CN1322349A - Head suspension with flexible circuit interconnect for reduced moisture permeability - Google Patents

Abstract

This invention provides a disk drive suspension device, including a transmission head electrically connected to the suspension device via a flexible circuit. The flexible circuit has a dielectric layer and at least one conductive material layer. At least one main surface of at least one layer of the circuit has at least one layer of diamond-like carbon (DLC). The DLC coating has a thickness of 300-3000 angstroms and a hardness of 8-9. The DLC layer can be directly coated onto the dielectric material substrate, or it can be a capping layer, or both.

Description

Head Suspension with Flexible Circuit Interconnect for Reduced Moisture Permeability

Background of the invention

field of invention

The present invention relates to disk drive suspensions utilizing head suspension assemblies interconnected by flexible circuits having reduced moisture permeability and thus warpage, wherein at least one major surface of said circuits is selectively or continuously coated with at least one layer Films of diamond-like carbon.

Description of prior art

A head suspension assembly for spinning data storage is a spring structure that holds and positions a flying head just a few nanometers away from a rapidly spinning data storage such as a magnetic hard disk drive or optical disk drive. This assembly includes components such as the suspension (usually made of metal, such as stainless steel), springs, load beams, and gimbal fixtures, the latter two of which must have rigid and flexible spring areas, as well as the magnetic head, The magnetic head consists of a highly sensitive transducer attached to an air bearing head or "slider".

The gimbal is one of the most critical parts of the suspension. The closer the suspension assembly can float (or "fly") on a cushion of air on the surface of the data storage, the denser the information that can be stored on the storage. However, it is critical that the suspension assembly should not come into contact with the disk, as a collision with a spinning disk will not only destroy the suspension, but also damage the storage disk and the data stored on it. Therefore, the suspension components and gimbal must be precisely balanced. The gimbal must be responsive in order to maintain a height above the many tiny peaks and valleys of the data storage disk. The gimbal must also resist static pitch and roll.

A conventional gimbal is formed from a single piece of material and includes a pair of flexible outer arms that surround a central hole. See, eg, US Patent 4,167,765. When the head reads and writes data memory, it receives and sends out electrical pulses of information. These pulses are coupled to appropriate amplification and processing circuitry. Discontinuous conductors are usually used to connect the amplification system and the heads on the head suspension assembly. This approach is advantageous because the amplification system and the magnetic head can be manufactured separately and can be adapted to various configurations. However, as technology develops, this "interconnect" system must be able to carry multiple signals, requiring multiple conductors and other components. However, as the number of conductors increases, the interaction of the interconnection system creates unpredictable biases and loads that may change the positioning of the head or make it difficult for the suspension assembly to respond to changes in the disk surface. This can significantly affect the density of stored information and the reliability of reading information. The interconnect system can also adversely affect the flexibility of the gimbal area by adding rigidity to this critical area of the gimbal. Furthermore, brittle conductors can be damaged during the production steps. Various improvements have been attempted to increase the quality of interconnected systems without reducing data storage density or increasing the likelihood of read/write errors.

Flexible circuits have also been used for interconnection systems in suspensions. The flex circuit follows the topography of the suspension assembly and can be connected at multiple locations. They provide options for increasing or decreasing resistance along the flex circuit path. They are less visible than discontinuous conductors and have controlled impedance. However, their use creates at least two significant problems. Moisture absorption can cause undesirable stress and warping, and film substrates give flexible circuits high rigidity.

A variety of flexible circuits are known in the industry. A flexible circuit is an electrical circuit formed on a flexible substrate. A flexible circuit is an electrical circuit formed on a flexible dielectric substrate, such as a polymeric material, including one or more conductive layers as described in US Patent 4,231,154. The circuit may also contain other layers, including additional insulating layers, including adhesive layers, encapsulation layers, stiffeners, and the like. The means used to interconnect a flex circuit to, for example, another flex circuit, a printed wiring board, etc. also vary. Circuitry may be on only one major surface, or on both major surfaces, see eg US Patent 4,480,288.

US Patent 4,819,094 discloses a damped head suspension assembly in which flexible circuits are bonded to a suspension to provide damping and appear less visible than discontinuous conductors.

US Patent 4,996,623 discloses a suspension assembly having an interconnection system comprising a sheet of polyimide material sandwiched between two metal layers. Multiple conductors can be formed within the copper layer.

US Patent No. 5,491,597 discloses a gimbal having an electrical interconnection assembly which is strong and supportive, it is this interconnection system which can be shaped and used as a gimbal when connected to a head suspension assembly. The gimbal includes a set of traces, preferably formed of beryllium copper, coated with a thin dielectric layer. The traces can be laminated to the load beam with an adhesive, which can also be a dielectric material.

British patent application GB 2295918A discloses a head suspension system for supporting a magnetic read/write head (slider), which has two curved arms with two flexible fingers. The electrical interconnection system, including electrical conductors connecting the heads, is placed along one or both of the flexure arms, or adjacent to but outside the second flexure arm so as not to increase the rigidity of the second flexure arm. The conductor consists of a laminate comprising a conductive layer, a dielectric layer and a support layer.

One difficulty with flexible circuits that has not been addressed by previous efforts, however, is the hygroscopicity of the dielectric materials used for flexible circuits. When a dielectric material absorbs water it swells, causing it to warp. This changes the static pose of the suspension components, which can lead to loss of signal. Data storage, such as disk drives, needs to operate in wet environments. Therefore, it is hoped that the flexible circuit will preferably absorb less water and warp less. Humidity changes between the location of the disk drive assembly and the location of use can create moisture-induced problems in the suspended flex circuit.

Diamond coatings and diamond-like carbon films are known. These products provide a hard coating and improved abrasion resistance and are typically applied to rigid surfaces subject to friction and wear, such as glass in eyeglass lenses, windshields, and the like.

US Patent No. 5,508,071 discloses a diamond coating for the annular inner surface to improve wear resistance. The coating is deposited on a substrate such as a metal, alloy or ceramic. Because chemical vapor deposition (CVD) of diamond layers occurs at very high temperatures, it cannot be used on many polymer substrates, such as polyimides, that degrade at the high temperatures at which diamonds are formed. In addition, the polycrystalline nature of CVD diamond coatings determines that it is a very hard and brittle coating with very little flexibility. The term "diamond-like carbon" generally refers to amorphous materials, especially those in which tetrahedral diamond bonds predominate.

US Patent 4,576,964 discloses barrier films made of a flexible polymeric substrate having an amorphous carbon coating bonded thereto.

US Patent 5,508,092 discloses an optically clear, very abrasion resistant coated substrate comprising a matrix substrate, one or more intermediate layers and a topcoat of diamond-like carbon or other low coefficient of friction material.

The inventors of the present invention have now found that one or more layers of well-conforming diamond-like carbon deposited on a flexible circuit not only provide protection against abrasion, but also reduce damage caused by increased humidity by effectively blocking moisture. The warpage phenomenon, and provides a means of stability and material robustness, which will greatly reduce the damage of these delicate materials during processing. Finally, a coating of diamond-like carbon can provide a non-outgassing covercoat or insulation for conductors.

The flexible circuit can be selectively coated so that the bonded areas remain uncoated, or fully coated and then peeled away from the bonded areas. In addition, the well-conforming diamond-like carbon layer can be applied directly to the non-conductive side of the flexible circuit substrate, or it can be applied after circuit steps such as development, etching, and plating, or it can be applied after each of these steps. Apply one coat in one step.

Summary of the invention

The present invention provides a disk drive suspension, which includes a flexible circuit with at least one diamond-like carbon (diamond like carbon) layer deposited on at least one of its major surfaces. The present invention also provides a flexible circuit for manufacturing to the frame method.

Disk drive suspension device of the present invention comprises:

a) at least one layer of polymeric dielectric material with at least one layer of electrically conductive material thereon, each layer having two major surfaces, at least one of said layers having at least one aperture,

b) at least one diamond-like carbon layer on at least one major surface of at least one layer, said carbon layer having a thickness of 300-3000 Angstroms and a hardness of 8-9.

In one embodiment, a layer of diamond-like carbon is deposited over the entire flexible circuit. In another embodiment, a layer is deposited only in selected areas, such as around vias or vias. A layer of diamond-like carbon may also be deposited on the inner walls of, for example, vias or vias. In another embodiment, multiple layers of diamond-like carbon are deposited on a plurality of layers as the circuit structure is formed.

The flex circuit can be attached to the suspension with a continuous layer of adhesive, or it can have adhesive only at predetermined locations (point bonding). In embodiments where a continuous adhesive layer is used to bond the circuit to the suspension, a layer of diamond-like carbon is only required on the side of the circuit that is opposite the adhesive layer. In cases where the circuits are joined by spot bonding, a layer of diamond-like carbon should be applied to both sides of the circuit. This provides the added benefit of providing a barrier against moisture between the circuit and the component. Reducing or substantially eliminating moisture in this area can significantly improve floatation characteristics and dimensional stability.

The following terms have the following meanings when used in the present invention.

1. The term "gimbal assembly" refers to the part of the suspension that makes the head roll and pitch.

2. The term "suspension" refers to the assembly used to support the head and provide a spring force that opposes the buoyancy of the air to maintain the head at a constant height above the disk.

3. The terms "diamond-like carbon" and "carbon-rich film" are synonymous and interchangeable in the industry, as in Ed. J. Mort and F. Joanne 1986 CRC Press, Boca Raton, FL As described in the book "Plasma-Deposited Films", they refer to carbon films that are composed mainly of carbon and have no long-range atomic order.

4. The term "hole" refers to an opening in a layer of flexible circuitry. The hole may extend through a portion of the layer, or may extend completely through the layer. Holes can be formed by various techniques including mechanical punching, chemical etching, and laser ablation.

5. The term "via" means a hole that extends completely through a layer of a flex circuit that has metal traces exposed on one side.

6. The term "via" refers to a metallized via that connects one conductive trace to another conductive trace or plane.

7. The term "blind via" refers to a metallized hole that does not extend completely through one layer of a flex circuit.

8. The terms "etching" and "milling" are used as synonyms and include mechanical, chemical and optical processes for removing material, including chemical etching, laser ablation, mechanical milling, and the like.

All ratios, parts and percentages used herein are by weight unless otherwise indicated.

Brief description of the drawings

Figure 1 is a perspective view of the disk drive suspension of the present invention.

Figure 2 is a side sectional view of the disk drive suspension of the present invention.

Figure 3 is a side sectional view of a disk drive suspension.

Figure 4 is a perspective view of another embodiment of the disk drive suspension of the present invention.

Figure 5 is a cross-sectional view of a conventional substrate used for disk drive suspension interconnects.

Figure 6 shows a substrate having performance enhancing diamond-like carbon applied to both sides of a polyimide film.

Figure 7 shows a substrate having performance-enhancing diamond-like carbon applied to both sides of a copper-clad polyimide film.

Figure 8 shows a test fixture for measuring warpage due to hygroscopicity.

Detailed Description of the Invention

The disk drive suspension of the present invention includes at least one layer of well-conforming diamond-like carbon (DLC). Diamond-like carbon coatings, also known as "thin-film carbon-rich coatings", contain two types of carbon-carbon bonds, triangular graphite bonds (sp2) and tetrahedral diamond bonds (sp3), with tetrahedral bonds predominating . Thus, the film exhibits many of the properties of diamond, such as being very hard, chemically inert, corrosion-resistant, and impermeable to water vapor and oxygen, and also some of the properties of graphite, such as smoothness and strong adhesion to polymeric materials. It also has extremely low electrical conductivity and excellent optical transparency over a broad wavelength range. The thickness of the diamond-like carbon layer used for the flexible circuit carrier and the flexible circuit of the present invention is 300-3000 angstroms, preferably 1000-2000 angstroms.

Further comparative tests on uncoated polyimide under the same conditions found that coating a layer of diamond-like carbon with a thickness of 1000 Angstroms on a 50 μm (2 mil) polyimide can substantially eliminate Warpage; 95% improvement in concentricity and registration of laser-ablated holes; 92% and 93% reduction in water and oxygen permeability; loss of adhesion between polyimide and copper 96% reduction; 43% increase in flexural stiffness.

In some cases, one or more layers of diamond-like carbon are deposited on a bare substrate dielectric material (typically a polyimide film) prior to forming the circuit. In other cases, it is appropriate to apply one or more intermediate layers of diamond-like carbon between or between each of the circuit forming steps such as development, etching and plating. It is also possible to coat a layer of diamond-like carbon on any dielectric layer (such as a metal layer). The desired properties of the resulting structure dictate the design, placement and number of diamond-like carbon layers. Any additional layers must also be soft enough to bend with the flex circuit for easy handling, manipulation and assembly without causing breakage or damage to the circuit.

The diamond-like carbon layer included in the circuit of the present invention can be coated by any of the various methods (ion beam, plasma, etc.) that have been developed to deposit such a carbon layer including the use of a solid carbon or hydrocarbon source . The deposition method can be batch deposition or continuous deposition, of course, in terms of production efficiency, continuous deposition is better.

In preferred articles and methods of the present invention, the diamond-like carbon layer is deposited using a continuous plasma process in combination with ion acceleration. Applying a high-frequency electric field to the carbon-containing environment, generally by energizing the rotatable electrode element, can generate a carbon-rich plasma. Ions within the carbon-rich plasma are accelerated toward the electrodes, striking the substrate in contact with the rotating electrodes.

Diamond-like carbon can also be functionalized with materials such as fluorine, silicon, oxygen, sulfur, nitrogen, copper, chromium, titanium, and nickel. The initial stage of deposition can be carried out with a mixture of appropriate gases by gradually increasing the concentration of each gas and decreasing the concentration of the hydrocarbon precursor before the final stage of diamond-like carbon deposition. This process is called "doping".

Flexible circuit substrates are flexible polymeric dielectric films that can be partially cured or substantially fully cured. Useful organic polymers include polyimides and modified polyimides such as polyesterimides and polyimide esters, polysiloxaneimides, and polyamides, polymethylmethacrylate , Polyesters such as polyethylene terephthalate, polycarbonate, polytetrafluoroethylene, and mixtures thereof. Preferred are polyimides, particularly preferred are polyimide polymers made from pyromellitic anhydride and 4,4-diaminodiphenyl ether, commercially available from EI DuPont de Nemours and Company under the tradename for ^Kapton® . Variants include Kapton(R) H, Kapton(R) E and Kapton(R) V. Another useful polyimide precursor also available from DuPont is Pyralin ^™ . Also useful herein are Upilex ^™ from Ube Industries, Ltd. and Apical ^™ from Kamaka Films.

The conductive layer is typically formed of conductive metals such as tin, gold, silver, copper, and the like. The thickness and arrangement of these layers are closely related to the specific type of circuit or electronic package required.

The flex circuit may also include interlayers, stiffeners, ground planes, and insulating layers, which are discussed below in conjunction with the preferred embodiment.

The diamond-like carbon (DLC) layer or coating deposited on the flexible circuit used in the disc drive of the present invention provides several benefits to the circuit and the disc drive suspension. The thin-film carbon-rich coating contains two types of carbon-carbon bonds, triangular graphite bonds (sp2) and tetrahedral diamond bonds (sp3), with the tetrahedral bonds predominating. Thus, the film exhibits many of the properties of diamond, such as being very hard, chemically inert, corrosion-resistant, and impermeable to water vapor and oxygen, and also some of the properties of graphite, such as smoothness and strong adhesion to polymeric materials. It also has extremely low electrical conductivity and excellent optical transparency over a broad wavelength range.

Useful diamond-like carbon layers have a thickness of 300-3000 angstroms, preferably 500-2000 angstroms.

Manufacturing process description of carriers and circuits

The fabrication process of the flexible circuit and carrier of the present invention includes the step of depositing at least one layer of diamond-like carbon thereon, which may be combined with steps such as metal sputtering, plating resist lamination, resist exposure, development, etching and Various known processes such as plating are used together. The order of these processes can be changed according to the specific application. Multiple deposition steps can be used when it is desired to deposit more than one layer of DLC, for example, a layer of DLC deposited directly on the substrate to improve planarity and create vias, and deposited on the metal layer A layer of diamond-like carbon on top to provide benefits such as wear resistance and thermal modulation. The process described here works equally well for carriers and flex circuits.

An additive process for fabricating a flexible circuit or carrier has a typical series of steps as follows:

First, a well-adhered diamond-like carbon layer is deposited on the polymer (such as polyimide) side of the film substrate. The substrate can be fabricated by various methods such as bonding a cured polymer layer to a copper foil, coating a liquid polyimide onto a copper foil, and the like. Typically, the substrate consists of a 25 micron to 125 micron polymeric film layer with a copper layer 1 to 5 microns thick. Coatings can be deposited on one or both sides of the polyimide.

Next, a polyimide film (or a film coated with diamond-like carbon) is sputtered with a seed layer of chromium and copper. A photoresist, which can be water-based or solvent-based , and can be negative or positive photoresist. The thickness of the photoresist is from 35 to 50 microns. The photoresist on both sides is then exposed to ultraviolet light or similar radiation through a mask or photographic tool to cause crosslinking of the exposed portions of the photoresist. The unexposed portions of the photoresist are then developed with a suitable solvent, and in the case of aqueous photoresists, a dilute aqueous solution such as 0.5-1.5% sodium or potassium carbonate solution is applied until the The desired pattern is obtained on both sides of the laminate. The photoresist is then removed, allowing the copper side of the laminate to be further plated to the desired circuit thickness. If desired, one or more layers of diamond-like carbon can also be deposited on top of the copper, or on its backside.

The laminate is then placed in a concentrated alkaline solution at a temperature ranging from 50°C to 120°C, which etches away the portions of the polymeric film not covered by the cross-linked etchant. This exposes certain areas of the original thin copper layer. The resist is then removed from both sides of the laminate in a 2-5% alkaline metal hydroxide solution at 20-80°C, preferably 20-60°C. Next, the original thin copper layer at the exposed locations is etched away with an etchant that will not damage the polymeric film (eg, Perma-etch ^(R) available from Electrochemicals, Inc.). The final circuit has copper circuitry on one side, a diamond-like coating on the polyimide surface on the opposite side, and optional diamond-like carbon layers between internal layers or on individual parts.

The circuit is then converted and inspected; ie the substrate is cut into smaller strips and checked for quality.

In another process called the subtractive process, a polyimide film is sputtered with a seed layer of chromium and copper. A water-processable photoresist is laminated on both sides of a substrate with a polymer film side and a thick copper side using standard lamination techniques. The substrate used in this process consists of a polymer film layer with a thickness of 12-125 microns and a copper layer with a thickness of 12-40 microns (in the additive process, the copper layer is 1-5 microns thick).

The photoresist on both sides is then exposed to ultraviolet light or similar radiation through a suitable mask to cause crosslinking of the exposed portions of the photoresist. The pattern is then developed with a dilute aqueous solution until the desired pattern is obtained on the trace side of the laminate. The circuit is then etched away from the thick copper layer, thereby exposing portions of the polymer layer.

Then, another layer of aqueous photoresist is laminated on top of the first resist on the copper side and exposed to a radiation source in large quantities to cause crosslinking to prevent the exposed polymeric film surface (on the copper side) ) are further etched. The area of the polymer film (on the film side) not covered by the cross-linked resist is then etched away with concentrated alkali at 70-120° C., and then the photoresist is removed from both sides with a dilute alkaline solution. The circuit is then transformed and inspected; that is, the substrate is cut into smaller strips and checked for quality.

As with the additive process, the diamond-like carbon layer can be applied after the initial lamination and sputtering steps.

Previous methods applied a diamond-like carbon layer over the entire substrate. In the selective coating process, the diamond-like carbon layer is applied only on the areas of the polyimide surface that need to be protected. These processes are usually completed after circuitization. An optional process is the mechanical masking process, which applies a diamond-like carbon layer through an appropriate mask after a quick etch and conversion step.

These processes may also include other steps such as soaking the membrane in hot water before or after etching baths, rinsing steps, and the like. An acid bath can also be used for neutralization after etching, and the blank cleaning step can follow the plating step.

To produce the final flexible circuit, other layers can be added and processed, the copper plated layer can be plated with gold, tin or nickel according to conventional means for subsequent soldering processes, etc. The diamond-like carbon layer may also be applied after this final plating step when using a selective coating process such as a mechanical masking process.

The means of interconnecting the flex circuit to the suspension of the suspension may be selected from any conventional means, connecting pads or other connection points including solder balls, reflow soldering, thermocompression bonding, wire bonding, internal wire bonding, and the like.

The embodiments discussed in the figures are illustrative and are not intended to limit the scope of the invention, which is indicated only by the claims.

Detailed description of the drawings

1 shows a hard disk drive suspension in which a conventional stainless steel flexure 100 has attached thereto a flex circuit 120 with traces 130 on its exposed surface and a bond head 140 attached to the flex circuit. connect. There is no diamond-like carbon coating on the exposed surface of the flexible circuit 120 .

Figures 2 and 3 show side cross-sectional views of different embodiments of the invention. In FIG. 2, the circuit has two continuous diamond-like carbon layers 260, one on each side of a polyimide substrate 280. In FIG. Circuit traces 270 are then formed. The embodiment of Figure 3 has a continuous diamond-like carbon coating 360 on the side of the polyimide adjacent to the stainless steel suspension. On top of the circuit traces 350 has been optionally coated another diamond-like carbon layer 360 .

4 shows a disk drive suspension in which a stainless steel suspension 400 has attached thereto a flex circuit 420 with traces 430 on its exposed surface and a bond head 440 connected to the flex circuit. In this embodiment, on the exposed surface of the flex circuit 420 is a coating 460 of diamond-like carbon that has been deposited.

FIG. 5 shows a conventional substrate 500 for disk drive suspension interconnection having a layer of polyimide 510 with a layer of copper 520 plated thereon. In the substrate 600 shown in FIG. 6, both sides of the polyimide 610 have been modified with a diamond-like carbon coating 630 to reduce moisture and oxygen transmission, and this layer 630 is used to separate the copper layer 630 from the polyimide imide 610 apart.

The substrate 700 shown in FIG. 7 has a polyimide layer 710 and a copper clad layer 720 . Both sides of the copper clad film have diamond-like carbon 730 coated thereon.

Figure 8 shows a test apparatus 800 in which a sample 810 is in a chamber of known humidity and temperature. This enables measurement of warpage due to moisture absorption. The sample is placed in a clamp 820 at one end of the apparatus in an environment with a higher humidity than previously described. The distance the sample deviates from the surface of the test appliance is measured, and this measurement is the warpage. When the sample without any diamond-like carbon coating was at 100% relative humidity (RH), the warpage was about 5mm. When samples similar to those shown in Figures 6 and 7 were coated at 100% RH, no warping was observed.

Claims (8)

1. A disk drive suspension, comprising: A delivery head electrically connected to the suspension by a flexible circuit comprising: a) at least one layer of polymeric dielectric material having two major surfaces, b) at least one layer of electrically conductive material having two major surfaces located thereon, c) at least one diamond-like carbon layer on at least one major surface of at least one layer, said coating having a thickness of 300-3000 Angstroms and a hardness of 8-9. 2. The disk drive suspension of claim 1 wherein said flexible circuit has a layer of diamond-like carbon coated on at least a portion of at least one major surface of said layer of polymeric dielectric material. 3. The disk drive suspension of claim 2, wherein said layer is coated on both major surfaces of said layer of polymeric dielectric material. 4. The disk drive suspension of claim 1, wherein said flexible circuit has a layer of diamond-like carbon coated on at least one major surface of said conductive material of said flexible circuit. 5. The disk drive suspension of claim 1, wherein said flexible circuit has a coating on at least one major surface of said conductive material and at least one major surface of said polymeric dielectric material. diamond-like carbon layer. 6. The disk drive suspension of claim 1 wherein said flexible circuit has said diamond-like carbon layer continuously coated on at least one surface. 7. The disk drive suspension of claim 1 wherein said flexible circuit has said diamond-like carbon layer selectively coated on at least one surface. 8. The disk drive suspension of claim 1, wherein said layer of conductive material is bonded to said layer of polymeric dielectric material.

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