Ultrasonically enabled Low Temperature Electroless and Immersion Metallisation

AmirahKassimThe Functional Materials Applied Research Group

Coventry University

Ultrasonically enabled Low Temperature Electroless and Immersion Metallisation

• 3 year project, started in October 2010• Funded by: IeMRC• Objective: to reduce the plating temperatures used in electroless and immersion plating by the application of ultrasound

Ultrasonically enabled Low Temperature Electroless and Immersion Metallisation

Electroless and Immersion plating processes used in Photovoltaic and Printed Circuit Board manufacture

Process Temperature (oC) Comments

Electroless Copper 30 – 70 Contains formaldehyde

Electroless Nickel 50 – 90

High temperatures required for good

deposition rate Immersion

Silver 40 – 90

ImmersionGold 80 – 90

The effect of sound on the chemistry of a solution

103 104 105 106 107

HumanHearing

Diagnostic Ultrasound

20kHz – 2MHzExtended range ultrasound

for sonochemistry

Ultrasound

COMPRESSIONCOMPRESSION COMPRESSION COMPRESSION

RAREFACTION RAREFACTIONRAREFACTION RAREFACTION RAREFACTION

BUBBLE FORMS

GROWS IN SUCCESSIVE CYCLES

REACHES UNSTABLE SIZE

VIOLENT COLLAPSE

5000oC2000 ats

Acoustic cavitation in a liquid: NEAR A SURFACE

Solid Surface

Boundary Layer

Asymmetric collapse Rush of liquid from one side produces a powerful jet of

liquid targeted at the surface.

Video courtesy of University of Twente, Netherlands.and Shimadzu Europa GmbH, Duisburg, Germany

Acoustic cavitation in a liquid: NEAR A SURFACE

Electrochemical Effects

• Thinning of diffusion layer

• Improved mass transport

• Improved solution movement

• Electrode cleaning

• Degassing

Acoustic cavitation in a liquid: NEAR A SURFACE

Effect of ultrasound on plating rate

Workers Plated Metal Reducing Agent Plating rate

Increase Decrease

Abyaneh et al

Nickel Hypophosphite complexing agent= low

complxingagent = high

Mallory Nickel Hypophosphite lowthioureaconcentration

highthioureaconcentration

Masuoka and Hayashi

Nickel Hypophosphite

Yang et al Nickel Hypophosphite

Touyeras et al Copper Formaldehyde

Zaho et al Copper Formaldehyde

Ultrasonic frequency = 530kHz

Data reproduced by kind permission of;F. Touyeras et alUltrasonics Sonochemistry 10 (2003) 363–368

• Improved mass transport

• Thinning of diffusion layer

• Crystal structure formed under ultrasonic conditions

in the initial stages of plating (Abyaneh et al)

• Localised heating in the diffusion layer (Toyeras et al)

• Activation of catalyst

• Degassing

Rinse 5 minutes

Conditioner 5 minutes 30-60oC

Pre-dip 1 minute RT

Accelerator 2 minutes RT

Catalyst 5 minutes 30-50oC

Rinse 5 minutes

Rinse 3 minutes

Electroless Copper 25- 60 minutes 30-70oC

Pd

Pd

Pd

Pd

Pd

Pd

Pd

Pd

HCHOOxidation

Cu2+Reduction

2e-

Copper deposit on the surface of the substrate.

Electroless Copper – 3350-1 (Chestech Ltd) – EDTA based

Solution subjected to ultrasonic irradiation of different frequencies continuously for 4 hours.

UltrasonicFrequency

Copper Conc.after 4 hours

Copper Conc. after 48 hours

Control(Mechanical Stirring) 100% 100%

20 kHz(20 % Amplitude) Precipitated NA

40 kHz 100% 100%

582 kHz 100% 70%

863 kHz 100% Precipitated

20 kHz - Precipitates copper from solution after 4 hoursEffects of cavitation destroy chemicals in electroless formulation e.g. EDTA, stabilizers

40 kHz – Stable after 4 hours and 48 hours

High frequency (583/850 kHz) – unstableRadical formation – oxidation of chemicals in electroless formulation

Precipitate, 70% Cu in Solution

100% Cu in solution

Precipitated

Experimental Procedure

• Electroless Copper – 3350-1 (Chestech Ltd) – 1 litre

• Catalyst – 3344 (Chestech Ltd – Pd/Sn colloid)

• Substrate – Unclad epoxy laminate (IsolaDuraver 104)

• Temperature - 25, 30, 35, 40, 45 and 50 ºC

• Plating Time – 25 minutes

• Agitation - Ultrasound (40 kHz) or Magnetic StirringUltrasonic Bath – Langford, Model 475TT 40 kHz Power density 90.3 W/litre

Responses

1. Weight gain

Dry epoxy panels in oven for 24 hours at 120 ºCWeighProcess through Electroless CopperWeighDry epoxy panels in oven for 24 hours at 120 ºC

From weight gain calculate plating rate.

2. Morphology

Scanning Electron Microscope

Plat

ing

rate

µm

/25m

in

Main effect on plating rate is temperature – particularly above 40 ºCUltrasound had a slight negative effect at low temperatureSlight improvement at 35 ºC and above

Pd

Pd

Pd

Pd

Pd

Pd

Pd

Pd

Ultrasonic Microjets •Ultrasound scrubs surface and

removes catalyst

•Lowers plating rate

•Particularly problematic at low temperature – longer initiation time

•At high temperature (50 ºC) ultrasound always produces good results – short initiation time

Plat

ing

rate

µm

/25m

in

Plating rate rises as delay time increases to 7 minutes 10 minute delay causes plating rate to drop since almost half the plating time is without agitation

After 10 minutes ultrasound most of catalyst removed from surface

Plat

ing

rate

µm

/25m

in

7 minute delay – best plating results ‘with ultrasound’ at all temperaturesUltrasound with 7 minute delay at 40 ºC gives plating rates higher than ‘silent’ plating at 45 ºC

Ultrasound40 ºCSilent

Conditions: Frequency= 40kHz7 minute delay

Ultrasound reduces plating temperature

Reduces energy needed

Thank You

www.ultiemet.co.uk/