Abstract:
A substrate for mounting a preamp chip thereupon, fabricated using a stiffener layer made of a conductive material; an insulating layer provided over the circuitry area of the substrate; a circuitry made of a conductive material provided over the insulating layer; and a flap which is an extension of the stiffener layer having no insulating layer provided thereupon. The flap is fabricated to fold over the preamp chip to remove heat therefrom.

Description:
CROSS-REFERENCES TO RELATED PATENT APPLICATIONS 
     This is a Continuation Application of application Ser. No. 11/548,681, filed Oct. 11, 2006 the disclosure of which is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The subject invention relates to hard drives and, more particularly for controlling the heat generated by the hard disk drive heads preamp. 
     2. Related Art 
       FIG. 1   a  depicts a prior art hard drive  100  with the cover removed, while  FIG. 1   b  depicts an enlarged image of the preamp area. The hard disk  100  uses rotating platters (disks)  110  to store data. Each platter is rotated by a spindle (not shown) and has a smooth magnetic surface on which digital data is stored. Information is written to the disk by applying a magnetic field from a read-write head (not shown) that is attached to an actuator arm  120 . For reading, the read-write head detects the magnetic flux emanating from the magnetic bits that were written onto the platter. Since the signals from the read/write head is very faint, a preamp  130  is provided in close proximity to the head. The preamp  130  is a chip that is mounted on a substrate  140 . The substrate  140  is mounted onto a carrier plate  150 , that connects to the actuator arm assembly  120 . The flexible circuit loop  160  is connected to the substrate  140 , to transfer signals between the preamp  130  and the associated electronics (not shown). The associated electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. 
       FIG. 2  depicts a prior art preamp sub-assembly, showing a carrier plate  250 , upon which the substrate  240  is mounted. The preamp  230  is attached to the substrate  240  and makes electrical connections to tap points on the substrate  240 . As shown in the cross-section inside the broken-line callout, the substrate is generally made of a stainless steel or aluminum backing, generally referred to as a stiffener,  215 , an insulating polyimide layer  225 , and copper conducting contacts and lines  235 . The “legs” of the preamp chip  230  (or bumps in case of a flip chip) are soldered to the copper contacts  235 . In the case depicted, substrate  240 , having its own stiffener  215 , folds back a top carrier plate  250 . Carrier plate  250  and stiffener  215  can be made from a common metal layer. Alternate designs integrate the function of the carrier plate into the stiffener, eliminating the need for the carrier plate. The substrate is generally made using a sheet of stiffener material, upon which several substrates are formed, as illustrated in  FIG. 3 . As depicted in  FIG. 3 , a sheet of stiffener material, such as stainless steel or aluminum,  315 , serves as a starting material for fabricating the substrate  345 . For each substrate  345 , a polyimide layer  325  is deposited on top of the stiffener  315  to serve as an electrical insulator. On top of the polyimide various conductive elements  335  are deposited to form contacts and transmission lines. The fabrication of these layers is done using conventional photolithography techniques. Both subtractive and additive flexible circuit fabrication processes are commonly employed in hard disk drives. To maximize the available real estate, the substrates  345  are fabricated so as to “nest” with each other, and after the fabrication is completed the substrates  345  are cut out of the stiffener sheet  315 . 
     As the physical size of the hard drive decrease, the heat generated by the preamp affects performance and reliability of the hard drive. 
     SUMMARY 
     The present invention has been made by observing a problem in the prior art, in that the heat generated by the preamp is not readily dissipated. While the carrier arm  250  can be used as a heat sink, the inventors of the subject application have discovered that little heat passes from the preamp  230  to the carrier arm  250 . The inventors have postulated that the reason for the low heat transfer is that the polyimide layer  225  of the substrate  240  acts as a heat barrier. Notably, polyimide has a thermal conductivity of 0.12 W/m-K, which is thermally insulative. Additionally, conductive pads  235  provide a very limited conductive heat release means, and suffer as well from the thermal isolation of the polyimide layer. Accordingly, the inventors have invented schemes to better remove heat from the preamp by providing a thermal conduit from the top of the preamp to the carrier arm. 
     According to an aspect of the invention, a substrate for mounting a preamp chip thereupon is provided, comprising a stiffener layer made of first conductive material; an insulating layer provided over circuitry area of the substrate; a circuitry of a second conductive material provided over the insulating layer; and a flap comprising an extension of the stiffener layer having no insulating layer provided thereupon, and wherein the flap is fabricated to fold over the preamp chip. According to one aspect, the first conductive material comprises stainless steel or aluminum. According to another aspect, the second conductive material comprises copper. The flap may comprise fins. The flap may also comprise cutout configured for injective adhesive thereupon. 
     According to another aspect of the invention, an actuator assembly for a hard disk drive is provided, comprising: an actuator arm; a circuitry substrate mounted onto the arm; a preamp chip mounted onto the circuitry substrate; and, wherein the substrate comprises a flap folded over top of the preamp ship. The substrate may comprises: a stiffener layer made of first conductive material; an insulating layer provided over circuitry area of the substrate; a circuitry of a second conductive material provided over the insulating layer; and, wherein the flap comprises an extension of the stiffener layer having no insulating layer provided thereupon. An adhesive may be provided between the preamp chip and the flap. The flap may comprise a cutout for an adhesive injected via the cutout. The adhesive may comprise a heat conducting epoxy. The flap may comprise fins. 
     According to yet another aspect of the invention, a method for manufacturing a substrate for supporting an integrated circuit chip thereupon is provided, comprising: providing a sheet of stiffener comprising a first conductive material; providing an insulating layer on defined sections of the stiffener, each section defining a circuitry area of one substrate; providing contacts on the insulating layer, the contacts made of a second conductive material; and, cutting each substrate out of the sheet according to a designed outline, the designed outlined comprising the circuitry area and a flap, the flap comprising a section of the sheet of stiffener having no insulating layer thereupon. The method may also comprise cutting a cutout in the flap. 
     According to a further aspect of the invention, a method for manufacturing a preamp assembly for a hard drive is provided, comprising: providing a substrate, the substrate comprising a stiffener conductive layer, an insulating layer provided on the stiffener and defining a circuitry area, and a plurality of contacts provided on the insulating layer, and a flap comprising a section of the stiffener having no insulating layer thereupon; mounting the preamp on the circuitry area of the substrate so as to form electrical connection to at least some of the contacts; and folding the flap over the preamp. The method may further comprise injective adhesive between the preamp and the flap. The flap may comprise a cutout and the method may further comprise injecting adhesive onto the cutout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects and features of the invention would be apparent from the detailed description, which is made with reference to the following drawings. It should be appreciated that the detailed description and the drawings provide various non-limiting examples of various embodiments of the invention, which is defined by the appended claims. 
         FIG. 1   a  depicts a prior art hard drive  100  with the cover removed, while  FIG. 1   b  depicts an enlarged image of the preamp area. 
         FIG. 2  depicts a prior art preamp subassembly. 
         FIG. 3  depicts a sheet of stiffener having several substrates fabricated thereupon. 
         FIG. 4  depicts a preamp subassembly with a substrate according to an embodiment of the invention. 
         FIG. 5  depicts a preamp subassembly with a substrate according to another embodiment of the invention. 
         FIG. 6  illustrates a sheet of stiffener having several substrates fabricated thereupon according to an embodiment of the invention. 
         FIG. 7   a  depicts the resulting temperature distribution in the preamp for the prior art assembly without the flap, while  FIG. 7   b  depicts the temperature distribution in the chip with the flap according to the embodiment of  FIG. 5 . 
         FIG. 8   a  depicts a finite element simulation run of the model with the flap, but without the epoxy, while  FIG. 8   b  depicts a run of the model with the flap and epoxy. 
         FIG. 9  depicts another embodiment of the invention. 
         FIG. 10  is a plot depicting simulated temperature reductions due to the flap design versus heat rise in the preamp chip. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  depicts a preamp subassembly showing carrier plate  450  with a substrate  440  according to an embodiment of the invention. Notably, a flap  465  of stiffener material has been added to the fabrication of the substrate  440 . The flap  465  is folded over the preamp  430 , so as to remove heat from the top of the preamp  430 . The heat removed is conducted by the flap  465  to stiffener  415 , then to carrier plate  450  and to the actuator arm assembly (not shown), which acts as the conduction heat sink. 
     That is, as can be understood from  FIG. 2 , the stiffener layer  215  of the substrate  240  is in physical contact with the carrier plate  250 . However, as noted above in the prior art, the heat from the preamp  230  is not conducted to the carrier plate  250  because the polyimide layer  225  of the substrate  240  acts as a barrier for heat conduction. On the other hand, the flap  465  of the embodiment of  FIG. 4  has no polyimide deposited thereupon. Consequently, heat from the top of the preamp  430  can be easily conducted to the flap  465 . Since the flap  465  is fabricated as an integral part of the substrate  440 , the heat from the flap  465  is easily conducted onto the carrier plate  450 . Convective heat transfer also takes place from the exposed surface area of the flap  465  to ambient air. 
     To further improve heat conductance to the flap, optionally a conductive adhesive  475  can be provided between the preamp  430  and the flap  465 , as is illustrated by the broken-line callout  475 . In practice, an air gap between flap  465  and  430  may exist due to geometric tolerances and forming uncertainties, so the conductive adhesive  475  is useful in filling that poor conductive path. On the other hand, to ease assembly of the preamp and substrate, in  FIG. 5  a cut-out  585  has been made in the flap  565 . In this embodiment, once the preamp  530  is attached to the substrate  540 , the flap  565  is folded over the preamp, as shown in  FIG. 5 . Then a conductive adhesive is injected into the cutout  585 , so as to spread under the flap  565  and onto the preamp  530 , as is illustrated by broken-line callout  575 . 
       FIG. 6  illustrates a sheet of stiffener having several substrates fabricated thereupon according to an embodiment of the invention. In  FIG. 6 , the starting material is a sheet of stiffener material, such as stainless steel  615 . Several substrates  645  are fabricated on sheet  615  in a nested arrangement. Each substrate  645  has a “circuit” region, defined by the polyimide layer  625  and shown in solid line, and a flap  685 , which is a differently processed area of the stainless steel. That is, flap  685  is not covered with a polyimide, but is rather bare stiffener material. When the substrate is cut out of the sheet material, the cut is made so that the flap is an integral part of the substrate. This ensures that heat conducted onto the flap would be immediately conducted to the entire stiffener layer of the substrate. Since the stiffener contacts the carrier plate, the heat would be conducted to it and to the actuator arm assembly, which acts as a heat sink. 
     During assembly, preamp  530  is attached to the substrate  645 , substrate  645  is folded along dash-dotted line  696  so as to mate carrier plate  550  and stiffener  515 , as shown in  FIG. 5 . Incidentally, as shown in  FIG. 5 , in this embodiment at the area of fold  696  there is no stiffener material, but rather only a polyimide layer. The flap  685  is then folded along dotted line  698  over the preamp  530 . Then, when a cutout is used, conductive adhesive is injected into cutout  585 . When no cutout is provided, the adhesive may be injected from the sides, or injected prior to folding the flap over the preamp. One type of adhesive that is suitable for use with the embodiments described herein is TIGA HTR-815 epoxy, available from Resin Technology Group of South Easton, Mass. This epoxy has thermal conductivity of 1.15 W/m-K, which is an order of magnitude higher than the polyimide. 
     The embodiment depicted in  FIG. 5  has been entered into a free convection thermal finite element simulation (hereon referred to as model) assuming a 25° C. ambient temperature and a fixed self heat generation magnitude in the preamp chip volume. The model was run assuming a fixed film coefficient for all exposed surfaces of the chip and surrounding sub-assembly bodies, allowing heat transfer to the ambient air by convection. For the first run, the exposed surfaces were set to have a film coefficient of 1 e −4 W/mm 2 -K and the preamp self heat generation magnitude assumed was 0.05 W/mm 3 , or 250 mW.  FIG. 7   a  depicts the resulting temperature distribution in the preamp for the prior art assembly without the flap, while  FIG. 7   b  depicts the temperature distribution in the chip with the flap according to the embodiment of  FIG. 5 . For illustration purposes, the surrounding structures are hidden in  FIGS. 7   a  and  7   b . The maximum temperature observed in  FIG. 7   a  was 60.1° C., while for  FIG. 7   b  with the flap it was 54.7° C. Additionally, without the flap, a large area of high temperature was observed on the preamp with the gradient increasing towards the center of the preamp, while with the flap, the center of the preamp was cooler than the edges, tending to show that heat is conducted to the flap via the epoxy. Therefore, it is believed that large coverage of epoxy over the preamp would lead to improved results. 
     The model was also run with the exposed surfaces set to have film coefficient of 2.0 e −4 W/mm 2 -K and the same heat generation magnitude. For this case the maximum observed preamp temperature was 52.0° C. without the flap and 47.1° C. with the flap. This tends to show that even when improved convection to ambient air is present, the flap still provides the benefits of heat removal from the chip. 
       FIG. 8   a  depicts a run of the model in  FIG. 5  with the flap, but without the epoxy, while  FIG. 8   b  depicts a run of the model with the flap and epoxy. As can be seen from  FIG. 8   a , there is poor thermal conductivity between the preamp and the flap due to an air gap placed intentionally between them, when no epoxy is present. This exemplifies the prior art configuration to an extent, because heat is not being removed from the top of the chip. On the other hand, the center of the preamp is cooler at the center when the epoxy is added. Both the maximum chip temperature and average temperature within the chip volume, are reduced. Consequently, it can be seen that if no epoxy is provided, physical contact between the flap and the preamp must be assured, which may increase manufacturing tolerances. The epoxy enables relaxing these tolerances and ease manufacturing. 
     Another embodiment is depicted in  FIG. 9 . In  FIG. 9 , heat removal from the preamp  930  is enhanced by adding fins  995  to the flap  965 . In this manner, heat is dissipated from the preamp to the flap, and from the flap to the carrier plate by conduction and to the ambient atmosphere via enhanced convection from the fins. Of course, other designs of fins can be made and those shown in  FIG. 9  are provided only as one example. 
       FIG. 10  is a plot depicting expected temperature reduction due to the flap design versus heat rise in the preamp chip. To simulate this trend, the preamp heat generation is varied from 0.05 to 0.2 W/mm 3 . It is shown, as the observed heat differential rises in the chip, the benefit provided by the flap increases. For example, for a chip that operates at about 35 degrees above ambient, the flap should provide a reduction of 5 degrees, as opposed to a design without a flap. On the other hand, a more typical chip operating temperature is about 100 degrees above ambient, for which the flap is expected to provide over 15 degrees reduction in maximum temperature. 
     Thus, while only certain embodiments of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention. Further, certain terms have been used interchangeably merely to enhance the readability of the specification and claims. It should be noted that this is not intended to lessen the generality of the terms used and they should not be construed to restrict the scope of the claims to the embodiments described therein.