LEAD FRAME WITH PLATED LEAD TIPS

A lead frame is formed with exposed lead tips. The leads are not attached at their tips to any of a tie bar, a dam bar or an end bar, so when the lead frame is plated, the lead tips are plated. During packaging, after die attach and molding, when the lead frame is cut from the frame assembly, the lead tips are not cut, so the plating remains on the tips. This improves solder joint reliability when the package is mounted on a PCB. The lead frame has connection bars that run parallel to the leads from the tie bar to the end bar. The connection bars provide stability to the leads during wire bonding, but are cut from the lead frame after wire bonding.

BACKGROUND

The present invention generally relates to a lead frame used for assembling a semiconductor device and a method for assembling a semiconductor device using the lead frame, and, more particularly, to a lead frame with plated lead tips.

FIG. 1is a top plan view of part of a conventional lead frame strip10, showing two lead frames12and14, during assembly of a semiconductor device. The lead frame strip10typically comprises copper or copper foil that is plated or at least partially plated with metals such as tin, nickel, and/or palladium, which inhibit corrosion. The lead frames12and14each comprise a die pad16surrounded by a plurality of leads or lead fingers18. The lead fingers18have a proximal end20near to the die pad16, a distal end22spaced from the die pad16, and a central portion24that connects the proximal and distal ends20and22. The die pad16is attached to the central frame with tie bars26. A dam bar28extends perpendicular to the leads18and is connected to the central portion24of the leads18, and an end bar30also extends perpendicular to the leads18and is connected to the distal ends22of the leads18.

During the assembly process, dies32are attached to the lead frames12and14, and electrodes on the dies32are electrically connected to respective ones of the leads18. The lead frame14shows bond wires34being used to electrically connect the die32with the proximal ends20of the leads18. After wire bonding, the die32, bond wires34and proximal ends20of the leads18are covered with a molding compound (not shown), where the molding compound will extend to the dam bar28. The molding compound provides electrical and mechanical protection to the semiconductor die32, the lead frame14and the connections34therebetween. After molding, the lead frame14is trimmed, whereby the dam bar28is cut away, and the distal ends22of the leads18are separated from the end bar30by cutting along the dashed line A-A.

FIG. 2shows a conventional semiconductor device40being mounted on a printed circuit board (PCB)42, which comprises a molded body44and the distal ends22of the leads18extending outwardly from the molded body44. The leads18are electrically connected to pads46of the PCB42with solder48. However, since the leads18were cut from the lead frame14along the line A-A, the tips50of the leads18were cut and exposed. That is, the bare copper of the lead frame is exposed, and as is well known, bare copper is readily corroded and therefore may not hold the solder48, which in turn can make for a weak solder joint.

One solution is to plate the lead frame tips50after separating the assembled devices40from the lead frame strip10, but this requires an additional plating step, which adds to the manufacturing time and cost. Accordingly, it would be advantageous to be able to assemble semiconductor devices where the copper base of the lead frame is not exposed during trim and form.

DETAILED DESCRIPTION

In one embodiment, the present invention provides a lead frame for a semiconductor device. The lead frame has a central area for receiving an integrated circuit die, and a plurality of leads that extend away from at least one lateral side of the central area. The leads each have a proximal end near to the central receiving area, a distal end, and a central portion connecting the proximal and distal ends. A dam bar extends generally perpendicular to the leads and connects the leads at the central portions thereof. An end bar extends generally perpendicular to the leads and parallel to the dam bar. The end bar is located near to, but spaced from, the distal ends of the leads. There also are one or more connection bars extending from the end bar to the dam bar.

In another embodiment, the present invention provides a method of assembling a semiconductor device. The method includes providing a lead frame having a central area for receiving an integrated circuit die, and a plurality of leads that extend away from at least one lateral side of the central area. The leads each have a proximal end near to the central receiving area, a distal end, and a central portion connecting the proximal and distal ends. The lead frame further comprises a dam bar that extends generally perpendicular to the leads and connects the leads at the central portions thereof, an end bar that extends generally perpendicular to the leads and parallel to the dam bar, and one or more connection bars extending from the end bar to the dam bar. The end bar is located near to, but spaced from, the distal ends of the leads. At least the distal ends of the leads are plated with a non-corroding material.

Referring now toFIG. 3, an enlarged top plan view of portion of a lead frame strip100for a semiconductor device in accordance with an exemplary embodiment of the present invention is shown. The lead frame strip100includes a plurality of lead frames, two of which are shown102and104. The lead frames may be arranged is a single strip or as an array, as are known in the art. The lead frame112and the lead frame114are shown during assembly of a semiconductor device.

The lead frame strip100may comprise a conductive base layer, such as copper or copper foil, that is plated or at least partially plated with metals or metal alloy, such as tin, nickel, and/or palladium, which inhibit corrosion and provide for a good, solderable surface. In the presently preferred embodiment, the lead frames112and114are comprised of copper, and at least the distal ends of the leads are plated with a non-corroding material, such as Nickel and Palladium.

The lead frames102and104each comprise a central die receiving area106, which in this embodiment is a die pad. The die pad106is sized and shaped to receive a semiconductor die and thus, the size of the die pad generally is based on the size of the die. In the embodiment shown, the die pad106is rectangular, but this is not a requirement. The lead frames102and104also have a plurality of leads108that extend away from at least one lateral side of the central area or die pad106. In the embodiment shown, the leads108extend away from two of the lateral sides of the die pad106. However, as will be understood by those of skill in the art, the leads108may surround the die pad106, thus extending away from all four sides of the die flag106. The leads108each have a proximal end110near to but spaced from the central receiving area106, a distal end112, and a central portion114connecting the proximal and distal ends.

The lead frames102and104include at least one tie bar that extends from a side of the central area106that is adjacent to the at least one lateral side of the central area106from which the leads108extend, in order to provide support to the die pad106. In the embodiment shown, the die pads106are attached to the central frame with tie bars116. A dam bar118extends perpendicular to the leads108and is connected to the central portion114of the leads108, and an end bar120also extends generally perpendicular to the leads108and parallel to the dam bar118. The tie bar116is generally perpendicular to the leads108and parallel to the dam bar118and the end bar120. However, unlike the conventional lead frames102and104ofFIG. 1, the distal ends112of the leads108are not connected or attached to the end bar120. Rather, the end bar120is located near to, but spaced from, the distal ends112of the leads108. The lead frames102and104also include one or more connection bars122that extend from the end bar120to the dam bar118. In the presently preferred embodiment, the connection bars122are temporary, and are cut away during trim and form.

During the assembly process, dies124are attached to the die receiving areas106of the lead frames102and104using a die attach material, such as an adhesive or adhesive tape, as is known in the art. In one embodiment, the adhesive is thermally conductive, so that heat generated by the semiconductor die124can be dissipated through the die pad106. In another embodiment, the adhesive is both electrically and thermally conductive for providing additional connection between the semiconductor die124and the die pad106. In one embodiment, the adhesive comprises an epoxy paste that is printed onto the die pad106. After the semiconductor die124is attached to the die receiving area106, the adhesive is cured so that the semiconductor die124is securely fastened to the receiving area106.

The semiconductor die124may be any type of die, such as a sensor die, a power die, an application specific integrated circuit (ASIC), etc. The semiconductor die124may have an active region on one side thereof and a non-active region on an opposite side. In the presently preferred embodiment, the semiconductor die124is placed on the die pad106such that the non-active region side faces the die pad106. In another embodiment, the active region side of the semiconductor die124can be configured to face the die pad106. In applications where the semiconductor die124generates heat (e.g., a power die), the die pad106can be used to dissipate the heat through contact between the active region side of the semiconductor die124and the die pad106.

When the semiconductor die124is mounted on the die pad106with its non-active region side attached to the die pad106, then bond wires126are used to electrically connect the semiconductor die124to the leads108. That is, the electrodes on the active side surface of the semiconductor die124are electrically connected to the proximal ends110of the leads108with the bond wires126. The bond wires126can be any kind of bond wires, such as copper or gold, and may be coated or uncoated.

It will be understood by those of skill in the art that the electrical connection of the semiconductor die124to the leads108is not limited to the above-mentioned wire bonding. In alternative embodiments, clip bonding, flip-chip, etc. also may be used. For example, in one embodiment the semiconductor die124is attached to the lead frame104with the die bond pads facing the proximal ends110of the leads108, and electrically connected thereto with conductive adhesive.

After attaching and electrically connecting the semiconductor die124to the lead frame104, the die124, bond wires126and proximal ends110of the leads108are covered with a molding compound (not shown), where the molding compound extends to the dam bar118. The molding compound provides electrical and mechanical protection to the semiconductor die124, the lead frame104and the connections126therebetween. In one embodiment, the molding compound comprises an epoxy-resin composition, for example a C-stage plastic material (Resite). The molding compound is applied such that it covers and seals the semiconductor die1124and at least part of the lead frame104. The molding compound is subsequently cured to be physically hard, so that the semiconductor die124, the proximal ends110of the leads108, and the bond wires126covered by the molding compound are protected from potential environmental influences like moisture and dust, as well as mechanical damage. The molding compound may be formed over the die124using known methods, such as transfer molding.

After molding, the lead frame104is trimmed, whereby the dam bar118is cut away, and the end bar120is separated from the assembly such as by cutting along line B-B, which separates the connection bars122from the end bar120. The connection bars122also preferably are cut away from the dam bar118at the same time as when the dam bar118is trimmed (when separating the leads108).

FIG. 4shows a semiconductor device130in accordance with an embodiment of the present invention being mounted on the PCB42. The semiconductor device130comprises a molded body132and leads134that extend outwardly from the molded body132. That is, the leads134comprise the distal ends112of the leads108shown inFIG. 3. The leads134are electrically connected to the pads46of the PCB42with solder136. Since the distal ends112of the leads108were not cut from the end bar120, the lead tips were not trimmed and thus, the lead tips remain plated and the underlying copper base is not exposed. Therefore, the leads134are readily soldered to the solder pads46of the PCB42. As illustrated, the solder136covers the lead tip.

FIG. 5is a flow diagram140illustrating steps of assembling a semiconductor device in accordance with an exemplary embodiment of the present invention. In a first step,142, a first sub-step144of providing a lead frame is performed. The lead frame preferably is one of a strip of lead frames, and includes a central area for receiving an integrated circuit die, and a plurality of leads that extend away from at least one lateral side of the central area, as shown and described above with reference toFIG. 3. As previously discussed, the leads each have a proximal end near to the central receiving area, a distal end, and a central portion connecting the proximal and distal ends. The lead frame further comprises a dam bar that extends generally perpendicular to the leads and connects the leads at the central portions thereof, an end bar that extends generally perpendicular to the leads and parallel to the dam bar, wherein the end bar is located near to, but spaced from, the distal ends of the leads, and one or more connection bars extending from the end bar to the dam bar. At sub-step146, the lead frame strip or at least the distal ends of the leads of each lead frame are plated with a non-corroding material, such as NiPd. It will be understood by those of skill in the art that the plated lead frame in accordance with embodiments of the present invention may be received from a lead frame supplier rather than the device assembly factory actually forming the lead frames on a strip or in an array and then plating the strip or array.

At step148, an integrated circuit die is attached to the central area with a die attach adhesive, and then electrodes on a planar surface of the integrated circuit die are electrically connected to the proximal ends of the leads, such as with bond wires, at step150.

At step152, the integrated circuit die, the electrical connections, and the proximal ends of the leads are covered with a molding compound. At step154, the connection bars122(seeFIG. 3) of the lead frame are cut away, such as with a saw, while the dam bar also is cut away, such as by stamping. Cutting away the connection bars122separates the lead frame assembly from the end bars too. At step156, the portions of the leads that extend outwardly from the molded body are formed to a desired shape, such as by a gull-wing shape, by bending. At step158, the lead frame assemblies are separated from adjacent assemblies by cutting the tie bars.

Finally, at step160, the fully assembled semiconductor device may be attached to a PCB, as described above with reference toFIG. 4. And because the distal ends of the leads are not cut or trimmed as part of the assembly process, the lead tips remain coated with the NiPd layer, and thus, good solder joints may be formed when connecting the device to the PCB.

In order to once again highlight the difference between the conventional lead frame and a lead frame of the present invention,FIG. 6is an enlarged drawing of a portion of a conventional lead frame170, andFIG. 7is an enlarged drawing of a portion of a lead frame180in accordance with an embodiment of the present invention. InFIG. 6, distal ends172of the leads are physically connected to an end bar174, while inFIG. 7, distal ends182of the leads are not connected to an end bar184. Instead, the lead frame180is provided with stability by having connection bars186that extend between the end bar184and a dam bar188.

Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the scope of protection sought is defined by the claims as set forth hereinafter together with any equivalents thereof entitled to. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illustrate the subject matter and does not pose a limitation on the scope of the subject matter unless otherwise claimed. The use of the term “based on” and other like phrases indicating a condition for bringing about a result, both in the claims and in the written description, is not intended to foreclose any other conditions that bring about that result. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention as claimed.