Solder control features for a disc drive head flex interconnect

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.

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.

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. 1is a plan view of a flex on suspension (FOS)10circuit according to the prior art. The FOS10circuit includes a head gimbel24region, a load beam26, a tail28, flying leads22and a shunt bar48. The tail28has a first end34and a second end36. The second end36comprises a first edge74, a second edge94, and a third edge96. The third edge96is located opposite the first edge74. The FOS10may also contain a loopback46, a flapper38, a shark fin40and a foot44. The head leads12are connected to the read-write components of a disc drive assembly. Signals are passed back and forth from the read-write head through the FOS10circuit. An electrical connection path20is formed between the head leads12and the flying leads22running from the head leads12through the head gimbel24region through the load beam26and the along the tail28to the flying leads22. The FOS10is used to make the connection between the read-write heads and the main circuitry of a disc drive.

The second end36of the tail28may also contains structural elements which assist the FOS10in staying in place. The flapper38prevents one FOS10from shorting to another. The flapper38is located on the third edge96of the tail28. During the manufacturing process, the flapper38is folded up to protect the flying leads22. The shark fin40adds depth to the FOS10engagement during the manufacturing process. The shark fin40is located on the third edge96of the tail28. The shark fin40also helps to hold the FOS10in place. The foot44helps to hold the FOS10in place. The foot44is designed so that it does not allow the FOS10to sag after final assembly. The foot is located on the second edge94of the tail28. The loopback46contains circuitry which is electrically continuous with the electrical connection paths20from the head leads12through the flying leads22. The loopback46is used for testing electrocontinuity of the electrical connection paths20during the manufacturing process. The shunt bar48is also a part of the loopback46and is used to assist placement of the flying leads22to be welded during the solder reflow process. The shunt bar48is removed after the electrocontinuity tests are performed.

FIG. 2is a perspective view of a FOS10circuit shown inFIG. 1coupled to a PCC18. The tail28is inserted into the slot50with the circuit leads52facing away from the surface54of the tail28which contacts the PCC18. The flying leads22are folded over onto the surface54of the PCC18such that each flying lead22is positioned substantially over a solder pad30. The shunt bar48extends substantially perpendicular from the surface54of the PCC18.

Because there is no physical constraint on the flow of the solder56as it is melted during the solder reflow process, it can wick and contact the PCC18stiffener58, causing a short. This short results in a defective head assembly. The reflowed solder56can also wick into contact with the shunt bar48. The solder56that contacts the shunt bar48can also bond to the surface54of the PCC18. After the flying leads22are positioned, the shunt bar48is removed during final assembly. In areas where the solder56forms a bridge between the shunt bar48and the surface54of the PCC18, the surface54of the PCC18can delaminate when the shunt bar48is torn off during final assembly. The result is a defective head assembly. Shorts due to solder56bridging between the flying leads22and the stiffener58of the PCC18and lift-off of the surface54of the PCC18during shunt bar48tear off are the two main sources of yield reduction due to the solder reflow process.

FIG. 3is a perspective view of a FOS10circuit according to a preferred embodiment of the present invention coupled to a PCC18. In the preferred embodiment of the present invention, a dam90is fabricated as part of the FOS10during the manufacturing process. Subsequently in the manufacturing process, the tail28is inserted into the PCC18slot50oriented so the electrical connection path20is oriented on the surface54of the tail28located away from the tail's28point of contact with the PCC18. The dams90restrict the solder56, which flows during the solder reflow process, by physically limiting where the solder56can flow. The dams90can be located on either surface54of the flying leads22. The dams90on the surface54of the flying leads22positioned between the flying lead and the surface54of the PCC18are formed from the polyimide substrate from which the FOS10circuit is fabricated. The darns90on the opposite surface54of the flying leads22are integrated into the cover coat pattern during fabrication. It can be seen that during the solder reflow process the dams90physically constrain the travel of solder56that is melted during the solder reflow step in the manufacturing process. Because of the physical restrictions on flow, solder56does not flow into the shunt bar48area, and bridging between the shunt bar48and the PCC18surface54is eliminated. Similarly, the dams90between the PCC18slot50and the solder pads30physically inhibit the solder56from flowing and bridging between the flying leads22and the PCC18stiffener58, thereby reducing or eliminating shorts.

FIG. 4is a plan view of the flying lead region of the FOS circuit shown inFIG. 3. The dam90spans the flying leads22from a first70flying lead22to a last72flying lead22wherein the last72flying lead22is located most distant from the first70flying lead22. The flying leads22extend from a first edge74of the tail28in a substantially perpendicular orientation with respect to the first edge74. The flying leads22are also connected to the shunt bar48. The dam90is patterned on the FOS10circuit during the imaging process whereby the features of the FOS10circuit are formed. The dam90spans the flying leads22with its long edge roughly parallel to the first edge74of the second end36of the tail28. The FOS10circuit also contains a flapper38which prevents one FOS10from shorting to another. The FOS10may also contain a shark fin40.

FIG. 5shows a front view of a FOS10circuit embodying the current invention as coupled with the PCC18. The tail28is positioned into a slot50on the PCC18. The PCC18also has a stiffener58to increase its rigidity. The stiffener58is usually made from a conductive material. The surface92of the tail28containing the electrical connection path20is placed away from the surface54of the PCC18and PCC18stiffener58. After the tail28is positioned into a PCC18slot50, the flying leads22are folded over such that the flying leads22are in contact with the solder pads30. The shunt bar48extends roughly perpendicular from the surface54of the PCC18during this step and keeps the flying leads22in place during the solder reflow process. The electroconnection between the flying leads22and the solder pads30is then accomplished by having a solder reflow head pass over the solder pads30, thereby melting or reflowing the solder pads30. After the solder head reflows the solder56, it passes from the area allowing the solder56to cool. Upon cooling, a continuous electrical connection is made between the solder pads30and the flying leads22. During the solder reflow process the dam90physically restricts the melted solder56from flowing over the surface54of the PCC18card and down into the slot50. Shorts due to solder56bridging between the solder pad30and the PCC18stiffener58are eliminated because solder56cannot physically pass the dam90. Similarly a dam90positioned between the shunt bar48and the solder pads30physically restricts the solder56from flowing to the shunt bar48. Because excess solder56does not attach, the shunt bar48is more easily removed during tear off. Delamination of the surface54of the PCC18is reduced or eliminated.

FIG. 6is a perspective view of a plurality of FOS10circuits according to a preferred embodiment of the present invention coupled to a PCC18. The tail28is placed into the slot50of the PCC18and the PCC18stiffener58. The flying leads22are folded over so that the flying leads22rest upon the solder pads30located on the surface54of the PCC18. The shunt bar48extends substantially perpendicular from the surface54of the PCC18and helps locate the flying leads22over the solder pads30during the solder reflow process. The solder reflow mechanism is passed over the solder pads30, which are melted. As a result of the solder reflow process, the flying leads22are immersed or surrounded by the molten solder56. Without the darns90there is no restriction on the flow of solder56during this process. It may only be controlled by restrictions on solder56volume or other design tradeoffs. As was discussed previously, there are inherent disadvantages to these approaches. Without the darns90in place, solder56from the solder pads30may flow into the slot50on the PCC18and connect with the PCC18stiffener58thereby causing a short resulting in a defective apparatus. Similarly, solder56from the solder pad30can flow to the shunt bar48, thereby increasing the amount of force needed to tear off the shunt bar48after the flying leads22are welded into position. The darns90provide a physical barrier to the flow of solder56during the solder reflow process thereby eliminating shorts caused by bridging of the solder56between the flying leads22and the PCC18stiffener58. Defects due to delaminating of the PCC18surface54are also reduced because the shunt bar48can be torn off with less force than required if there is a solder56bridge is between solder pads30and the shunt bar48.

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.