Glob top encapsulation using molding tape

A lead frame used to assemble a semiconductor device, such as a smart card, has a first major surface including exposed leads and a second major surface including a die receiving area and one or more connection pads surrounding the die receiving area. The connection pads enable electrical connection of an Integrated Circuit (IC) die to the exposed leads. A molding tape sized and shaped like the lead frame is adhered to and covers the second major surface of the lead frame. The molding tape has a die receiving area cut-out that exposes the die receiving area and the connection pads on the second major surface of the lead frame and forms a cavity for receiving an encapsulant. The cut-out has an elevated sidewall for retaining the encapsulant within the cavity.

BACKGROUND

The present invention relates generally to techniques for assembling integrated circuit (IC) devices and, more particularly, to techniques performing a molding or encapsulating process in IC device assembly.

Encapsulation is a standard semiconductor process used to provide physical, chemical, and electrical protection to an IC die. Typically, the encapsulation process covers the entire surface of the die as well as any electrical connections to the die. One encapsulation process commonly used for smart card assembly, for example, is glob top molding, which is an inexpensive and easily automated process. In glob top molding, there is a single resin glob top process and a two-resin, “Dam and Fill” process. In the single resin process, a thixotropic glob top material is dispensed over the die and wirebonds and cured, while in the dam and fill process, a highly thixotropic “dam” material is dispensed around the area to be encapsulated and then the “dam” is filled with a lower viscosity material that covers the die and wirebonds.

FIG. 1Ais a top plan view of a conventional lead frame10used in smart card assembly. The lead frame10has a first side with exposed metal contacts12.FIG. 1Bis a bottom view of the lead frame10, which has an epoxy surface14, conductive traces that form an antenna16and holes18that are used to provide a path to the metal contacts12. There also is a die receiving area20located within an area defined by the holes10.FIG. 1Cshows the bottom side of the lead frame10after a die has been attached and electrically connected to the contacts12with bond wires and then covered with an encapsulant22. However, as can be seen, after dispensing, the size and shape of the glob top material, i.e., the encapsulant22has not been controlled very well, and as shown in the inset, one of the bond wires24is exposed.

Both the single resin and two resin glob top processes often encounter problems like the one shown inFIG. 1C, where the size and shape of the glob top is not uniform or well controlled, because the low viscosity resin material is not easy to control. Some other issues encountered are incomplete glob top, exposed die, exposed wire, glob top dimensions do not conform to package outline specifications, and the glob top may be offset.

Accordingly, it would be advantageous to have an improved glop top encapsulation process that overcomes the above-listed defects.

DETAILED DESCRIPTION

Detailed illustrative embodiments of the invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the invention. The invention may be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention.

In one embodiment, the present invention is a lead frame having a first major surface with a plurality of exposed contacts and a second major surface having a die receiving area and one or more connection pads surrounding the die receiving area. The connection pads enable electrical connection of an Integrated Circuit (IC) die to the exposed contacts. A molding tape sized and shaped like the lead frame is adhered to and covers the second major surface of the lead frame. There is a die receiving area cut-out in the molding tape that exposes the die receiving area and the connection pads on the second major surface of the lead frame and forms a cavity for receiving an encapsulant. The cut-out includes an elevated sidewall for retaining the encapsulant within the cavity.

In another embodiment, the present invention is a smart card comprising a lead frame, a molding tape, an IC die and an encapsulant. The lead frame has a first major surface with a plurality of exposed contacts and a second major surface having a die receiving area and one or more connection pads surrounding the die receiving area. The connection pads enable electrical connection of the IC die to the exposed contacts. The molding tape is sized and shaped like the lead frame, and is adhered to and covers the second major surface of the lead frame. The molding tape includes a die receiving area cut-out that exposes the die receiving area and the connection pads on the second major surface of the lead frame and forms a cavity for receiving an encapsulant. The cut-out includes an elevated sidewall for retaining the encapsulant within the cavity. The IC die is attached to the die receiving area and bonding pads on an upper surface of the IC die are electrically connected with respective ones of the connection pads using bond wires such that the IC die is electrically connected to the exposed contacts. The encapsulant is dispensed within the cavity and covers the IC die and the bond wires and thereby forms a glob top. The elevated sidewall prevents the encapsulant from spilling out of the cavity and onto the second major surface of the lead frame.

In yet another embodiment, the present invention provides a method of assembling a smart card. The method includes attaching an IC die to a die receiving area of a lead frame, where the lead frame has a first major surface with a plurality of exposed contacts and a second major surface having the die receiving area and one or more connection pads surrounding the die receiving area. The connection pads enable electrical connection of the IC die to the exposed contacts. A molding tape sized and shaped like the lead frame is adhered to and covers the second major surface of the lead frame. The molding tape includes a die receiving area cut-out that exposes the die receiving area and the connection pads on the second major surface of the lead frame and forms a cavity for receiving an encapsulant. The cut-out includes an elevated sidewall for retaining the encapsulant within the cavity. The method further includes electrically connecting the IC die with the exposed contacts by way of the connection pads and dispensing a liquid encapsulant into the cavity such that the encapsulant covers the IC die and the electrical connections of the IC die to the connection pads.

Referring now toFIGS. 2A and 2B, top and bottom views of a lead frame assembly100in accordance with an embodiment of the present invention are shown. The lead frame assembly100includes a lead frame, such as the lead frame10shown in FIGS.1A and1B, and a molding tape.FIG. 2Ashows a contact side of the lead frame assembly100. The contact side comprises a first major surface of the assembly100and has a plurality of exposed contacts102. The lead frame and lead frame assembly100also have a plurality of holes104that are used to position the lead frame and lead frame assembly during processes such as die attach and wire bonding.

FIG. 2Bshows an encapsulation side of the lead frame assembly100, which is a second major surface of the lead frame like the conventional lead frame10shown inFIG. 1Bexcept that here, the second major surface is covered with a molding tape106. The molding tape106, which is sized and shaped like the lead frame, has an adhesive on one side such that it adheres to the second major surface of the lead frame. The molding tape106includes a die receiving area cut-out that exposes a die receiving area108and connection pads110on the second major surface of the lead frame. The die receiving area cut-out also forms a cavity for receiving an encapsulant, as will be discussed in more detail below. Also, as will be discussed in more detail below, the die receiving area cut-out includes an elevated sidewall for retaining the encapsulant within the cavity.

The connection pads110enable electrical connection of an IC die to the exposed contacts102on the contact side of the lead frame. In some embodiments, the lead frame includes an antenna112and in such embodiment, the molding tape106includes another cut-out to expose the antenna.

The lead frame may be a multi-layer lead frame, for instance, comprising a layer of conductive metal, one or more layers of non-conductive material, and conductive metal traces. In the lead frame assembly100, the first major surface comprises exposed conductive metal and the second major surface comprises a non-conductive material, such as epoxy resin and/or epoxy glass. The conductive metal typically comprises Copper that is plated with other metals such as Nickel, Gold and/or Palladium.

FIG. 2Cshows an IC die114attached to the die receiving area108and electrically connected to the connection pads110with bond wires116, such that the die114is in electrical communication with the exposed contacts102. An encapsulant118, which is dispensed within the cavity, covers the IC die114and the bond wires116and forms a glob top. In the drawings, the encapsulant118is shown as transparent, but this is just so that the underlying components of the assembly may be visualized. The encapsulant118typically is non-transparent. The elevated sidewall of the cut-out prevents the encapsulant118from spilling out of the cavity and onto the second major surface of the lead frame.

FIG. 3Ashows the molding tape106being peeled off the encapsulation side of the assembly100andFIG. 3Bshows just the molding tape106. In the embodiment shown inFIGS. 3A and 3B, the lead frame does not include an antenna, so the molding tape does not include a corresponding cut-out for an antenna. Thus, in this embodiment, the molding tape106includes a die receiving area cut-out120and a cut-out sidewall122. InFIG. 3A, it can be seen that the placement of the encapsulant118is controlled by the molding tape106, with the molding tape106preventing the encapsulant118from spilling onto the second major surface of the lead frame.

FIG. 3Cis a greatly enlarged view of the elevated sidewall122, which in this embodiment, is shown in the inset to be L-shaped cross-section. The molding tape106has a first height labeled C, the sidewall has a second height labeled B, and the sidewall has a thickness labeled A. In the presently preferred embodiment, the second height B is about twice the first height C, and twice the thickness A. In one example, the first height C is 0.05 mm or greater, the second height B is 0.1 mm or greater, and the thickness A is 0.05 mm or greater.

FIGS. 4A, 4B and 4Cshow alternative embodiments of the sidewall122. InFIG. 4A, the angle formed where the sidewall122meets the body of the tape is curved or beveled instead of at 90°. InFIG. 4B, the angle formed where the sidewall122meets the body of the tape is greater than 90°, such that there is a slope between the sidewall and the main body of the tape. InFIG. 4C, although the angle formed where the sidewall122meets the body of the tape is around 90°, the inner part of the sidewall122that is exposed at the cavity is angled such that where the tape106meets the encapsulation side of the lead frame near to the die receiving area has an acute angle. In other words, an inner wall of the molding tape that defines the cavity is slanted.

FIGS. 5A and 5Bare perspective views of lead frames with a molding tape partially peeled away (for ease of understanding the invention), where inFIG. 5Athe die receiving area cut-out130is rectangular and inFIG. 5Bthe die receiving area cut-out132is hexagonal. It should be apparent that the molding tape is used to control the encapsulation shape and size on the encapsulation side of the lead frame.

FIG. 6illustrates a method of assembling a smart card in accordance with an embodiment of the present invention. Starting at the upper left side ofFIG. 6, in a first step150, a lead frame for assembling a smart card is provided. The lead frame is similar to a conventional smart card lead frame and has a first side152with exposed contacts and a second, encapsulation side, except that the encapsulation side is covered with a molding tape154. The molding tape154is sized and shaped like the lead frame and is adhered to and covers the second major surface of the lead frame. The molding tape154includes a die receiving area cut-out that exposes the die receiving area and the connection pads on the second major surface of the lead frame and forms a cavity for receiving an encapsulant. The cut-out includes an elevated sidewall for retaining the encapsulant within the cavity. In one embodiment, the tape is applied to the lead frame by a lead frame supplier when the lead frame is manufactured.

At step156, an adhesive or die attach film (DAF) is placed in the die receiving area on the encapsulation side of the lead frame, and at step158a die is attached in the die receiving area using the adhesive or DAF. The adhesive is cured so that the die is securely attached to the lead frame. At step160, the die is electrically connected to the connection pads on the lead frame. In the presently preferred embodiment, a standard wire bonding process is used to attach bond wires to die bonding pads and respective lead frame connection pads. At step162, a glob top encapsulation process is performed whereby a liquid encapsulant164is dispensed into the cavity such that the encapsulant covers the IC die and the electrical connections (i.e., the bond wires) of the IC die to the connection pads. The encapsulant164is retained within the cavity by the elevated sidewalls so that the encapsulant does not spill over onto the second major surface of the lead frame. At step168, which is optional, the molding tape may be removed. That is, the tape can be removed when the encapsulation material is completely cured. The tape can be removed by a peeling tape machine, which is a conventional device well known by those of skill in the art in semiconductor device assembly. Of course, the tape also could be removed other ways, such as manually. Finally, at step170, a finished device is expected.

FIG. 7Ais a perspective view of a lead frame strip180with a molding tape182adhered to one side of the lead frame strip, andFIG. 7Bis a top plan view of a lead frame strip184that comprises the lead frame strip180and the molding tape182. Thus, it will be apparent to those of skill in the art that the invention is practiced on multiple lead frames simultaneously.

As will be apparent to those of skill in the art, the concept of the present invention is to attach a molding tape with guiding holes and a cavity with elevated sidewalls to control better either a single or two-resin glob top encapsulation operation to better control the resulting size and shape of the glob top. In one embodiment, the molding tape is removed after temperature and UV curing. It will be understood by those of skill in the art that the size and shape of the tape can vary depending on the size and shape of the lead frame and the die(s) to be attached to the lead frame. Use of the tape allows low viscosity or liquid materials to be applied to a smartcard package, or other types of packages, such as MEMS packages. The molding tape is sized to conform to the outline of the finished package.

The molding tape allows for vastly improved shape, size and thickness for glob top encapsulation. The cut-out may be circular, rectangular, triangular or even a complex shape for a desired glob top process.

It will be further understood that various changes in the details, materials, and arrangements of the parts that have been described and illustrated in order to explain embodiments of this invention may be made by those skilled in the art without departing from embodiments of the invention encompassed by the following claims.