Non-leaded semiconductor package and a method to assemble the same

A method to assemble a non-leaded semiconductor package is disclosed. In one embodiment, a carrier tape is attached to a metal foil. A plurality of leadframes are formed in the metal foil, each leadframe including a die pad laterally surrounded by a plurality of contact leads. A semiconductor die, including an active surface with a plurality of die contact pads, is attached to each die attach pad and electrically connected to the leadframe by a plurality of bond wires connecting the die contact pads and the lead contact areas of the contact leads. A plurality of leadframes, each including a wire bonded semiconductor die, are encapsulated with mold material. The carrier tape is removed and the non-leaded semiconductor packages separated.

FIELD OF THE INVENTION

The invention provides a non-leaded semiconductor package and to a method of assembling a non-leaded semiconductor package.

BACKGROUND

U.S. Pat. No. 6,498,099 discloses a method to produce a leadless semiconductor package by half etching one or both sides of the leadframe strip. After the wire bonding and molding processes, a further etching process is performed to isolate and expose the contact pads.

This process includes many processes and, in particular, many etching processes. Etching is a slow and, therefore, expensive manufacturing process.

SUMMARY

One embodiment of the invention provides a non-leaded semiconductor package and a simpler and more cost-effective method for producing the package.

DETAILED DESCRIPTION

In one embodiment, a non-leaded semiconductor package is assembled using a leadframe strip assembly. A method to produce the leadframe strip assembly according to the invention includes the following processes. A metal foil is provided and a carrier tape attached to one surface. A plurality of leadframes is then formed in the metal foil, each leadframe having a die pad or die attach pad laterally surrounded by a plurality of contact leads. Each leadframe includes the design of the desired semiconductor package. Preferably, the plurality of leadframes is formed by an etching process which is, more preferably, performed from one side of the metal foil. A plurality of isolated individual leadframes is formed.

By performing only one etching process the manufacturing process according to the invention is simplified. Performing the etching process from only one side of the metal foil leads to a simplification of the equipment required to form the leadframes and lower manufacturing costs.

In one embodiment, a plurality of leadframes is formed in the metal strip by laser cutting or stamping or any method known in the art. In this embodiment of the invention the leadframes are attached to each other by thin metal joining strips and form a continuous leadframe strip. The leadframe strip is then attached to the adhesive coating of a carrier tape. The metal joining strips are then removed by, for example, etching or by laser cutting to form a plurality of isolated leadframes attached to the carrier tape. The etching process is performed from only one side of the leadframe strip. This method has the advantage that the laser cutting or stamping process is relatively fast so that the manufacturing time is reduced.

The leadframe strip assembly according to the invention therefore includes a metal foil attached to a carrier tape. The metal foil includes a plurality of leadframes preferably arranged in a regular array of columns and rows in the metal foil. Each leadframe includes a die attach pad laterally surrounded by a plurality of contact leads. The arrangement of the contact leads and the die attach pad relates to the desired package design.

In one embodiment, the die pad and contact leads of each leadframe of the leadframe strip include anchorage features. The side walls of the die pad and inner side walls of the contact leads include protruding portions formed by an approximately square cut-out section at the base of the side walls. The protruding portion therefore extends approximately horizontally from the side walls of the die attach pad and inner side walls of the contact leads and typically has an approximately square cross-section. The upper surface of the protruding portion lies on approximately the same plane as the upper surface of the die attach pad and contact leads. This protruding portion is the anchorage feature which advantageously improves the reliability of the package.

In one embodiment, the die pad and contact leads of each leadframe of the leadframe strip assembly are spatially isolated from each other and, preferably, each leadframe of the metal foil of the leadframe strip assembly is spatially isolated from its neighbor. The die pad and contact leads of each leadframe are laterally isolated and are not attached to each other. Each leadframe is also laterally isolated and is not attached to the neighboring leadframes. The isolated leadframes are mechanically supported by the carrier tape. This arrangement of the leadframe strip assembly according to the invention is advantageous in that the cutting and stripping processes used in conventional leadframe manufacturing processes are avoided. This manufacturing line is therefore simplified.

By isolating the contact leads, a space is created between adjoining leadframes. Therefore, during the singulation process the contact leads are not cut which is extremely advantageous.

In one embodiment, the carrier tape includes a polyimide film with a silicone adhesive coating. This carrier tape material has the advantage that it has good heat resistance and, therefore, provides good mechanical support to the leadframes through the manufacturing process and, in particular, during the molding process. Also the adhesive can be cleanly removed from the bottom surface of the molded leadframe module or panel at the end of the manufacturing line just prior to the singulation of the individual semiconductor packages. This reduces the complex and costly cleaning processes.

In one embodiment, the metal foil includes copper or aluminum or one of their alloys and more preferably includes oxygen free high conductivity (OFHC) copper. These materials have good electrical conductivity, are relatively inexpensive and can be easily processed.

In one embodiment, the metal foil includes a thickness of approximately 1 mm to approximately 0.01 mm or more preferably approximately 0.25 mm to approximately 0.1 mm. The leadframe strip assembly according to the invention is advantageous in that the thickness of the metal foil which is used to form the plurality of leadframes can be thinner than that used in conventional processes as the carrier tape provides mechanical support. Also, in the method according to the invention the whole surface area of the metal strip is not thinned by an etching process to create leadframes of the desired thickness. The materials cost and manufacturing time is, therefore, reduced.

The leadframe strip may be partly or completely covered by an electro-plated coating. Different parts, such as the die attach pad, the contact leads and the contact areas of the contact leads, of a leadframe may be coated in different materials. The electro-plated coating may include silver, nickel/palladium/gold or nickel/nickel phosphorous.

In the next stage of the process a semiconductor die, which includes an active surface with a plurality of die contact pads and a passive surface, is attached to each die attach pad of the leadframe strip assembly. Each die is electrically connected to the leadframe by a plurality of bond wires connecting the die contact pads and the lead contact areas of the contact leads.

The plurality of dies attached to the leadframe strip assembly, contact leads, wire bonds and upper surface of the carrier tape are then encapsulated with mold material forming a panel or molded leadframe module. The mold material is typically a polymeric-based material.

A method to assemble a non-leaded semiconductor package according to the invention includes the following processes. The panel or molded leadframe module formed by the leadframe strip assembly process is provided and the carrier tape attached to the bottom surface is removed. The individual non-leaded semiconductor packages are singulated from the panel by sawing.

The sawing process may be performed by a saw blade, water jet or laser. The leadframe strip assembly according to the invention provides a panel in which the packages are singulated by cutting through only the mold material as the leadframe strip assembly includes isolated leadframes, die attach pads and contact leads. This greatly simplifies the sawing process.

If a saw blade is used to cut through two different materials, cracks can form between the two materials, burr formation is a problem and the wear rate of the saw blade is increased. Water jet technology is extremely slow and, therefore, may be impractical for high volume production as it is too costly. If a laser beam is used to cut through two types of material then the intensity must be adjusted which leads to a complicated process. These problems are avoided by the leadframe strip assembly according to the invention.

The leadframe strip assembly and the method of the invention is advantageously used to produce very thin quad flat non-leaded (VQFN) packages and particularly for packages of the pancake design leadframe which include a plurality of adjoining leadframes.

FIG. 1illustrates a non-leaded semiconductor package1according to the invention. The package1includes a semiconductor die2and a leadframe3. The leadframe3includes a die attach pad4in approximately its lateral centre which is laterally surrounded by a plurality of contact leads5. The die attach pad4and contact leads5have essentially the same thickness and lie on essentially the same lateral plane. The inner ends of the contact leads5include lead contact areas6. The side walls of the die attach pad4and the inner side wall of the contact leads5include a protruding portion19whose upper surface lies on approximately the same plane as that of the upper surface of the die attach pad or contact lead. The protruding portions19have an approximately square cross-section. The leadframe3includes oxygen-free high conductivity copper.

The semiconductor die2includes an active surface with a plurality of die contact pads7and a passive surface. The passive surface of the die2is attached approximately in the lateral centre of the die attach pad4by die attach material8. The die2is electrically connected to the leadframe3by a plurality of bond wires9which connect the die contact pads7and the lead contact areas6.

The upper surface of the die2, contact leads5, bond wires9and space between the die pad4and contact leads5is encapsulated with mold material10. The bottom surface11of the non-leaded package1includes mold material10and the bottom surfaces of the die attach pad4and contact leads5on an essentially common plane. The outer side surfaces of the contact leads5are covered by a thin coating of the mold material10. The outer sides of the semiconductor package1are essentially vertical. The bottom surfaces of the contact leads5provide the external contact areas of the package1.

FIG. 2illustrates a copper foil12attached to a carrier tape13in the first process of the method according to the invention to manufacture very thin quad flat non-leaded (VQFN) packages. The copper foil includes oxygen-free high conductivity copper and includes a thickness of between approximately 0.25 mm and approximately 0.1 mm. The copper strip12is attached to the adhesive coating17on the upper surface of a carrier tape13which includes a polyimide film substrate with a layer of silicone adhesive17on its upper surface.

FIG. 3illustrates the next stage of the method according to the invention in which a plurality of leadframes3are formed in the copper foil1. The leadframes3formed by a selective etching process which takes place on one side, from the top as illustrated inFIG. 2, of the copper strip12. The upper surface of the carrier tape13acts as an etch stop.

The leadframes3are laterally arranged in a regular array of rows and columns in the copper foil12. Each leadframe3includes a die attach pad4in the centre which is laterally surrounded by a plurality of contact leads5. The lateral arrangement of the plurality of leadframes and the lateral arrangement of each individual leadframe cannot be seen in the cross-sectional views of the figures.

The contact leads5are separate from the die attach pad4and are not connected to each other. Each leadframe3is laterally isolated from the neighboring leadframes in the copper foil12and the bottom surface of each leadframe is attached to the adhesive coating17of the carrier tape13. The carrier tape13provides the mechanical support to the isolated leadframes3, each including an isolated die attach pad4and a plurality of isolated contact leads5during the next stages of the manufacturing process up to just prior to the singulation process.

In the next step in the process, illustrated inFIG. 4, a semiconductor die2is attached using die attach material8to the die pad4of each leadframe3in the copper foil12. The semiconductor die2includes an active upper surface including a plurality of die contact pads7. The inner portion of each contact lead5of the leadframe3also includes a contact area6. The electrical connection between the contact pads7of the semiconductor die2and the contact areas6of the contact leads5of the leadframe3is formed by wire bonds9.

FIG. 5illustrates the molding process of the method according to the invention. The plurality of leadframes3is encapsulated by mold material10to form a molded leadframe module or panel14. The die2, die attach pad4, contact leads5, wire bonds9and areas between the contact leads5, die attach pads4and the upper surface of the carrier tape13of each of a plurality of leadframes3are encapsulated by a single mass of mold material10. The upper surface of the carrier tape13acts as the bottom surface of the mold. The mold material is then given an appropriate curing treatment.

FIG. 6illustrates the final process of the process in which the carrier tape13is removed from the molded leadframe module14. The bottom surface16of the molded leadframe module14includes mold material10and isolated areas having the bottom metal surfaces of the die attach pads4and isolated contact leads5on an essentially common surface. The individual non-leaded packages1are singulated from the molded leadframe module14by sawing through the mold material10between the individual leadframes3as indicated by the dotted lines15and saw blade18.

Contacting means, such as solder balls, are then attached to the bottom surface of the contact leads5and provide the external contacts from the non-leaded package1to, for example, a printed circuit board. Alternatively, the solder balls may be attached to the contact leads5while the package forms part of the molded leadframe module. The packages1are then tested, packages and then mounted on, for example, a printed circuit board.