN

Effect of Water on Activity


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Effect of Water on Activity

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80 90 100

% Water

Wetting

Force/mN

Without wetting

agent

100% water with

wetting agent

Multicore MF101


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Multicore MF101

High Activity, No-clean & VOC-

free High activity

Wide process window

Excellent top-side fillet formation

Halide free

Passes J-STD and Bellcore SIR

Passes Bellcore electromigration

Multicore MF101


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Multicore MF101 Sustained activity

Specially formulated for low solder balling

0.

55

0.

65

0.

75

0.

85

0.

95

1.

05

1.

15

1.

25

0

30

5

10

15

20

25

30

Average

number of

solder ballsper PCB

PTH Diameter / mm

% Additive

Advantages of Novel


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Formulation

0

2

4

6

8

10

12

Normal VOC

Free

Re-

formulated

VOC Free

Novel

Formulation

Low Solids

Rosin

1.25

1.05

0.85

0.65

Hole diameter

Multicore MF101


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u t co e 0

Validated for lead free processes

Thermally stable resin system suited to hotter bath

temperatures and varying pre-heat demands

Multicore MF101


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Higher reliability

Natural electrical insulating properties of rosin

Bellcore GR-78-CORE SIR results after 5 days

(35C, 85% RH, pattern down)

Without rosin - 9.9 x 104

M

With rosin - 2.5 x 105 M


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Any Questions?

Ian Wilding

Henkel Electronics

+44 (0)7780 738300

[email protected]

Lead Free Wave Soldering

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