Abstract:
A FOS circuit and a method for fabricating the same that reduces the possibilities of shorts or damage to a circuit board during assembly due to solder reflow. The FOS circuit has a tail, a shunt bar, a plurality of flying leads, and a dam. The tail has a first and second end. The shunt bar is located adjacent to the second end of the tail. The plurality of flying leads project substantially perpendicular from the first edge of the second end of the tail. The plurality of flying leads are substantially parallel to one another and extend between the second end of the tail and the shunt bar. A plurality of electrical paths are formed through the tail to the flying leads. The dam intersects the flying leads and extends from a first flying lead to a last flying lead and is substantially parallel with the first edge of the second end of the tail.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of provisional application Ser. No. 60/117,792 (Seagate No. 9029) entitled “Solder Control Features For A Disc Drive Head Flex Interconnect” filed on Jan. 29, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to flex on suspension (FOS) circuits, and more particularly to an improved design of a FOS circuit. 
     BACKGROUND OF THE INVENTION 
     Disc drive systems are well-known. They include data heads including transducer elements for reading or writing data to a recordable disc. Transducer elements of the data heads are electrically coupled to the main drive circuitry through a head interconnect circuit. Conductive paths on the head interconnect circuit electrically couple head leads coupled to transducer elements on the head to circuit leads connected to drive circuitry. 
     Heads are supported relative to a disc surface by a head actuator. A drive circuit is mounted on the head actuator and circuit leads on the head interconnect circuit are coupled to lead connectors or solder pads on the drive circuit via a flex on suspension (FOS) circuit. Leads are supported along an edge of a lead tip of the head interconnect circuit and connectors or solder pads are aligned along a slot or edge of the drive circuit. The lead tip is inserted into the slot or aligned with the edge to couple the circuit leads to connectors. Leads are soldered to connectors to electrically connect transducer elements of the head to main drive circuitry. 
     Prior to soldering, leads are aligned with the connectors or solder pads to assure desired electroconnection for read and write operations. Drive circuits on a head actuator include a conductive metal substrate supporting a printed circuit. During the soldering operation, solder can wick from the solder pad or connector to surrounding features. Solder that wicks to a metal substrate can short the electroconnection between the data heads and drive circuitry making the data heads defective. Similarly, solder can bridge from the solder pads to the shunt bar. The shunt bar is removed from the FOS circuit during assembly and after the flying leads are soldered to the solder pads. If solder attaches to the shunt bar, extra force is required to remove it. This can result in the surface of the printed circuit delaminating and result in a defective circuit. The present invention addresses these and other problems, and offers other advantages over the prior art. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a dam is fabricated coupled to the flying leads of the FOS circuit. The flying leads project from a tail in a roughly perpendicular direction to a first edge of the tail. The dam spans the flying leads substantially parallel to the edge of the tail. The dam restricts solder flow during the solder reflow process in which the flying leads are soldered to the printed circuit card (PCC) of a disc drive circuitry. 
     In another aspect of the invention, two dams are fabricated coupled to the flying leads of the FOS circuit. The additional dam further restricts solder travel during the solder reflow process in which the FOS circuit flying leads are soldered to the solder pads of the PCC. 
     In another aspect of the invention, a non-metallic substrate is coated with conductive layer. A mask is deposited on the conductive layer defining the FOS circuit. Also included in the mask is a dam which intersects a plurality of flying leads of the circuit. The mask is then developed and etched such that a FOS circuit is formed with a dam. 
     In another aspect of the invention, a non-metallic substrate material is coated with a conductive layer. A mask is deposited on the conductive layer to define a FOS circuit and also a plurality of dams intersecting a plurality of flying leads on the circuit. The mask is then developed and etched to form a FOS circuit with a plurality of dams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a prior art FOS circuit. 
         FIG. 2  is a perspective view of a FOS circuit shown in  FIG. 1  coupled to a PCC. 
         FIG. 3  is a perspective view of a FOS circuit according to a preferred embodiment of the present invention coupled to a PCC. 
         FIG. 4  is a plan view of the flying lead region of a FOS circuit shown in  FIG. 3 . 
         FIG. 5  is a front view of a FOS circuit shown in  FIG. 3  coupled to a PCC. 
         FIG. 6  is a perspective view of a plurality of FOS circuits circuit according to a preferred embodiment of the present invention coupled to a PCC. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A FOS circuit, or FOS, is used to interconnect between the read-write head of a disc drive and the main control circuitry of the disc drive. One of the steps in the assembly is connecting the FOS to the data flex circuit, also known as the PCC, by a solder reflow process. The solder used to make the interconnect between the FOS and the data flex circuit is not well controlled during the reflow process. Because of this solder can wick onto a shunt bar, which is in a region which is supposed to be compliant thereby making it non-compliant. This also makes the shunt bar more difficult to remove, thus increasing the time required in assembling the FOS. In addition, the PCC may delaminate during removal of the shunt bar resulting in defective heads. To compensate for solder bridging between the solder pads and the shunt bar, shunt removal can be facilitated by making the FOS leads narrower at the tear off location. However, this leads to a more fragile circuit and decreases yield during manufacturing processes. 
     Solder shorts are also caused between the flying leads and the PCC stiffener. The PCC stiffener is used to make the PCC more rigid. The PCC stiffener is usually made from a electrically conductive material such as aluminum. Because the PCC stiffener is conductive, if solder bridges the gap between the flying leads and the PCC stiffener, a short is caused making the head defective. 
     One way to control the shorting is by restricting the volume of solder which will reflow during the solder reflow process. Similarly, the tip of the solder reflow instrument impacts the travel of the solder during the assembly process and varying the design can help reduce defects caused by uncontrolled solder flow. However, neither of these methods provide an absolute restriction on solder travel. Other methods that impact the defects due to unrestricted solder travel may be used, although in the existing methods there is no absolute restriction on solder flow. 
     The present invention reduces the disadvantages associated with solder flow by providing a FOS with a unique design. More particularly, the present invention provides a means to physically limit solder flow during reflow soldering. A dam is pattered in the cover coat and/or the polyimide base on either side of the FOS leads. The dam provides a physical barrier which limits the flow of solder during the reflow process, thereby reducing the possibility of shorts because of solder bridging between the leads and the PCC stiffener and allowing the FOS leads to be made broader, thereby increasing the strength at the tear off location of the shunt. 
     As way of background,  FIG. 1  is a plan view of a flex on suspension (FOS)  10  circuit according to the prior art. The FOS  10  circuit includes a head gimbel  24  region, a load beam  26 , a tail  28 , flying leads  22  and a shunt bar  48 . The tail  28  has a first end  34  and a second end  36 . The second end  36  comprises a first edge  74 , a second edge  94 , and a third edge  96 . The third edge  96  is located opposite the first edge  74 . The FOS  10  may also contain a loopback  46 , a flapper  38 , a shark fin  40  and a foot  44 . The head leads  12  are connected to the read-write components of a disc drive assembly. Signals are passed back and forth from the read-write head through the FOS  10  circuit. An electrical connection path  20  is formed between the head leads  12  and the flying leads  22  running from the head leads  12  through the head gimbel  24  region through the load beam  26  and the along the tail  28  to the flying leads  22 . The FOS  10  is used to make the connection between the read-write heads and the main circuitry of a disc drive. 
     The second end  36  of the tail  28  may also contains structural elements which assist the FOS  10  in staying in place. The flapper  38  prevents one FOS  10  from shorting to another. The flapper  38  is located on the third edge  96  of the tail  28 . During the manufacturing process, the flapper  38  is folded up to protect the flying leads  22 . The shark fin  40  adds depth to the FOS  10  engagement during the manufacturing process. The shark fin  40  is located on the third edge  96  of the tail  28 . The shark fin  40  also helps to hold the FOS  10  in place. The foot  44  helps to hold the FOS  10  in place. The foot  44  is designed so that it does not allow the FOS  10  to sag after final assembly. The foot is located on the second edge  94  of the tail  28 . The loopback  46  contains circuitry which is electrically continuous with the electrical connection paths  20  from the head leads  12  through the flying leads  22 . The loopback  46  is used for testing electrocontinuity of the electrical connection paths  20  during the manufacturing process. The shunt bar  48  is also a part of the loopback  46  and is used to assist placement of the flying leads  22  to be welded during the solder reflow process. The shunt bar  48  is removed after the electrocontinuity tests are performed. 
       FIG. 2  is a perspective view of a FOS  10  circuit shown in  FIG. 1  coupled to a PCC  18 . The tail  28  is inserted into the slot  50  with the circuit leads  52  facing away from the surface  54  of the tail  28  which contacts the PCC  18 . The flying leads  22  are folded over onto the surface  54  of the PCC  18  such that each flying lead  22  is positioned substantially over a solder pad  30 . The shunt bar  48  extends substantially perpendicular from the surface  54  of the PCC  18 . 
     Because there is no physical constraint on the flow of the solder  56  as it is melted during the solder reflow process, it can wick and contact the PCC  18  stiffener  58 , causing a short. This short results in a defective head assembly. The reflowed solder  56  can also wick into contact with the shunt bar  48 . The solder  56  that contacts the shunt bar  48  can also bond to the surface  54  of the PCC  18 . After the flying leads  22  are positioned, the shunt bar  48  is removed during final assembly. In areas where the solder  56  forms a bridge between the shunt bar  48  and the surface  54  of the PCC  18 , the surface  54  of the PCC  18  can delaminate when the shunt bar  48  is torn off during final assembly. The result is a defective head assembly. Shorts due to solder  56  bridging between the flying leads  22  and the stiffener  58  of the PCC  18  and lift-off of the surface  54  of the PCC  18  during shunt bar  48  tear off are the two main sources of yield reduction due to the solder reflow process. 
       FIG. 3  is a perspective view of a FOS  10  circuit according to a preferred embodiment of the present invention coupled to a PCC  18 . In the preferred embodiment of the present invention, a dam  90  is fabricated as part of the FOS  10  during the manufacturing process. Subsequently in the manufacturing process, the tail  28  is inserted into the PCC  18  slot  50  oriented so the electrical connection path  20  is oriented on the surface  54  of the tail  28  located away from the tail&#39;s  28  point of contact with the PCC  18 . The dams  90  restrict the solder  56 , which flows during the solder reflow process, by physically limiting where the solder  56  can flow. The dams  90  can be located on either surface  54  of the flying leads  22 . The dams  90  on the surface  54  of the flying leads  22  positioned between the flying lead and the surface  54  of the PCC  18  are formed from the polyimide substrate from which the FOS  10  circuit is fabricated. The darns  90  on the opposite surface  54  of the flying leads  22  are integrated into the cover coat pattern during fabrication. It can be seen that during the solder reflow process the dams  90  physically constrain the travel of solder  56  that is melted during the solder reflow step in the manufacturing process. Because of the physical restrictions on flow, solder  56  does not flow into the shunt bar  48  area, and bridging between the shunt bar  48  and the PCC  18  surface  54  is eliminated. Similarly, the dams  90  between the PCC  18  slot  50  and the solder pads  30  physically inhibit the solder  56  from flowing and bridging between the flying leads  22  and the PCC  18  stiffener  58 , thereby reducing or eliminating shorts. 
       FIG. 4  is a plan view of the flying lead region of the FOS circuit shown in  FIG. 3 . The dam  90  spans the flying leads  22  from a first  70  flying lead  22  to a last  72  flying lead  22  wherein the last  72  flying lead  22  is located most distant from the first  70  flying lead  22 . The flying leads  22  extend from a first edge  74  of the tail  28  in a substantially perpendicular orientation with respect to the first edge  74 . The flying leads  22  are also connected to the shunt bar  48 . The dam  90  is patterned on the FOS  10  circuit during the imaging process whereby the features of the FOS  10  circuit are formed. The dam  90  spans the flying leads  22  with its long edge roughly parallel to the first edge  74  of the second end  36  of the tail  28 . The FOS  10  circuit also contains a flapper  38  which prevents one FOS  10  from shorting to another. The FOS  10  may also contain a shark fin  40 . 
       FIG. 5  shows a front view of a FOS  10  circuit embodying the current invention as coupled with the PCC  18 . The tail  28  is positioned into a slot  50  on the PCC  18 . The PCC  18  also has a stiffener  58  to increase its rigidity. The stiffener  58  is usually made from a conductive material. The surface  92  of the tail  28  containing the electrical connection path  20  is placed away from the surface  54  of the PCC  18  and PCC  18  stiffener  58 . After the tail  28  is positioned into a PCC  18  slot  50 , the flying leads  22  are folded over such that the flying leads  22  are in contact with the solder pads  30 . The shunt bar  48  extends roughly perpendicular from the surface  54  of the PCC  18  during this step and keeps the flying leads  22  in place during the solder reflow process. The electroconnection between the flying leads  22  and the solder pads  30  is then accomplished by having a solder reflow head pass over the solder pads  30 , thereby melting or reflowing the solder pads  30 . After the solder head reflows the solder  56 , it passes from the area allowing the solder  56  to cool. Upon cooling, a continuous electrical connection is made between the solder pads  30  and the flying leads  22 . During the solder reflow process the dam  90  physically restricts the melted solder  56  from flowing over the surface  54  of the PCC  18  card and down into the slot  50 . Shorts due to solder  56  bridging between the solder pad  30  and the PCC  18  stiffener  58  are eliminated because solder  56  cannot physically pass the dam  90 . Similarly a dam  90  positioned between the shunt bar  48  and the solder pads  30  physically restricts the solder  56  from flowing to the shunt bar  48 . Because excess solder  56  does not attach, the shunt bar  48  is more easily removed during tear off. Delamination of the surface  54  of the PCC  18  is reduced or eliminated. 
       FIG. 6  is a perspective view of a plurality of FOS  10  circuits according to a preferred embodiment of the present invention coupled to a PCC  18 . The tail  28  is placed into the slot  50  of the PCC  18  and the PCC  18  stiffener  58 . The flying leads  22  are folded over so that the flying leads  22  rest upon the solder pads  30  located on the surface  54  of the PCC  18 . The shunt bar  48  extends substantially perpendicular from the surface  54  of the PCC  18  and helps locate the flying leads  22  over the solder pads  30  during the solder reflow process. The solder reflow mechanism is passed over the solder pads  30 , which are melted. As a result of the solder reflow process, the flying leads  22  are immersed or surrounded by the molten solder  56 . Without the darns  90  there is no restriction on the flow of solder  56  during this process. It may only be controlled by restrictions on solder  56  volume or other design tradeoffs. As was discussed previously, there are inherent disadvantages to these approaches. Without the darns  90  in place, solder  56  from the solder pads  30  may flow into the slot  50  on the PCC  18  and connect with the PCC  18  stiffener  58  thereby causing a short resulting in a defective apparatus. Similarly, solder  56  from the solder pad  30  can flow to the shunt bar  48 , thereby increasing the amount of force needed to tear off the shunt bar  48  after the flying leads  22  are welded into position. The darns  90  provide a physical barrier to the flow of solder  56  during the solder reflow process thereby eliminating shorts caused by bridging of the solder  56  between the flying leads  22  and the PCC  18  stiffener  58 . Defects due to delaminating of the PCC  18  surface  54  are also reduced because the shunt bar  48  can be torn off with less force than required if there is a solder  56  bridge is between solder pads  30  and the shunt bar  48 . 
     The above specification provides a complete description of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereafter appended.