Laminate for HDD suspension and its manufacture

Details Laminate for HDD suspension and its manufacture Laminate for HDD suspension and its manufacture

1999-02-19

2001-03-20

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Composite

Of metal

C156S320000, C156S327000, C156S330900, C427S388100, C428S473500

Reexamination Certificate

active

06203918

ABSTRACT:

FIELD OF TECHNOLOGY
This invention relates to laminates for use in HDD suspensions and also to a process for preparing the same.
BACKGROUND TECHNOLOGY
An HDD (hard disk drive) suspension has been prepared by etching stainless steel foil, and a magnetic head, such as a thin-film inductive head is mounted onto the tip of such suspension and wire bonded with gold wire. In recent years, however, concentrated studies are under way aiming at reduced size, higher density and higher capacity of HDD and, as the studies progress, reduction of flying height of a slider onto which a magnetic head is mounted has become a theme of the utmost importance. From this viewpoint, the conventional gold wires have presented an obstacle to reduction of flying height because of their resistance to air and rigidity. Moreover, wire bonding with such gold wires presents difficulties in automating the step for connecting gold wires with a magnetic head.
As a solution to the problems related to wire bonding with a gold wire, a process such as disclosed in Japan Kokai Tokkyo Koho Sho 60-246015 (1985) proposes to effect patterning with electrically insulating polyimide directly on a stainless steel foil and form a circuit with copper on the polyimide patterns in order to form signal lines directly on a suspension in place of the aforementioned gold wire. A suspension on which signal lines are formed directly in this manner or the so-called integrally-wired suspension does not cause problems of interference with reduction of flying height of a slider due to resistance to air and rigidity of signal lines and also makes it possible to automate the connecting process of a magnetic head.
On the other hand, Japan Kokai Tokkyo Koho Hei 5-131604 (1993) discloses a process for laminating polyimides to a thin metal sheet. However, no attention is paid to the thermal expansion of the polyimides in use and high values of the thermal expansion coefficient concretely disclosed there suggest that deflection tends to occur readily after removal of the metal by etching from the laminate. Moreover, lamination performed as disclosed tends to show variation in adhesive strength between the metal sheet and the polyimide layer and, in addition, gives problematical heat resistance, which makes application of this process difficult to HDD suspensions. Thus, development of materials free of the aforementioned problems was looked for.
Accordingly, an object of this invention is to provide laminates which produce minimal deflection during etching and are suitable for integrally-wired suspensions and to provide a process for preparing said laminates.
After intensive studies of the aforementioned theme, the present inventors have found that the formation of polyimide with specified linear expansion coefficient and adhesive strength on the base material gives laminates suitable for HDD suspensions and completed this invention.
DISCLOSURE OF THE INVENTION
This invention is related to laminates for use in HDD suspensions comprising a stainless steel base material and successive layers formed thereon of polyimides and an electrical conductor wherein the linear expansion coefficient of the layer of polyimides is in the range of 1×10
−5
/° C. to 3×10
−5
/° C. and the adhesive strength between the stainless steel base material and the layer of polyimides and that between the layers of polyimides and the electrical conductor are 0.5 kg/cm or more respectively.
This invention is also related to a process for preparing laminates for use in HDD suspensions comprising applying a solution of polyimide precursors or polyimides in one layer or more to a stainless steel base material with a thickness of 10 to 70 &mgr;m, drying, performing heat treatment at 250° C. or higher to form a layer of polyimides with a thickness of 3 to 20 &mgr;m and a linear expansion coefficient of 1×10
−5
/° C. to 3×10
−5
/° C., and contact-bonding said layer of polyimides under heat to a layer of an electrical conductor with a thickness of 3 to 20 &mgr;m.
Furthermore, this invention is related to a process for preparing laminates for use in HDD suspensions comprising applying successively a solution of polyimides with high thermal expansion 1 or a solution of polyimide precursors with high thermal expansion 1, a solution of polyimide precursors with low thermal expansion, and a solution of polyimides with high thermal expansion 2 or a solution of polyimide precursors with high thermal expansion 2 to a stainless steel base material with a thickness of 10 to 70 &mgr;m, drying, performing heat treatment at 250° C. or higher to form a layer of polyimides with a thickness of 3 to 20 &mgr;m and a linear expansion coefficient of 1×10
−5
/° C. to 3×10
−5
/° C., and contact-bonding said layer of polyimides under heat to a layer of an electrical conductor with a thickness of 3 to 20 &mgr;m.
There is no specific restriction to a stainless steel base material to be used in the laminates for HDD suspensions of this invention. However, it is preferably SUS 304 or more preferably SUS 304 tension-annealed at 300° C. or higher from the viewpoint of spring characteristics and dimensional stability required for suspensions. The thickness of the stainless steel base material is preferably 10 to 70&mgr;, more preferably 15 to 51 &mgr;m. As a base material for HDD suspensions, the thickness is preferably in the range from 18 to 30 &mgr;m from the viewpoint of reduction in size and weight and rigidity of HDD suspensions. A stainless steel base material with a thickness of less than 10 &mgr;m tends to cause large deflection of the laminate and leads to a loss in reliability when used in suspension. On the other hand, a base material with a thickness in excess of 70 &mgr;m shows excessively large rigidity and a slider onto which the base material is mounted is not amenable to reduction of flying height.
A stainless steel base material to be used in this invention preferably has a linear expansion coefficient in the range of 1.75×10
−5
/° C. to 2.0×10
−5
/° C. in each of the transverse and longitudinal directions with the difference between the two directions kept at 0.15×10
−5
/° C. or less, it preferably shrinks 0.025% or less in both directions when heated at 330° C. for 30 minutes in air, and the average roughness (R a) along center line on the surface to be laminated to the layer of polyimides is preferably 20 to 300 nm. When the linear expansion coefficient or the shrinkage after heat treatment exceeds the aforementioned range or value, the laminates tend to produce large deflection. On the other hand, when the average roughness (R a ) along center line is less than 20 nm, it becomes difficult to obtain sufficient adhesion between the stainless steel base material and polyimides. Conversely, an average roughness in excess of 300 nm produces the possibility of adversely affecting the flying behavior as a suspension.
A layer of polyimides useful for this invention means a layer of polymers containing imide linkages in their structure such as polyimides, polyamideimides, and polyetherimides. In this case, the layer of polyimides may have a multilayer structure consisting of plural kinds of polyimides. The thickness of the polyimide layer is 3 to 20 &mgr;m, preferably 5 to 15 &mgr;m, or more preferably 7 to 12 &mgr;m. When the thickness of the polyimide layer is less than 3 &mgr;m, the electrical insulation becomes less reliable and at the same time the electrical characteristics deteriorate. On the other hand, a thickness in excess of 20 &mgr;m makes it difficult to perform patterning of polyimides with high precision.
The linear expansion coefficient of polyimides to be used in this invention is 1×10
−5
/° C. to 3×10
−5
/° C., preferably 1.5×10
−5
/° C. to 2.5×10
−5
/° C. When the linear expansion coefficient of the polyimides is either less than 1×10
−5
/° C. or more than 3×10
−5
/° C., the laminate tends to deflect

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