AUTOCATALYTIC ALLOY PLATING PROCESSES
FOR THIN-FILM MEMORY DISCS
By
Donald W. Baudrand and Michael Malik
ABSTRACT
Electroless (autocatalytic) plating processes can greatly increase the 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- performance memory device with large storage capacity.
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 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 (500-800 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.23,4 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 resistance, 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 51/4" disc. The limit of capacity is much larger with redesign of the
recording heads — over 50,000 pages. With the next generation 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 plating, 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 finished 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 millimetreradius 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.
PROCESS CYCLE FOR
PLATING ELECTROLESS NICKEL
ON 5086 ALUMINUM ALLOY
Typical Process Cycle
1.Alkaline clean in a mild, non-etch cleaner.
2.Rinse.
3.Acid clean and deoxidize in a mild non-etch cleaner/deoxidizer.
4.Rinse.
5.Zincate 20-25 seconds at room temperature.
6.Thorough rinse.
7.Strip the zinc deposit in 60% by vol. nitric acid (42° Be').
8.Rinse.
9.Zincate (15 seconds at room temperature). Must be a separate zincate.
10.Thoroughly rinse (double rinse, counter-flow).
11.Neutralize in a solution of 4 oz/gallon (30 g/L) sodium bicarbonate.
12.Rinse (DI water).
13.Electroless nickel plate.
14.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 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 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.
PROCESS CYCLE FOR
PLATING ELECTROLESS COBALT
ON ELECTROLESS NICKEL
A typical process cycle for plating an electroless cobalt medium on the electroless nickel undercoat is as follows:
1.Polish electrOless nickel undercoat to a high finish.
2.Clean in a mild alkaline cleaner.
3.Rinse (DI water).
4.Activate. Mild acids are often used (proprietary solutions vary from mild alkaline to acidic solutions).
5.Rinse (DI water).
6.Electroless cobalt plate for 1-2 minutes at a bath temperature of 150-170° F (65-76° C).
7.Rinse.
8.Dry.
9.Apply lubricant overcoating (carbon, silicon dioxide, fluocarbons, oxidizing treatments and other processes). This step is optional.
10.Test for magnetic characteristics.
A thin layer (0.04 to 0,08micrometres) of carbon is often deposited over the cobalt medium using sputtering techniques. This serves as a lubricant.
AbstractElectroless (outocato/yhc) plating processes can greatly increase the storage capacity of memory discs used in Winchester- type disc drives A nonmagnetic electroless nickel layer is deposited onto a carefully prepared aluminurn substrate, polished to a near perfect finish, activated, and plated with an electroless cobalt-Phosphorus allay. The resultant disc is a durable, high-performance memory device with large storage capacity.
This paper describes the deposit 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 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.'
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 equivalent 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. The limit of capacity is much 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.
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. nonetchcleonerideoxidizer.
16.Rinse.
Zincate 20 to 25 seconds at room
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 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 required.
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
lurzinc is applied from a special zinc- ate 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, polishablebase 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
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.Clean in a mild alkaline cleaner.
3.Rinse (Dl water).
4.Activate. Mild acids are often used (proprietary solutions vary from mild alkaline to acidic solutions).
5.Rinse (DI water).
6.Electroless cobalt plate for 1 to 2 minutes at a bath temperature of 150 to 170°F (65 to 76°C).
7.Rinse.
8.Dry.
9.Apply lubricant overcooting (carbon, silicon dioxide, fluorocarbons. oxidizing treatments and processes). This step is optional.
Test for magnetic characteristics.