Backup material for small bore drilling

Abstract

A backup material for small bore drilling in hard .[.crystalline.]. materials, such as multilayer circuit boards containing polyimide/glass, polyimide kevlar or modified epoxy BT/glass, includes at least one layer of hard crystalline material. Standard LCOA material with an additional layer selected from the group comprising polyimides, fluorocarbons and high temperature polycarbonates has proven successful.

What is claimed is: 
     
       1. In a backup sheet for backing a work sheet during a small bore drilling process, said work sheet containing at least one layer of .[.hard crystalline.]. .Iadd.hard .Iaddend.material selected from the group comprising polyimide/glass, polyimide/kevlar and modified epoxy BT/glass, said backup sheet having a core, the improvement comprising an additional layer of .[.hard crystalline.]. .Iadd.hard .Iaddend.material selected from the group comprising polyimides, fluorocarbons and high temperature polycarbonates fixed to opposite sides of said core. 
     
     
       2. A backup sheet according to claim 1 wherein said additional layer consists of a layer of polyimide of the order of about 0.002 inches thick fixed to said core with a layer of acrylic adhesive of the order of 0.001 inches thick. 
     
     
       3. A backup sheet for backing a work sheet during a small bore drilling process, said work sheet containing at least one layer of .[.hard crystalline.]. .Iadd.hard .Iaddend.material, said backup sheet comprising a core constructed of a compressed organic material and a binder, said organic material being selected from the group comprising wood chips and sawdust, said core having opposite planar surfaces; a first aluminum layer adhered to one of said planar surfaces of said core and a second aluminum layer adhered to the other of said planar surfaces of said core; and a first layer of .[.hard crystalline.]. .Iadd.hard .Iaddend.material adhered to said first aluminum layer and a second layer of .[.hard crystalline.]. .Iadd.hard .Iaddend.material adhered to said second aluminum layer, said first and second layers of .[.hard crystalline.]. .Iadd.hard .Iaddend.material being selected from the group comprising polyimides, fluorocarbons and high temperature polycarbonates. 
     
     
       4. A backup sheet according to claim 3 wherein said first and second layers of .[.hard crystalline.]. .Iadd.hard .Iaddend.materials each consist of a layer of polyimide of the order of about 0.002 inches thick and are adhered to the respective first and second aluminum layers with a layer of acrylic adhesive of the order of about 0.001 inches thick.