PCB, FPC surface treatment process categories
With the continuous improvement of human living environment requirements, the environmental problems involved in the PCB production process are particularly prominent. The topic of lead and bromine is the most popular, and lead-free and non halogenated will affect the development of PCB in many ways.
Although the changes in the surface treatment process of PCB and FPC are not very large at present, it seems to be a relatively distant matter, but it should be noted that long and slow changes will lead to great changes. In the case of increasing environmental protection, the surface treatment process of PCB and FPC will certainly change greatly in the future.
The basic purpose of surface treatment is to ensure good solderability or electrical properties. Since copper in nature tends to exist as an oxide in air, it is unlikely to remain as primary copper for a long time, so copper needs to be treated in other ways. Although the majority of copper oxides can be removed by strong flux in the subsequent assembly, the strong flux itself is not easy to remove, so the industry does not use strong soldering flux.
There are many PCB surface treatment processes, such as hot air leveling, organic coating, electroless nickel plating / gold dipping, immersion silver and tin dipping. The following five steps will be introduced.
1, hot air leveling (HASL) (no tin, tin)
Hot air leveling aka hot air solder leveling (commonly known as tin spray), it is in the PCB coated on the surface of molten tin solder (lead) and compressed air for heating the whole flat (blowing) process, the formation of a layer of both anti oxidation of copper, and can provide good solderability of coating layer. During the hot air, solder and copper combine to form copper tin intermetallic compounds. When the PCB is hot air, it sinks in the molten solder; the wind knife blows the liquid solder before the solder solidifies; the wind knife minimizes and prevents solder from bridging the solder on the copper surface.
2. Organic solderability protection agent (OSP)
OSP is a process for the surface treatment of printed circuit board (PCB) copper foil in accordance with the requirements of RoHS. OSP is the abbreviation of Organic Solderability Preservatives. It is also called "organic solder protection film". It is also called "copper protecting agent". It is also called Preflux. Simply put, OSP is on the surface of cleaning bare copper, chemically grown a layer of organic film.
The film has oxidation resistance, thermal shock resistance, moisture resistance, for protecting the surface of copper in the normal environment no longer continue to rust (oxidation or curing); but in the welding heat in the follow-up, this protective film must be very easy to be the flux rapidly cleared, so that clean surface exposed to copper in a very short period of time and the molten solder immediately combined into a solid solder joint.
Organic solderability protection agent (OSP, organic, solderability, preservatives) was called heat resistant flux (preflux) in the early stage. In essence, it is a (ABI, alkyl, benzimidazole) compound with high heat resistance, and its decomposition temperature is generally required at above 300 degrees celsius. Therefore, it can protect the fresh copper surface from oxidation and contamination very well. When soldering at high temperature, the OSP is exposed to the surface of the copper due to the action of the solder, and the solder is quickly welded to the solder. But it is not suitable for reflow soldering.
3, full plate nickel plated gold (plated soft gold)
The gold plated with nickel plated on the PCB surface conductor is first plated with nickel and then plated with a layer of gold. The nickel plating is mainly to prevent the diffusion of gold and copper. There are two kinds of electroplated nickel gold: soft gold plating (pure gold, gold surface does not look bright) and hard gold (smooth and hard surface, wear resistant, cobalt and other elements, the surface of gold looks brighter). Soft gold is mainly used for gold wire in chip packaging; hard metal is mainly used in non welding electrical interconnection.
4, sink, nickel, gold
The deposit of gold is a thick, well made nickel gold alloy on copper surface, which can protect PCB for a long time. Besides, it has the environmental endurance that other surface treatment processes do not have. In addition, the deposit can also prevent the dissolution of copper, which will be beneficial to lead-free assembly.
5, sink tin
Since all solders are based on tin, the tin layer matches any type of solder. Tin process can form metal flat copper tin intermetallic compounds, this characteristic makes tin with hot air leveling as good weldability and no flatness of the hot air leveling headache; Shen tin can not be stored for a long time, the assembly must be according to the order of tin.
6, sink Silver
Heavy silver process between organic coating and electroless nickel / gold, process is simple and rapid; even when exposed to heat, humidity and pollution of the environment, the silver is still able to maintain good weldability, but will lose luster. Heavy silver does not have the good physical strength of electroless nickel / gold deposits because there is no nickel below the silver layer.
7, ENEPIG
ENEPIG is between nickel and gold palladium compared with a layer of gold, palladium can prevent the corrosion phenomenon caused by the replacement reaction, fully prepared for gold. Gold is tightly covered with palladium to provide a good interface.
8, electroplating hard gold
In order to improve the wear resistance of products, increase the number of plugging and electroplating hard gold. Electric gold wear resistance is relatively high, with the integration of IC more and more high, IC feet more and more closely. The pad blowing flat and vertical tin spray process is difficult to be fine, which gives the SMT mount brought difficulty; also HASL inactive life (shelf life) is very short. The gold plate just solves these problems: 1 for surface mount technology, especially for 0603 and 0402 ultra compact surface, because the pad flatness is directly related to the solder paste printing process |