Introduction:
Flexible printed circuit boards are used in a wide range of applications, almost all types of electronic products are involved, and the emergence of rigid-flex PCBs for the complex electronic packaging interconnection provides a better way to produce perhaps the most complex interconnection structure of the PCB products. The rigid-flex PCB includes two technologies what are the flexible printed circuit board technology and the rigid printed circuit board technology. For example, by laminating the rigid substrate circuit and the flexible substrate circuit together, and then connected by PTH, so as to constitute a simple rigid-flex PCB.
Drilling Technology:
The use of the subplate and cover plate is also very important for the quality of the drilling hole. In an experiment, we used 0.2mm thick aluminum sheet as cover plate and 1.5mm thick subplate (the subplate has the function of protecting the table surface and preventing burrs under the board).Both materials are free of resinous binders and provide excellent heat dissipation and reduce drilling fouling.
Due to the complex structure of the rigid-flex PCB, the quality of the drill bit and the optimal drilling parameters are very important to obtain a good hole wall in the drilling process. In order to prevent the nail head phenomenon of inner copper ring and flexible substrate, sharp drill bits should be selected and replaced according to the number of holes drilled. The rotation speed and feed speed of the drill bit are the most important process parameters. If the feed speed is too slow, the temperature will rise rapidly and a lot of drilling dirt will be produced. On the contrary, too fast feeding speed, it is easy to cause the phenomenon of drilling group and nail head.
Desmear:
In the manufacturing process of PCB, both flexible PCB and rigid PCB will produce drilling contamination on the hole wall after drilling, and the degree of drilling contamination is also different according to the composition of different materials. Rigid-flex PCB is composed of flexible material and rigid material. During drilling process, epoxy resin, epoxy glass fiber cloth and rubber film will be melted and then attached to the hole wall with the debris of copper or PI film to form pollution. If this pollution is not cleaned before the PTH process, the hole wall will probably fail in plating a copper layer, which will affect the binding force of the electroless plating and the conduction between the hole wall and the inner layers.
There are many ways to remove dirt, including dry and wet methods. Dry method is in the vacuum environment through the plasma to remove the hole wall drilling dirt. Wet process includes concentrated sulfuric acid, potassium permanganate and PI adjustment treatment. Concentrated sulfuric acid method and potassium permanganate treatment method are mainly used to treat rigid PCB.PI adjustment method is used to deal with simple flexible plates. The plasma treatment method requires special equipment to deal with drilling and fouling, and the treatment cost is high. Therefore, it is generally used to deal with rigid-flex PCBs, flexible multilayer PCBs and PCBs with small aperture.
The parameters of plasma treatment mainly include gas flow rate, power, vacuum degree and processing time. The gas ratio is an important parameter to determine the plasma-forming activity. The high power increases the ionization degree of the gas and the reaction speed. Plasma decontamination is a complex physical and chemical process, which is affected by many factors, including process parameters, quality of drilling, structure of product materials and size of product aperture.
Electroless Plating:
A layer of conductive metal is deposited in the hole of the plate after drilling by chemical method, and the metal layer is thickened by full plate electroplating to achieve the metallization in the hole. It is a kind of self-catalyzed chemical oxidation and reduction reaction. In the process of chemical copper plating, copper ions get electrons and are reduced to copper, and the reducing agent releases electrons and itself is oxidized.
In the rigid-flex PCB, due to the poor adhesion between the coating layer and the acrylic bonding sheet, when subjected to thermal shock, it is easy to cause the separation of the coating and the hole wall, in addition to the complete removal of drilling dirt, copper thickness in the hole is also a key factor. The thickness of copper in the hole must be more than 20 um to improve the heat resistance of the metallized hole.
Conclusion:
The PTH process is one of the most important processes in PCB manufacturing technology. It is related to the inherent quality of multilayer PCBs. Its quality is controlled by three processes, namely drilling technology, desmear technology and electroless copper plating technology. In the complex process of making multilayer rigid-flex PCB, it uses the glass fiber epoxy resin board, polyimide film, acrylic resin binder and other materials. Under the action of high temperature of drilling, these resins and debris are adhered to the hole wall to form drilling stains. So it is very important to control the process parameters and methods of drilling in the process technology of rigid-flex PCBs.
