ASPIS Nickel & Gold solderable finishes from Ionic Liquids Dr - - PowerPoint PPT Presentation

Dr Karl S. Ryder Dr Andrew Ballantyne, Gregory Forrest

Scionix Laboratory, Department of Chemistry, University of Leicester, Leicester, LE1 7RH,UK

k.s.ryder@le.ac.uk

ASPIS – Nickel & Gold solderable finishes from Ionic Liquids

Contents

• ENIG and Current Problems
• Ionic liquids
• Why immersion gold from ionic liquids onto electroless

nickel

• Standard aqueous sample
• Comparison of aqueous and ionic liquid methods
• Further work

ENIG and current problems

• ENIG – Electroless Nickel Immersion Gold
• Thick electroless Ni/P deposit (c.a. 6-8 µm

thick)

• Thin immersion Au coating (>50 nm thick)
• Wets solder well, good planarity, long shelf life and

high reliability

• Problems: Black Pad
• Immersion Au inherently a corrosive process
• Accelerated corrosion of Ni/P substrate

during immersion Au plating

• Related to acid medium required for aqueous

plating Fibre Board Cu Ni Au

Ionic Liquids

• Organic cations with halide anions and various complexing

agents to make an anionic complex

• R1R2R3R4N+ [X-· z Y]

cation anion complexing agent

• Ionic liquid used here Ethaline

200

• 2:1 molar ratio ethylene glycol

and choline chloride

• Environmentally benign/cheap

materials

• Unusual solvation properties
• f metal salts

Why use ionic liquids?

• Problems with ENIG associated with hypercorrosion of

nickel substrate during immersion Au

• Ionic liquid metal speciation and behaviour at metal

surfaces considerably different to molecular solvent i.e. high solubility of metal oxides

• Possibility of breakdown of passive Ni layer from neutral

ionic liquid media

• High quality Au deposits from considerably more benign

environment

Standard Aqueous Sample

• Developed standard aqueous electroless Ni to study

difference in Au plating solutions

• Electroless Ni plating solution
• 0.1 M NiCl2
• 0.25 M Na2PO2H2
• 0.1 M malonic acid
• 0.4 M glycine
• pH 4.5
• Plating at 80 ºC for 45 mins
• Aqueous immersion Au solution
• 5 mM KAu(CN)2
• 0.1 M citric acid
• pH 4.5
• Plating at 80 ºC for 45 mins

Potential (V)

• 1.0
• 0.5

0.0 0.5 1.0 1.5

Current (µA)

• 1

1 2 3 4 5 6 AuCl AuCN KAu(CN)2

Various Au salts in Ethaline 200

• High Cl- content in Ethaline 200 (c.a. 5

M) gives rise to novel Au salt species

• Use of AuCl, AuCN and KAu(CN)2
• Sequential introduction of CN into Au

species in ionic liquid

• Electrochemical behaviour of these

species remarkably varied

Plating from Au cyanide salts

• IL solutions of AuCl, AuCN or

KAu(CN)2 or standard aqueous sample

• 40 mins @ 80 ºC

AuCl AuCN KAu(CN)2

Plating from Au cyanide salts: Plating Rate

• in situ measurement of mass deposited by

Quartz Crystal Microbalance (QCM)

• quartz crystal resonates at specific

frequency in response to A.C. current

• frequency varies dependent on mass on

crystal surface Sauerbrey Equation

Plating from Au cyanide salts: Plating Rate

Time (s)

200 400 600 800 1000 1200 1400 1600 1800

Mass (µg)

• 2

2 4 6 8 10 12 14 16 18

Aqueous AuCl AuCN KAu(CN)2 (b) Frequency (MHz)

8.20 8.22 8.24 8.26 8.28 8.30 8.32

Admittance (mS)

0.0 0.2 0.4 0.6 0.8 1.0

(a)

Salt Plating Rate (nm min-1) Aq KAu(CN)2 1.75 AuCl 1.16 AuCN 0.76 KAu(CN)2 0.37 Na3Au(S2O3)2 1.45

AuCl frequency response Mass deposited with time

Plating from Au cyanide salts: Comparative morphologies

Aq AuCl AuCN KAuCN

2

Plating from Au cyanide salts: Comparative morphologies

Evidence of hypercorrosion of aqueous immersion Au sample

Plating from Au cyanide salts: Comparative morphologies

Aq AuCl KAu(CN)2 AuCN

Plating from Au cyanide salts: Comparative morphologies

Aq AuCl AuCN KAu(CN)2

Plating from Au cyanide salts: Comparative surface roughness

Salt % Surface area difference (%SAD) @ 20 µm % Surface area difference (%SAD) @ 2 µm

Aq KAu(CN)2 4.13 9.27 AuCl 12.6 180 AuCN 1.94 4.03 KAu(CN)2 2.65 3.78

Plating from Au cyanide salts: Stripped Au surface

Aqueous AuCl in Eth 200 AuCN in Eth 200 KAu(CN)2 in Eth 200

Ils show less evidence of “mud cracking” Aqeous shows a number of corrosion features which are not present in the IL samples.

Plating from Au cyanide salts: Plating of standard tokens

Immersion Au from AuCN or KAu(CN)2 in Ethaline 200 onto aqueous electroless Ni/P AuCN gives bright uniform coatings KAu(CN)2 gives very thin coatings

Time (s)

2 4 6 8

Force (mN)

• 1

1 2 3 4

Solder Wetting Balance Measurements

1 2 3 t1/2 Rate at which solder wets surface and reliability with which it does is a good indicator of standard of ENIG coatings

t1/2 = time for force measurement to reach half of its maximum value

Solder Wetting Balance Measurements

Aqueous AuCN KAu(CN)2

t1/2 (s)

1 2 3 4

Coatings from ILs wet faster and more reliably than the model aqueous process

Comments and Future directions

• Extensive studies on effect of including cyanide in immersion bath;
• Improved coating with increasing cyanide content.
• Rate of deposition decreases with increasing cyanide content.
• AuCN and KAu(CN)2 coatings have lower roughness than aqueous.
• Good coatings onto standard tokens.
• Coatings from AuCN and KAu(CN)2 in Ethaline 200 wet faster and more

reliably on a solder wetting balance.

• Investigate use of additives on coating quality
• Ageing studies on solder wetting balance and XPS
• Metal ion speciation studies by UV and EXAFS (proposal submitted)