Multilayer printed wiring boards are commmonly manufactured by the subtractive technique. In the conventional subtractive process, the inner layers are prepared on thin, copper clad, epoxy glass laminates, typically 0.1 mm to 0.2 mm thick, by etching away the unwanted copper. The inner layers are assembled in a stack with B-staged epoxy prepreg sheets between the layers and laminated together with sheets of copper foil on the outside surfaces of the stack. Holes are drilled through the multilayer laminate and the hole walls are plated to establish plated through hole connections to the internal layers. Then, the outer layers of copper foil are etched to provide the outer layer conductive patterns.
In the "mass molded" multilayer process, the operations of etching the custom conductive patterns for the inner layers and the lamination of the inner layers together with the outer layers of copper foils are carried out in central laminating plants. Then, the laminated package is sent to individual printed wiring board manufacturers who perform the operations of drilling, forming plated through holes and etching the outer surface conductor patterns to complete the multilayer board. When a completed "mass molded" board is examined by a purchaser or user, there is no obvious difference in appearance, form or function from the multilayer boards made by the standard multilayer process.
Additive multilayer boards have been made by the "mass molding" technique. The conductive patterns for the inner layers were etched in a subtractive process. The inner layers were laminated together as in the regular "mass molding" technique, but the outer surfaces were C-staged epoxy coated glass cloth, not copper. These "mass molded" packages were finished by additive printed wiring board manufacturers who applied first a plating adhesive to each surface, next drilled the through holes, and then applied a plating resist and plated the conductive pattern on the outer surfaces and through the holes to complete the multilayer board. This manufacturing procedure did not offer significant price or functional advantages over fully subtractive "mass molded" multilayer board manufacturing and has not been widely adopted. A multilayer board made by an additive process and the "mass molding" technique has a different appearance from multilayer boards made by the subtractive processes.
Other methods of making multilayer printed wiring boards start with a thicker inner layer laminate (0.2 mm to 1 mm thick) which is copper clad on both sides. The inner layer conductive patterns are etched. Instead of laminating all the layers together in a laminating press, the layers are built up sequentially on the thick inner layer laminate by adding in sequence, an insulating layer, and then another conductive pattern layer. The conductive pattern is added either by fully-additive, semi-additive or subtractive processes. In the fully-additive processes, the conductive pattern is plated directly. In the semi-additive and subtractive processes, a complete layer of copper is applied over the insulating layer and then the conductive pattern established by plating and etching. Multilayer boards made sequentially by either fully-additive processes or semi-additive processes have a distinct different appearance which is obvious to the purchaser or user.
Multilayer printed wiring boards are commonly provided with internal ground and power planes. These internal planes are frequently solid sheets of copper only interrupted by clearance holes (the perforations required for electrically isolating the through hole pattern of the printed wiring board). These ground and power planes provide power voltage and current and ground connections for the components of the multilayer printed circuit. A second function of the ground and power planes is to provide electromagnetic shielding for the multilayer printed circuit board and reduce the electromagnetic and radio frequency interference. Multiple ground and power planes and additional ground planes or shields on the surface layers with the conductive pattern are common.
When components are mounted on a multilayer printed wiring board and mass soldered in place at temperatures in the range of 275.degree. C., a severe thermal shock is applied to the insulating layers placed between two copper planes, such as the insulating layer between an internal ground plane and ground shield on the surface surrounding the conductor pattern. Frequently, delamination will occur and blisters will form between the ground shield on the surface and the internal ground or power plane. Delamination and blistering has been a problem with multilayers made by the fully-additive, semi-additive or subtractive sequential processes.
In the multilayer printed wiring board, an application of strongly adherent oxide layers on copper has been adopted to enhance the bond between the copper conductive patterns and the insulating layers. The oxide layers are used in the press laminating processes as well as the sequential processes. Such strongly adherent oxide layers are usually applied by immersing the copper surface in hot (40.degree.-110.degree. C.), strongly alkaline, hypochlorite solutions. This immersion produces an adherent, black, dendritic, oxide layer with a high surface area for adhering to organic films, coatings and laminated layers. In the printed wiring industry, this oxide layer is commonly called "black oxide".
The black oxide layer is subject to attack by solutions which dissolve copper oxides. Use of such solutions are necessary in multilayer board manufacturing. In multilayer board manufacturing, the inner copper planes are coated with black oxide, and the outer layers of insulator and copper laminated over them. When holes are drilled through the multilayer laminate and the hole walls are plated to create electrical connections to the inner copper planes, the plating and cleaning solutions dissolve the black oxide surrounding the holes and leave a non-adherent ring around the hole. This is known in the industry as "pink ring" because a pink ring of copper is visible in the pattern of black oxide coated copper. At the pink ring, there is no adhesion between the copper plane and the laminated insulating layer over it. Ionic contamination and failure of insulation between holes occur where pink ring is found. Pink ring has been a severe problem for additively and sequentially manufactured multilayers.
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