Nickel barrier end termination and method

Abstract

A method of providing nickel barrier end terminations for a zinc oxide semiconductor device with exposed body surfaces and end terminal regions, in which the device is controllably reacted with a nickel plating solution only on an exposed end terminal region and thereafter provided with a final tin or tin-lead termination.

What is claimed is:  
     
         1 . A method of making a semiconductor device, the body of the semiconductor device having an exposed zinc oxide surface and nickel end terminations, the method comprising the steps of: 
 (a) providing a semiconductor body having electrically conductive plates interleaved with zinc oxide layers;    (b) providing a selected nickel plating solution for an intended method of nickel plating; and    (c) controllably contacting an end of the semiconductor body with the nickel plating solution in order to form a desirably thick nickel barrier cap over the end of the semiconductor body without forming a nickel barrier cap over the entire semiconductor body.    
     
     
         2 . The method of  claim 1 , wherein the temperature of the nickel plating solution is uncontrolled and remains at approximately room temperature.  
     
     
         3 . The method of  claim 1 , wherein the pH of the nickel plating solution is maintained between about 2 and about 6.  
     
     
         4 . The method of  claim 1 , wherein contact between the semiconductor body and the nickel plating solution is maintained for a period of approximately 10 to 120 minutes.  
     
     
         5 . The method of  claim 4 , where contact between the semiconductor body and the nickel plating solution is maintained until the thickness of the nickel barrier cap is between approximately 1 and 3 um.  
     
     
         6 . The method of  claim 1 , further comprising the step of forming an solderable contact by partially immersing the nickel barrier cap in an acid solution comprising one or more of Alkyl-Tin, Alkyl-Tin-Lead, Tin-Lead sulphuric acid, or Tin sulphuric acid with a pH between about 3 to about 6 at room temperature.  
     
     
         7 . The method of  claim 6 , wherein the immersion of the nickel barrier cap in the acid solution is for a period of about 10 to about 120 minutes.  
     
     
         8 . The method of  claim 7 , further including the application to the nickel barrier cap of a biasing current of approximately 0.3 to 2.0 A/dm 2 .  
     
     
         9 . The method of  claim 7 , wherein the immersion of the nickel barrier cap in the acid solution continues until a solderable contact having a thickness of 3 to 6 um is formed.  
     
     
         10 . The method of  claim 1 , wherein the nickel plating solution is a room temperature solution comprising one or more of nickel sulphate, dimethylamineborane, lactic acid, ammonium citrate, and ammonia.  
     
     
         11 . The method of  claim 10 , wherein the zinc oxide layers have a resistivity in the range from about 10 10  to about 10 12  Ohms/cm 2 .  
     
     
         12 . The method of  claim 1 , wherein the contact is by partial immersion in the nickel plating solution.  
     
     
         13 . The method of  claim 12 , including the further steps of applying a termination material comprising silver and glass frit onto the end of the semiconductor body; and 
 firing the semiconductor body to mechanically bond the termination material with the end of the semiconductor body.    
     
     
         14 . The method of  claim 13 , wherein the termination material is essentially free of platinum and palladium; and 
 wherein the termination material is fired at a temperature between about 550° and 800° C.    
     
     
         15 . The method of  claim 13 , wherein the nickel plating solution includes one or more of (i) nickel sulphate or nickel chloride, (ii) boric acid, (iii) a wetting agent, and (iv) a stress relieving agent at a temperature of about 50° to 70° C.  
     
     
         16 . The method of  claim 15 , including the further step of applying a biasing current of about 0.3 to about 2.0 A/dm 2  during nickel plating.  
     
     
         17 . The method of  claim 16 , wherein the biasing current is variably dependent on the area of the end of the semiconductor to be coated.  
     
     
         18 . The method of  claim 12 , wherein the immersion depth of the semiconductor body ia controlled to thereby selectively control the distance that the barrier cap extends upwardly from the end of the semiconductor body.  
     
     
         19 . The method of  claim 1 , wherein the controllable contact is by impregnated absorbent material.  
     
     
         20 . A method of providing a semiconductor device having a body with an exposed zinc oxide surface and electrically conductive, solderable metal end terminations, the method comprising the steps of: 
 (a) providing a semiconductor body having electrically conductive plates interleaved with zinc oxide layers;    (b) applying a termination material comprising silver and glass frit onto opposing ends of the semiconductor body;    (c) mechanically bonding the termination material to the ends of the semiconductor body by firing;    (d) providing at a temperature of about 50° to 70° C. a nickel plating solution comprising one or more of (i) nickel sulphate or nickel chloride, (ii) boric acid, (iii) a wetting agent, and (iv) a stress relieving agent;    (e) coating a silver terminated end of the semiconductor body by selectively partially immersing the end of the semiconductor body in the nickel plating solution for a period of about 15 to about 120 minutes while applying a biasing current of about 0.3 to 2.0 A/dm 2  to thereby form a desirably thick nickel barrier cap in contact with the silver terminated end which extends a selected distance up the body of the semiconductor device;    (f) providing a final termination solution of one or more of alkyl-tin, alkyl-tin-lead, tin-sulfuric acid or tin-lead-sulfuric acid, having a pH from about 3 to about 6 and an uncontrolled temperature; and    (g) forming a desirably thick, electrically conductive, solderable contact end termination over the nickel barrier cap by selectively partially immersing the end of the semiconductor body into the final termination solution for a period of about 10 to about 120 minutes while applying a biasing current of about 0.3 to about 2.0 A/dm 2 .    
     
     
         21 . The method of  claim 20 , wherein the pH of the nickel plating solution is maintained between about 2 and about 6.  
     
     
         22 . The method of  claim 20 , wherein the silver termination material is provided free of platinum and palladium and is fired onto the semiconductor body at a temperature between about 550° and about 800° C.  
     
     
         23 . The method of  claim 20 , wherein the partial immersion of the semiconductor body in the nickel plating solution is continued until the thickness of the nickel coating is between about 1 and about 3 um.  
     
     
         24 . The method of  claim 20 , wherein the solderable contact is about 3 to about 6 um thick.  
     
     
         25 . The method of  claim 20 , wherein the distance that the barrier cap extends from the end of the semiconductor body is controlled by controlling the immersion depth.  
     
     
         26 . The method of  claim 20 , wherein the biasing current is varied as a function of the area of semiconductor to be coated.  
     
     
         27 . A method of providing metal end terminations to a semiconductor device without the use of a plating resist comprising the steps of: 
 (a) providing a semiconductor body having a zinc oxide exterior with electrically conductive elements interleaved between ceramic layers consisting principally of zinc oxide;    (b) providing a nickel plating solution comprising one or more of nickel sulphate, dimethylamineborane, lactic acid, ammonium citrate, and ammonia at room temperature;    (c) positioning one end of the semiconductor body a selectable distance into the nickel plating solution for a period of about 15 to about 120 minutes to thereby form a desirably thick nickel barrier cap over the end of the semiconductor body;    (d) providing a metal termination solution of either: alkyl-tin, alkyl-tin-lead, tin-sulfuric acid, or tin-lead-sulfuric acid, having a pH between about 3 to about 6; and    (e) forming a metal termination over the nickel barrier cap by partially immersing an end of the semiconductor body into the metal termination solution for a period of about 10 to about 120 minutes while applying biasing current of about 0.3 to about 2.0 A/dm 2 .    
     
     
         28 . The method of  claim 27 , wherein the pH of the nickel plating solution is maintained between about 2 and about 6.  
     
     
         29 . The method of  claim 27 , wherein the semiconductor body is immersed in the nickel plating solution until the thickness of the nickel coating is between about 1 and about 3 um.  
     
     
         30 . The method of  claim 27 , wherein the nickel barrier cap is coated with a solderable contact 3 to 6 um thick.  
     
     
         31 . The method of  claim 27 , further including the step of providing a silver fired termination on the end of the semiconductor body prior to partial immersion in the nickel plating solution.  
     
     
         32 . The method of  claim 27 , wherein the zinc oxide resistivity is between about 10 10  to about 10 12  Ohms/cm 2 .  
     
     
         33 . A method of providing a metal termination on a semiconductor device comprising the steps of: 
 (a) providing a semiconductor body having a zinc oxide exterior with electrically conductive elements interleaved between ceramic layers consisting principally of zinc oxide;    (b) providing at room temperature a nickel plating solution comprising one or more of nickel sulphate, dimethylamineborane, lactic acid, ammonium citrate, and ammonia;    (c) impregnating an absorbent material with the nickel plating solution;    (d) positioning one end of the semiconductor body in contact with absorbent material for a period of about 15 to about 120 minutes to thereby form a desirably thick nickel barrier cap covering that end of the semiconductor body and a selected portion of the body of the semiconductor device immediately contiguous thereto;    (e) providing a metal termination solution of one or more of: alkyl-tin, alkyl-tin-lead, tin-sulfuric acid, or tin-lead-sulfuric acid having a pH between about 3 and about 6; and    (f) forming a desirably thick metal termination over the nickel barrier cap by partially immersing an end of the semiconductor body into the metal termination solution for a period of about 10 to about 120 minutes while applying a biasing current of about 0.3 to about 2.0 A/dm 2 .    
     
     
         34 . The method of  claim 33 , wherein the semiconductor body is maintained in contact with the absorbent material for a period sufficient to form a nickel barrier thickness of about 1 to about 3 um.  
     
     
         35 . The method of  claim 33 , including the further step of moving the semiconductor body relative to the absorbent material.  
     
     
         36 . A varistor comprising: 
 a body of interleaved resistive plates and zinc oxide layers having an external surface of zinc oxide free of any passivation material; and    nickel barrier caps on opposing ends of the body, the nickel barrier caps terminating with naturally formed edges.    
     
     
         37 . The varistor of  claim 36 , further comprising a silver barrier between the body and the nickel barrier.  
     
     
         38 . The varistor of  claim 36 , where a nickel barrier cap is between about 1 and about 3 um thick.