There are four main quality characteristics of circuit board prepregs:
Resin content (RC), fluidity (RF), gel time (also called geltime, GT)), volatile content (also called volatile content, VC) .
In the military standard MIL-P-13949, there is also a control index for dicyandiamide crystallization. In recent years, FR-4 and its prepreg production factories have also begun continuous automatic monitoring of the “resin curing rate” project on the production line.
The characteristic index of the prepreg after lamination refers to the laminated sheet with a nominal thickness of 0.8mm, which is pressed according to the process parameters recommended by the manufacturer, to test electrical, thermal shock and flame retardancy.
The main appearance requirements of the prepreg used in the multilayer board are: the cloth should be flat, no oil, no stains, no foreign impurities or other defects, no cracks and excessive resin powder, but turtle cracks are allowed.
Surface stains and dust must not be greater than 0.25.
Within 0.093m2 area:
The stain and dust at 1 place should be less than 0.25mm;
50mm creases are not more than 2;
100mm crease is not more than 1;
No more than one fiber is loose or skipped;
The cloth edge crack should be less than 3mm from the edge.
In the production of multilayer boards in circuit board factories, when selecting a certain specification and brand of prepreg, the general principles are:
First, the resin can fill the gap between the printed wires when the multilayer board is pressed;
Second, it can exclude air and low molecular volatiles between laminations during pressing;
Third, it can provide the necessary resin / glass cloth ratio for multilayer boards.
At the same time, the actual situation of the thickness of the laminate, the wiring density, the number of layers, the method of pressing and the processing conditions must also be considered.