AUTOCATALYTIC ALLOY PLATING PROCESSES
FOR THIN-FILM MEMORY DISCS
By
Donald W. Baudrand and Michael Malik
ABSTRACT
E l e c t r o l e s s ( a u t o c a t a ly t i c ) p l a t i n g p r o c es s e s c a n g r e a t l y i n c r ea s e t h e
storage capacity of memory discs used in Winchester -type disk drives. A nonmagnetic electroless nickel layer is deposited onto a carefully prepared
aluminum substrate, polished to a near perfect finish, activated, and plated with
an electroless cobalt-phosphorus alloy. The resultant disc is a durable, high p e r f o r m a n c e m e m o r y d e v i c e w i t h l a r g e s t ora g e c a p a c i t y .
This paper describes the deposit properties required for the non -magnetic
electroless nickel undercoat and the electroless cobalt alloy recording media;
and the test methods used for evaluation of the finished thin-film memory discs.
Introduction.
Electroless (autocatalytic) plating processes have been widely used in the manufac ture
of electronic devices, and their areas of application continue to expand.) Electroless
deposits were initially commercially developed for use on industrial equipment and
hardw are becau se of their h igh hardness (500 - 800 VHN) and chemical resistance
(especially those consisting of nickel-phosphorus alloys). These properties, along with
uniform thickness, diffusion barrier characteristics, specific resistance, and the magnetic
characteristics attainable with some alloys make electroless coatings desirable for many
electronic applications. 2 3 , 4 Nickel-boron, nickel-boron-phosphorus, and nickel-boron
with one or more additional alloying metals can be used to produce deposits with good
solderability, favorable wire and die bonding characterisitcs, good heat re sistance, and
high as-plated hardness (700-800 DPH1
Electroless cobalt-phosphorus coatings deposited from certain solution formulations
have "hard" magnetic characteristics that are desirable for magnetic recording on thin -film
memory discs used in Winchester disk drives. These deposits can outperform the gamma ferric oxide medium on discs currently in use, increasing the memory storage capacity and
thereby extending the capabilities of the computer.
Computer logic and memory capacity have enjoyed quantum leaps in the last decade.
They have become smaller, more reliable, and faster, and this trend is continuing.1
The hard disk drive (Winchester type) resulted in a major advance in memory storage
capacity. The use of "thin-film plated discs" makes another advance possible. For example,
a "floppy disc" may have a capacity equivalent to about 250 typewritten pages, a "hard
disc" using iron oxide medium will hold about 10,000 -15,000 pages, and the plated disc
about 25,000 pages per 5 1 /4" disc. The limit of capacity is much larger with redesign of the
recording heads — o ver 50,000 pages. With the next gener atio n of heads now in
laboratories which utilize vertical magnetization, the storage capacity can be many times
greater.
Manufacturing Procedure
The "hard" disc is made from aluminum, presently a 5086 alloy. The aluminum disc is
machined (diamond turned) or polished to a very flat, very smooth surface. One typical
machining sequence begins with an aluminum disc that has a fine diamond turned finish of
0.0127 micrometres maximum arithmetic average surface deviation. After nickel pl ating,
the surface typically has 0.0305-0.0660 micrometres average surface deviation; it is then
polished back to within 0.0152-0.0203 micrornetres, then cobalt plated. The cobalt deposit
is so thin that further machining is generally not required.
A proposed ANSI specification on surface finish for the nickel undercoat calls for a
surface roughness of less than 0.025 micrornetres arithmetic average, with a maximum
protrusion in height of 0.178 micrometres. A proposed ANSI specification for the finis hed
magnetic surface roughness calls for less than 0.025 micrometres arithmetic average, with
a maximum deviation in height of 0.25 micrometres as measured with a 2.54 millimetre
radius stylus and 750 micrometre cutoff range.
Because of the tight operating clearances, the surface condition is critically important
(Figure 1). It should be free of pits, inclusions and other surface imperfections.
The disc should be cleaned in very mild, non-etching cleaning solutions (see Table I for
the process cycle). This is followed by a mild, non-etching acid treatment to remove surface
oxides. An immersion zinc is applied from a special zincate solution, removed in nitric acid
and re-applied in a second zincate bath to produce a very thin zinc film.
TABLE I.
P R OCE SS CYCLE FOR
P L A T I N G E LE CT R O LE SS NI CK E L
ON 5086 ALUMINUM ALLOY
T ypical P ro cess Cycle
1. A lk a l in e c l e an i n a m i l d, n o n - e tc h c l e an er .
2. R i n s e .
3. Acid c lean an d deo x i diz e in a m i ld no n - et ch cleaner / deo x id iz er .
4. R i n s e .
5.
6.
7.
8.
Z in c at e 2 0 - 2 5 s e co n ds a t r o o m te m p er at ur e.
Thorough rinse.
Strip the zinc deposit in 60% by vol. nitric acid (42° Be').
Rinse.
9. Zincate (15 seconds at room temper ature) . Must be a separ ate zincate.
10. T h o r o u g h l y r i n s e ( d o u b l e r i n s e , c o u n t e r - f l o w ) .
11.
12.
Neutr alize in a so lu tio n of 4 o z/ gallon ( 30 g/L) so dium bicar bonate.
Rinse (DI water).
13.
14.
Electroless nickel plate.
Dry.
Be careful rinsing and neutralizing, the discs are transferred to an electroless nickel - phosphorus
plating bath designed to produce a non-magnetic, smooth, pit-free deposit. Typically 0.0127 to
0.0254 millimetres of nickel-phosphorus alloy (9.5-11% P by wt.) is deposited. The deposit is
then often heat treated, and must remain non-magnetic up to at least 250° C (bake test is usually
1-3 hrs.). This heat resistance assures that the deposit will remain non-magnetic for the life of the
device, and that if heat operations are subsequently per f o r m ed, s uc h as w h en sp ut te r e d c o b a l t
a l lo y m e d i a ar e em p lo y ed i n s te a d o f electrolessly deposited cobalt-phosphorus, the nickel
deposit and its magnetic characteristics will not change.
It is important that the electroless nickel alloy is non-magnetic so that it cannot exert an influence
on the very thin (0.05 to 0.10 micrometres) recording layer. Slightly magnetic undercoatings
manifest themselves as distorted hysteresis loops, loss of signal, noise and overwrite.
The electrolessly deposited nickel phosphorus alloy serves as a corrosion resistant, hard,
polishable base for the recording medium. The nickel-phosphorus alloy is polished to an even finer
finish than can be achieved on bare aluminum. A very slight texture polish follows which creates
air flow patterns used for floating the recording head at 0. 2-0.4 micrometres above the
surface of the disc.
The polished, textured nickel alloy plated disc is cleaned and activated, then plated with an
electroless cobalt-phosphorus alloy. See Table II for a typical process cycle.
TABLE II.
P R O CE SS CY C LE F OR
PLATING ELECTROLESS COBALT
ON E LECTR OLE SS NICK EL
A typical process cycle for plating an electroless cobalt medium on the electroless nickel
undercoat is as follows:
1. P o lish e lectr O less n i c k el u n der co at to a hi gh f inish.
2. C l e a n i n a m i l d a l k a l i n e c l e a n e r .
3. R in se (DI w ater ).
4. Activate. Mild acids are often used (proprietary solutions vary from mild alkaline to acidic
solutions).
5. R inse (DI w ater ).
6. Electroless cobalt plate for 1-2 minutes at a bath temperature of 150-170° F (65-76° C).
7. R i n s e .
8. D r y .
9. Apply lubricant overcoating (carbon, silicon dioxide, fluocarbons, oxidizing treatments and
other processes). This step is optional.
10. T e s t f o r m a g n e t i c c h a r a c t e r i s t i c s .
A thin layer (0.04 to 0,08 micrometres) of carbon is often deposited over the cobalt medium
using sputtering techniques. This serves as a lubricant .
Abstract
Electroless (outocato/yhc) plating processes can greatly in crease the storage capacity of
memory discs used in Winchestertype disc drives A nonmagnetic
electroless nickel layer is deposi te d o nt o a c a re fu ll y p re pare d
aluminurn substrate, polished to a
n e a r p e rf e c t f i n is h , ac t i va te d ,
and plated with an electroless cobalt-Phosphorus allay. The resultant disc is a durable, high-perf o rm a nce m em o ry d e vice wi t h
large storage capacity.
This paper describes the de posit properties required for the
nonmagnetic electroless nickel
undercoat ono the electroless cobalt alloy recording media, and
also the test methods used for
evaluation of the finished thin-film
memory discs
INTRODUCTION
E
lectroless (autocatalytic) plating
processes have been widely
used in the manufacture of electronic devices, and their areas of
application continue to expand.'
Electroless deposits were initially
commercially developed for use
on industrial equipment and hardware because of their high hardness (500 to 800 VHN) and chemical resistance (especially those
consisting of nickel-phosphorus alloys).
These properties, along with uniform thickness, diffusion barrier
characteristics, specific resistance,
and the magnetic characteristics
attainable with some alloys make
electroless coatings desirable for
many electronic applications. 2T4
Nickel-boron, nickel-boron-phosphorus, and nickel-boron with one
or more additional alloying metals
can be used to produce deposits
with good solderability, favorable
wire and die bonding characteristics, good heat resistance, and
high as-plated hardness (700 to 800
Electroless cobalt-phosphorus
coatings deposited from certain
solution formulations have "hard"
magnetic characteristics that are
desirable for magnetic recording
on thin-film memory discs used in
Winchester disc drives. These deposits can outperform the gammaferric oxide medium on discs currently in use, increasing the memory storage capacity and thereby
extending the capabilities of the
computer.
C o mp ut e r l ogic an d me mo ry
capacity have enjoyed quantum
leaps in the last decade. They have
become smaller, more reliable,
and faster, and this trend is continuing.'
6.
7.
8. Thoroughly rinse (double rinse.
counter-flow).
9. Neutralize in a solution of 4 oz/gal
(30 gIL) sodium bicarbonate.
10. Rinss (Dl water).
11. Electroless nickel plate.
12. Dry.
13. Alkaline clean in a mild, nonetch
cleaner.
14. Rinse.
15. Acid clean and deoxidize in a
mild. nonetch cleonerideoxidizer.
16. Rinse.
Zincate 20 to 25 seconds at room
The hard disc drive (Winchester
type) resulted in a major advance
in memory storage capacity. The
use of "thin -film plated discs"
makes another advance possible.
For example, a "floppy disc" may
have a capacity equi va lent to
about 250 typewritten pages, a
"hard disc" using iron oxide medium will hold about 10,000 to
15,000 pages, and the plated disc
about 25,000 pages per 51/4" disc.
Th e li mit of ca paci t y is mu c h
larger with redesign of the recording heads — over 50,000 pages.
With the next generation of heads
Table I. Typical Process Cycle for
Plating Electroless Nickel on 5086
Aluminum Alloy
1. temperature.
2. Thorough rinse.
3. Strip the zinc deposit in 60% v.
nitric acid (42° Be '),
4. Rinse.
5. Zincate (t5 seconds at room
temperature). Must be a separate
zincate.
Table I. Typical Process Cycle for
Plating Electroless Nickel on 5086 Aluminum Alloy
1. Alkaline clean in a mild, nonetch cleaner.
2. Rinse.
3. Acid clean and deoxidize in a mild. nonetch cleonerideoxidizer.
4. Rinse.
5. Zincate 20 to 25 seconds at room temperature.
6. Thorough rinse.
7. Strip the zinc deposit in 60% v. nitric acid (42° Be '),
8. Rinse.
9. Zincate (t5 seconds at room temperature). Must be a separate zincate.
10. Thoroughly rinse (double rinse. counter-flow).
11. Neutralize in a solution of 4 oz/gal (30 gIL) sodium bicarbonate.
12. Rinss (Dl water).
13. Electroless nickel plate.
14. Dry.
now in laboratories which utilize
vertical magnetization, the storage
capacity can be many times
greater.
MANUFACTURING PROCEDURE
The "hard" disc is made from aluminum, presently a 5086 alloy. The
aluminum disc is machined (diamond turned) or polished to a very
flat, very smooth surface.
One typical machining se quence begins with an aluminum
disc that has a fine diamond turned
finish of 0.0127 micrometers maximum arithmetic average surface
deviation. After nickel plating, the
surface typically has 0.0305 to
0,0660 micrometers average surface deviation: it is then polished
back to within 0.0152 to 0.0203 micrometers, then cobalt plated. The
cobalt deposit is so thin that further
machining is generally not re quired.
A proposed ANSI specification on
surface finish for the nickel undercoat calls for a surface roughness
of less than 0.025 micrometers arithmetic average, with a maximum
protrusion in height of 0.178 micrometers. A proposed ANSI specification for the finished magnetic
surface roughness calls for less
than 0.025 micrometers arithmetic
lur
zinc is applied from a special zincate solution, removed in nitric acid
and reapplied in a second zincate
bath to produce a very thin zinc
film.
After careful rinsing and neutralizing, the discs are transferred to on
electroless nickel-phosphorus plating bath designed to produce a
nonmagnetic, smooth, pit-free deposit. Typically 0.0127 to 0.0254
millimeters of nickel-phosphorus
alloy (9.5-11% wt F)) is deposited.
The deposit is then often heat
treated, and must remain nonmagnetic up to at least 250°C (bake test
is usually one to three hours). This
heat resistance assures that the deposit will remain nonmagnetic for
the life of the device, and that if
heat operations are subsequently
performed, such as when sputtered
cobalt alloy media are employed
instead of electrolessly deposited
cobalt-phosphorus, the nickel deposit and its magnetic characteristics will not change.
It is important that the electroless
nickel alloy is nonmagnetic so that
it cannot exert an influence on the
very thin (0.05 to 0.10 micrometers)
recording layer. Slightly magnetic
undercoatings manifest themselves
as distorted hysteresis loops, loss of
signal, noise and overwrite.
The electrolessly deposited nickel phosphorus alloy serves as a corrosion resistant, hard, polishable
base for the recording medium.
The nickel-phosphorus alloy is polished to an even finer finish than
can be achieved on bare alumi num. A very slight texture polish follows which creates air flow patterns
used for floating the recording
average, with a maximum deviation in height of 0.25 micrometers
as measured with a 2.54-millimeter
radius stylus and 750-micrometer
cutoff range.
Because of the tight operating
clearances, the surface condition
is critically important (Fig. 1). It
should be free of pits, inclusions
and other surface imperfections.
The disc should be cleaned in
very mild, nonetching cleaning
solutions (see Table I for the process
cycle). This is followed by a mild, nonetching acid treatment to remove
surface oxides. An immersion
Table II. Typical Process Cycle for
Plating Electroless Cobalt on
Electroless Nickel
1. Polish electroless nickel
undercoat to a high finish.
2. C l e a n i n a m i l d a l k a l i n e
c l e a n e r.
3. Ri ns e (Dl wa t e r).
4. Activate. Mild acids are often
used (proprietary solutions vary
from mild alkaline to acidic
solutions).
5. Ri ns e (DI wa t er).
6. Electroless cobalt plate for 1 to 2
minutes at a bath temperature of
150 to 170°F (65 to 76°C).
7. Ri ns e.
8. D r y .
9. Apply lubricant overcooting
(carbon, silicon dioxide,
fluorocarbons. oxidizing
treatments and processes). This
step is optional.
T e s t f o r m a g n e t ic
c h a ra c t e ri s t ics .