With the continuous improvement of human requirements for living environment, the environmental problems involved in PCB production are particularly prominent.Lead and bromine are the hottest topics, and lead-free and halogen-free will affect PCB development in many ways.
Although changes in the surface treatment process of PCB are not significant at present and seem to be relatively distant, it should be noted that the slow change in the long term will lead to great changes.PCB surface treatment technology is bound to change dramatically in the future as environmental concerns grow.
The most basic purpose of surface treatment is to ensure good weldability or electrical properties.Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as raw copper for a long time, so additional treatment of copper is required.Although a strong flux can be used to remove most copper oxides in subsequent assembly, the strong flux itself is not easily removed, so strong flux is generally not used in the industry.
There are many PCB surface treatment processes, such as hot air leveling, organic coating, electroless nickel/gold, silver and tin.
1. Hot air leveling (tin spraying)
Hot air leveling (also known as hot air solder leveling (commonly known as tin spraying), it is the process of coating molten tin (lead) solder on the surface of PCB and heating compressed air leveling (blowing), so that it forms a coating layer that is not only anti-copper oxidation, but also provides good weldability.Thermally fluidized solder and copper form an intercalated copper-tin compound at the junction.PCB should be immersed in molten solder during hot air conditioning.Blow-flat liquid solder before the solder solidifies;The wind knife minimizes and prevents weld bridging in the curved month of the solder on the copper surface.
2. Organic weldability protectant (OSP)
OSP is a process for surface treatment of printed circuit board (PCB) copper foil in accordance with RoHS instructions.OSP is the abbreviation of Organic Solderability Preservatives.In a nutshell, OSP is a chemical process to grow a layer of organic skin on a clean bare copper surface.
This film has anti-oxidation, heat shock and moisture resistance to protect copper surface from rust (oxidation or vulcanization, etc.) in normal environment.However, at subsequent welding temperatures, the protective film must be easily removed by the flux, so that the exposed clean copper surface can be immediately combined with the molten solder to form a solid solder spot in a very short time.
3. All plates are plated with nickel gold
Nickel-gold plating is to coat the surface conductor of PCB with a layer of nickel and then a layer of gold. The main purpose of nickel plating is to prevent the diffusion between gold and copper.There are two types of electroplated nickel gold: soft gold (pure gold, with an unshiny surface) and hard gold (smooth and hard, wear-resistant, and containing other elements such as cobalt, with a shiny surface).Soft gold is mainly used for chip packaging with gold wire.Hard gold is mainly used for electrical interconnections in non-welds.
The deposit is a thick nickel gold alloy with good electrical properties on the copper surface, which can protect the PCB for a long time.It also has environmental tolerance that other surface treatment processes do not.Gold also prevents copper from dissolving, which would be beneficial for lead-free assembly.
5, sink a tin
Since all current solders are based on tin, the tin layer can match any type of solder.The process of settling tin can form a flat copper-tin intergeneric compound, which makes it as weldable as hot air leveling without the flatness problem of hot air leveling.The solder plate cannot be stored for too long and must be assembled according to the order of the solder plate.
6, heavy silver
The silver precipitation process is between organic coating and chemical nickel plating/gold precipitation.Even when exposed to heat, moisture and pollution, silver remains well weldable, but it loses its luster.Sunken silver does not have the physical strength of electroless nickel/sunken gold because there is no nickel below the silver layer.
7. Chemical nickel palladium
Chemical nickel palladium is a layer of palladium between nickel and gold, which can prevent corrosion caused by replacement reaction and make full preparation for alloy deposit.Gold covers palladium tightly, providing a good interface.
8. Hard gold electroplating
In order to improve the wear resistance of the product, increase the number of insertion and extraction and electroplating hard gold.