Plastic-packaged semiconductor device having wires with polymerized insulator skin

Abstract

The assembly of a chip ( 101 ) attached to a substrate ( 103 ) with wires ( 201 ) spanning from the chip to the substrate is loaded in a heated cavity ( 402 ) of a mold; the wire surfaces are coated with an adsorbed layer of molecules of a heterocyclic compound ( 302 ); a pressure chamber ( 404 ) of the mold is loaded with a solid pellet ( 410 ) of a packaging material including a polymerizable resin, the chamber being connected to the cavity; the vapor of resin molecules is allowed to spread from the chamber to the assembly inside the cavity during the time interval needed to heat the solid pellet for rendering it semi-liquid and to pressurize it through runners ( 403 ) before filling the mold cavity, whereby the resin molecules arriving in the cavity are cross-linked by the adsorbed heterocyclic compound molecules into an electrically insulating at least one monolayer of polymeric structures on the wire surfaces.

We claim: 
     
         1 . A semiconductor device comprising:
 a semiconductor chip assembled on a substrate and connected to the substrate by wire spans;   the chip and wire spans encapsulated in a compound including a polymerized resin mixed with inorganic fillers; and   the surface of the wire spans covered by an insulating layer including the polymerized resin and a heterocyclic compound, the layer being free of the fillers.   
     
     
         2 . The device of  claim 1  wherein the heterocyclic compound is selected from a group including pyrrole (C 4 H 5 N 1 ), pyrazole and imidazole (C 3 H 4 N 2 ), containing a five-membered di-unsaturated ring with two non-adjacent nitrogen atoms as part of the ring; triazole (C 2 H 3 N 3 ), containing a five-membered ring with three nitrogen atoms as part of the ring; pentazolde, thiazole, isothiazole, and their derivatives. 
     
     
         3 . The device of  claim 1  wherein the polymerizable resin is an epoxy resin selected from a group including bisphenol A and novalac. 
     
     
         4 . The device of  claim 1  wherein the compound further includes a curing agent (accelerator) selected from a group including amines, acid anhydrates, and phenol novalac resins. 
     
     
         5 . The device of  claim 4  wherein the inorganic fillers include grains of silicon dioxide, silicon nitride, and aluminum oxide. 
     
     
         6 . The device of  claim 1  wherein the substrate is a metal leadframe. 
     
     
         7 . A method for fabricating a packaged semiconductor device comprising the steps of:
 loading the heated cavity of a mold with an assembly including a semiconductor chip attached to a substrate and wires spanning from the chip to the substrate, the wire surfaces coated with an adsorbed layer of molecules of a heterocyclic compound;   loading the heated pressure chamber of the mold with a solid pellet of a packaging material including a polymerizable resin, the chamber being connected to the cavity; and   allowing the vapor of resin molecules to spread and be pushed from the chamber to the assembly inside the cavity during the time interval needed to heat the solid pellet for rendering it semi-liquid and to pressurize it through the mold runners before filling the mold cavity,   whereby the resin molecules arriving in the cavity are cross-linked by the adsorbed heterocyclic compound molecules into at least a monolayer of polymeric structure on the wire surfaces.   
     
     
         8 . The method of  claim 7  further including, prior to the step of loading the cavity, the step of dispensing a solution of a heterocyclic compound dissolved in ethanol onto the wires spanned from a semiconductor chip to the substrate, whereby a layer of molecules of the heterocyclic compound is adsorbed on the wire surfaces. 
     
     
         9 . The method of  claim 7  wherein the solution has a concentration of about 1 millimole per liter (mmol/L) heterocyclic compound in ethanol. 
     
     
         10 . The method of  claim 7  wherein the heterocyclic compound is selected from a group including pyrrole (C 4 H 5 N 1 ), pyrazole and imidazole (C 3 H 4 N 2 ), containing a five-membered di-unsaturated ring with two non-adjacent nitrogen atoms as part of the ring; triazole (C 2 H 3 N 3 ), containing a five-membered ring with three nitrogen atoms as part of the ring; pentazolde, thiazole, isothiazole, and their derivatives. 
     
     
         11 . The method of  claim 7  wherein the polymerizable resin is an epoxy resin selected from a group including bisphenol A and novalac. 
     
     
         12 . The method of  claim 11  wherein the liquefying temperature of the solid polymerizable resin is about 175° C. 
     
     
         13 . The method of  claim 7  wherein the packaging material further includes a curing agent selected from a group including amines, acid anhydrates, and phenol novalac resins. 
     
     
         14 . The method of  claim 13  wherein the packaging material further includes inorganic fillers. 
     
     
         15 . The method of  claim 7  wherein the substrate is a metal leadframe having an assembly pad for attaching the chip and leads for attaching the wires.