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

ASPIS – Nickel & Gold solderable finishes from Ionic Liquids 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

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 Au Ni • ENIG – Electroless Nickel Immersion Gold • Thick electroless Ni/P deposit (c.a. 6-8 µm Cu thick) Fibre Board • 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

Ionic Liquids • Organic cations with halide anions and various complexing agents to make an anionic complex • R 1 R 2 R 3 R 4 N + [X - · z Y] • Ionic liquid used here Ethaline 200 • 2:1 molar ratio ethylene glycol complexing and choline chloride agent • Environmentally benign/cheap cation anion materials • Unusual solvation properties of 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 NiCl 2 • 0.25 M Na 2 PO 2 H 2 • 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

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 6 • Electrochemical behaviour of these AuCl 5 AuCN species remarkably varied KAu(CN) 2 4 Current (µA) 3 2 1 0 -1 -1.0 -0.5 0.0 0.5 1.0 1.5 Potential (V)

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 18 (a) (b) 1.0 16 14 0.8 Mass deposited Aqueous AuCl frequency 12 with time response Admittance (mS) 10 0.6 Mass (µg) 8 AuCl 0.4 6 AuCN 4 0.2 2 KAu(CN)2 0 0.0 -2 8.20 8.22 8.24 8.26 8.28 8.30 8.32 0 200 400 600 800 1000 1200 1400 1600 1800 Frequency (MHz) Time (s) Salt Plating Rate (nm min -1 ) Aq KAu(CN) 2 1.75 AuCl 1.16 AuCN 0.76 KAu(CN) 2 0.37 Na 3 Au(S 2 O 3 ) 2 1.45

Plating from Au cyanide salts: Comparative morphologies Aq AuCl KAuCN AuCN 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 AuCN KAu(CN) 2

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

Solder Wetting Balance Measurements Rate at which solder wets surface and reliability with which it does is a good indicator of standard of ENIG coatings 4 t 1/2 3 3 Force (mN) 2 1 2 0 1 -1 0 2 4 6 8 Time (s) t 1/2 = time for force measurement to reach half of its maximum value

Solder Wetting Balance Measurements 4 3 t 1/2 (s) 2 1 0 Aqueous AuCN KAu(CN)2 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; o Improved coating with increasing cyanide content. o Rate of deposition decreases with increasing cyanide content. o AuCN and KAu(CN) 2 coatings have lower roughness than aqueous. o Good coatings onto standard tokens. o 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)