a. The dimensions of the metallized holes (see Figure 20.8). The metallized holes must be large enough to deposit the desired metal on the walls of the holes. The minimum aperture after plating should be 3/4 of the board thickness. According to this ratio, it is possible to deposit 0.0005 to 0.002 inch of metal on the wall of each hole by a standard plating apparatus. The amount of metal deposited on the surface of the plate will be twice that of the metal in the hole. Smaller holes can of course be plated, but the ratio of metal deposited on the surface to the metal in the hole will increase.
The relationship between the size of the metallized holes and the total thickness of the printed board depends on the maturity of the plating process. As described above, the thickness of the multilayer printed wiring board has an upper limit on the ratio of the metallized hole size (plating ratio). The minimum size of the metallized holes is determined by the total thickness of the plates, the component lead diameter, and the appropriate plating ratio. Generally, the strength and reliability of the metallized holes increase as the size of the holes increases.
The relationship between the minimum aperture of the metallized hole of the circuit board factory and the thickness of the multilayer printed board often depends on the maturity of the plating process. In general, pores that are small and deep in pore size are more difficult to plate than pores that are larger and shallower in pore size.
b. Pad and Hole Size The size of the pad is directly related to the manufacturability of the multilayer printed circuit board. A good rule of thumb is that the pad is at least twice the hole diameter, but in today's high-density processes, it is almost impossible to allow such ample ratio. 004〜0.007英寸。 The size of the drilled hole must be greater than the required nominal aperture of 0. 004~0.007 inches.
CNC drilling machines can drill 40 to 180 times per minute. If there are several sizes of holes on the printed circuit board, keep in mind that the CNC drill can drill approximately 1,700 holes for the time required to replace a cobalt head. Therefore, the smaller the size of the hole, the less downtime the drill hole is and the lower the cost. The size of the hole affects the other side of the cost. If the hole size exceeds 0.125 inches, it may be necessary to fix the printed board to a special fixture; with manual drilling, the manual drilling is costly and inaccurate.