Patent Publication Number: US-2010123258-A1

Title: Low Temperature Board Level Assembly Using Anisotropically Conductive Materials

Description:
BACKGROUND 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an enlarged, cross-sectional view of an integrated circuit in the process of being secured to a board in accordance with one embodiment; 
         FIG. 2  is an enlarged, cross-sectional view at a subsequent stage; 
         FIG. 3  is an enlarged, cross-sectional view at still a subsequent stage; 
         FIG. 4  is a greatly enlarged, cross-sectional view of the two components after joinder in accordance with one embodiment; and 
         FIG. 5  is a greatly enlarged, partial depiction of the connection between a solder ball and a pad in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     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  FIG. 1 , an integrated circuit  10 , in accordance with one embodiment, has protrusions  16  formed on a lower surface thereof. In one embodiment, the protrusions  16  may be solder balls. An interconnection layer  14  may include metallic layers to make connections between components within the integrated circuit chip  12  and the individual protrusions  16 . 
     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  FIG. 2 , the protrusions  16  are aligned with the contacts  22  using conventional techniques such as pick and place equipment. 
     Then, as shown in  FIG. 3 , compressive pressure is applied between the integrated circuit  10  and the substrate  20 . As a result of this compression, the anisotropically conductive adhesive  18 , which was originally non-conductive, becomes conductive. It becomes conductive because the individual metallic particles within the anisotropically conductive adhesive become trapped between the protrusion  16  and the contact  22 , as best shown in  FIG. 5 . 
     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&#39;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  FIG. 4 , the structure may include a substrate  20  coupled to an integrated circuit chip  12 . In one embodiment, the integrated circuit chip  12  is a phase change memory. Electrical connections may be made from the integrated circuit chip to the substrate  20  by attaching the integrated circuit chip  12  to a first surface  42  of an interposer  24 , wherein the interposer has at least one routing trace  44  extending from said interposer first surface  42  to an opposing interposer second surface  46 . Electrical connections are formed between the integrated circuit chip  12  to at least one interposer routing trace  44  on the interposer first surface  42 , such as through wire bonds  23 . Electrical connections are formed between the routing trace  44  on the interposer second surface  46  and the substrate contacts  22  through the protrusions  16  and the conductive particles  30 . 
     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.