Abstract:
A method of encapsulating materials to protect circuit components from the stress of thermal cycling includes applying a first substance to cover Wire bonds on a first layer, applying a second substance to wire bonds on a second layer, and curing the first and second substances by application of heat or radiation to bond the first and second substances together. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or the meaning of the claims.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims priority to the U. S. provisional patent application titled “Method of Encapsulating Materials to Reduce Thermal Cycling Stress” and having serial No. 60/646,801, which is hereby incorporated by reference in its entirety as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to optical, image projection and multi-component opto-electronic systems. Specifically, the present invention relates to the encapsulation of circuit components of such systems to reduce mechanical stress from thermal cycling.  
       BACKGROUND OF THE INVENTION  
       [0003]     In optical, imaging and multi-component opto-electronic systems, issues often arise with damage to certain circuit components such as wire bonds resulting from thermal cycling. Stress from thermal cycling causes mechanical stress on these circuit components due to differences in thermal characteristics of materials. When the circuit components are subjected to heating and/or cooling cycles, these differences cause mechanical stress, which when coupled with normal mechanical stresses encountered in handling results in damage to the wire bond. Damage to circuit components results in losses in signal integrity and in optical distortion in optical and imaging systems.  
         [0004]     Prior art encapsulation of circuit components uses one material as an encapsulant that is stress-matched to one of the components being assembled. Typically, this encapsulant is a hard or flexible material. However, net thermal cycling stress remains a problem because of additional components that must be encapsulated, and which are made and and/or mounted on different materials.  
       SUMMARY OF THE INVENTION  
       [0005]     Circuit assemblies having encapsulated wire bonds are implemented in micro displays in systems such as high definition television, liquid crystal on silicon (LCOS) displays, and other high performance optical or image projection systems. Rigid encapsulation is a necessary part of packaging to protect fragile opto-electronic components, such as wire bonds, from the elements and from handling.  
         [0006]     Thermal conditions under normal operating cycles often result in degradation of optical performance due to stress-induced distortion or birefringence caused by a thermal coefficient of expansion mismatch among various components and encapsulation materials involved. To reduce this stress, two or more encapsulation substances, at least one of which is a pliable material, are applied to surfaces of wire bonds on joining components to encapsulate them. This protects the components while providing a flexible interface that will reduce the effects of thermo-mechanical stresses.  
         [0007]     The foregoing and other aspects of the present invention will be apparent from the following detailed description of the embodiments, which makes reference to the several figures of the drawings as listed below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a perspective diagram of components of a circuit assembly according to one embodiment of the present invention;  
         [0009]      FIG. 2  is a side diagram of components of a circuit assembly according to the embodiment shown in  FIG. 1 ;  
         [0010]      FIG. 3  is a perspective diagram of components of a circuit assembly according to another embodiment of the present invention; and  
         [0011]      FIG. 4  is a side diagram of components of a circuit assembly according to the embodiment shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0012]     In the following description of the present invention reference is made to the accompanying drawings which form a part thereof, and in which is shown, by way of illustration, exemplary embodiments illustrating the principles of the present invention and how it may be practiced. It is to be understood that other embodiments may be utilized to practice the present invention and structural and functional changes may be made thereto without departing from the scope of the present invention.  
         [0013]     Thermal conditions under normal operating cycles often result in degradation of optical performance due to stress-induced distortion or birefringence caused by a thermal coefficient of expansion mismatch among various components and encapsulation materials involved. The present invention therefore contemplates that two or more encapsulation substances are applied to surfaces of wire bonds on joining components to encapsulate them and protect the components, while providing a flexible interface that will reduce the effects of thermo-mechanical stresses.  
         [0014]     Without conventional hard encapsulation of wire bonds, optical, imaging and multi-component opto-electronic systems that consist of two or more components requiring wire bond interconnection suffer from physical distortion, which can translate to optical distortion. In liquid crystal micro-displays, optical distortion occurs when the gap spacing between silicon and glass substrates, separated by some uniformly-sized spacer to define the distance between the two substrates, causes physically distortion relative to each other. This spacing is distorted by physical stress from the mismatched thermal coefficients of expansion of the materials systems. This stress-related optical distortion is typically measured as 1 micron of random distortion, or approximately 2 waves (frequency cycles). By encapsulating the wire bonds and wire connecting loop according the present invention, the optical distortion is reduced to a negligible amount, or less than 0.25 frequency cycles.  
         [0015]     On the other hand, when only a soft encapsulant, such as RTV (room temperature vulcanizing) is used to encapsulate the wire bonds of multi-component optoelectronic systems, the reliability of the wire bonds suffers due to the flexibility of the encapsulation. While providing protection against environmental elements such as dust and moisture, flexible encapsulation by itself allows the bonded joints to weaken during thermal cycles by allowing the joints to bend and stretch.  
         [0016]     In one embodiment, the present invention provides for the use of two (or more) materials with one material being applied primarily to the side of the wire bonds that attach to the bond pads on a silicon die, and the other material being applied primarily to the side of the wire bonds that attach to the bond pads on a circuit side. In general this circuit is usually a printed circuit which could be made from a flexible polymeric substrate, rigid polymer glass fiber substrate or ceramic substrate.  
         [0017]     By incorporating the methods described herein, net stress in thermal cycling of the assembly can be reduced by judicious selection of the two (or more) materials. As opposed to using one material as an encapsulant that may only be stress-matched to one of the components being assembled, this allows for an improved optimization taking into consideration the characteristics of the different materials being bonded together.  
         [0018]      FIG. 1  is a perspective diagram of components of a circuit assembly configured to reduce net stress due to thermal cycling as a result of a mismatch in thermal coefficients of expansion between layers of the circuit assembly and the substrate onto which the first and second layers are mounted, according to one embodiment of the present invention.  FIG. 2  is a side perspective diagram of components of a circuit assembly according to the embodiment depicted in  FIG. 1 .  
         [0019]     Referring to  FIG. 1  and  FIG. 2 , a circuit assembly  100  includes a first layer  110  and a second layer  120 . The first layer  110  may be a silicon die that is physically separated from the second layer  120 . The second layer  120  may be a flex cable that is electrically connected to the silicon die. The first layer  110  and the second layer  120  are mounted on a substrate  130 . In one embodiment, the substrate  130  is an aluminum mount. In another embodiment, the substrate  130  is a ceramic mount. In yet another embodiment, the first layer  110  is an integrated circuit chip.  
         [0020]     The first layer  110  includes a first plurality of wire bonds  140  which connect the first layer  110  to specific components thereon. The second layer  120  includes a second plurality of wire bonds  150 . Between the first plurality of wire bonds  140  and the second plurality of wire bonds  150  is a connecting wire loop  160 .  
         [0021]     Because of the mismatch in thermal coefficients of expansion between the first layer  110 , the second layer  120 , and the substrate  130 , in operation the circuit assembly  100  suffers from performance degradation due to temperature variations or thermal cycling. Thermal cycling creates mechanical stress on circuit components that often results in circuit components being worn down due to the stress of high and low temperature testing. To reduce this stress, one or more substances are applied to the surfaces of wire bonds on circuit components and to the connecting wire loop  160  to encapsulate them to protect the components from the effects of different applications of temperature.  
         [0022]     Accordingly, a circuit assembly  100  includes a first substance  170  encapsulating the first plurality of wire bonds  140  of the first layer  110 . The first substance  170  substantially encapsulates the surface areas of the first plurality of wire bonds  140  on the first layer  110 . Each wire in the first plurality of wire bonds  140  is entirely encapsulated. The first substance  170  may be hard encapsulant. One example of a commercially available hard encapsulant is an adhesive such as Hysol®. Other examples include UV epoxies such as those manufactured by Norland Products, Inc. Still further examples include two-part mixed epoxies.  
         [0023]     The circuit assembly  100  also includes a second substance  180  encapsulating the second plurality of wire bonds  150  on the second layer  120 . The second substance  180  substantially encapsulates the surface areas of the second plurality of wire bonds  150  on the second layer  120 . As with the first plurality of wire bonds  140 , each wire in the second plurality of wire bonds  150  must be entirely encapsulated.  
         [0024]     A third substance  190  encapsulates the wire connecting loop  160 . The wire connecting loop  160  connects wire bonds of the first plurality of wire bonds  140  and the second plurality of wire bonds  150 . On the substrate  130 , the first layer  110  and the second layer  120  are separated by a gap  200 , within which the wire connecting loop  160  is positioned. The third substance  190  is a flexible bridging material for filling the gap  200  between the first layer  110  and the second layer  120 . Additionally, the third substance must entirely encapsulate the wire connecting loop  160 . The third substance  190  is a flexible material due to a mismatch between thermal coefficients of expansion between the first layer  110 , the second layer  120 , and the substrate  150 . In one embodiment, the third substance is a flexible silicone encapsulant such as RTV (room temperature vulcanizing) that does not shrink and harden during curing to cause optical distortion. RTV undergoes a process of thermal curing, such that one embodiment of the present invention includes curing silicone adhesives with moisture or a catalyst.  
         [0025]      FIG. 3  is a perspective diagram of components of a circuit assembly according to another embodiment of the present invention.  FIG. 4  is a side diagram of components of a circuit assembly according to the embodiment depicted in  FIG. 3 . Referring to  FIG. 3  and  FIG. 4 , a hard encapsulant, shown as the second substance  180 , is matched by thermal coefficient of expansion to the bond pad material on the second layer  120 . This hard encapsulant therefore is used to encapsulate the second plurality of wire bonds  140  on the second, or flex layer  120 . A flexible encapsulant shown as the third substance  190  is used to encapsulate the wire connecting loop  160  in the gap  200  between the silicon layer  110  and the cable  120 . In this embodiment, selection of the hard encapsulant used to encapsulate the second plurality of wire bonds  140  on the flex layer  120  and selection of the flexible encapsulant  190  is made so that matches in thermal coefficients of expansion reduce thermal cycling stress so that signal integrity is not compromised.  
         [0026]     In the present invention, one method of encapsulating circuit components with a selected substance includes first dispensing the substance onto the circuit component, and then curing the substance by application of heat. In another embodiment, curing can be accomplished by application of radiation to the substance. Additionally, any other method of curing the selected substance to the desired circuit component so that the substance encapsulates the circuit component may also be used.  
         [0027]     Therefore, in another embodiment of the present invention, the second plurality of wire bonds  150  of the second or flex layer  120  are encapsulated with a hard encapsulant, or second substance  180 . In a next step, the first substance  170  (which also may be a hard encapsulant) is dispensed onto the first or silicon layer  110 . The first substance  170  is cured on the first plurality of wire bonds  140 . In this embodiment, the first substance  170  may be UV epoxy and the second substance  180  may be Hysol®. However, Hysol® may also be the first substance  170  if dispensed carefully on the first layer  110 . Once the wires of the first and second plurality of wire bonds  140  and  150  are entirely encapsulated by the first substance  170  and the second substance  180 , these first and second substances  170  and  180  are cured to a hard finish. The third substance  190  is then applied to the gap  200  between the first layer  110  and the second layer  120  and to cover the wire connecting loop  160  of the wire bonds.  
         [0028]     During the encapsulation process, dispensing the encapsulant onto the plurality of wire bonds requires precision so as not to further increase mechanical stress from thermal cycling. For example, when the first layer  110  surface is encapsulated, one must ensure that the first substance does not extend over a silicon edge of the first layer  110 . One must also ensure that the first substance  170  does not shrink during curing and cause further optical distortion.  
         [0029]     As discussed herein, the present invention contemplates that at least the first substance  170  be a hard encapsulant such as Hysol®) or UV curing adhesives manufactured by Norland Products, Inc., for example. These materials meet or exceed aggressive thermal cycling testing, which involves applying 10 cycles of rapid heating and cooling between 0° C. to 90° C. near the liquid crystal clearing temperature. Use of such testing to determine an appropriate encapsulant is effective because the thermal cycle range spans the entire operating temperature range of a typical liquid crystal material used for display applications.  
         [0030]     Additionally, when pressure is applied to the flex cable that exceeds what would normally break bonds on RTV encapsulated components, the materials can withstand physical stress without losing electrical connectivity while being heated to 75° C.  
         [0031]     It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the scope of the present invention. The foregoing descriptions of embodiments of the invention have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Accordingly, many modifications and variations are possible in light of the above teachings. For example, many different materials can be used to encapsulate circuit components according to the present invention to further refine matching between thermal coefficients of expansion. Additionally, multiple materials may be used to encapsulate particular wire bonds on the first or second layer depending on the heat characteristics of those components when connected to particular circuit components. It is therefore intended that the scope of the invention be limited not by this detailed description.