“Glass cloth acts as the main structural reinforcement in most laminates. The rigidity and strength offered by glass compliments the binding, encapsulating and insulating properties of the epoxy resin. The singular fibre-glass filament is the building block with which glass clothes are constructed. These thread-like fibres are put together to form a yarn or bundle. Subsequently, like weaving of any other type of cloth, numerous yarns are woven together in the manufacture of cloth. Various combination of filament and bundle diameters, filament counts and weave density, among other variables, will resist in a multiplicity of glass cloth thicknesses and weights. Finally, glass clothes are coated with a finish that facilities resin impregnation of and bonding to the cloth.”
According to a FBI document date 20 August 1990 (classified SECRET):
“The Scottish Police have determined, after extensive investigation, that the green circuit board (PT 35) is single sided and composed of nine (9) layers of glass cloth, type 7628.”
A Memo from Det. Insp. Williamson dated 3 september 1990 states that:
On 14th February 1990 George Wheadon, Chief Technical Manager and Mr Paul Boyle. Laboratory Manager New England Laminates Company Skelmersdale Lancashire. I t I carried out an examination of the laminate of PT35. New England Laminates are one of the top manufacturers of copper clad laminates in the world. Mr Boyle removed a small cross-section or PT35 which was examined under a microscope by Mr Wheadon and himself. Their findings were that the laminate is constructed with 9 layers of American standard 7628 glass cloth which is very commonly used in the industry.
The MEMO goes on:
Though standard FR4 (fire resistant rating) glass cloth has been used in the manufacture of the laminate of Production PT35 the number of layers of’ glass cloth used may be a feature of importance. PT35 is constructed on 9 layers of glass cloth, the most commonly used method of production for this type of laminate board is to use 8 layers of glass cloth. This feature was identified by Mr George Wheadon, Chief Technical Manager, Nelco Laminates.
Here is a photo of a 7628 sample:
Understanding the PCB Laminate (ISOLA)