PCB Technical - Post-processing and production of circuit boards

Post-processing and production of circuit boards

Most of the circuit boards are made in working size until the solder resist is completed. After that, post-processing procedures such as metal surface treatment and shape processing are carried out to match the specifications required for assembly. These are the scope of the post-processing process of the circuit board.

One post-processing process

In order to make the subsequent assembly of the circuit board smooth, slicing, shape processing and other size cutting treatments are necessary tasks, and in order to obtain a good assembly connection, appropriate metal treatments are made on the surface of the contact. Since most circuit board manufacturers have a large customer base, the post-processing procedures may have different results depending on the needs of each company. Mushroom is shown as an example of post-processing flow.

The circuit board with solder stop varnish will be treated with the metal surface of the contact or terminal, and then the working size circuit board will be cut into the appropriate size and shape according to the assembly requirements, and then cleaned or placed in the later stage of the metal treatment and inspection, Packaging and shipping.

Two metal surface treatment

The metal surface treatment of metal contacts and terminals is mainly for loading and connecting various electronic components.

Now the most commonly used solder for soldering is a 63/37 eutectic tin-lead composition, but due to environmental protection factors, lead-containing products will be banned in the future, so various alternatives have been proposed. Currently, there are Sn-Ag, Sn-Ag-Cu, Sn-Ag-Bi, Sn-Cu, etc., for lead-free solder. Although there are many types, all of them are solder pastes. In terms of assembly, it seems that materials are not a problem. But in terms of the metal surface treatment of the circuit board, it is impossible to find a fully compatible product.

(1) Spray tin (HASL -Hot Air Solder Leveling)

After the circuit board is coated with the solder resist, the copper surface exposed to the air must be protected. One of the methods is to attach a layer of solder to the copper surface. Since the solder generally used for electronic soldering has a melting point of about 200°C or less, if it is melted into a molten tin, it can be directly immersed on the copper surface with a layer of solder protective film. The composition of these solders is exactly the same as the tin used for soldering in the future, which is conducive to the assembly of components. However, the amount of tin deposition on the circuit board surface will be too high and uncontrolled by the way of soaking. Therefore, the excess tin amount on the surface is scraped off with high-pressure hot air and the residual tin in the through hole is blown out to protect the copper surface and the inner wall of the hole. the goal of.

The general tin spraying process is: degreasing-micro etching-pickling-drying-flux coating-HAL-cooling-cleaning-drying

In HASL, the circuit board is immersed in molten solder, and high temperature and high pressure air is blown on the panel with an air knife to control the thickness of the solder when it is pulled out. Since it is quite difficult to level the entire board surface with hot air in a short time, the thinner copper pads may cause installation problems when assembling components. Since the tin has not completely cooled and solidified at the moment when the tin is sprayed, the horizontal placement generally has a better thickness distribution. Of course, the thickness of the horizontal tin spraying and vertical tin spraying are not the same. The general experience is that the uniformity of the horizontal tin spraying is slightly better than that of the vertical tin spraying, but the maintenance of the horizontal tin spraying machine is more troublesome. In the near future, due to the lead-free requirements of environmental protection issues, there are doubts about whether the spray tin process will continue to be used, and the selection of solder has become a top priority.

(2) Organic protective film (0SP) Note 113

Covering the copper surface that is not covered by the solder resist with a heat-resistant organic protective film is another way of metal surface treatment. It is also called pre-flux because the next process is to solder components. Since only the fresh copper surface has solder-ability (Solder-ability), if the organic precipitation layer can maintain the fresh copper surface, the subsequent solderability can be maintained. In fact, not all organic protective films have flux resistance. Except for a few rosin series protective films, most protective films have only protective functions. Therefore, in the next soldering, the protective film must be compatible with the flux. Generally speaking, if an organic protective film is used, the activity of the flux used for soldering needs to be slightly stronger. A stronger flux can decompose the organic film in a hot environment and directly connect the tin to the copper substrate.

The current assembly often has more than one remelting process, so the organic film must pass a certain heat resistance test to be competent.

(3) Selective solder plating

In the process of making the circuit by the circuit electroplating method, the solder can be directly electroplated on the circuit area as an etching barrier layer. After etching, the photoresist film is peeled off, and then the second photoresist film is used to select the remaining solder area to cover, And then remove the uncovered area with tin stripping liquid, and reserve the area that needs soldering for soldering. This method must be implemented during circuit electroplating. If the circuit has been formed, it cannot be implemented if the wire is lost. Therefore, most of the solder coating is still made by the spray tin process.

Most solder electroplating probes use tin and lead fluoroborate electroplating systems, and some users use organic acid electroplating systems. The composition of electroplated solder is about 60/40 in tin-lead ratio.

The circuit made for welding must control the amount of precipitation on the copper pad, so the current density and distribution must be controlled, otherwise not only the thickness will deviate, but the composition will also deviate.

(4) Nickel/gold plating

For multi-layer circuit boards and high-density build-up boards, in some applications, bare die assembly and component assembly may be mixed. Recently, due to the gradual growth of organic packaging boards, packaging boards such as BGA, PGA, and CSP will require wire bonding. These circuit boards that need to be bonded must be fully plated with nickel and gold.

The thickness of the common plating layer is about 1-5 m for nickel, and about 0.05-0.75m for gold. The nickel sulfamate plating solution has been extensively explored because of the low plating stress.

Generally, the gold plating system used for gold-plated fingers is not suitable for wire gold plating. The plating solution with metal system additives will make the plating hard. Hard gold has good wear resistance in connector applications, and soft gold is similar to pure gold, which is more suitable for wire bonding. Because it is precipitation by electroplating, the electroplating area must be connected to the electrode, and then cut off after electroplating. Because the residual wire has an antenna effect in the circuit board, some manufacturers use photoresist to block the connection before electroplating. After electroplating, the photoresist is stripped and the pins are etched away. Therefore, there is a so-called Etch Back process. This is not the same thing as the Etch Back that the US military talked about in the early days.

It is not necessary to use electric current to make chemical nickel/gold^16, so there is no need for line connection, which greatly improves the flexibility of circuit board production, so it is valued. When most manufacturers carry out the chemical nickel process, they use hypophosphite as the reducing agent, and the catalyst is similar to the chemical copper system. Due to the use of phosphate system reducing agent, the precipitated nickel will have phosphorus eutectoid phenomenon, and the phosphorus content will affect the physical properties of the coating, so the eutectoid amount must be controlled.

The precipitation of chemical gold is basically divided into two types: replacement gold system and reduced gold system. Nowadays, most of the ones used are replaced chemical gold, which can produce thin gold plating with a thickness of about 0.05-0. 1m or less. The application of thick coating is still more suitable for reduced gold, and some applications reach 0.5m. During gold replacement, pinholes are formed due to ion exchange with the nickel surface, but reduced gold is precipitated using a catalyst, so this phenomenon is relatively absent. Electroless gold plating solutions are mostly cyano-based systems. Because such substances can damage the organic layer of the solder resist, some manufacturers are working hard to develop gold sulfite systems.

For package board assembly using gold wire assembly, a high-purity and thicker gold layer is required. As for product applications that are mainly soldered or aluminum wire, a lower gold plating thickness is required.

Three mechanical processing

In order to meet the final assembly requirements, the circuit board must be shaped and machined in terms of dimensions. The SMT processing and PCBA processing have a high degree of freedom and can adapt to a variety of needs. For the sake of assembly efficiency, the assembly work will be assembled first under the condition that many single circuit boards are integrated together. After the parts are assembled and tested, the work of dividing the individual pieces is carried out. In order to facilitate subsequent division, circuit boards are often processed with broken V-shaped grooves, broken holes drilling, etc., so as to facilitate post-assembly processing. For products with stricter product appearance requirements, the cutting will use a milling machine (Router) for outer frame processing, and for less strict products, there is also a punching mode. At this stage, assembly tool holes will be made at the same time, and some oversized non-plated through holes can also be processed here. As for interface card products, since they are often inserted and removed, they will be chamfered for smooth operation.

After the complete mechanical processing, there will be a lot of dust on the circuit board surface that must be removed. Therefore, the final cleaning action must be performed to remove the cutting powder or the dirt during processing. After drying, the finished product is processed through shipping inspection.