Electroplated Ni/Au: Limiting Ni
Diffusion
Jonathan Harris
CMC Laboratories, Inc.
Tempe, Arizona
Plating Sequence
Etch
Activation
H2SO4 for CuEtch off CuOxide
Ni Strike
(5 µinch)
(NiCl2·6H2O)
and
Ni(SO3NH2)2role to bond
well to layer
below (e.g.
Cu)
Ni Plate
(100-200
µinch)
Nickel
sulfamate
Ni(SO3NH2)2 –
role to form
thick Ni layer
Au Strike
(5 µinch)
K(Au(CN)2)
(99.9% Au
purity) role to
keep
impurities out
of the Au
plating bath
Au Plate
(20-100
µinch)
K(Au(CN)2)
type III (99.9%
Au purity) role
to plate high
purity Au layer
Metal Stack Up and Chemistry
Plated Ni (3-5 µm)
Plated Au (1-2 µm)
W Co-fire Metal
Typical Assembly Sequence
1. Plated package (Ni/Au)
2. Die attach (high power applications)
– AuSn at >280C for 3-5 minutes under forming gas
shroud
– AuSi at > 380 C for 3-5 minutes under forming gas
shroud
– GGI bonding at 250C for > 30 minutes in air
3. Wirebond to Au surface (Au or Al) –
wirebonding after heat exposure of plated layer
from die attach
Wirebond Yield vs. Ni(oxide) on Au
Surface
Au wirebond onto plated Ni/Au
surface
Wirebond lifts (%) vs. Atomic %
Ni on Au Surface (Auger
Analysis)
Data from Duane Endicott,
Motorola (Casey and Endicott,
Plating and Surface Finishing,
V67, July 1980, pg. 39)
Wirebonding requires less than
2% Ni on Au surface
Sources for Ni on Au Surface
• Ni “drag-out” impurities in Au bath that coplated with Au
– Ni plating solution that is not rinsed completely
and builds up as contamination in Au bath
– Present but can be minimized with effective Au
strike
• Ni from Ni under-layer that diffuse through
the Au layer during die attach heat exposure
Ni Diffusion in Au
• Ni diffusion into the Au layer driven by increase
in entropy
• Ni diffusion and concentration on the Au surface
driven by Ni reaction with the atmosphere
– Oxygen atmosphere: Ni + O2  NiO (ΔH= -244 KJ/m)
– Results in surface segregation of Ni on Au
– Forming gas shroud slows but does not eliminate this
reaction
Diffusion of various metals
in Au
Hall and Morabito, Thin
Film Solids, Vol 53, 1978
Grain boundary diffusion
rates 7 orders of
magnitude higher for Ni at
200C along grain
boundaries than through
bulk.
To limit Ni diffusion, must
limit grain boundary
diffusion.
To Limit Ni on Au Surface for
Wirebond Yield….
• Limit the level of Ni grain boundary diffusion
in the Au layer
• Limit the reactivity of the atmosphere during
the die attach with Ni (limit oxygen)
Altering Au Microstructure using
Electro-plating Conditions to
Minimize Ni Diffusion
How Do Electroplated Layers Grow?
• Diffusion of M+ ion to cathode where
electrochemical reduction occurs M+ + e M
• M atoms diffuse on cathode surface until critical
nuclei is formed
• Subsequent M atoms either grow on existing
nucleated grain or initiate new nuclei
• Intersection of growing grains generally form
grain boundaries
Grain nucleation and
growth study, Au film on
single crystal Fe
• Atoms deposit
randomly
• Diffuse until multiple
atoms collide to form
a cluster
• This represents the
film nucleation
• Clusters then grow to
form grains
• Cluster/grain
intersection points
become grain
boundaries
TEM of Au film growth on Fe, Kamasaki, 1974
For Larger Grain Microstructure…
• Limit the nucleation rate for the growing film
• “Encourage” incident atoms to add to existing
grains vs. nucleate new grains
• = Limit the rate of deposition of M atoms
• = Limit the diffusion rate of M+ ions to the
cathode surface
• = For example, lowering the overall plating
current density will increase grain size (but also
increase cost due to reduced plating rate)
Approach to Growing Large, Dense
Au Grains
• Acceptable plating rate
• Multiple controls over Au plating
process
Structure of the Electrolyte During
Plating
• Plating process is dynamic
• Deplete metal ions as M+
are reduced at the cathode
• Results in thin layer of (-)
ions very close to the
cathode
• Results in depletion of M+
in diffusion layer
Plating Potential
•
•
•
•
•
•
With no current flowing,
electrochemical potential is
established Eo
During plating Ep = Eo + η
η “electrochemical over-potential”
η (cathode) = η (diffusion) + η
(activation)
η (diffusion) = energy to diffuse
metal ion through electrolyte
diffusion layer to electrode surface
η (activation) = activation energy to
reduce the atom, atomic surface
diffusion to form nuclei
Plating Potential and Film Nucleation
Rate
• Increasing η (activation)
– Decreases nucleation rate
– Makes it more energetically unfavorable to initiate new
nucleation sites
– At some point will make plating inefficient
• Increasing η (diffusion)
– Decreases the nucleation rate
– Makes it more energetically unfavorable to move an M+ ion to
the cathode surface
– May also make plating inefficient if plating rate is limited to an
impractical level
Plating Scheme for Large Au Grains
• Add a Pb at ppm level to Au plating bath
– Pb2+ ion which will not be reduced during Au deposition
– Reside near the cathode and decrease the magnitude of
the M+ depletion during plating
– Increase η (diffusion) which will decrease Au2+ diffusion in
solution
– As Pb2+ builds up near cathode, becomes barrier to Au2+
diffusion to surface
– Decrease nucleation density
• To control this level of Pb2+ build up– implement
“pulse plating”
Pulse Plating
• Alternate cathodic pulse with no current flow period
• During “on” pulse
–
–
–
–
Ionic diffusion patterns form
M+ ions plate
Pb+ “grain refiners” align near the cathode
M+ ion become depleted near the cathode
• During “off” pulse
– Ionic diffusion patterns re-randomize
– Pb+ ions diffuse away from cathode surface
– M+ ions can diffuse back toward the cathode surface
• Pulse plating cycle can be used to mitigate and control impact of M+
depletion and Pb+ “grain refiner” build up
DC Plated Au
Pulse Plated with ppm Pb Additive
Ideal Au Grain Structure
Uniform but small grains- high Ni diffusion
due to very high Au grain boundary density
Non-uniform mixed small and large grains.
Some improvement in grain boundary
density
Large Uniform Au Grains
minimize Ni grain boundary
diffusion
Optimization of Au Plating ProcessKey Variable
Attribute
Too High
Too Low
Pb concentration
Effects bath function
Grains get small
Pulse duty cycle
Grains get small
Plating rate decreases
Pulse “on” current density
Grains get small
Plating rate decreases
Au concentration
Higher Au costs
Grains get small
Auger spectra, Au with
small grain size, 250C
for 10 minutes
Ni concentration is
9.2%
Ni concentration < 1.0%
Summary
• Combination of ppm level Pb addition to plating
bath and pulse plating can produce large, dense
Au grains
• Pb level, pulse magnitude and frequency can be
used to control Au microstructure
• Manipulate energetics of ion diffusion in solution
and reduction at the cathode
• Large grains effective barrier to Ni diffusion