One type of semiconductor package is referred to as a BGA package. BGA packages were developed to provide a higher lead count, and a smaller foot print, than conventional plastic or ceramic semiconductor packages. A BGA package includes an array of external ball contacts, such as solder balls, that permit the package to be surface mounted to a printed circuit board (PCB) or other electronic component. Some BGA packages have a foot print that is about the size of the die contained in the package. These BGA packages are also known as chip scale packages.
As BGA packages are made smaller, and with higher lead counts, the electrical connections between the die and the ball contacts for the package become more difficult to make. The component of the package that makes the electrical connections between the die and package is sometimes referred to as an interconnect.
One type of prior art BGA package uses an interconnect in the form of a multi layered polymer tape, such as TAB tape. The polymer tape can be provided as a "strip" or "coupon" of material for packaging several dice at a time. Typically, the polymer tape comprises a polyimide substrate having patterns of metal conductors formed thereon. In addition, portions of the conductors can comprise beam leads, formed in a configuration which allows the beam leads to be bonded to the bond pads on the die.
FIG. 1 illustrates a prior art BGA package 10. The BGA package 10 includes: a semiconductor die 12; a polymer tape 14 bonded to a face of the die 12; and an encapsulant 16 bonded to the face and sides of the die 12. The BGA package 10 also includes an adhesive layer 18 for bonding the polymer tape 14 to the die 12. In addition, the BGA package 10 includes a dense array of ball contacts 20 formed on the polymer tape 14. A solder mask 15 locates and electrically insulates the ball contacts 20 from one another.
The polymer tape 14 includes a pattern of metal conductors 21 that form separate electrical paths between bond pads 24 on the die 12 and the ball contacts 20. The conductors 36 include ball bonding pads 23 that are bonded to the ball contacts 20, and beam leads 22 that are bonded to the bond pads 24 on the die 12. The beam leads 22 are also encapsulated in the encapsulant 16.
A representative process for forming the BGA package 10 includes the initial step of bonding one or more dice 12 to a strip of the polymer tape 14. The beam leads 22 can then be bonded to the device bond pads 24. Next, the encapsulant 16 can be formed, and the ball contacts 20 bonded to the ball bonding pads 23. The individual BGA packages 10 can then be singulated from the strip of polymer tape 14 and tested.
Typically, a thermosonic bonding process, using gold, or gold plated materials, is employed to bond the beam leads 22 to the bond pads 24. Specialized bonding tools are required to make the bonds between the beam leads 22 and the bond pads 24. In addition, the beam leads 22 are subjected to stresses from the bonding and encapsulation processes, and during subsequent use of the BGA package 10. These stresses can cause the bonds to weaken and the beam leads to separate from the bond pads 24.
Because of these and other deficiencies in conventional BGA packages, improvements in BGA packages, and in methods for fabricating BGA packages, would be welcomed in the art. The present invention is directed to an improved interconnect for constructing BGA packages. The interconnect is constructed to facilitate formation of bonded connections with the die, and to provide improved bonded connections with the die.