Flip chip attach on flexible circuit carrier using chip with metallic cap on solder

Abstract

A structure and method is disclosed for directly attaching a device or package on flexible organic circuit carriers having low cost and high reliability. IC chips with a new solder interconnect structure, comprised of a layer of pure tin, deposited on the top of high melting Pb—Sn solder balls are employed for joining. These methods, techniques and metallurgical structures enables direct attachment of electronic devices of any complexity to any substrate and to any level of packaging hierarchy. Also, devices or packages having other joining technologies, eg. SMT, BGA, TBGA, etc. could be joined onto the flexible circuit carrier.

What is claimed is:  
     
         1 . A method of directly attaching an electronic device onto a flexible circuit carrier, said method comprising the steps of: 
 (a) providing said electronic device with at least one reflowed solder ball, wherein said reflowed solder ball has at least one coating of at least one low melting point metal to form a metallic cap,    (b) adhering at least one layer of at least one stiffener sheet with at least one thermo-plastic adhesive onto at least one surface of a flexible sheet,    (c) forming at least one electrically conductive metal line on said at least one surface of said flexible sheet,    (d) coating at least a portion of said flexible carrier with at least one insulator material, and removing selective portions of said insulator material and exposing selective portions of said metal line, and forming a flexible circuit carrier,    (e) screening eutectic solder paste to coat selective sites on said flexible circuit carrier,    (f) placing said flexible circuit carrier on an assembly fixture to hold said flexible circuit carrier,    (g) dispensing at least one solder flux at selective sites on said flexible circuit carrier,    (h) aligning and placing said electronic device onto said flexible circuit carrier, such that reflowed solder ball with metallic cap makes contacts with said solder flux, and upon heating forms an electrical connection between said electronic device and said flexible circuit carrier.    
     
     
         2 . The method of    claim 1   , wherein at least a portion of said electronic device is encapsulated with at least one epoxy.  
     
     
         3 . The method of    claim 1   , wherein at least a portion of said flexible circuit carrier is encapsulated with at least one epoxy.  
     
     
         4 . The method of    claim 1   , wherein at least a portion of the surface of said insulator material is roughened.  
     
     
         5 . The method of    claim 4   , wherein- said surface is roughened using oxygen plasma.  
     
     
         6 . The method of    claim 1   , wherein a portion of said flexible circuit carrier is screened and reflowed using eutectic solder.  
     
     
         7 . The method of    claim 6   , wherein said eutectic solder is deposited using a method selected from a group consisting of solder injection, electroplating, electroless plating, and decal placements.  
     
     
         8 . The method of    claim 1   , wherein a portion of said flexible circuit carrier is screened and reflowed using eutectic solder, and wherein said eutectic solder provides screened ball grid array electrical connection.  
     
     
         9 . The method of    claim 1   , wherein said electronic device is selected from a group consisting of an IC chip, a capacitor, a resistor, a circuit carrier card, a power supplier and an amplifying device.  
     
     
         10 . The method of    claim 1   , wherein said flexible sheet is selected from a group consisting of organic substrate, multi-layer organic substrate, ceramic substrate and multi-layer ceramic substrate.  
     
     
         11 . The method of    claim 1   , wherein said solder ball is selected from a group consisting of high melting point solder and low melting point solder.  
     
     
         12 . The method of    claim 1   , wherein said solder ball is on an electrically conductive feature of said electronic device.  
     
     
         13 . The method of    claim 12   , wherein material for said electrically conductive feature is selected from a group consisting of Au, Co, Cr, Cu, Fe, Ni, TiW, phased Cr and Cu, and alloys thereof.  
     
     
         14 . The method of    claim 12   , wherein said electrically conductive feature is in electrical contact with at least one internal electrically conductive feature.  
     
     
         15 . The method of    claim 1   , wherein said eutectic solder is selected from a group consisting of Pb, Bi, In, Sn, Ag, Au, and alloys thereof.  
     
     
         16 . The method of    claim 1   , wherein said solder ball comprises of a lead-tin alloy, and wherein said alloy contains between about 2 percent to about 10 percent tin.  
     
     
         17 . The method of    claim 1   , wherein said solder ball comprises of a lead-tin alloy, and wherein said alloy contains between about 90 percent to about 98 percent lead.  
     
     
         18 . The method of    claim 1   , wherein said solder ball is formed on said electronic device using a method selected from the group consisting of evaporation, electroplating and solder injection.  
     
     
         19 . The method of    claim 1   , wherein said at least one layer of at least one low melting point metal is formed on said solder ball by a method selected from a group consisting of Radio Frequency evaporation, E-beam evaporation, electroplating, electroless plating and injection.  
     
     
         20 . The method of    claim 1   , wherein said at least one low melting point metal is selected from a group consisting of bismuth, indium, tin and alloys thereof.  
     
     
         21 . The method of    claim 1   , wherein said at least one low melting point alloy is selected from a group consisting of lead, bismuth, indium, tin and alloys thereof.  
     
     
         22 . The method of    claim 1   , wherein said at least one low melting point metal completely envelopes said solder ball.  
     
     
         23 . The method of    claim 1   , wherein heat for said solder reflow is provided by at least one focused IR lamp.  
     
     
         24 . The method of    claim 1   , wherein average thickness of said at least one low melting point metal cap is between about 15 to about 50 micro-meters.  
     
     
         25 . The method of    claim 1   , wherein said at least one low melting point metal, caps between about 10 percent to about 90 percent of the exposed surface of said solder ball, and preferably caps between about 20 percent to about 80 percent of the exposed surface of said solder ball, and more preferably caps between about 30 percent to about 50 percent of the exposed surface of said solder ball.  
     
     
         26 . The method of    claim 1   , where thickness of said at least one low melting point metal cap is chosen to provide a eutectic volume of between about 5 percent to about 50 percent of the volume of said solder ball, and preferably between about 10 percent to about 30 percent of the volume of said solder ball.  
     
     
         27 . The method of    claim 1   , wherein at least a portion of said metal cap is secured to an electrically conductive feature on said flexible circuit carrier.  
     
     
         28 . The method of    claim 27   , wherein material for said electrically conductive feature is selected from a group consisting of Au, Co, Cr, Cu, Fe, Ni, Ta, Ti, TiW, phased Cr and Cu, and alloys thereof.  
     
     
         29 . The method of    claim 1   , wherein said flexible circuit carrier is selected from a group consisting of an interposer, a first level package, a PCMCIA card, a disc drive, a second level package, and a mother board.  
     
     
         30 . The method of    claim 1   , wherein maximum reflow temperature for said solder is between about 190 and about 230° C.  
     
     
         31 . The method of    claim 1   , wherein said step of solder reflow is above about 150° C. for between about 2 to about 5 minutes.  
     
     
         32 . The method of    claim 1   , wherein time for solder reflow at maximum temperature is between about 15 to about 90 seconds.  
     
     
         33 . The method of    claim 1   , wherein said solder reflow is performed in an environment selected from a group consisting of dry nitrogen, forming gas and hydrogen.  
     
     
         34 . The method of    claim 1   , wherein said flexible circuit carrier is either a flexible organic laminated card or a flexible inorganic laminated card.  
     
     
         35 . The method of    claim 34   , wherein material for said flexible organic circuit carrier is selected from a group consisting of polyimides, poly tetra flouro ethylene (PTFE), polyester and resin-impregnated fabrics.  
     
     
         36 . The method of joining an electronic device, having C4 solder balls with tin cap, on a flexible circuit carrier of    claim 1   , comprising the steps of: 
 (a) aligning C4 balls of said electronic device with corresponding openings in said passivation layer of said flexible circuit carrier,    (b) holding said electronic device in place by surface tension afforded by solder flux, and    (c) reflowing said C4 solder to bond said electronic device to the flexible circuit carrier.    
     
     
         37 . The method of    claim 36   , wherein at least a portion of said electronic device is encapsulated with at least one epoxy.  
     
     
         38 . A flexible electronic carrier comprising a flexible device carrier and at least one electronic device electrically-connected thereto by at least one solder ball, wherein said solder ball has a cap of at least one low melting point cap forming a eutectic.  
     
     
         39 . The carrier of    claim 38   , wherein said electronic card has at least one SMT secured thereto.  
     
     
         40 . The carrier of    claim 38   , wherein said electronic card has a ball grid array.  
     
     
         41 . The carrier of    claim 38   , wherein said electronic card has a SMT connection.  
     
     
         42 . The carrier of    claim 38   , wherein at least a portion of said electronic card has at least one coating of an insulator material.  
     
     
         43 . The carrier of    claim 38   , wherein at least a portion of said electronic device is encapsulated with at least one epoxy.  
     
     
         44 . The carrier of    claim 38   , wherein said flexible device carrier has at least one stiffener secured to at least one side.  
     
     
         45 . The carrier of    claim 44   , wherein said stiffener is selected from a group consisting of aluminum, molybdenum, silicon, tantalum and titanium.