Patent Application
20080157299
The invention relates to electronic semiconductor devices and manufacturing. More particularly, the invention relates to microelectronic semiconductor device package assemblies employing leadframes with cantilevered leads, and to methods for making chip-on-lead (COL) assemblies with cantilevered leads.
Microelectronic semiconductor device packages are subject to maximum size constraints at the system level, and minimum size constraints based on the semiconductor die, or chip, size. Of course, no package can be smaller than the chip itself. The maximum size of a chip that fits in a particular package varies with the package style. The leadframe provides mechanical support to the chip during its assembly into a finished product. Typically, the leadframe consists of a mounting pad, to which the chip is attached, and leads, which serve as the means for external electrical connection to the world outside the chip. The typical package is made by mounting a chip on an exposed mounting pad, leaving a necessary gap between the leads and the mounting pad. The planar area of the chip is generally less than that of the mounting pad by an amount determined by various manufacturing and reliability concerns. There is often also a minimum chip size for a given mounting pad size because the wires that extend from the chip to the leads must be limited in length for reasons of manufacturability and/or electrical performance. Since there is usually little flexibility in terms of combining mounting pads and chips of different sizes, typically a series of leadframes with various mounting pad sizes must be provided for use with various chip sizes.
In order to keep the bond wires that stretch from chip to lead short, cantilevered leads are known in the arts. Cantilevered leads, as heretofore practiced in the arts, are leads which project from the periphery of the leadframe inward toward a mounting pad location in the central region of the leadframe. A relatively small mounting pad designed for supporting a small chip is the norm. Significantly different chip sizes therefore require different leadframe designs with different cantilever lead lengths, each design requiring its own tooling and separate stocking. This lack of flexibility places practical cost limitations on the implementation of devices employing cantilevered leads.
Supporting the chip on the leads, called chip-on-lead or COL, is another design approach known in the arts. Chip-on-lead designs have historically been limited by the requirement that the leads be capable of withstanding the stresses generated by the chip attachment process. Generally, molded plastic packages using a COL design are limited to types that have full lead thicknesses, and not cantilevered leads, which typically have a thinner, partial-thickness inner portion. This is because, using common assembly processes, when the chip is pressed onto the leads, full thickness leads are supported from below across their whole area by the flat surface on which they are placed. Thus, the force of placing the chip on the leads is resisted by the full-thickness leads, whereas partial-thickness, cantilevered leads would have a tendency to bend. The cantilever leads used in the so-called leadless molded package type are not supported by an underlying flat surface. In most cases, the leads are not rigid enough to avoid bending when subjected to pressure during chip mounting processes.
Due to these and other technical challenges, improved packaged chip-on-lead (COL) semiconductor device assemblies and methods for their manufacture would be useful and advantageous in the arts. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems described above.
In carrying out the principles of the present invention, in accordance with preferred embodiments thereof, packaged microelectronic semiconductor devices and methods for their assembly use cantilevered leads in a chip-on-lead configuration. The cantilevered leads extend from the periphery of a leadframe toward a central region where a chip is affixed directly to the cantilevered leads. Temporary bracing preferably supports the cantilevered leads during chip mounting when needed.
According to one aspect of the invention, a method for assembling a packaged microelectronic semiconductor device includes steps of providing a leadframe that has numerous cantilevered leads extending from the periphery toward a central region. A chip is affixed to the central region of the leadframe on a plurality of the cantilevered leads.
According to another aspect of the invention, according to a preferred embodiment, a method for assembling a packaged semiconductor device also includes the step of providing tie bars extending from the outer corners of the leadframe supporting a mounting pad located in the central region of the leadframe. A chip is affixed to both the mounting pad and to a plurality of the cantilevered leads.
According to yet other aspects of the invention, in preferred embodiments, steps include the attachment of multiple chips to the mounting pad and cantilevered leads.
According to another aspect of the invention, a microelectronic semiconductor device package assembly has a leadframe with a plurality of cantilevered leads extending from the periphery of the package toward a central region and a chip attached to the cantilevered leads.
According to another aspect of the invention, a package assembly according to preferred embodiments has more than one chip affixed to cantilevered leads in the central region of a leadframe.
According to another aspect of the invention, a microelectronic semiconductor device package assembly includes a leadframe having tie bars supporting a mounting pad in the central region of the leadframe. The mounting pad is smaller in area than the chip, and is adjacent to cantilevered leads. One or more chips are affixed both to the mounting pad and to a number of the cantilevered leads.
The invention has numerous advantages including but not limited to providing methods and packaged semiconductor device assemblies offering one or more of the following; accommodating the use of a single leadframe for multiple die sizes while allowing similar wire lengths for the multiple die sizes, improved efficiency in the assembly process, and reduced costs. These and other features, advantages, and benefits of the present invention can be understood by one of ordinary skill in the arts upon careful consideration of the detailed description of representative embodiments of the invention in connection with the accompanying drawings.
The present invention will be more clearly understood from consideration of the following detailed description and drawings in which:
References in the detailed description correspond to like references in the various Figures unless otherwise noted. Descriptive and directional terms used in the written description such as first, second, top, bottom, upper, side, etc., refer to the drawings themselves as laid out on the paper and not to physical limitations of the invention unless specifically noted. The drawings are not to scale, and some features of embodiments shown and discussed are simplified or amplified for illustrating the principles, features, and advantages of the invention.
In general, the invention provides packaged microelectronic semiconductor devices and methods for their assembly using cantilevered leads in a chip-on-lead configuration. Referring primarily to
Now referring primarily to
In some instances, in preferred methods of assembling a COL package on cantilevered leads in accordance with the invention, a temporary brace may be used under the cantilevered leads. Again referring primarily to
Another example of a preferred embodiment of the invention is shown in
The invention provides advantages including but not limited to one or more of the following: permitting the mounting of chips larger in area than the maximum possible pad size in any given package size, allowing a greater range of chip sizes to be used with a given leadframe; permitting a reduction in the chip mounting pad area required for packaging associated IC components; increasing assembly and design process efficiency; and, reducing costs. While the invention has been described with reference to certain illustrative embodiments, the methods and systems described are not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the art upon reference to the description and claims.
