Methods for making semiconductor packages with leadframe grid arrays

#The present invention relates to grid array semiconductor packages and methods of assembling and evaluating the same. In particular, the present invention relates to leadframes for mounting a semiconductor chip for encapsulating to form a complete semiconductor package. The leadframe includes a plurality of leads having a similar length, with offset array pads forming a grid array on the surface of the package.

#Semiconductor chips or dies are typically enclosed in semiconductor assemblies, or packages, prior to use. These packages protect chips from the conditions of the surrounding environment and provide leads or other connection points, allowing a chip to be electrically accessed. Packages have typically included a semiconductor chip bonded to a leadframe, either seated on a die paddle or directly to the leads in a leads-over-chip (“LOC”) attachment. The contact pads on the semiconductor die are then electrically connected to the chip by wires in wirebond fashion. The connected leadframe and chip are then placed in a mold cavity and encapsulated in a mold compound to form a complete package. The leads extend out from the mold compound, allowing the chip to be electrically accessed. Typically, the leads extend laterally from the package in a flat array, which may be trimmed and formed into a desired conformation.

#As electronic devices have decreased in size, alternative methods of assembling and packaging semiconductor dies have been used. These methods decrease the “real estate” or area that is required to install the die on higher-level packaging, such as a printed circuit board. Flip-chip installation of a chip using a ball grid array (“BGA”) reduces the real estate used to an area the same as or only slightly larger than the chip dimensions, but introduces a number of difficulties and shortcomings into the manufacturing process. Attempts have been made in the art to provide a semiconductor assembly that includes the benefits of a flip-chip type of attachment while keeping the benefits of a conventional molded package.

#In the drawings, which depict the best mode presently known for carrying out the invention:

#FIG. 1 is a cutaway perspective view of one embodiment of a semiconductor assembly made in accordance with the principles of the present invention.

#FIG. 1A is a view of an alternative embodiment of an array offset in accordance with the principles of the present invention.

#Reference will now be made to FIGS. 1 and 2. FIG. 1 depicts a perspective cutaway view of one embodiment of a semiconductor assembly 10 made in accordance with the principles of the present invention. FIG. 2 depicts a sectional side view of a section of the embodiment of FIG. 1. A semiconductor assembly 10 includes a semiconductor chip 12 attached in leads-over-chip (“LOC”) fashion to a leadframe 14 by an adhesive element 16. In the depicted embodiment, the adhesive element 16 is an adhesive strip, such as a double-sided adhesive polyimide tape such as a KAPTON™ tape, but it will be appreciated that any suitable adhesive may be used, including a liquid or gel adhesive.

#Leadframe 14 includes a plurality of leads 18, such as leads 18A and 18B. Groups of leads 18 are organized into lead sets, such as first lead set 15 and a second lead set 17 on opposing sides of the longitudinal axis L of the semiconductor chip 12. Within a first lead set 15, the leads 18 have substantially similar lengths, running from a side 11 of the assembly 10 toward the longitudinal axis L. Each lead 18 comprises a lead shaft 20 that generally runs within a first common plane P1. Along its length the lead shaft 20 includes an offset 22 formed as the lead shaft 20 extends out of the first common plane and then returns to the first common plane. The offset 22 may include an array pad 24 having a flat surface, although any suitable array pad design may be used. The array pad may vary in width or shape from the remainder of the lead. For example, FIG. 1A shows an assembly 10A including leads with circular array pads 24A, which can be useful for forming solder balls therein. Desirably, the array pads 24 of the various leads 18 lie within a second common plane, although small variations such as two or more separate common planes may be used for specific applications. It will be appreciated that the leadframe may be constructed of any suitable material known presently, or in the future, to those skilled in the art, including aluminum, copper and alloys thereof as well as ferrous alloys.

#In the embodiment depicted in FIG. 1, leads 18 protrude from side 11 of molding compound 26 of semiconductor assembly 10. In fabricating assemblies 10 in accordance with the principles of the present invention, it may be advantageous to produce leadframe 14 as one of a number of leadframes 14 on a strip. A number of assemblies 10 may be fabricated on the strip, which assemblies 10 are then separated by cutting the strip. Protruding lead ends 19 of leads 18 may result from such a procedure. Alternatively, lead ends 19 may be trimmed flush at side 11. Embodiments where the lead ends 19 are enclosed within the molding compound 26 are also contemplated as within the scope of the invention, but may be somewhat more difficult to fabricate. In addition to providing electrical connection to chip 12, leads 18 may also act to conduct heat from assembly 10 during operation. The leads 18 are all of substantially similar length, and two opposite lead sets 15 and 17 extend across the majority of the active surface of the chip 12 to be in contact with and accessible to bond pads thereon. Exposed lead ends 19 increase the ability of leads 18 to conduct heat from the assembly 10, increasing the potential functional life of the assembly 10.