Patent Application
20100123258
This relates generally to securing components to substrates. In the formation of electronic devices, integrated circuits are typically coupled to a substrate, such as printed circuit boards, flexible film interconnects, sockets, and the like.
In the formation of electronic devices, integrated circuits are typically coupled to substrates. Techniques for joining components to substrates include surface mounting, soldering, and frictional connections. Soldering involves the application of relatively high heat to join contacts using solder. Surface mounting also involves temperatures above 180° C. and the softening of solder-like materials to cause heat-based joints. Frictional connections involve the use of pins or other mechanical components which frictionally engage sockets or the like.
Each of these connection techniques has disadvantages in terms of board level reliability. Surface mount techniques may be subject to thermally induced cracking or cracking due to dropping the component. Solder techniques involve sufficiently high temperatures that may cause damage to some components to be joined. Frictional securement may raise reliability problems because the joints may come undone.
In accordance with some embodiments, integrated circuits may be mounted on substrates using an anisotropically conductive adhesive. An anisotropically conductive adhesive is a material that, prior to the application of pressure, is non-conductive. It may be formed of a non-conductive resin base with electrically conductive particles dispersed therein. The size and concentration of the electrically conductive particles is tailored so that normally the material is not conductive prior to the application of pressure. This means that the conductive particles do not touch prior to the application of compressive force to two components to be joined.
In accordance with some embodiments, the resin system used in the anisotropically conductive adhesive is one that cures at a temperature of 150° C. or less. Particularly, it is desirable for the cure to occur at a temperature of 150° C. or less, in less than 15 seconds or at still lower temperatures even if longer cure times are needed. The use of low temperature and anisotropically conductive materials means that the board level interconnects can be accomplished in a fashion that reduces the heat related damage to the components to be joined in some embodiments. This makes the technique particularly amenable to joining integrated circuits that are heat sensitive, such as phase change memories, to substrates.
The type of electrical contact between the integrated circuit and the board is subject to considerable variation.
For example, solder balls, bumps, or protrusions may be used on one component that mate with planar lands or other structures on the other component. Generally, it is advantageous to have a protrusion on one component and a flat surface to be joined on the other component. However, the present invention is not so limited.
Referring to
The integrated circuit 10 may be generally aligned over a substrate 20. In one embodiment, the substrate 20 includes flat land type contacts 22. A layer of anisotropically conductive adhesive 18 is formed over the pertinent lands 22. In one embodiment, the anisotropically conductive adhesive is an anisotropically conductive film that has been secured over the substrate 20.
As another embodiment, an anisotropically conductive paste may be utilized. Anisotropic paste may be applied by screen printing or other deposition techniques. Alternatively, the structure may be dipped into a bath of anisotropically conductive adhesive. As still another alternative, ink jet printing may be utilized to apply the anisotropically conductive adhesive. Other techniques may be utilized as well.
It is advantageous, in some embodiments, that the anisotropically conductive adhesive 18 be applied to the contacts 22 prior to the time that the integrated circuit 10 is joined to the substrate 20.
Next, as shown in
Then, as shown in
Specifically, the conductive particles 30 become lodged in the interface and remain as a rigid separators between the protrusions 16 and the contacts 22. The surrounding resin 32 extrudes out from between the pressurized, confined interface, leaving only the rigid conductive particles 30 to form the electrically conductive joint between the protrusions 16 and the contacts 22.
In accordance with some embodiments, the particles 30 may be conductive particles having dimensions on the order of two to ten microns. They may be nickel particles in one embodiment. Alternatively, gold-coated, nickel particles may be utilized. As still other examples, a polymer core with a nickel finish may be utilized or, as yet another alternative, a nickel gold finish may be utilized.
The resin material 32 is generally a non-conductive adhesive that is activated at a temperature below 150° C. in less than 15 seconds (or longer at lower temperatures). One suitable resin is acrylic resin using monomers containing an acryloxy group or a methacryloxy group, together with peroxide curing agents, having a viscosity of 10 to 100 Pa·s at room temperature. The cured resin may have a Young's modulus from 500 MPa to 10 GPa, and a coefficient of thermal expansion from 20 ppm to 100 ppm below the glass transition temperature Tg in some embodiments. The acrylic resin may include major components of base resin, curing agents, catalysts and coupling agents. In one embodiment, the concentration of metallic particles within the resin is from about 1 wt % to about 10 wt % of the polymer resin. The adhesive bond may have a die shear strength of greater than 0.5 MPa in one embodiment. As another example, epoxy polymers may also be used.
In some embodiments, the coefficient of thermal expansion of the particles may generally match that of the components to be joined such as the protrusions 16 and the contacts 22.
Without being limited by theory, it is believed that the use of an anisotropically conductive adhesive that is activated at temperatures below 150° C. may result, in some embodiments, in better board level reliability. This may be due to one or more of the reduction in joining temperature, the compliance or resilience of the resin material, the strength of the resin bond, and the conductivity achieved by the particles 30.
Thus, referring to
References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
