Microactuated suspension with spring bias acting on conductive adhesive bond for improved reliability

In an electrical connection to a microactuator in a disk drive suspension, an electrical lead is adhered to a microactuator using conductive adhesive and is also mechanically pressed up against the microactuator using a bias mechanism. The bias mechanism may be a spring finger that is welded to the suspension, or it may be a stainless steel finger that is formed integrally with the trace gimbal assembly. The resulting bias force that presses the contact against the microactuator surface reduces the small failure rate that can occur when the conductive adhesive separates from the microactuator's surface as a result of stress such as induced by thermal cycling.

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 in 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. A disk drive suspension is the assembly that aligns 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 actuator arm with the slider affixed to the distal end. In a DSA suspension, as 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 upon the suspension in order to effect fine movements of the magnetic head slider thereby maintaining proper alignment over the appropriate data track on the spinning disk. The microactuator(s) provide much finer control and a higher bandwidth of the servo control loop than does the voice coil motor alone which is capable of effecting 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. PZTs are often used as the microactuator motor, although other types of microactuator motors are possible.

Various structures and methods have been proposed for making the required electrical connections to the PZT microactuators. One structure and method is to bond a gold plated copper contact pad that is part of the suspension's flexible circuit to a gold plated (metalized) surface of a PZT using conductive epoxy.

In the discussion that follows, it will be assumed without explicitly reciting that the copper contact pad is coated with gold or some other protective metal, and that the PZT surface is metalized with gold or some other relatively non-corroding metal.

Another structure and method is disclosed in U.S. Pat. No. 8,570,688 issued to Hahn et al, which is owned by the assignee of the present invention. That structure and method use a stainless steel spring to press the copper contact pad against the PZT surface optionally with conductive grease between those two surfaces, and/or with a protective pad between the spring finger and the contact pad in order to eliminate or reduce fretting. The copper contact pad and the PZT's electrode are thus physically pressed together and physically held together. The conductive grease enhances the electrical connection between them. In that proposal the bias pressure provided by the spring eliminates the need for conductive epoxy between the two parts.

The industry continues to seek ways to improve the quality of the electrical connection to the PZT microactuator, to improve the reliability of that connection, and to reduce the cost of making it.

SUMMARY OF THE INVENTION

The connection method disclosed in U.S. Pat. No. 8,570,688 involving a bias spring and conductive grease has not shown to be 100% satisfactory under all conditions, and thus has not displaced the use of conductive epoxy for the contact pad-to-PZT connection. Furthermore, a conductive epoxy connection has also been shown to be less than 100% satisfactory under all conditions. The inventors of the present application have discovered that the conductive epoxy connection from gold plated copper to gold plated PZT material suffers from reliability problems particular upon thermal cycling in which the temperature is varied from −50° to +150° C. and the difference in coefficients of thermal expansion between the PZT material and the epoxy causes physical stresses to be placed on the epoxy bond, sometimes creating a small gap between the silver fibers in the conductive epoxy compound and the circuit's gold surface, thus increasing the electrical resistance of the bond sufficiently to produce a very small albeit measurable defect rate in the parts after thermal cycle testing.

The inventors have further discovered a way to reduce the incidences of this failure mechanism. According to a first aspect of the invention, the electrical and mechanical bond is made using a conductive adhesive such as conductive epoxy, and a spring bias is applied to press together the gold plated copper and the gold plated PZT (or stainless steel) with the epoxy therebetween, with the spring bias forcing pressing those components together before, during, and/or after the epoxy curing step. Applying a bias before and/or during the epoxy curing step presses the silver fibers up against the gold surface and thus creates a stronger bond than in the absence of such a bias. Furthermore, applying a bias after the epoxy curing step and during operation of the device helps to hold the silver fibers against the gold surface, thus allowing those fibers to better resist tearing away from the surfaces during operation especially during thermal cycling.

In one embodiment an additional spring structure comprising a spring metal is welded to the suspension.

In another embodiment a spring finger or arm can be integrally formed with the trace gimbal. It is important to manufacture the device so that the spring finger is at a consistent height above the PZT and presses against the PZT with a consistent force. By laser adjusting the height of the end of the spring finger, and using automated optical inspection (AOI) feedback to achieve the desired height and the desired spring tension, the repeatability and hence reliability of those parameters and the bond joints can be improved.

Regardless of whether the spring is an external structure that is affixed to the suspension such as by welding or is integrally formed with the trace gimbal, or takes the form of some other structure, the spring configuration and hence spring bias can be adjusted as desired using laser adjustment techniques and automated optical inspection feedback, thus improving the repeatability and reliability of the resulting bond joints.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The bias force for use in the invention can be provided using structures and methods such as disclosed in U.S. Pat. No. 8,570,688 issued to Hahn et al. which is hereby incorporated by reference in its entirety.FIG. 1is a top perspective view of a DSA suspension assembly5having a PZT14and a copper contact pad22biased against the PZT according to a first exemplary embodiment of the invention, andFIG. 2is a sectional view thereof taken along section line2-2. Contact pad22including copper contact pad32lies at the end of circuit finger20which is part of the suspension's flexible circuit18. A copper signal trace within flexible circuit18carries the PZT driving voltage to copper contact pad32. Other traces within flexible circuit18define the signal lines to and from the read/write head. The trace gimbal assembly16includes the flexible circuit18and the gimbal (not shown) which supports the read/write head for writing data to, and reading data from, the data storage media such as a spinning magnetic hard disk. A bent stainless steel finger40having a dimple42formed in it acts as a spring and is welded to the suspension at weld points41. Spring finger40helps to press copper contact pad32against the top surface PZT14. Spring40provides the desired bias force to bias copper contact pad32against conductive adhesive38and towards the electrode of PZT14. Optional insulating coverlayer34can be included over copper contact pad32. Optional protective pad36helps to protect against fretting. An additional ground spring80can be welded to the suspension body part3at weld point81to press against the ground electrode of PZT14. Conductive epoxy15also provides a ground path from the PZT's bottom electrode to the grounded suspension part3, which in this particular illustration is the base plate but could also be the load beam7or the gimbal. Also seen in the figure are suspension spring or hinge6and load beam7. These figures of the present application are similar to FIGS. 4 and 5 of U.S. Pat. No. 8,570,688 although the present version employs conductive adhesive such as conductive epoxy instead of the conductive grease shown and described in that patent.

Similarly, the arrangement shown in FIGS. 6 and 7 of U.S. Pat. No. 8,570,688 in which a dimple is used as a spring feature, could be adapted for use with the present invention, again with conductive epoxy being applied between the copper contact pad and the PZT electrode surface.

FIG. 3Ais a top plan view of a suspension in which two spring fingers20are integrally formed with the trace gimbal assembly (TGA)16.FIG. 3Bis a detailed view of the area indicated inFIG. 3A.FIG. 4is a sectional view of the suspension ofFIG. 3Ataken along section line4-4. Trace gimbal assembly16includes the gimbal (not shown) which supports the head slider and allows it to freely rotate in each of three rotational axes, and also includes the flexible circuit18. Flexible circuit18typically comprises a metal support layer28such as stainless steel, an insulative layer30such as polyimide, and a conductive layer such as copper or copper alloy for carrying electrical signals including the read/write signals to and from the head slider, and the activation voltages for activating the PZT microactuators14.

Spring finger50is integrally formed of the metal support layer28, typically stainless steel, of flexible circuit18. More generally, spring finger50could be formed of any spring metal. The spring finger50constitutes a second finger.

First and signal-carrying finger20includes a copper contact pad32formed of the signal conductive layer of the flexible circuit, and polyimide30or other insulator from the flexible circuit's insulative layer. At the end of finger20a stainless steel ring29constitutes an isolated island of stainless steel electrically isolated from most of the rest of the stainless steel layer28of trace gimbal assembly18. That stainless steel layer28is typically grounded to the suspension body. Copper contact pad32is part of copper signal conductive layer. Conductive adhesive38completes the electrical path from copper contact pad32at the end of the copper signal conductor to the positive or driven electrode of PZT14. The opposite surface of the PZT which constitutes the ground electrode is grounded to the base plate3, the load beam7, or whichever metal part of the suspension to which it is mounted, typically via conductive adhesive15such as conductive epoxy. The PZT14mounting may include non-conductive structural epoxy19. Typically, a cover layer (not shown) is applied over the copper conductor for electrical insulation and anti-corrosion purposes. Signal-carrying finger20carries the driving voltage that activates PZT14. In a typical suspension the two PZTs14on opposite lateral sides of the suspension assembly are poled or are arranged oppositely, such that a single common voltage is applied to both PZTs thus causing one to expand in the x-direction while the other contracts in the same direction. That arrangement creates a push-pull effect to rotate the portion of the suspension that is distal to the PZTs and thus moves the head slider in an arc over the disk platter.

In this embodiment, spring finger50is separate from signal-carrying finger20, and both fingers are generally curved and extend generally in parallel with each other until they meet at their ends, with the end of spring finger50pressing down against copper contact pad32, pressing that contact pad down against the conductive adhesive38and toward PZT14.

The stainless steel or other spring metal of which spring finger50is formed must be electrically isolated from copper contact pad32in order to avoid shorting the PZT's driven electrode to grounded suspension body part3. One way to accomplish the required electrical isolation is shown in the figures, in which the stainless steel portion58of spring finger50is electrically isolated from stainless steel ring29, and is physically connected to it by a short copper finger52with polyimide30electrically and physically separating the copper and stainless steel layers. Thus, spring finger20includes both a majority portion58thereof that includes stainless steel, and a short minority portion52thereof of copper, and another section having both stainless steel and copper separated by polyimide30, with all of those materials integrally formed with, and as part of, the suspension's trace gimbal assembly16including its flexible circuit18. The physical bias force provided by spring finger50is transferred at least in part through copper section52. Copper section52could be reinforced by being made thicker than the rest of the copper layer, especially if the TGA is made using an additive method in which case it would be relatively easy to add additional copper or some other material to copper section52in order to transmit the bias force from spring finger50to copper contact pad32.

Spring finger50can have one or more bends57,59formed therein to adjust both the height of the ends of the springs and the bias force that will be applied against the contact pad. The bends can be formed and adjusted using mechanical bending, by laser adjustment, or other adjustment techniques before and/or after the PZT attachment step. Laser adjustment involves using a laser to locally heat and partially melt a piece or section of metal, and then cooling the metal, to impart a permanent bend in the metal without applying a mechanical force to bend it. Such techniques are disclosed generally in the literature including U.S. Pat. No. 5,228,324 to Frackiewicz which is incorporated herein by reference in its entirety. By laser adjusting the height of the end of the spring finger50, and using automated optical inspection (AOI) feedback to achieve the desired height and hence the desired spring tension, the repeatability and hence reliability of the bond joints can be improved.

Although the stainless steel material used in the trace gimbal assembly is relatively thin, springs made from the stainless steel layer of the trace gimbal assembly after laser adjustment can provide several mg of bias force, which is enough to make a significant difference in the reliability of the bond of the copper bond pad which has a surface area of only around 0.09 mm2. The total bias force applied by the stainless spring or other bias mechanism is preferably at least 1 mg which is approximately 1×10−5N, and more preferably at least 2 mg which is approximately 2×10−5N. The bias pressure applied at the copper bond pad is preferably at least 10 Pa, and more preferably at least 20 Pa.

Laser adjust techniques and other adjustment techniques can also be applied after the conductive adhesive on the PZT has been partially hardened and/or fully hardened in order to provide a desired amount of bias during operation of the device.

Although the invention has been described with reference to piezoelectric microactuators, it will be appreciated that the invention is applicable more generally to other types of microactuators, and indeed to more generally still to making electrical connections to various types of electronic components than just microactuators.