Method and apparatus for applying a protective over-coating to a ball-grid-array (BGA) structure

Abstract

A vacuum-application and coating apparatus for applying a protective coating to at least one ball-grid-array assembly is provided. The apparatus comprises an upper plate having at least one injection port forming the upper chamber wall, and a lower plate having at least one vacuum port forming the lower chamber wall of the vacuum-application and coating apparatus when assembled. A compliant layer of material is provided on the chamber-side surface of the upper plate and a sealing mechanism for enabling a vacuum seal is also provided. At least one ball-grid-array assembly is placed on the chamber surface of the lower plate during assembly of the vacuum-application and coating apparatus, which forms a vacuum chamber. The ball-grid-array assemblies held in the chamber are protected from receiving any coating on the upper portions of connected solder balls during processing by virtue of intimate contact between the solder balls and the compliant layer of material. In other aspects methods are provided for adding a protective coating to ball-grid-array assemblies and subsequently providing opening for access to the die pads. In another aspect a process is provided for completely encapsulating balls, then exposing a portion and applying a new grid array.

What is claimed is:  
     
         1 . A coating mold for forming a protective coating on a ball-grid-array assembly having solder balls extending above a base surface of the ball-grid-array assembly, comprising: 
 a first portion having a substantially flat area for supporting the ball grid array assembly on a back surface;    a second portion having a substantially flat compliant layer;    an injection port passing through one or the other of the first and second portions;    a vacuum pumping port passing through one or the other of the first and second portions; and    a sealing mechanism for sealing the first portion to the second portion, enclosing the ball-grid-array assembly;    characterized in that with the first portion closed on the second portion the compliant layer contacts the solder balls of the ball grid array, and a space is formed between the second portion and the base surface of the ball-grid-array.    
     
     
         2 . The coating mold of  claim 1  wherein the sealing mechanism is an o-ring.  
     
     
         3 . The coating mold of  claim 1  further comprising a clamping or bolting mechanism for keeping the mold closed in process.  
     
     
         4 . The coating mold of  claim 1  wherein the compliant layer is a flexible polymer material.  
     
     
         5 . A method for applying a protective coating to a ball-grid-array assembly having solder balls extending above a base surface, comprising steps of: 
 (a) placing the ball-grid-array assembly, including solder balls, into a coating mold having a first surface for supporting the ball-grid-array assembly on a back surface and a second portion having a substantially flat compliant layer;    (b) closing and sealing the mold such that the compliant layer contacts the balls of the ball grid array, leaving a space between the compliant layer and the base surface of the ball-grid -array assembly;    (c) creating a vacuum in the space formed in step (b);    (d) injecting a polymer-based coating into the space formed; and    (e) curing the polymer material, such that, when opened the ball-grid-array assembly is coated while leaving an upper portion of each of the solder balls exposed.    
     
     
         6 . The method of  claim 5  wherein the mold closes on an o-ring seal.  
     
     
         7 . A method for providing a protective polymer coating to a ball-grid-array assembly, before placing solder balls on the die pads of the assembly, comprising the steps of: 
 (a) overcoating the assembly with a polymer material; and    (b) opening each die pad area to the die pad through the polymer material by a removal process to expose the die pads for placement of the solder balls.    
     
     
         8 . The method of  claim 7  wherein, in step (b) the removal process comprises laser machining.  
     
     
         9 . The method of  claim 7  wherein the removal process comprises chemical etching.  
     
     
         10 . The method of  claim 7  wherein the removal process comprises physical etching.  
     
     
         11 . A method for providing a protective polymer coating to a ball-grid-array assembly, before placing solder balls on the die pads of the assembly, comprising the steps of: 
 (a) screen printing photoresist onto the ball-grid-array assembly;    (b) masking the ball-grid-array assembly to protect areas of photoresist over the die pads;    (c) developing and removing the photoresist exposed through the mask, leaving the photoresist over the die pads as protective photoresist islands;    (d) applying a protective coat to the ball-grid-array assembly to the thickness of the photoresist coating; and    (e) developing and removing the remaining photoresist islands to expose the underlying die pads.    
     
     
         12 . A method for protecting and strengthening a ball-grid-array assembly having solder balls extending above a base surface, comprising steps of: 
 (a) applying a protective material layer over the solder balls to a level at or above the level of the top of the solder balls, providing thereby a new upper surface for the assembly;    (b) removing a portion of the new upper surface to an extent that a portion of each of the original solder balls is exposed as a flat region in a planar upper surface; and    (c) applying new solder material over each of the flat exposed solder ball regions.    
     
     
         13 . The method of  claim 12  wherein, in step (b), removal is by machining.  
     
     
         14 . The method of  claim 12  wherein, in step (a) the protective material is applied by one of screening, spraying, or dispense and spinning.  
     
     
         15 . The method of  claim 12  comprising an additional step (d) for reflowing the new solder material.