Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – Of specified material other than copper
Reexamination Certificate
1998-10-27
2001-07-03
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
Of specified material other than copper
C257S666000, C257S766000, C257S764000
Reexamination Certificate
active
06255723
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to connection components useful in electrical assemblies such as in connecting semiconductor chips to substrates, and to methods of making such connection components.
BACKGROUND OF THE INVENTION
Semiconductor chips typically are connected to external circuitry through contacts on the surface of the chip. The contacts may be disposed in a grid on the front surface of the chip or in elongated rows extending along the edges of the chip's front surface. Each such contact must be connected to an external circuit element such as a circuit trace on a supporting substrate or circuit panel. The rapid evolution of the semiconductor art has created continued demand for incorporation of progressively greater numbers of contacts and leads in a given amount of space. With such closely spaced contacts, the leads connected to the contacts of the chip must be extremely fine structures, typically less than about 0.1 mm wide, disposed at center-to-center spacing of about 0.1 mm or less. Handling and connecting such fine, closely-spaced leads poses a formidable problem.
Leads are commonly bonded to contacts on semiconductor chips or other microelectronic elements by a process such as ultrasonic bonding or preferably thermocompression or thermosonic bonding. In the bonding process, the bonding region of each lead is engaged by a bonding tool which bears on the top surface of the lead in the bonding region and forces the lead downwardly into engagement with the contact. Energy supplied through the bonding tool causes the lead to joint with the contact.
After the lead is bent into a vertically-curved configuration and bonded to the semiconductor chip contacts, the region adjacent the bonding region is formed. This region is commonly referred to as the “heel” of the lead, i.e., the upwardly curving region close to the contacts on the bond side of the lead. The heel of the lead is typically the most fatigue-susceptible region of the lead. Other curved portions of the lead are also susceptible to fatigue.
Methods of making various lead connections involve deformation of the lead, forming curved portions in the leads. Examples include conventional tape automated bonding (“TAB”), and the methods disclosed in U.S. Pat. Nos. 5,489,749, 5,491,302, 5,629,239, and 5,518,964, the disclosures of which are hereby incorporated by reference herein. As further discussed in these patents, leads can be provided on dielectric layers having gaps such that the leads extend into or across the gaps. To form connections, the leads can be bent downwardly towards contacts on another surface. The leads have the bent configuration as depicted in the drawings. The leads may include a polymer layer. Preferably, the polymer layer is absent in the bond region, or any part of the bond region engaged by the bonding tool, to permit sufficient energy coupling between the tool and the bond interface. Combined metal and polymer lead structures are shown in the above-mentioned '749 Patent and in U.S. patent application Ser. No. 08/715,571, the disclosure of which is also hereby incorporated by reference herein.
As described in the aforementioned patents, the leads may extend on either side of the dielectric layer included in the support structure. Thus, the lead may extend on the top surface of the dielectric layer, remote from the chip or other element having contacts to which the leads are bonded. However, the lead may also extend across the dielectric layer on the bottom surface. Also, the support structure need not include a dielectric layer, but instead may include a metallic lead frame which is used to hold leads temporarily and which is removed from the leads during or after bonding.
Connection components formed as discussed above typically have a reduced fatigue life. It is therefore desirable to provide leads with a structure which reinforces the lead, particularly in the most fatigue-susceptible regions of the lead. The most fatigue-susceptible regions of the lead are those regions which were most distorted in the fabrication of the component. Reinforcing at least these regions enhances the fatigue life of the completed assembly. It is therefore desirable to provide a lead structure which resists distortion. It is also desirable to provide a lead structure which is reinforced against fatigue and which promotes more efficient coupling of energy between the bonding tool and the bond interface between the bottom of the lead and the contact. This in turn allows reduced bonding force, bonding energy and/or bonding time, or provides a stronger bond with the same force, energy and time so that connection components can be fabricated more economically.
SUMMARY
The present invention addresses these needs.
Leads, including flexible leads, used in semiconductor chip assemblies and other microelectronic assemblies can be provided with substantially enhanced fatigue resistance by providing a layer of a fatigue-resistant alloy. The lead typically includes a structural material which may be copper, gold, alloys of these metals or other metals. The lead is provided with a thin layer of the fatigue-resistant alloy. This layer desirably extends on the bonding or bottom side of the lead, i.e., the side of the lead which is bonded to a contact. The fatigue-resistant alloy layer may extend on other sides of the structural material. Most preferably, the layer of fatigue-resistant alloy is provided at least in the bond region, i.e., the region of the lead which is bonded to the contact in use. Preferably, the fatigue-resistant alloy is also provided in the adjacent region of the lead. This adjacent region forms which is commonly referred to as the “heel” of the bond, i.e., the region close to the contact on the bond side of the lead. The fatigue-resistant alloy optionally may extend on other parts of the lead as well. For example, it is advantageous to provide the fatigue resistant alloy in the “shoulder” region of the lead, or other bent regions.
Typically, the fatigue-resistant alloy does not provide an optimum surface on the bond side of the lead for engagement with the contact during the bonding process. Therefore, a layer of a readily bondable material such as gold, palladium or other metal compatible with the contact to which the lead is to be bonded is applied on the bond side of the lead covering the fatigue-resistant alloy at least in the area of the lead which will engage the contact during use.
The fatigue-resistant alloy and particularly nickel titanium alloys, can also serve as a diffusion barrier to retard diffusion of materials from the contact into the structural metal of the lead or vice-versa. Such diffusion can result in formation of brittle intermetallic compounds. Similarly, where the lead includes a structural material such as copper on one side of the fatigue-resistant metal and a more bondable material such as gold on the other side, the fatigue-resistant metal retards mingling of the two metals by diffusion prior to bonding.
Leads in accordance with another aspect of the present invention are provided with an asymmetrical distribution of bonding metal at least in the bond region of the lead. The lead incorporates a layer of a structural metal, desirably copper, copper-based alloy or other relatively low-cost metal. A first layer of a readily bondable metal is provided on the bottom surface of the structural metal layer. To provide the asymmetrical distribution of bonding material, the top surface of the structural metal layer may be devoid of the bonding metal in the bond region or else may have a second layer of bonding metal which is thinner than the first layer of bonding metal. Typically, the first layer of bonding metal is thinner than the structural metal. Preferably, a layer of bonding material surrounds the lead on all surfaces, as opposed to just the top and bottom surfaces of the layer of structural material.
A layer of a barrier metal which is adapted to retard alloying of the structural metal and the bonding metal by diffusion optionally may be provid
Baker David R.
DiStefano Thomas H.
Light David
Smith John W.
Wang Hung-Ming
Clark Sheila V.
Lerner David Littenberg Krumholz & Mentlik LLP
Tessera Inc.
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