Most of the PCBA are manufactured in working dimensions until the Solder Resist job is finished, and then post-processing procedures such as metal surface treatment and profiling processing are performed to meet the specifications required for assembly. These are the areas covered by the post-processing of PCBA.

1. Post-processing procedure

In order to make the subsequent assembly of the circuit board smooth, dimensional cutting processing such as slicing and outline processing is necessary. In order to obtain a good assembly connection, the metal surface of the contact is appropriately treated. Depending on the customer's needs, post-processing procedures may have different results. Example of post-processing flow:

The circuit board that has completed the solder resist will be subjected to the metal surface treatment of the contacts or terminals, and the working size circuit board will be cut into an appropriate size and shape according to the assembly needs, and then cleaned, or metal processing, inspection and packaging shipment.

2. 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 is a 63/37 eutectic tin-lead composition, but due to environmental factors, lead-containing products will be banned in the future, so various alternatives have been proposed. The solutions currently seen for lead-free solders include Sn-Ag, Sn-Ag-Cu, Sn-Ag-Bi, Sn-Cu, and the like. However, in terms of metal surface treatment of circuit boards, it is impossible to find fully compatible products.

2.1. HASL -Hot Air Solder Leveling

After the circuit board is coated with solder resist, the copper surface exposed to the air must be protected. One method is to attach a layer of solder to the copper surface. Because the solder used for electronic soldering generally has a melting point below 200 ° C, if it is melted into a furnace to melt the tin, it can be directly coated with a layer of solder protection film by immersion. The composition of these solders is similar to the tin used for subsequent soldering, which is beneficial to the assembly of components. Due to the immersion method, the amount of tin deposited on the circuit board surface will be too high and uncontrolled, so the excess tin on the surface is scraped with high pressure hot air and the residual tin in the through holes is blown out to protect the copper surface and the inner wall of the hole the goal of.

The general HASL process is: degreasing→microetching→ acid pickling→drying→flux coating→HAL→cooling→cleaning→drying

HASL is to immerse the circuit board in molten solder, and blow high temperature and high pressure air on the panel with an air knife when pulling out to control the thickness of the solder. Because it is very difficult to level the entire board surface in a short period of hot air, the thin copper pads have assembly problems when assembling components. The moment of spraying tin, the tin has not completely cooled and solidified, so it will generally have a good thickness distribution when placed horizontally. Of course, the thickness of the HASL is different from the thickness of the vertical spray and the uniformity of the horizontal spray is slightly better than the vertical, but the maintenance of the horizontal spray is more troublesome. Due to the lead-free demand for environmental protection issues, doubts have been raised as to whether the sprayed tin process will continue to be used, and the selection of solder has become an urgent task.

2.2. 0SP

Covering the copper surface that is not covered by the solder resist with a heat-resistant organic protective film is another method of metal surface treatment, which is also called pre-flux. Because the next process is welding components. Since the fresh copper surface has Solder-ability, if the fresh copper surface can be retained by the organic precipitation layer, the subsequent solderability can be maintained. In fact, not all organic protective films are solderable. Except for a few rosin series protective films, most of the protective films only have a protective function. Therefore, in the subsequent soldering, the protective film must be compatible with the flux. If an organic protective film is used, the flux used for soldering needs to be slightly stronger. A stronger flux can decompose the organic film in a thermal environment and directly connect tin and copper substrates.

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

2.3. Selective solder plating

In the production process of the circuit plating method, the solder can be directly plated on the circuit area as an etching stopper. After the etching, the photoresist film is peeled off, and then the second photoresist film is selected to cover the solder area to be retained. After that, the uncovered area is removed with a stripping solution, and the area requiring soldering is reserved for soldering. This method must be performed during the plating of the circuit. If the circuit has been formed, it cannot be performed without the wire. Therefore, most of them still use solder spraying to make solder coating.

Most solder plating uses tin and lead fluoroborate plating systems. Some users use organic acid plating systems. The composition of the solder is about 60/40 tin-lead ratio.

The circuit made for soldering 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 may also deviate.

2.4. Nickel / Gold Plating

Multi-layer circuit boards and high-density build-up boards may have mixed die assembly and component assembly in some applications. Recently, due to the gradual growth of organic packaging boards, BGA, PGA, CSP and other packaging boards will have wire bonding requirements. These circuit boards that need to be bonded must be plated with nickel-gold plating.

For common coating thickness, nickel needs about 1-5 m, and gold needs about 0.05-0.75 m. Nickel sulfamate plating solution is widely used because of less plating stress.

The gold plating system generally used for gold-plated fingers is not suitable for Wire Bonding gold plating. The plating solution with metal system additives will make the coating hard. Hard gold has good abrasion resistance in connector applications, and soft gold is similar to pure gold, which is more suitable for Wire Bonding. Because it is deposited by electroplating, the plating area must be connected to the electrode and then cut off after plating. Because the residual wire has an antenna effect in the circuit board, some manufacturers use a photoresist to block the connection before electroplating. After electroplating, the photoresist is stripped and the pins are etched. Therefore, there is a so-called Etch Back process.

It is not necessary to use current when making chemical nickel / gold, so there is no need for line connection, and the flexibility of circuit board production is greatly improved, so it is valued. Most manufacturers use hypophosphite as the reducing agent in the chemical nickel process, and the catalyst is similar to the chemical copper system. Due to the use of a phosphate system reducing agent, the precipitated nickel will have the phenomenon of phosphorus eutectoid, and the phosphorus content will affect the physical properties of the coating, so the amount of eutectoid must be controlled.

Chemical gold precipitation is basically divided into two types of replacement gold system and reduced gold system. Now most of the users are replacement chemical gold. The thickness of gold that can be produced is about 0.05-0. 1m or less. The application of thick plating is still more suitable for reducing 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 rarely precipitated using a catalyst. Electroless gold plating solutions are mostly cyano-based systems. Since such materials can harm the organic layer of the solder stop paint, some manufacturers are working hard to develop gold sulfite systems.

For packaging board assembly using gold wire assembly, high-purity and thick gold ore layers will be required. As for welding or aluminum wire product applications, lower gold plating thickness will be required.

3. Machining

In order to meet the final assembly requirements, the circuit board must be formed and machined. The SMT processing and PCBA processing have a high degree of freedom to meet a variety of needs. For assembly efficiency issues, assembly operations are performed prior to the assembly of many single circuit boards. After the assembly test of the parts, the work of dividing the single piece is performed. In order to make subsequent division easier, the circuit board is often processed by breaking V-grooves and breaking holes by drilling, so as to make the post-assembly processing easy. For products with stricter product appearance requirements, the router will be used for the cutting of the frame. For less stringent products, there is also a punch processing mode. At this stage, the fabrication of assembly tool holes is also performed, and some oversized non-plated through holes can also be processed here. As for the interface card products, chamfering will be performed for smooth operation because it is often inserted and removed.

After the complete body machining, a lot of dust on the circuit board surface must be removed, so the final cleaning action must be performed to remove the cutting powder or dirt during processing. After drying, it is finished after the shipment inspection.