PCB Technical - The importance of electroplating to circuit board production

The importance of electroplating to circuit board production

PCB factory editor: On the printed circuit board, copper is used to interconnect the components on the substrate. Although it is a good conductor material for forming the pattern of the conductive path of the printed circuit board, if it is exposed for a long time In the air, it is also easy to lose luster due to oxidation, and lose weldability due to corrosion. Therefore, various technologies must be used to protect copper traces, vias, and plated through holes. These technologies include organic paint, oxide film, and electroplating.

Organic paint is very simple to apply, but it is not suitable for long-term use due to changes in its concentration, composition and curing cycle. It even leads to unpredictable deviations in weldability. Oxide film can protect the circuit from corrosion, but it cannot maintain solderability. Electroplating or metal coating processes are standard operations to ensure solderability and protect circuits from corrosion, and play an important role in the manufacture of single-sided, double-sided, and multilayer printed circuit boards. In particular, plating a layer of solderable metal on the printed wire has become a standard operation for providing a solderable protective layer for the copper printed wire.

In the interconnection of various modules in electronic equipment, it is often necessary to use a printed circuit board socket with spring contacts and a printed circuit board with connecting contacts designed to match it. These contacts should have a high degree of wear resistance and very low contact resistance, which requires a layer of rare metal plating on them, of which the most commonly used metal is gold. In addition, other coated metals can be used on the printed lines, such as tin plating, plating, and sometimes copper plating in certain areas of the printed lines.

Another type of coating on the copper printed line is organic, usually a solder mask, where there is no need for soldering, screen printing technology is used to cover a layer of epoxy resin film. This process of applying a layer of organic solder preservative does not require electronic exchange. When the circuit board is immersed in the electroless plating solution, a nitrogen-resistant compound can stand on the exposed metal surface and will not be absorbed by the substrate. .

The sophisticated technology required by electronic products and the strict requirements of environmental and safety adaptability have promoted considerable progress in electroplating practice. This is clearly reflected in the manufacturing of high-complexity, high-resolution multi-substrate technology. In electroplating, through the development of automated, computer-controlled electroplating equipment, the development of high-complex instrument technology for chemical analysis of organics and metal additives, and the emergence of technologies for precise control of chemical reaction processes, electroplating technology has reached a high level. s level.

There are two standard methods for growing metal build-up layers in circuit board wires and through holes: circuit electroplating and full-board copper plating, which are described as follows.

1. Line plating

In this process, only the copper layer generation and etching resist metal plating are accepted where the circuit patterns and through holes are designed. During the circuit electroplating process, the increased width of each side of the circuit and the solder pad is roughly equivalent to the increased thickness of the electroplated surface. Therefore, it is necessary to leave a margin on the original film.

In circuit electroplating, most of the copper surface must be masked by resist, and electroplating is only performed where there are circuit patterns such as circuits and solder pads. Since the surface area that needs to be plated is reduced, the required power supply current capacity is usually greatly reduced. In addition, when using contrast reverse photopolymer dry film plating resist (the most commonly used type), the negative film It can be made with a relatively inexpensive laser printer or drawing pen. The copper consumption of the anode in the circuit electroplating is less, and the copper that needs to be removed during the etching process is also less, so the analysis and maintenance costs of the electrolytic cell are reduced. The disadvantage of this technique is that the circuit pattern needs to be plated with tin/lead or an electrophoretic resist material before etching, and it is removed before the solder resist is applied. This adds complexity and an additional set of wet chemical solution treatment processes.

2. Copper plating on the whole board

In the process, all surface areas and drilling holes are copper-plated, and some resist is poured on the unneeded copper surface, and then an etching resist metal is plated. Even for a medium-sized printed circuit board, this requires an electric excavator that can provide a considerable amount of current to make a smooth, bright copper surface that is easy to clean and can be used in subsequent processes. If you do not have a photoelectric plotter, you need to use a negative film to expose the circuit pattern, making it a more common contrast inversion dry film photoresist. Etching the full-board copper-plated circuit board will remove most of the material plated on the circuit board again. As the copper carrier liquid in the etchant increases, the burden of additional corrosion on the anode is greatly increased.

For the manufacture of printed circuit boards, circuit plating is a better method, and the standard thickness is as follows:

1) Copper

2) Nickel 0.2mil

3) Gold (connector top) 50μm

4) Tin-lead (circuits, solder pads, through holes)

The reason why such parameters are maintained in the electroplating process is to provide the metal plating layer with high conductivity, good solderability, high mechanical strength, and can withstand the terminal panel of the components and fill the copper from the surface of the circuit board into the plated through hole. Required ductility.