Abstract:
A flexible electrical circuit such as for use in a hard disk drive dual stage actuated (DSA) suspension has a dimple or other raised feature such as a jog formed in an electrical contact pad. The dimple raises up at least part of the contact pad in height so as to reduce the distance that an electrical bridging component or material such conductive epoxy, solder paste, or jet dispensed solder must traverse in order to complete an electrical connection from the contact pad to an adjacent piezoelectric microactuator or other electrical component for which the electrical circuit carries an electrical signal or electrical power. The reduced distance improves the cleanliness and reliability of the electrical and physical bond, and can allow for electrical connection types that would otherwise be impractical.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application No. 61/951,506 dated Mar. 11, 2014. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of dual stage actuated (DSA) suspensions for disk drives. More particularly, this invention relates to the field of electrical connections to microactuators on DSA suspensions. 
     2. Description of Related Art 
     Magnetic hard disk drives and other types of spinning media drives such as optical disk drives are well known. Disk drive suspensions are the assemblies that hold the read/write head over the correct place on the spinning data disk, in order to write data to, and read data from, the desired data track on the disk. 
     Both single stage actuated disk drive suspensions and dual stage actuated (DSA) suspension are known. In a single stage actuated suspension, only a voice coil motor moves the disk drive suspension. In a DSA suspension, as for example in U.S. Pat. No. 7,459,835 issued to Mei et al. as well as many others, in addition to the voice coil motor which moves the entire suspension, at least one secondary actuator, often referred to as a microactuator, is located on the suspension in order to effect fine movements of the magnetic head slider to keep it properly aligned over the data track on the spinning disk. The secondary DSA microactuator(s) provide much finer control and much higher bandwidth of the servo control loop than does the voice coil motor alone which effects relatively coarse movements of the suspension and hence the magnetic head slider. Lead zirconium titanate is one of the broadly used intermetallic inorganic compounds possessing piezoelectric properties and is commonly referred to as PZT. PZT devices are often used as the microactuator motor, although other types of microactuator motors are possible. Examples of a dual stage actuated suspension, a PZT microactuator often referred to simply as a PZT for short, and various methods of electrically and mechanically integrating the PZT into the suspension, are disclosed in U.S. Pat. No. 8,570,688 to Hahn, and in copending U.S. patent application Ser. No. 14/045,773, which are owned by the assignee of the present application. Other mechanical and electrical connections have been proposed. 
     Various structures and methods have been proposed for making the required electrical connections to the PZT microactuators. One structure and method that was developed by the assignee of the present application is shown in  FIG. 1 , and in  FIG. 2  which is a cross sectional view taken of the suspension of  FIG. 1  taken along section line  2 - 2 . In this example, suspension  10  includes slider  12  mounted to gimbal  14 . The PZT  20  is connected to gimbal  14  through gimbal arms  16 . The PZT  20  typically includes PZT material  22 , a thin top metalized electrode  24 , and thin bottom metalized electrode  26 . A flexible circuit  30  typically includes a metal support layer such as stainless steel layer  32 , insulating layer such as polyimide layer  34 , and a copper signal conductor layer including copper electrical contact pad  37  covered by an anti-corrosion layer  38  such as nickel followed by gold. Anti-corrosion layer  38  will be referred to hereafter simply as gold layer  38 . Non-conductive adhesive such as non-conductive epoxy  50  physically attaches PZT  20  to the flexible circuit  30  on one side. For the electrical connection, bottom electrode  26  of PZT  20  is physically and electrically connected to the ground potential of the suspension body by conductive epoxy  18  bonded to grounded suspension body  16  which is stainless steel. The driving voltage potential is provided through conductive epoxy or solder balls  52  that forms a physical and electrical bridge from a copper contact pad  37  and its anti-corrosion layer  38  on the flexible circuit  30  to the metalized top surface of the PZT which defines the top electrode  24 . 
     SUMMARY OF THE INVENTION 
     The electrical connection of  FIGS. 1 and 2  that employs conductive epoxy may be prone to dry joint issues. Using solder paste instead of conductive epoxy might be possible; such a technique, however, is considered less than ideal due to the non-wetting properties of the solder paste on the non-conductive epoxy in  FIG. 2 . Other potential techniques such as solder jet bonding are at least theoretically possible. In solder jet bonding, tiny balls of molten solder are ejected from a print head and directed to the target using inkjet printing technology. Forming a bridge from the copper signal pad to the PZT top electrode using solder jet technology, however, would require multiple solder dots in order to establish the electrical bridge over the required distance involved and is more prone to fatigue failure. 
     The present invention provides an improved connection structure and method that reduces the distance from the copper contact pad to the electrode that is the top surface of the PZT. In an exemplary embodiment of the invention, part or all of the electrical contact pad is formed such as by stamping or other techniques to give it a raised profile compared to the part of the suspension that is directly underneath the PZT. In one embodiment, a dimple is stamped into a portion of the flexible circuit that includes the copper contact pad so as to raise at least part of the copper contact pad up to a higher level, namely, a level that is closer to the level of the top of the PZT. In another embodiment, the flexible circuit has a jog or step in it that raises the copper contact pad up to a higher level. 
     Exemplary embodiments of the invention will be further described below with reference to the drawings, in which like numbers refer to like parts. The drawing figures might not be to scale, and certain components may be shown in generalized or schematic form and identified by commercial designations in the interest of clarity and conciseness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a prior PZT electrical connection method. 
         FIG. 2  is a sectional view of the PZT connection of  FIG. 1  taken along section line  2 - 2 . 
         FIG. 3  is a top perspective view of a PZT connection to an electrical contact pad according to a first illustrative embodiment of the invention in which the raised portion defines a curved bump. 
         FIG. 4  is a sectional view of the PZT connection of  FIG. 3  taken along section line  4 - 4 . 
         FIG. 5  is a cross sectional view of an additional illustrative embodiment of the invention in which the raised portion defines a jog. 
         FIG. 6  is a cross sectional view of an additional illustrative embodiment of the invention in which the raised portion includes one acute angle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3  is a top perspective view of a PZT connection to an electrical contact pad according to a first illustrative embodiment of the invention, and  FIG. 4  is a sectional view thereof taken along section line  4 - 4 . The flexible circuit  30  of the suspension includes a support layer  32  such as a stainless steel (SST) substrate, an insulating layer  34  such as polyimide, and a conductive signal-carrying layer  36  such as copper or copper alloy (generally Cu). Most of the copper layer of the flexible circuit is covered by an insulating coverlay  40  such as polyimide in order to prevent corrosion of the copper and to prevent short circuiting. The copper is covered by a protective metal layer  38  such as nickel followed by gold but is otherwise exposed at an area that defines an electrical contact pad  37  and where an electrical connection to the contact pad is to be made. As used herein and in the claims, the contact pad  37  is “exposed” if it is not covered by an electrical insulator, although it may be covered by other electrically conductive layers such as anti-corrosive nickel and/or gold layers, such that it is electrically accessible. 
     A raised or vertical feature such as a convex curved bump, dimple or dome  60  has been formed in the supporting layer  32  and in the conducting layer  36  and its contact pad  37 , thus creating both a supporting layer bump  64  and a conducting layer bump  62  in the flexible circuit, in order to raise at least part of the contact pad  37  up to a higher z-height. At least part of the copper contact pad  37  is therefore raised up to a vertical height that is closer to, or even on the same height as or above, the top metallized surface  24  of the PZT that defines the PZT&#39;s top electrode, than it would be in the absence of the vertical feature. Furthermore, at least part of the copper contact pad  37  is therefore raised up to a vertical height that is closer to the top metallized surface of the PZT than it is to the bottom surface or bottom electrode  26  of the PZT. In this embodiment, as in the additional embodiments disclosed herein, the raised part of the copper contact pad  37  can be raised up to: closer to the top electrode  24  of the PZT  20  than to its bottom surface  26 ; as high as or even higher than the top electrode  24  of the PZT; higher than the unbent portion of the copper signal trace  36  that is adjacent the bump  60 ; and/or at least as high as a midplane MP of the PZT  20 . In all cases, the copper contact pad  37  and its convex surface is effectively raised to higher than it would be, and closer to the top surface and top electrode  26  of the PZT, than it would be in the absence of the vertical feature  60 . 
     An electrically conductive bridge  152  is then established between the PZT&#39;s top (positive) electrode or surface  24  and electrical contact pad  37 , such as by conductive adhesive  152  such as conductive epoxy, or perhaps some other flowable and subsequently hardened conductive material such as solder paste. The presence of the bump thus reduces the physical and electrical distance that must be traversed by conductive bridge  152  from copper contact pad  37  to the PZT&#39;s driven electrode  26  on its top surface. 
     In the illustrative embodiment a portion  33  of the supporting layer  32  not covered by copper layer  36  extends underneath bottom surface  26  of PZT  20 . A section  35  of polyimide layer  34  acts as a dam to help prevent the undesired spread of non-conductive epoxy  50 . 
     The result of the formed and raised structure  60  is that the physical and electrical distance from the PZT&#39;s top electrode  24 , or more generally the distance from some other electrical component to which an electrical connection is to be established, to the copper contact pad  37  is reduced as compared to what that distance would be in the absence of the formed vertical feature. This reduction in distance has several advantages. First, less bridging material is needed, whether that bridging material is conductive epoxy or any other material. Second, the distance that the bridging material must traverse over the non-conductive epoxy, which is relatively non-wetting, is reduced, and at the same time the surface area of the bridging material on the more wetting copper contact pad is increased, thus strengthening the resulting bond. Third, because the distance that must be bridged is reduced, and because the distance that must be bridged over the relatively non-wetting non-conductive epoxy is significantly reduced, the conductive epoxy  152  can now be replaced by materials that have better manufacturing process characteristics such as solder paste or solder jet bonding. Solder paste and solder jet bonding are generally superior to non-conductive epoxy at least for the reasons that those materials are more easily dispensed, harden much more quickly, and do not require elevated temperatures for hardening. Elevated temperatures such as used for epoxy hardening can have negative effects on other components of the suspension including the PZT and its poling. 
     The invention provides the additional advantage that a dimpled or otherwise raised contact pad acts as an effective dam against undesired spreading of the bridging material to the right in  FIG. 4 , whether that bridging material is conductive epoxy, solder paste, or other material. 
     In another feature of the invention, a patterned mask of insulating material such as polyimide can be formed on a portion of the top surface of the PZT and/or the copper contact pad. The patterned mask of insulating material defines a top insulating layer that acts as a dam against undesired spread and smearing of the solder or other bridging material over the PZT and/or the copper contact pad. On the bumped contact pad  37 , polyimide coverlay  40  acts as a dam to help prevent the unwanted spread of conductive epoxy  152  over the flexible circuit. On the PZT  20 , polyimide layer  28  acts as a dam to prevent unwanted spread of conductive epoxy  152  over the PZT. 
       FIG. 5  is a cross sectional view of an additional illustrative embodiment of the invention in which the raised portion defines a jog. In this embodiment the copper contact pad  37  including its protective nickel/gold layer  38  (if present) is raised up in z-height by a jog or step  70  that is formed in the flexible circuit including the SST support layer  32  and the copper conducting layer  36 . Such a jog structure could provide additional physical support for the copper contact pad during the soldering operation by acting as a stiffening rib. The jog can be in either the x-direction (the longitudinal direction of the suspension) or the y-direction (the lateral direction of the suspension), but preferably the cross-section exhibits a jog of approximately 0=90°, with the circuit having two approximately 90° individual bends as seen in the figure. More precisely, the figure shows four 90° bends in the circuit. More generally, the jogs could have angles of 70°-110°, or other angles. 
       FIG. 6  is a cross sectional view of an additional illustrative embodiment of the invention in which the raised portion includes one 90° bend and one acute bend angle 0&lt;90°. More generally, the raised feature could have one bend angle of between 70° and 110°, and one bend having an acute angle, that is, less than 90°. 
     Although the invention has been described with reference to piezoelectric microactuators which are a type of transducer, it will be appreciated that the invention is applicable more generally to other types of microactuators, and indeed more generally still to making electrical connections to various types of electronic components other than just microactuators including but not limited to other transducers, sensors, heaters, and in environments other than disk drives and suspension assemblies for disk drives. 
     It will be understood that the terms “generally,” “approximately,” “about,” “substantially,” and “coplanar” as used within the specification and the claims herein allow for a certain amount of variation from any exact dimensions, measurements, and arrangements, and that those terms should be understood within the context of the description and operation of the invention as disclosed herein. 
     It will further be understood that terms such as “top,” “bottom,” “above,” and “below” as used within the specification and the claims herein are terms of convenience that denote the spatial relationships of parts relative to each other rather than to any specific spatial or gravitational orientation. Thus, the terms are intended to encompass an assembly of component parts regardless of whether the assembly is oriented in the particular orientation shown in the drawings and described in the specification, upside down from that orientation, or any other rotational variation. 
     It will be appreciated that the term “present invention” as used herein should not be construed to mean that only a single invention having a single essential element or group of elements is presented. Similarly, it will also be appreciated that the term “present invention” encompasses a number of separate innovations which can each be considered separate inventions. Although the present invention has thus been described in detail with regard to the preferred embodiments and drawings thereof, it should be apparent to those skilled in the art that various adaptations and modifications of the present invention may be accomplished without departing from the spirit and the scope of the invention. Accordingly, it is to be understood that the detailed description and the accompanying drawings as set forth hereinabove are not intended to limit the breadth of the present invention, which should be inferred only from the following claims and their appropriately construed legal equivalents.