Ball grid array resistor network

Abstract

A ball grid array resistor network has a planar substrate formed of an organic material. The substrate preferably is a printed circuit board. The substrate has a top and bottom surface. A ball pad is located on the bottom surface. A low temperature resistor is located on the bottom surface and is connected to the ball pad. A solder mask is located over the first surface except for the ball pads. A conductive ball is attached to the ball pad. A reflowed solder paste connects the conductive ball to the ball pad. Several embodiments of the invention are shown.

1 . A method of making a resistor network comprising the steps of: 
 a) providing a conductive layer and a resistive layer laminated to a first surface of an organic substrate, the resistive layer located between the organic substrate and the conductive layer;    b) forming at least one ball pad from the conductive layer;    c) forming at least one resistor from the resistive layer;    d) coating the first surface with a solder mask except for the pad;    e) screening solder paste onto the pad;    f) placing a solder sphere onto the pad; and    g) reflowing the solder paste such that the solder sphere is joined to the pad.    
   
   
       2 . The method of  claim 1 , wherein forming the ball pad and resistor further comprises: 
 a) depositing a first layer of photoresist over the conductive layer;    b) exposing the ball pads and resistors onto the first layer of photoresist;    c) developing the photoresist to form a first pattern;    d) etching the conductive and resistive layers;    e) striping the photoresist;    f) depositing a second layer of photoresist over the conductive layer and the substrate;    g) exposing the resistor onto the second layer of photoresist;    h) developing the second layer of photoresist to form a second pattern;    i) etching the conductive layer; and    j) removing the photoresist.    
   
   
       3 . The method of  claim 1 , wherein the resistor is cut with a laser to adjust the resistance of the resistor.  
   
   
       4 . A method of making a resistor network comprising the steps of: 
 a) providing a conductive layer mounted to a first surface of an organic substrate;    b) etching a first and second ball pad from the conductive layer;    c) electroplating at least one resistor between the ball pads;    d) laser trimming the resistor to a pre-determined resistance value;    e) coating the first surface with a solder mask except for the pads;    f) screening solder paste onto the pads;    g) placing a solder sphere onto the pads; and    h) reflowing the solder paste such that the solder sphere is attached to the pad.    
   
   
       5 . The method of  claim 4 , wherein forming the ball pads and resistor further comprises: 
 a) depositing a first layer of photoresist over the conductive layer;    b) exposing a first pattern of ball pads onto the first layer of photoresist;    c) developing the photoresist to form the first pattern;    d) etching the conductive layer;    e) removing the photoresist;    f) depositing a second layer of photoresist over the conductive layer and the substrate;    g) exposing a second pattern onto the second layer of photoresist;    h) developing the second layer of photoresist;    i) plating the resistor; and    j) removing the second layer of photoresist.    
   
   
       6 . A method of making a resistor network comprising the steps of: 
 a) providing a conductive layer;    b) screen printing a plurality of resistors on the conductive layer;    c) firing the conductive layer and resistors in an oven;    d) depositing a laser reflective layer over the conductive layer and resistors;    e) laminating the conductive layer and resistors to a substrate;    f) etching a first and second ball pad from the conductive layer;    g) coating the first surface with a solder mask except for the ball pads;    h) screening solder paste onto the pads;    i) placing a solder sphere onto the pads; and    j) reflowing the solder paste such that the solder sphere is attached to the pad.    
   
   
       7 . The method of  claim 6 , wherein an adhesive layer is used to laminate the conductive layer and the resistors to the substrate.  
   
   
       8 . The method of  claim 6 , wherein the resistors are cut with a laser to adjust the resistance of the resistor.