Conductors located several layers below the top are connected by using a STEPPED-DOWN HOLE
PROCESS. Before assembly of a three-level board, a clearance hole is drilled down to the first layer to be
interconnected. The first layer to be interconnected is predrilled with a hole smaller than those drilled in
layers 1 and 2; succeeding layers to be connected have progressively smaller clearance holes. After
assembly, the exposed portion of the conductors are interconnected by filling the stepped-down holes
with solder, as shown by hole B in figure 1-40. The larger the number of interconnections required at one
point, the larger must be the diameter of the clearance holes on the top layer. Large clearance holes on the
top layer allow less space for components and reduce packaging density.
PLATED-THROUGH-HOLE METHOD.The PLATED-THROUGH-HOLE method of
interconnecting conductors is illustrated in figure 1-41. The first step is to temporarily assemble all the
layers into their final form. Holes corresponding to required connections are drilled through the entire
assembly and then the unit is disassembled. The internal walls of those holes to be interconnected are
plated with metal which is 0.001 inch thick. This, in effect, connects the conductor on the board surface
through the hole itself. This process is identical to that used for standard printed circuit boards. The
boards are then reassembled and permanently bonded together with heat and pressure. All the holes are
plated through with metal.
Figure 1-41.Plated through-hole interconnection.
LAYER BUILD-UP METHOD.With the LAYER BUILD-UP method, conductors and
insulation layers are alternately deposited on a backing material, as shown in figure 1-42. This method
produces copper interconnections between layers and minimizes the thermal expansion effects of
dissimilar materials. However, reworking the internal connections in built-up layers is usually difficult, if