Patent Application
20070138240
The present invention generally relates to electronic circuit packaging, and more particularly, to a method for joining electronic circuit substrates to lead frame material to form leadframe assemblies.
Electronic components and devices are employed in a wide variety of systems and applications. These devices are typically implemented as integrated circuits (ICs) that include multiple logical functions or sub-devices integrated into a single device. Because most electronic systems require the interconnection of a number of integrated circuits in order to perform a given function, leaded integrated circuit packages have been developed to facilitate electrical circuit interconnection. Examples of leaded integrated circuit packages include Quad Flat Pack (QFP), and Plastic Leaded Chip Carrier (PLCC). Various other package types exist.
The leads in leaded integrated circuit packages are designed to conduct electrical signals between the integrated circuit package and the electronic circuitry that is mounted in or on the integrated circuit package. In a typical package configuration, a semiconductor die is mounted on a copper leadframe having multiple leads. The leadframe may also be mounted on a substrate to provide, among other things, structural support and heat transfer capability. Electrical contacts on the semiconductor die are then connected to individual leads, typically by a wire-bonding process. After the wire-bonding process, the device is typically encapsulated in a material (such as plastic) to provide support and protection for the device, and to keep out moisture. The end result of the process is a device having leads capable of carrying electrical signals from the exterior pins or contacts of the device to the interior circuitry, or vice versa. Due to the small geometries involved, the process for making packaging structures onto which semiconductor devices are mounted can require a great deal of precision and can be quite expensive.
In integrated circuits having a leadframe attached to a substrate, the assembly is generally formed by a process that secures the leadframe to the substrate. In one conventional process for forming a leadframe assembly, a substrate material is provided. The substrate material is typically an electrically insulating material having a metallized coating on its surface. A metallic leadframe is then attached to the substrate by soldering. Prior to being attached to the substrate, the leadframe can be etched or stamped to form a pattern of leads and a die-mounting pad. In one exemplary soldering process, solder paste is deposited on the substrate, in a process called solder paste printing, before the leadframe is placed on the substrate. Next, a high-temperature solder reflow process melts the solder paste, securing the leadframe to the substrate. While this conventional soldering method for connecting a leadframe to a substrate does have utility, the nature of the solder paste and reflow process can make the joints connecting the leadframe to the substrate subject to failure due to cracking. When cracking of the joints occurs, the reliability of the device, and the system in which the device is used, can be negatively impacted.
In another conventional process for connecting a leadframe to a substrate, the leadframe is attached to the substrate by a brazing process. In this conventional process, a substrate, such as a ceramic, is first provided. The substrate could optionally be pre-scribed. The substrate material may have a metallized coating on its surface. Next, a brazing alloy is printed on various locations of the surface of the substrate. Copper leadframes are then placed on top of the areas of the substrate that have been printed with brazing alloy. Next, the entire structure (including the substrate, brazing alloy, and leadframes) is heated in an inert atmosphere, causing the brazing alloy to bond the leadframes to the surface of the substrate. Finally, individual devices are removed from the resulting structure by snapping or cutting the individual devices apart and scraping away adjacent substrate material underneath the leadframe.
This conventional brazing process often results in a device with higher reliability than a device made by the soldering process discussed above, due primarily to the elimination of the solder joints. In addition, devices made using the brazing process typically have better thermal and electrical conductivity than devices made using a soldering process. However, the conventional brazing process is often more expensive than the soldering process. This is due in part to the limited substrate size of certain substrate materials (typically no more than six inches long by six inches wide for ceramic substrates). Smaller substrates mean that fewer devices can be formed on a given substrate. The higher cost of the conventional process is also due to the fact that excess substrate material is frequently removed to form the circuit structures and leads, thereby wasting costly substrate material.
Although some conventional processes for connecting leadframes and substrates can be relatively low-cost, this low cost often comes at the expense of the reliability of the connections formed. While some processes offer more reliable connections and improved thermal and electrical conductivity, this improved reliability often comes at an increased cost. It is therefore desirable to provide for a cost-effective process for connecting leadframes and substrates that will result in leadframe assemblies having more reliable connections between the substrate and leadframe, and improved electrical and thermal conductivity.
In accordance with one aspect of the present invention, a method for forming a leadframe assembly is provided. The method includes the steps of providing a sheet comprising leadframe material, depositing a brazing alloy on a first surface of the sheet, and providing at least one substrate having a first surface that is primarily planar. The method also includes the steps of placing the first surface of the at least one substrate adjacent to the first surface of the sheet so that at least a portion of the substrate first surface overlaps and contacts the brazing alloy, and heating the brazing alloy to bond the substrate to the first surface of the sheet.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring to
In
In
The leadframe material sheet 10 and brazing alloy 12 are further shown provided with substrates 16 in
A fourth step of the method 100 is shown in
Next, the structure shown in
The opposite bottom planar surface of sheet 10 is shown in
After the etching step has been completed, individual leadframe assemblies 22 may be separated from sheet 10 by cutting, breaking, or etching, in a seventh step of the method 100.
It should be appreciated that various steps in the exemplary process illustrated in
Referring to
In one specific embodiment of the method 100, the leadframe material sheet 10 is made of copper, and has a length and width much greater than the thickness of the sheet 10. The copper sheet 10 is placed adjacent to a screen printing device, which then screen prints patterns of active brazing alloy 12 in multiple locations on a first surface of the copper sheet 10. Next, the screen printed copper sheet is positioned adjacent to an automated pick-and-place machine, which positions multiple ceramic substrates 16 above the first surface of the copper sheet 10, such that each substrate 16 at least partially overlaps brazing material 12.
The substrates 16 are then placed directly on the first surface of the copper sheet 10 by the pick-and-place machine, such that each substrate at least partially overlaps the patterned brazing material 12. The sheet 10 is then placed in a furnace, where it is heated until the brazing material 12 bonds the substrates 16 to the first surface of the copper sheet 10. After being removed from the oven, at least some of the copper sheet 10 is etched away to form leads 20 and expose portions of substrates 16, circuitry 17, and brazing alloy 12 to form circuitry. Finally, each individual leadframe assembly 22 is separated from the copper sheet 10 by cutting.
In another specific embodiment of the method 100, leadframe assemblies 22 are further processed prior to being separated from sheet 10 into individual leadframe assemblies. In yet another embodiment of method 100, sheet 10 is separated into manufacturable sections, each section containing multiple leadframe assemblies 22, for further processing (e.g., surface mounting of semiconductor devices onto assemblies 22), prior to separating individual leadframe assemblies 22 from copper sheet 10. For example, a thirty-six inch by thirty-six inch sheet of leadframe assemblies 22 could be divided into twelve inch by twelve inch sections, each containing leadframe assemblies 22. The separated twelve inch by twelve inch sections could then be further processed (e.g. surface mounting semiconductor devices onto assemblies 22) prior to separating individual leadframe assemblies 22 from each twelve inch by twelve inch section.
By using the method described above, leadframe assemblies can be formed without the use of less reliable solder joints. The method also enables cost-effective manufacturing of four-sided leadframe geometries. In addition, leadframe structures and circuit elements can be formed by etching away less expensive copper material, rather than by removing more expensive substrate material. The method also advantageously provides for cost-effective high-volume manufacturing of leadframe assemblies that offer reliable connections and contain multiple devices.
The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.
