SnCu Based Alloy Design for Lower Copper Dissolution

SnCu Based Alloy Design for Lower Copper Dissolution

SnCu Based Alloy Design for Lower Copper

Dissolution and Better Process Control

Peter Biocca

Senior Engineer, Technical Manager

Kester ITW, Itasca, Illinois, U.S.A.

Material Concepts for Alternative Alloys

To meet the market demand for a best-in-class, low-cost leadfree alloy for wave, selective and dip soldering

„

SAC305 is the industry standard but higher in cost due to Silver content

„

New material had to have the following attributes:

„

„

„

„

„

„

Low cost, Silver free

Low drossing, low oxide potential

Shiny joints without shrink holes

Minimized dissolution of Copper and other metals

Low solder maintenance

Good wetting behavior on popular lead-free finishes

SAC305 Lead-Free Alloy

Industry standard lead-free alloy for SMT, wave, rework

3% Silver

Æ High Cost

Benefits:

„

Mass Production – Industry Standard alloy

„

Prevalence of Reliability Data

„

Lower Melting Temperature than SnCu systems

„

Increased Wetting Speed vs. SnCu systems (temperature dependent)

„

Perceived compatible in reflow soldering using SAC

SAC305 Lead-Free Alloy

Concerns:

„

Cost (3% Ag may add $6/pound to metals cost)

„

High Rate of Copper Dissolution

„

Dull or Matte Finish Solder Joints

„

Hot Tear / Shrink Hole Defects

„

Industry needs new materials to resolve these issues

Alloy Cost Comparative and new alloy design

Alloy

Sn63

K100

LD

SAC305

Composition

Sn63Pb37

Sn99.3Cu0.7 + Ni + Bi

Sn96.5Ag3.0Cu0.5

Relative Cost

(approx)

1x

1.5x

3x

Addition of bismuth and other elements in lead-free solders

Bismuth can be added in small amounts to certain lead-free solder alloy compositions to improve the wetting ability and slightly reduce the melting temperature of the solder. As much as 1% bismuth is soluble in solid tin.

The much lower surface tension of bismuth compared to tin helps wetting.

ƒ

Bismuth acts synergistically with Nickel to reduce copper dissolution further than nickel alone.

ƒ

Bismuth reduces surface tension of the SnCuNi alloy.

ƒ

Addition of phosphorus less than 0.010% reduces oxidation, usual practice.

Lower costs

K100

LD -

reduced costs for wave and selective systems

„

Silver-free alloy is ~50% less in metals cost vs. SAC305

„

Low Dissolution of Copper means lower pot maintenance and fewer defects

„

Shiny joints means minimal operator training and AOI recalibration costs

„

Minimal dross means lower maintenance & dross-handling costs

Typically seen with SAC solders in wave, selective and hand-soldering

SAC shrinkage on a wave joint

Many assemblers are concerned about hot tear inspection and long term effects.

SAC after 500 thermal cycles, photographs iNemi Lead-free Wave Project 2006, initial work.

Surface Cosmetics

SAC SnCuNi+Bi

Alloy properties summary

Melt Point

Pasty Range

Appearance

Shrink Holes

Copper Dissolution (Sn63 = 1)

Pot Management

Reactivity to Equipment

Suggested Pot Temperature

Approximate Relative Cost (Sn63 = 1)

Additive

K100

LD

~227C

0

Shiny

No

0.8

Easiest

Low

255 – 265 o C

1.5

K100LDa

SAC305

217-220C

3C

Dull

Yes

2.1

Difficult

High

250 – 260 o C

3.0

SAC300

SnCuNi+Bi surface finish after wave soldering

Low Dullness

K100

LD

is both doped with a small amount of Nickel to prevent surface shrinkage

Benefits:

„

Shininess means that operators don’t need inspection training and and AOI equipment doesn’t require recalibration

„

Lack of shrink holes reduces possibilities of reliability risk

Why is Copper Dissolution Important?

With many lead-free alloys,

Copper level in solder pot increases quickly over time

Æ

Melt point of alloy increases as Copper level increases

Æ

More Copper in the alloy makes it more sluggish

Æ

A more sluggish alloy will cause hole-fill defects increase!

Additionally, alloys that dissolve Copper quickly may completely erode Copper terminations during the soldering process

Why is Copper Dissolution Important?

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By maintaining the Copper level through a low dissolution alloy, Copper levels are practically constant, producing consistent soldering performance

„

This reduces insufficient defects

„

No issues with complete erosion of Copper terminations

„

Low dissolution also means less maintenance and less use of “additive” bars to lower Copper content in the solder pot

Copper Dissolution Test

Objective:

To determine the Copper Dissolution time in a Solder Alloy

Equipment/Apparatus/Reagent:

SWET 2100 Wetting Balance

0.6 diameter copper wire (U bend wire)

RMA flux #186

Stop Watch

Wetting Balance Parameter Setting:

Test temperature: 300degC (need to ensure solder pot temperature is 300degC by using an external digital thermometer)

Dipping depth: 5mm

Speed: 2mm/sec

Test Method:

1. Preclean the copper wire using #5520 and rinse with water and IPA

2. Prepare the copper wire by bending it into a U bend wire [refer to appendix A Picture 1 to 7 for method of bending]

3. Melt the test specimen solder alloy into the inner pot

4. Attached the U bend copper wire onto the holder and dip about 1cm into #186 [refer to Appendix A picture 8]. Attached the holder to the wetting balance.

5. Before the start of the test, stirred the solder alloy in the inner pot for 10 times to prevent segregation of the elements in the solder alloy

6. Start the test which is similar to wetting balance test for chemical flux. Press ‘Solder’ and then press ‘Start’

7. Once the solder pot is raised to the maximum height, press ‘STOP’ and press the stopwatch simultaneously.

8. Every 5 minutes stirred the solder alloy in the inner pot for 10 times to prevent segregation of the elements in the solder alloy. Take care not to disturbed the copper wire.

9. Note the time taken for the U bend copper wire to disconnect

10. Press ‘START” for the solder pot to return to the original position.

11. minimum 3 samples per test

Result:

Reading

1

2

3

Ave 1

Std Dev 1

Ave 2

Std Dev 2

Ave 3

Std Dev 3

Note:

SnCuNi1

Operator A

Set A Set B

Operator B

Set A Set B

865

858

901

874.7

850

882

903

878.3

888

863

913

888.0

844

876

907

875.7

23.07

876.5

26.69

22.40

879.17

23.47

25.00

881.8

26.32

31.50

Operator A

Set A

SAC305

Set B

Operator B

Set A Set B

460

500

503

487.7

486

470

476

477.3

413

445

540

466.0

427

452

491

456.7

24.01

482.5

16.99

8.08

471.92

35.32

66.05

461.3

46.77

32.25

Operator A

Set A

K100LD

Set B

Operator B

Set A Set B

888

973

930

930.3

882

945

1020

949.0

902

958

1000

953.3

908

961

1010

959.7

42.50

939.7

52.31

69.09

948.08

47.32

49.17

956.5

44.94

51.01

Operator A

Set A

SnCuNi2

Set B

Operator B

Set A Set B

642

694

691

675.7

700

672

722

698.0

668

648

723

679.7

652

686

722

686.7

29.19

686.8

27.24

25.06

685.00

29.06

38.84

683.2

35.00

33.29

Lowest Dissolution of Copper

Minimizing Copper Dissolution is critical with the conversion to lead-free soldering.

Other lead-free alloys dissolve Copper much faster than K100

LD

:

Alloy

K100

LD

Sn63

SnCu+Ni

SAC+Bi

SAC305

SnCu

SnAg

Pure Tin

Relative Rate of Copper Dissolution

0.8

1.0

1.0

1.6

2.1

2.2

2.3

2.4

Celestica Independent Study

Copper dissolution on board copper in rework operation

Top is SnPb, blue green, red are SnCuNi, SnCuNi+Bi, SnCu+Co

Low Defects

K100

LD

is designed to give excellent wetting to through-hole and bottom-side SMT components

Dopants in K100

LD

promote fluidity and proper surface tension to yield good hole-fill without bridges

K100

LD

will work with all board and component finishes

Benefits:

„

Easy implementation of lead-free process

„

Reduction in rework costs and reliability risk

Diminish the 5D’s

K100

LD

– Alloy that will Diminish the 5D’s

Lowest Dissolution of Copper

„

Prevents Copper Erosion and Yields Consistent Soldering Results

Low Dullness

„

Produces Shiny, Smooth Solder Joints

Low Defects

„

Bridge-free with Excellent Top-Side Fillets

Low Dross

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Anti-Drossing Additive Lowers Drossing by 20% vs. Sn63Pb37

Low Dollars

„

Silver-Free Alloy is ~50% Lower Metal Cost than SAC305

Comparing to SAC305, SnCuNi, K100LD

All 0.063” AgImm but similar behavior observed with OSP, SnImm, ENIG

Typical results obtained using no-clean ROL0

LF Implementation at a Major Contractor Level

They built 12 board types for Nautilus Europe with K100LD, NO-CLEAN ROLO

FLUX and SAC305 ROL0 NO-CLEAN solder paste

Mixed technology board with top and bottom-side SMDs, 0.063” SN100CL

Bottom-side SMDs and PTHs done with K100LD and N/C flux

The boards exhibited no defects and bright joints

K100LD Excellent Top-side Fillets; No Dullness, No Shrinkage

SAC305 N/C used top-side

K100LD and low solids no-clean flux ROLO

0.063” SN100CL Finish

K100LD and NO-CLEAN ROL0 Flux with SAC305 NO-CLEAN ROL0

Top-side reflow, 0.093” Thick SN100CL Finished

K100LD excellent defect-free bottom-side and top hole-fill

Low Dross

Lead-free alloys generally dross more than leaded counterparts

Due to combination of higher-Tin alloys and higher processing temperatures

Dross formation with lead-free can be 100% greater than traditional leaded process if not controlled via inert environment or anti-drossing technology

Low Dross

K100

LD

is designed with anti-drossing technology to reduce dross rate in wave soldering applications

Anti-dross additive can lower dross rate to 20% less than untreated Sn63

Benefits:

„

Lower maintenance time & costs

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Reduced solder usage

„

Lower recycling costs & dross handling

„

Increased process robustness

Lead-free Wave Soldering Liquid Flux Compatibility

SnCuNi+Bi is compatible with all lead-free fluxes

No-Clean, Low Solids, No Rosin

No-Clean, Low Solids, With Rosin

VOC-Free

(water is solvent)

Best for LF *

N/A

Alcohol-based

Not suitable for LF

Suitable for LF

Organic Acid (Water washable residues)

Best for LF * Suitable for LF

Rosin-based N/A Suitable for LF

* Best selections for lead-free wave soldering, most popular global options today.

SnCuNi+Bi Cored Wire is used for hand-soldering

Testing of tip erosion is ongoing to determine if this alloy erodes tips to a lesser extent than SAC305.

ƒ

Compatible with SnCuNi and SnCuNi+Bi solder

ƒ

Being used to touch up SAC joints, no problems reported

ƒ

Flux percentage in is 3% by weight

ƒ

Excellent hole-fill at 700-800°F tip temperatures

Thank-you.

Further information is available.

Contact [email protected]

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