Abstract:
An insulating sheet is manufactured by attaching a buffer layer and a resin film using an adhesive layer that is not adhesive at a normal ambient temperature. Since there is no need to use a release liner when manufacturing the insulating sheet, there is no release of a silicone component. Accordingly, when installing a hard disk printed wiring board, the printed wiring board and electronic circuits inside the hard disk will not be damaged by the released silicone.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a divisional application of U.S. application Ser. No. 10/652,495 filed on Sep. 2, 2003, now abandoned, which is a Continuation of International Application No. PCT/JP02/01798 filed Feb. 27, 2002. The entire disclosures of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The present invention relates to insulating sheets, and particularly to an insulating sheet suitable for a hard disk device. 
     A hard disk device (HDD) has disks with a recording layer formed on a surface, and comprises a head for writing to these disks, an arm for supporting the head, and an actuator for causing movement of the arm. 
     These disks are housed inside a case, and a printed wiring board for carrying out control of the actuator and input and output of data to and from the disks is located outside the case.  FIG. 5  is a schematic cross sectional view of a HDD, with reference numeral  110  representing the actual HDD, reference numeral  111  representing the case of the HDD  110 , and reference numeral  112  representing the printed wiring board. Electronic components are mounted on the printed wiring board  112  and an electronic circuit made up of these electronic components is connected to the bottom  117  of the actuator using a flexible wiring board  114 . 
     An insulating sheet  113  is located between the printed wiring board  112  and the case  111 , and the printed wiring board  112  is screwed to the case  111  in this state. The electronic circuit formed on the printed wiring board  112  is insulated from the case  111  by this insulating sheet  113 . 
     The insulating sheet  113  has a buffer layer  121 , an adhesive layer  122  and a resin film  123 . The buffer layer  121  and the resin film  123  are glued together using the adhesive layer  122 . 
     The adhesive layer  122  conventionally uses adhesive, and in order to maintain its characteristics, this film is supplied interposed between release liners  131  and  132 , as shown in  FIG. 6 . 
     When this type of adhesive layer  122  is used, the release liner  131  is first torn off to expose one surface of the adhesive layer  122 , and this exposed surface is pressed against the resin film  123 . The adhesive layer  122  is a pressure sensitive resin and it will attach to the resin film  123  when the adhesive layer is pressed. Next, the other release liner  132  is torn off to expose the opposite surface of the adhesive layer  122 . The adhesive layer  122  is pressed against the buffer layer  121 . The resin film  123  and the buffer layer  121  will attach together by the adhesive layer  122 , thus resulting in an insulating sheet  113 . 
     Once the insulating sheet  113  is interposed between the printed wiring board  112  and the case  111 , screw fastened, and the flexible wiring board  114  attached, the HDD  110  shown in  FIG. 5  is completed. 
     As described above, with the HDD  110  of the related art, since the adhesive layer  122  is pressure sensitive and has adhesive properties at a normal temperature, the release liners  131  and  132  are indispensable when the adhesive layer  122  is rolled up for transportation or storage. 
     Since the release liners  131  and  132  are required to have a non-adhesive property, a material having silicone oil coated on the resin films as the base material of the release liners  131  and  132  is used. Therefore, a silicone component is attached to the adhesive layer  122 , and if the silicone component is released from the insulating sheet  113  by generation of heat during driving of the HDD  110 , this is likely to have a detrimental effect of the electronic circuit of the printed wiring board  112  and the electronic components inside the case  111 . 
     Non-silicone type release liners are expensive and have poor applicability. Even if a non-silicone type release liner is adopted, it is necessary to dispose of the release liner, and this is not desirable from the point of view of reducing waste. 
     The present invention has been created in order to solve the above described drawbacks in the related art, and the object of the present invention is to provide an insulating sheet that does not release a silicone component. 
     SUMMARY 
     A first aspect of the present invention is an insulating sheet comprising a buffer layer having buffering attributes, an adhesive layer that is not adhesive at normal ambient (room) temperature but exhibits adhesiveness when heated, and an insulating resin film, wherein the buffer layer and the resin film layer are attached together using the adhesive layer. 
     A second aspect of the present invention is an insulating sheet, wherein an adhesive layer of the insulating sheet is formed of resin containing no silicone component. 
     A third aspect of the present invention is an insulating sheet wherein a resin film of the insulating sheet is formed from polyethylene terephthalate. 
     A fourth aspect of the present invention is an insulating sheet, wherein a buffer layer of the insulating sheet is formed of urethane foam. 
     A fifth aspect of the present invention is a hard disk device comprising a case for housing a hard disk; a printed wiring board provided with a circuit for controlling the hard disk; and an insulating sheet interposed between the printed wiring board and the case, the insulating sheet comprising a buffer layer having buffering attributes, an adhesive layer that is not adhesive at normal ambient temperature but exhibits adhesiveness when heated, and an insulating resin film, wherein the buffer layer and the resin film layer are attached together using the adhesive layer. 
     A sixth aspect of the present invention is a hard disk device having an adhesive layer formed of resin that does not contain a silicone component. 
     A seventh aspect of the present invention is a hard disk device having a resin film formed from polyethylene terephthalate. 
     An eighth aspect of the present invention is a hard disk device having a buffer layer formed from urethane foam. 
     A ninth aspect of the present invention is a method of manufacturing an insulating sheet including the steps of: bringing an adhesive layer of a sheet comprising insulating resin film and an adhesive layer attached on the insulating resin film, wherein the adhesive layer has a property of exhibiting adhesiveness when heated, into contact with a buffer layer having buffering attributes; heating the adhesive layer while pressing against the buffer layer; and attaching the resin film to the buffer layer. 
     The insulating sheet of the present invention has the structure as described above, where the adhesive layer of the insulating sheet is not adhesive at normal ambient temperature. Accordingly, it is possible to wind the insulating sheet up in roll form even if the release liner is not used. As a result, there is no attachment of a silicone component released from the release liner to the adhesive layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a film having a two-layer structure according to an embodiment of the present invention; 
         FIG. 2  illustrates a roll obtained by rolling up a two-layer structure film; 
         FIG. 3  illustrates an insulating sheet in an embodiment of the present invention; 
         FIG. 4  illustrates one example of a HDD using the insulating sheet of the present invention; 
         FIG. 5  illustrates a HDD of the prior art; and 
         FIG. 6  illustrates an insulating sheet of the prior art. 
     
    
    
     In each drawing, reference numeral  8  represents an adhesive layer. Reference numeral  10  represents a hard disk. Reference numeral  13  represents an insulating sheet. Reference numeral  21  represents a buffer layer. Reference numeral  23  represents a resin film. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference numeral  8  in  FIG. 1  is an adhesive layer used with an insulating sheet of the present invention, and is attached in advance to a resin film  23 . 
     This adhesive layer  8  is formed of a resin that is not adhesive at normal ambient temperature, but exhibits adhesiveness when heated, and even if an exposed surface of the adhesive layer  8  comes into contact with the resin film  23 , there will not be adherence at normal ambient temperature. 
     For this reason, the two-layer structure film  18  can be directly rolled up without the use of a release liner so as to be stored or transported in the form of a roll  19 , as shown in  FIG. 2 . 
     The two-layer structure film  18  is unwound from this roll  19 , such that the surface of the adhesive layer  8  is brought into contact with the buffer layer  21  and pressed while heating. The heating temperature is such that the adhesive layer  8  exhibits adhesiveness without deterioration of the buffer layer  21 . As shown in  FIG. 3 , if the resin film  23  is attached to the buffer layer  21 , the insulating sheet  13  of the present invention is obtained. 
     Reference numeral  10  in  FIG. 4  is a HDD using the insulating sheet  13  of the present invention. A buffer layer  21  side of the insulating sheet  13  is brought into contact with the printed wiring board  12 . A resin film  23  side of the insulating sheet  13  is brought into contact with the case  11 , and the printed wiring board  12  is screw fastened to the case  11 . The printed wiring board  12  and an electrical circuit inside the case  11  are electrically connected to a lower part  17  of an actuator using a flexible wiring board  14 . 
     Further, the buffer layer  21  is formed of a hard foam such as polyurethane and is elastic. After vibration and heat occurring in a motor inside the case  11  have been absorbed by the buffer layer  21 , the buffer layer  21  is applied to the printed wiring board  12 . 
     Embodiments 
     Insulating sheets of the first to third embodiments of the present invention are shown in Table 1 below. PET is an abbreviation for polyethylene terephthalate. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Material properties and evaluation results of insulating sheets of first 
               
               
                 to third embodiments of the present invention 
               
             
          
           
               
                   
                 Embodiment 1 
                 Embodiment 2 
                 Embodiment 3 
               
               
                   
                   
               
             
          
           
               
                 material property 
                 PET 
                 PET 
                 PET 
               
               
                 of resin film 
               
               
                 material property of 
                 saturated polyester 
                 flame resistant 
                 halogen free flame 
               
               
                 adhesive layer 
                 resin type adhesive 
                 polyester resin type 
                 resistant polyester 
               
               
                   
                   
                 adhesive 
                 resin type adhesive 
               
               
                 Tg of adhesive layer 
                 45° C. 
                 45° C. 
                 30° C. 
               
               
                 flame resistance of 
                 non flame resistant 
                 VTM-0 
                 VTM-0 Equivalent 
               
               
                 adhesive film 
               
               
                 buffer layer property/ 
                 flame resistant 
                 non-flame resistant 
                 non-flame resistant 
               
               
                 flame resistance 
                 urethane foam/ 
                 urethane foam 
                 urethane foam 
               
               
                 grade UL94″ 
                 HF-1 
               
               
                 range of urethane 
                 Δ 
                 ◯ 
                 ◯ 
               
               
                 amount and cost of 
               
               
                 buffer layer 
               
               
                 edge tack after 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 die-cutting 
               
               
                 silicone amount 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 cost 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 overall evaluation 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
               
             
          
         
       
     
     Tg of the adhesive layer in Table 1 is a glass transition point. A method of measuring Tg is to use a viscoelasticity measuring device to measure a maximum point of tan δ of 35 Hz, to give Tg. When the adhesive layer  8  of the first to third embodiments are attached to the buffer layer  21 , the adhesive layer  8  is heated to 80° C. 
     The item ‘edge tack after die-cutting’ in the table is a determination as to whether or not adjacent insulating sheets  13  attach together after die-cutting. A sample for this item uses an insulating sheet  13 , after being made by attaching to each other the buffer layer  21  and the resin film  23  using the adhesive layer  8 . The insulating sheet  13  is die-cut in the shape of the case  11  of the HDD  10  and the printed wiring board  12 . In this state, the adhesive layer  8  is exposed at the edge of the insulating sheet  13 , and if the edge of the insulating sheet  13  is adhesive, adjacent insulating sheets will attach together when brought into contact with each other and operability is therefore reduced. The symbol ∘ in Table 1 represents the fact that there is absolutely no attachment. 
     The item ‘silicone amount’ in the table represents a calculated result for amount of silicone content, in a state of the two-layer structure film  18  before attaching to the buffer layer  21 , washing exposed surface of the adhesive layer  8  using N-hexane, enriching the obtained N-hexane and then evaluating the enriched N-hexane by FTIR (Fourier transform infrared spectroscopy). The symbol ∘ represents the fact that silicone was not detected. 
     For the sake of comparison, characteristics of insulating sheets of the prior art are shown in Table 2 below as comparative examples 1-3. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Material property and evaluation results for insulating 
               
               
                 sheets of comparative examples 1 to 3 of the prior art 
               
             
          
           
               
                   
                 comparative 
                 comparative 
                 comparative 
               
               
                   
                 examle 1 
                 examle 2 
                 examle 3 
               
               
                   
                   
               
             
          
           
               
                 material property 
                 PET 
                 PET 
                 PET 
               
               
                 of resin film 
               
               
                 material property of 
                 acrylic adhesive 
                 acrylic adhesive 
                 flame resistant 
               
               
                 adhesive layer 
                   
                   
                 acrylic adhesive 
               
               
                 Tg of adhesive layer 
                 −10° C. 
                 −10° C. 
                 −20° C. 
               
               
                 flame resistance of 
                 non flame resistant 
                 non flame resistant 
                 VTM-0 
               
               
                 adhesive film 
               
               
                 material property of 
                 low silicone type 
                 non-silicone type 
                 low silicone type 
               
               
                 release liner 
               
               
                 peeling force of 
                 ◯ 
                 Δ~X 
                 ◯ 
               
               
                 release liner 
               
               
                 buffer layer property/ 
                 flame resistant 
                 flame resistant 
                 low-flame resistant 
               
               
                 flame resistance 
                 urethane foam/ 
                 urethane foam/ 
                 urethane foam/ 
               
               
                 grade UL94″ 
                 HF-1 
                 HF-1 
                 HF-2 
               
               
                 range of urethane 
                 Δ 
                 Δ 
                 ◯ 
               
               
                 amount and cost of 
               
               
                 buffer layer 
               
               
                 edge tack after 
                 Δ 
                 Δ 
                 Δ 
               
               
                 die-cutting 
               
               
                 silicone amount 
                 Δ~X 
                 ◯ 
                 Δ~X 
               
               
                 cost 
                 Δ 
                 Δ~X 
                 Δ 
               
               
                 overall evaluation 
                 Δ~X 
                 Δ~X 
                 Δ~X 
               
               
                   
               
             
          
         
       
     
     The item ‘release force of release liner’ in Table 2 represents less than 0.5N/5 cm as ∘, from 0.5 to 1N/5 cm as Δ, and in excess of 1N/5 cm as X. The symbol ∘ for edge tack after die-cutting represents the fact that there was slight attachment of adjacent insulating sheet edges, but they detach under their own weight. In the evaluation for silicone amount, a large amount of silicone contained in the N-hexane is represented by the symbol X, and less than X is represented by the symbol Δ. 
     As will be understood from Table 1 and Table 2, in comparative examples 1-3, a silicone component of a release liner attaches to an adhesive layer, whereas in the insulating sheets  13  of the first to third embodiments of the present invention, silicone is not attached to the adhesive layer  8 . 
     Also, in comparative examples 1-3, adjacent edges become attached together, whereas in the insulating sheets  13  of the first to third embodiments of the present invention, there is no attachment of adjacent edges and operability is high. 
     With the insulating sheet  13  of the present invention, there is a possibility of flame resistant additives being contained in the adhesive layer  8 , and it is possible to broaden the material properties and compositional selection for the buffer layer  21 . 
     It is possible to manufacture an insulating sheet without using a release liner.