Views: 0 Author: Site Editor Publish Time: 2024-06-03 Origin: Site
Comparison of different PCB surface treatment processes, their costs are different, of course, the occasions used are also different, which is why there are so many processes to let us choose. Each process has its own characteristics, the existence of both reasonable, the key is that we have to know them to use them.
Scope of application: tin spraying process had been in the PCB surface treatment process in a dominant position, especially for the size of the larger components and spacing larger wires, but it is a very good process.
Advantages: Lower price, good soldering performance. In the higher density of the PCB, tin spraying process will affect the flatness of the subsequent assembly; therefore, HDI boards generally do not use tin spraying process.
Disadvantages: not suitable for welding fine gap pins and too small components, this is because the surface flatness of the tin spray board is poor. Easy to produce tin beads in the PCB processing, the fine gap pin components are more likely to cause a short circuit. Used in double-sided SMT process, this is because the second side has been a high-temperature reflow soldering, it is very easy to happen to spray tin re-melting and produce tin beads or similar to the effect of gravity into a drop of spherical tin point, resulting in the surface is more uneven and thus affect the welding problem.
Practice: In the PCB surface coated with molten tin-lead solder and heated compressed air levelling (blowing) process, so that the formation of a layer of both resistance to copper oxidation and can provide good solderability of the coating layer. Hot air levelling level solder and copper in the combination of copper and tin metal compounds, the thickness of about 1 ~ 2 mil.
Scope of application: It is estimated that at present about 25% -30% of the PCB using OSP process, the proportion has been rising (it is likely that the OSP process has now exceeded the spray tin and in first place).OSP process can be used in low-tech PCB, can also be used in high-tech PCB, such as single-side TV with PCB, high-density chip packaging with the board. For BGA, OSP application is also more. PCB if there is no surface connection functional requirements or storage period of the limit, OSP process will be the most ideal surface treatment process.
Advantages: All the advantages of bare copper soldering, expired (three months) boards can also be re-do surface treatment, but usually limited to one time.
Disadvantages: susceptible to acid as well as humidity. When used for secondary reflow, it has to be done within a certain period of time, and usually the results of the second reflow will be worse. If the storage time is more than three months, it must be re-surfaced. The OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer before contacting the needle point for electrical testing.
Practice: On the clean surface of bare copper, a layer of organic skin film is chemically grown. This film has anti-oxidation, thermal shock resistance, moisture resistance, used to protect the copper surface in the normal environment will not continue to rust (oxidation or sulphide, etc.); at the same time must be in the subsequent soldering of high temperature, can be very easy to be quickly removed by the flux to facilitate soldering.
Scope of application: it is mainly used in the surface of the board with connection functional requirements and longer storage period, such as mobile phone key area, router shell edge connection area and chip processor elastic connection of the electrical contact area. ENIG was widely used in the 1990s due to the flatness problems of the tin spray process and the flux removal problems of the OSP process; the use of the ENIG process has since decreased due to the emergence of black discs and brittle nickel-phosphorus alloys, although almost every high-tech circuit board manufacturer currently has an ENIG line.
Considering that the solder joints become brittle when copper-tin intermetallic compounds are removed, a lot of problems will occur at the relatively brittle nickel-tin intermetallic compounds. Therefore, portable electronic products (such as mobile phones) almost always use OSP, Immersed Silver or immersed tin to form the copper-tin intermetallic compound solder joints, while the use of Immersed Gold to form the key area, the contact area and the shielding area of the EMI, the so-called selective ENIG process. It is estimated that at present about 10-20% of PCBs use chemical nickel plating / immersion gold process.
Advantages: not easy to oxidise, can be stored for a long time, the surface is flat, suitable for soldering fine gap pins as well as solder joints smaller components. Preferred PCB boards with keys (such as mobile phone boards). Can be repeated many times over the reflow soldering is not likely to reduce its solder ability. Can be used as a COB (Chip On Board) to play the line of the base material.
Disadvantages: Higher cost, poorer solder strength due to the use of electroless nickel plating process, prone to black disc problems. The nickel layer will oxidise over time, making long-term reliability a problem.
Practice: in the copper surface wrapped in a thick layer of good electrical properties of nickel-gold alloy and can be long-term protection of the PCB, unlike the OSP only as a rust barrier layer, which can be useful in the process of long-term use of the PCB and achieve good electrical properties. In addition, it also has other surface treatment process does not have the tolerance of the environment.
Scope of application: Immersed Silver is cheaper than Immersed Gold, if the PCB has a connection functional requirements and the need to reduce costs, Immersed Silver is a good choice; coupled with the good flatness of Immersed Silver and contact, it would be even more desirable to choose the Immersed Silver process. In communications products, automotive, computer peripherals, Immersed Silver applied to a lot of high-speed signal design in the Immersed Silver has also been applied.
It is also used in high frequency signalling because it has good electrical properties unrivalled by other finishes and is recommended by EMS because it is easy to assemble and has good inspect ability. However, the growth of immersion silver has been slow (but not declining) due to defects such as loss of lustre and solder joint voids. It is estimated that there are about 10-15% of PCBs using the immersion silver process.
Characteristics: Between OSP and chemical nickel plating / immersion gold, the process is simpler and faster. Exposed to heat, humidity and contamination, it still provides good electrical properties and maintains good solder ability, but loses its lustre. This is due to the fact that there is no nickel underneath the silver layer, so immersion silver does not have all the good physical strength of chemical nickel/impregnated gold.
Scope of application: Nickel plating is used as a backing layer for precious metals and base metals on PCBs, and is commonly used as a top layer for some single-sided printed boards. For some surfaces subject to heavy wear and tear, such as switch contacts, contacts or plug gold, nickel is used as a backing layer for gold, which can greatly improve wear resistance.
When used as a barrier layer, nickel effectively prevents diffusion between copper and other metals. Matte nickel/gold combination coatings are often used as etch-resistant metal coatings and can be adapted to the requirements of hot-press and brazing. The only only nickel can be used as a resist coating containing ammonia etchant, and do not need hot press soldering and require bright PCB plating, usually using light nickel / gold plating. Nickel coating thickness is generally not less than 2.5 microns, usually 4-5 microns.
Advantages: can greatly improve the wear resistance, and effectively prevent the diffusion between copper and other metals.
Disadvantages: the colour is not bright enough, slightly inferior to the appearance of sunken gold.
Practice: in the PCB surface conductor first plated on a layer of nickel and then plated on a layer of gold, nickel plating is mainly to prevent the diffusion between gold and copper. Now there are two types of nickel plating: soft gold plating (pure gold, gold shows that it does not look bright) and hard gold plating (smooth and hard surface, wear-resistant, containing cobalt and other elements, the surface looks brighter). Soft gold is mainly used for chip packaging when playing gold wire; hard gold is mainly used in non-welded electrical interconnections (such as gold fingers).
The introduction of Immersion Tin into the surface treatment process has been a matter of recent decades, and the process has emerged as a result of the requirements of production automation. Immersion Tin does not introduce any new elements at the soldering point and is particularly suitable for communication backplanes. Outside the storage period of the boards tin loses its solder ability and thus Immersion Tin requires good storage conditions. In addition, the use of Immersion Tin is restricted due to the presence of carcinogenic substances in the process. It is estimated that about 5-10 per cent of PCBs currently use the Immersion Tin process.
Other Surface Treatment Processes
The application of other surface treatment processes is relatively small, the following look at the application of a relatively large number of electroplating nickel gold and chemical palladium plating process. Electroplating nickel gold is the originator of the PCB surface treatment process, since the emergence of PCB it appeared, and then slowly evolved into other ways. It is in the PCB surface conductor first plated with a layer of nickel and then plated with a layer of gold, nickel plating is mainly to prevent the diffusion of gold and copper. Now there are two types of electroplated nickel gold: plated soft gold (pure gold, the gold surface does not look bright) and plated hard gold (smooth and hard surface, wear-resistant, containing cobalt and other elements, the gold surface looks brighter). Soft gold is mainly used in chip packaging to strike gold wires; hard gold is mainly used for electrical interconnections at non-soldered places. Considering the cost, the industry often performs selective plating by image transfer method to reduce the use of gold.
The use of selective gold plating continues to increase in the industry today, primarily due to the difficulty of controlling the electroless nickel/dip gold process. Normally, soldering causes embrittlement of the electroplated gold, which will shorten its life, so soldering on electroplated gold is avoided; however, with electroless nickel/impregnated gold, embrittlement is rare due to the thinness and consistency of the gold. The process of chemical palladium plating is similar to that of chemical nickel plating. The main process is through the reducing agent (such as sodium dihydrogen hypophosphite) so that palladium ions in the catalytic surface reduction into palladium, the newborn palladium can become a catalyst to promote the reaction, and thus can get any thickness of palladium coating. The advantages of chemical palladium plating are good soldering reliability, thermal stability and surface flatness.
Choose two or more surface treatment methods for surface treatment, common methods are: immersion nickel gold + anti-oxidation, electroplating nickel gold + immersion nickel gold, electroplating nickel gold + hot air levelling, immersion nickel gold + hot air levelling.
