Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
ELECTROLESS NICKEL AND COBALT FOR MEMORY DISCS
by HENK DIESBERGEN and DON BAUDRAND
Computers use different types of memory discs to store information. Today's surface technologies for producing these discs will be discussed with emphasis on thin film memory discs. Electroless nickel and electroless cobalt are very important for- the manufacturing of these modern memory devices. Operating parameters as well as treatment cycles for these processes will be discussed along with quality requirements and post-treatments.
Electroless nickel deposits were initially commercially developed for use on industrial equipment and hardware because of their high hardness (500 -800 VHN) and chemical resistance. It did not take long until electroless nickel was used for many applications in the electronics industry. The deposit was welcome here because of characteristics like uniform thickness diffusion barrier, solder ability, wire and die bonding , heat resistance etc. For the manufacturing of memory discs the nonmagnetic properties of electroless nickel and the magnetic properties of cobalt are of very much interest in addition to hardness, wear resistance and corrosion protection.
A LITTLE BIT OF HISTORY
At this point we like to look back a few (!) years be-fore addressing today's situation.
The history of computers started as far back as the scientific revolution, beginning in the mid-sixteenth century with the Copernican view of the universe. In 1642 Blaise Pascal-, a French mathematician and philosopher, devised a calculating machine. In 1760 the Industrial Revolution began to take shape in England. In 1804 Jacquard, a French inventor developed the first "programmed" weaving loom using coded information punched into paper cards. In 1880 Hollerith took the idea of the punched card a step further and built machines that could read the cards. These electrical accounting machines are probably still in use today. Punched cards were the first memory devices. In 1952 a major step towards further progress was made by IBM's use of magnetic tape, followed in 1955 by the first magnetic memory disc.
WHAT IS A MEMORY DISC
A memory disc is a high speed magnetic medium used in a computer that can store and allow retrieval of information. A magnetic disc drive is the device that can either read a disc or write onto a disc. Each disc drive has a read/write head that the computer uses to access the disc for either reading or writing. Because disc drives read data electronically by sensing magnetized areas, and write data electronically by magnetizing areas, discs may be processed at very high speeds. A disc drive can contain more than one disc. Discs can store large amounts of information in a condensed area.
TYPES OF DISCSFLEXIBLE DISCS
Flexible discs, also called floppy discs, are low-cost storage devices, made of Mylar coated with a magnetic medium, usually iron---oxide. The floppy disc system works well but has some limitations, one of which is storage capacity. A 5.25 " floppy disc has a storage capacity of 250-500 pages of information.
HARD or RIGID DISCS
Hard discs became popular after IBM introduced the Winchester drive with its sealed head/disc assembly. Hard discs can store at least ten times more information than floppy discs. The so called head does not ride directly on the iron oxide coating as does the head on a floppy disc. Instead the head "flies" at a distance of about 0.2- 0.5 micrometer above the coating on a cushion of air created by the disc's rotation.
Generally hard discs are rigid, polished aluminum discs with thin coatings of iron oxide. In relation to their distance from the operating level of the head, these iron oxide coatings are thick. This can create problems.
These discs have a storage capacity of 10.000 - 15,000 pages of information.
Since the early 1980's the memory storage industry is moving rapidly to a "thin-film" cobalt-based medium because it offers advantages over the thicker iron oxide coating. The thickness of the cobalt coating is only in the order of 0.1 micrometer, whereas the iron oxide is 3-10 micrometer. One thin film technology with good promise involves the use of a special non-magnetic electroless nickel-phosphorus undercoat. The cobalt recording medium can be applied in different ways: electrolessly, electrolytically or through sputtering. In this presentation we will only discuss electroless cobalt deposits.
The main advantages of the new technology are; higher storage density and faster access. Therefore thin film discs are well suited for small but high capacity disc drives.
The storage capacity of these thin film discs can be 20,000 - 50,00O pages of information for a 5.25 " disc,
The thin film rigid (hard) disc is made from an aluminum base that is machined smooth and flat and is then plated with nonmagnetic electroless nickel-phosphorus. The nickel-phosphorus deposit is polished and textured and plated with electroless cobalt. The cobalt layer is then coated with a lubricant and/or a protective coating.
Most popular disc sizes are 5.25 " and 3.5 " in diameter. Other sizes are 8 " and 14 ".
Because thin-film medium cobalt does not require fillers and binders, there is more magnetic material in a smaller volume. Thin- film media are also more durable and abrasion resistant than is the standard iron oxide medium.
In the United States the production of thin film memory discs will be 10 million in 1937. In 1986 this figure was 8 million.
Of the total hard disc market 30% is thin film, 70% is iron oxide. In 1988 the share of thin film may be 507...
WHY USE ELECTROLESS NICKEL AND COBALT FOR MEMORY DISCS 2
Electroless nickel is needed for corrosion protection and as a machinable base for the thin-film cobalt medium-It provides a hard, pi•-free, flawless surface' for the cobalt . To avoid interference with the magnetic properties of the cobalt layer, the electroless nickel also has to be non-magnetic.
The advantages of cobalt in general have already been mentioned. There is a difference of opinion as to which application method is the best. Plating, especially electroless plating, seems to be more economical because it is fast and lends itself readily to high volume production.
The magnetic properties of electroless cobalt can be excellent for read/write storage devices. Coercivity of 350-1300 Oersted can be obtained with all other parameters favorable for good high density recording.
Aluminum is used as substrate for hard discs. Alloy 5036 is the most common but also 5186, 7075 and CW 66 are used.
The alloy can be H-24 or 0 temper. This refers to the hardness and stability of the alloy. 0 temper is preferred for plating because it produces smoother surfaces.
The alloy has to be of high quality with the highest possible finish. To obtain this finish, polishing and diamond turning are used. After this the surface roughness has to be less than 0.013 micrometer.
The surface condition is critically important because the head "flies" at a distance of 0.2 - 0.5 micrometer above the surface of the disc during read and write operations. As comparison : a human hair has a diameter of 75 micrometers, a smoke particle has a dimension of 6.25 micrometer.
Polishing and handling defects of the aluminum substrate can appear as pits in the electroless nickel plated surface. Some pits will be leveled by the electroless nickel plating; others, deeper and larger scratches or dents, remain after plating. Some of these which will be removed by polishing.
The use of certain plastics has been studied as replacement for aluminum. So far this has not been successful because of adhesion problems and perhaps more importantly because an etch treatment is required to obtain good adhesion between the electroless nickel and the plastic. This etching produces rough surfaces. As mentioned above, rough surfaces cannot be used for memory discs.
HOW ARE ELECTROLESS NICKEL AND ELECTROLESS COBALT DEPOSITS APPLIED?
CHEMICAL TREATMENT BEFORE ELECTROLESS NICKEL
The alloy used to produce magnetic memory discs is a magnesium containing alloy. Magnesium tends to congregate at the grain boundaries, providing electrochemical cells that could lead to pitting of the aluminum if severe processing steps were used Therefore a mild low-etching alkaline cleaner is required in the initial stage of the preplate cycle. An extremely mild acidic treatment is then used to remove oxide films from the aluminum disc so it can properly receive the zincate treatment.
TYPICAL PROCESS CYCLE
l Alkaline clean in a mild, low-etch cleaner.
3. Acid clean and deoxidize in a mild non-etch
5. Zincate 20-25 seconds at 25 Dec.
6. Thoroughly rinse.
7. Strip the zinc deposit in 6074 by vol. nitric acid (42 Be).
9. Zincate (15 sec. at 25 Degree. C). Must be a separate zincate tank from step 5.
10. Thoroughly rinse (double rinse, counter-flow).
11. Neutralize in a solution of 30 g/1 Sodium Bicarbonate.
12. Rinse (DI water)
13. Electroless nickel plate.
Filtering of all solutions is recommended
THE ELECTROLESS NICKEL PLATING SOLUTION FOR MEMORY DISCS.
Composition, characteristics and operation of the electroless nickel bath must be such that very smooth; pit free, non-magnetic deposits can be produced in a most economical way. The non-magnetic properties shall not be affected when exposed to a heat treatment at 250 Degree C for a period of 1-3 hrs. This heat resistance assures that the deposit will remain non-magnetic for the life of the device. It also prevents the magnetic characteristics from changing if an operation like sputtering is carried out at a later stage.
It almost goes without saying that the room where the plating is done has to be a so called clean room (category 1000 or better).
Operating parameters of an electroless nickel solution meeting the above requirements:
Bath temperature - 99 den C
Bath pH - 4.6 - 4.8
Bath replenishment - Automatic chemical feed based on nickel content and pH analyses to maintain bath at near 100 'Z.
pH adjustment with potassium carbonate results in longer bath life compared to using ammonium hydroxide.
Plating time 90 - 120 minutes
Filter through a 1 - micron depth filter, followed by an absolute filter of 0.25 - 0.45 micron. The filtration rate should be 7 x solution volume/hr.At frequent intervals carbon filtration should be used followed by the 0.25 - 0.45 micron filtration. The best available filter system should be used.
Tanks of 1600 liter are in use.
Typical load is 150-250 5.25 " discs per tank or 0.4-1.3 dm 2 of disc surface/liter of bath solution.
Disc rotation 8 -20 rpm depending on solution movement.
Tank material: Polypropylene or stainless steel with liners. Liners have to
be leached be-for use in 2 -4 H2SO4 1 hr. 80
Degree c. Liner material is normally PVC.
Nickel thickness after plating : 12 -20 micrometers nickel-phosphor alloy with 10-12% phosphorus.
Polishing and/or diamond turning will remove about 4 micrometers of nickel.
Pitting : With use of a well selected electroless nickel solution pitting and roughness will seldom be a problem if+ proper filtration, for instance a 1 micron filter followed by a 0.2-0.5 micron absolute filter is used .The assumption is made here, that the substrate did meet the necessary quality standards.
Also other housekeeping aspects should be in order; do not use compressed air for agitation. To "platers" this is well known, but in many cases "memory disc companies" have no prior experience in plating and therefore this should be mentioned.
Edge pull back ; This refers to the phenomenon where some areas on the disc, especially the outer diameter, may have a thin nickel layer or no nickel deposit at all. The cause for edge pull back can be : too much agitation, rough edges (microscopically) due to a dull chamfering tool, and organic or metallic impurities.
Poor adhesion: not using proper cleaning cycle and conditions or. for instance, incorrect operating temperatures of the zincate solutions.
There are other potential problems like poor bath stability. magnetic deposits instead of non-magnetic deposits, low rate of deposition. These symptoms however depend more on the quality of the electroless nickel solution than on housekeeping by the user.
EXPECTED QUALITY OF ELECTROLESS NICKEL DEPOSIT
Surface roughness should be 0.03 - 0.06 micrometer, or- less. Practically no pits or roughness.
No gassing after immersion in 15% hydrochloric acid. No pits after the required mechanical treatment.
Non-magnetic even after heat treating at 250 dir. for 3 hrs. Thickness specification as prescribed by customer.
There are other ,obvious requirements like good adhesion and uniform plate.
MECHANICAL TREATMENT OF ELECTROLESS NICKEL DEPOSIT
The condition of the nickel surface prior to cobalt plating can have a considerable effect on the magnetics resulting from the subsequent cobalt deposit. This fact is the reason that in many cases a special texturing is applied to the electroless nickel deposit to improve the coercively of the cobalt layer.
Before the texturing operation the electroless nickel is polished to a high finish. The surface roughness of such a textured nickel layer is in the order of 0.025 micrometer. Because the cobalt layer is very thin (0.06 micrometer) the nickel surface texture remains.
Texturing increases the coercivity by 50-100 Oersted. The texture is also important because it creates an air cushion when the disc rotates.
CHEMICAL TREATMENT OF THE ELECTROLESS
NICKEL DEPOSIT BEFORE
ELECTROLESS COBALT PLATING
Activating the polished electroless nickel surface is important because adhesion, dropout rate and magnetic properties of the electroless cobalt are influenced by the activation. Ideally. activation and plating should take place immediately after polishing and texturing the electroless nickel plated disc surface. This is not always possible.
The following treatment cycle can be used
1. Low etch alkaline cleaner 2-4 min , 60 degree C
3. 20 % HCL until gassing( approx. 1 min) optional
5. 5 % H2SO4, room temp, 1 min..
7. Electroless cobalt.
ELECTROLESS COBALT SOLUTION OPERATING PARAMETERS
The solution must be easy to operate and control. The solution should not rely on ammonia for pH control, resulting in less fluctuation of pH during plating. Magnetic properties thus remain constant and predictable. A common problem with electroless cobalt solutions has been that the plating time to obtain the thin coating was very short ( 20 sec.), making it difficult to use these processes. The solution described here has a "long" plating time of 1-3 minutes. This also provides a more dense, pore-free deposit.
Electroless cobalt coatings are harder than electrolytic cobalt deposits.
Liners used for the plating tank have to be carefully selected. Organic impurities leached from the liner van cause plating defects.
Thickness of the cobalt layer is 0.045 - 0.06 micrometers. Bath loading 0.5-1.25 dm2/ltr
Bath loading is not critical as far as plating performance is concerned. However, large loads can cause significant cooling of the bath and as the temperatures is lowered a longer plating time is required. Because magnetic characteristics change with the thickness of the cobalt layer, it is important to monitor the bath temperature and/or plating time.
Sometimes it is good practice to dummy plate the cobalt solution with steel sheets or rejected discs before production starts.
Continuous 1 micron filtration is recommended. Occasional carbon filtration can be beneficial.
Bath temperature 65-82 Degree C. Plating time 1-2 min.
Coating thickness has an influence on coercivity, therefore the plating rate of the electroless cobalt solution should be predictable. Thinner coatings have higher coercivity and
DUALITY CONTROL OF ELECTROLESS COBALT
Coercivities of 600-800 are typical. It is possible to produce a range of coercivities with one solution. For instance, a bath on the market with a nominal coercivity of 600 Oersted can create discs with values of 600-800 Oersted. The Oersted value can be controlled by changing the ratios of the make-up components of the electroless cobalt solution. By use of altered formulations, coercivities up to 1300 Oersted are possible.
Electrolessly deposited recording media provide a superior finish, completely uniform thickness, and greater versatility than other media processes.
Some of the more important tests carried out on a finished disc:
A B-H meter evaluates the strength and magnitude of the magnetic layer.
A Certifier checks the memory response and performance of the disc.
POST-TREATMENT OF ELECTROLESS COBALT
Protective coatings like carbon are used on top of the cobalt layer. These coatings also can provide lubricity. Carbon is applied by sputtering.
Other possible coatings : silicon dioxide, fluorocarbons, oxidizing treatments.
These coatings are very thin, for instance 0.08 micrometer.
OTHER APPLICATIONS FOR COBALT MEDIUM
The following are additional potential applications for thin-film cobalt media:
1 Read/write heads.
2 Mylar film for tapes and floppy discs.
WHAT IS NEXT ?
Presently the read/write heads are such that magnetic orientation parallel to the surface is preferred. Using "thin film' heads, the next generation of head technology, which offers faster switching and will handle higher coercive force and signal amplitude, will allow the use of vertically oriented magnetic domains. This vertical orientation will further increase the capacity of thin-film discs, and can be accomplished by slight modification of the solution composition and operating parameters, along with changing the type of under layers supporting the cobalt phosphorus alloy recording medium.
The authors wish to thank Allied-Kelite Div., Witco Corporation for its support in providing time and resources necessary to the preparation of this paper.
Baudrand, D.W. and Malik, M. "Autocatalytic alloy plating processes for thin-film memory discs".
Baudrand. D.W., "Electroless Nickel for Thin-Film Magnetic Memory Disc-s".
R.A.Stern and N.B.Stern "Principles of Data Processing", John Wiley & Sons, New York.
Henk P D Diesbergen is Manager of International Operations for Allied-Kelite Div., Witco Corporation. He received his BSc in chemical engineering from the HTS in Dordrecht, The Netherlands, in 1952. He was assistant plant manager in the metal finishing department of N.. V. Philips' Gloeilampenfabrieken in Eindhoven from 1954-1959.He has been marketing Allied-Kelite's products since 1940 in Switzerland and since 1964 in Europe. He was assigned his present position in 1979.
Donald W Baudrand is Vice-President of New Market Development for Allied-Kelite Div. Witco Corporation. He has been with AlliedKelite since 1966 serving as Western Regional Manager and Vice- President Research & Development. Prior to joining the firm he was president of Electrochemical Laboratories, founded by him in 1955.
You may download this article FREE in .pdf form, save it or share it with a colleague. Click Here