When we send the designed PCBA drawings to the PCB factory for sampling or mass production, we usually attach a PCB processing technology document, which includes a section that specifies which PCB surface treatment process to use. Different PCB surface treatment processes will have a significant impact on the final PCB processing quotation. Different PCB surface treatment processes will have a significant impact on the final PCB product, and the cost will vary greatly. Then, how to choose the right surface finish for your PCB?
First, you need to know why do you need to perform special treatment on the surface of PCB?
Because copper is easily oxidized in the air, the oxide layer of copper has a great impact on welding, and it is easy to form false welding and virtual welding. When it is serious, it will cause the solder pad and the component to be unable to be welded. Therefore, in the production of PCB, there is a process to coat (plate) a layer of material on the surface of the solder pad to protect it from oxidation.
Currently, the surface treatment processes for PCBs in domestic board factories include: tin spraying (HASL, hot air solder leveling), tin plating, silver plating, OSP (anti-oxidation), chemical gold plating (ENIG), electroplating gold, and so on. Of course, there are also some special PCB surface treatment processes for special applications.
Compared with different PCB surface treatment processes, their costs are different, and of course, they are used in different situations. Only by choosing the right one instead of the expensive one, there is currently no perfect PCB surface treatment process that can be suitable for all application scenarios (here we are talking about cost-effectiveness, that is, the lowest price can meet all PCB application scenarios). Therefore, there are so many processes for us to choose from. Of course, each process has its own advantages, and the key is that we need to understand them and use them well.
So, when you need to choose the right surface finish for our PCB boards, we need to compare its own functional advantages of each different surface finish and its cost.
Physical Properties | HASL (Hot Air Solder Leveling) | Immersion Nickel | ENIS (Electroless Nickel Immersion Silver) | OSP (Organic Soldering Preservative) | Electroplated Ni-Au | ENIG (Electroless Nickel Immersion Gold) |
Storage Life(Month) | 12 | 12 | 12 | 12 | 6 | 6 |
Reflow Times | 4 | 5 | 6 | 4 | 4 | 4 |
Cost | Medium | Medium | Medium | Low | High | High |
Manufacture Difficulty | High | Medium | Medium | Low | High | High |
Manufacture Tem. | 250℃ | 50℃ | 70℃ | 40℃ | 55-60℃ | 80℃ |
Plating Thickness(um) | 1–25 | 0.05–0.2 | 0.8–1.2 | 0.2–0.5 | NI: 4–5 AU>0.05 | NI: 3–5 AU>0.05–0.2 |
The advantages of different surface finish for PCB boards in details:
1. Bare copper board.
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Advantages: low cost, smooth surface, good welding performance (when not oxidized).
Disadvantages: easily affected by acid and humidity, cannot be stored for a long time, must be used within 2 hours after unpacking, because copper is easily oxidized when exposed to air; cannot be used for double-sided boards, because the second side has already oxidized after the first reflow soldering. If there are test points, solder paste must be added to prevent oxidation, otherwise it will not be able to make good contact with the probe in subsequent testing.
2. HASL (Hot Air Solder Levelling) Also Called Tin spraying board.
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Advantages: lower cost, good welding performance.
Disadvantages: not suitable for welding fine-pitch pins and small components, because the surface flatness of the tin spraying board is poor. Solder beads are easily generated during PCB processing, which can cause short circuits for fine-pitch pin components. When used in double-sided SMT process, because the second side has already undergone high-temperature reflow soldering, it is very easy to cause the tin spraying to re-melt and produce solder beads or similar droplets of spherical tin points affected by gravity, which will make the surface even more uneven and affect the welding.
Tin spraying technology used to dominate the PCB surface treatment process. In the 1980s, more than three-quarters of PCBs used tin spraying technology, but in the past decade, the industry has been reducing the use of tin spraying technology. The tin spraying process is relatively dirty, smelly, and dangerous, so it has never been a popular process, but it is an excellent process for larger components and wider wire spacing. In high-density PCBs, the flatness of the tin spraying process will affect subsequent assembly; therefore, the tin spraying process is generally not used for HDI boards.
With the advancement of technology, the industry has now developed tin spraying technology suitable for assembling smaller pitch QFP and BGA, but it is rarely used in practice. Currently, some factories use OSP and immersion gold processes instead of tin spraying technology; technological developments have also led some factories to use tin plating and silver plating processes. In addition, the trend towards lead-free has further limited the use of tin spraying technology. Although so-called lead-free tin spraying has now emerged, this may involve compatibility issues with equipment.
3. OSP(Organic Soldering Preservative)
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Advantages: has all the advantages of bare copper board welding, and expired (three months) boards can be reprocessed, but usually limited to one reprocessing.
Disadvantages: easily affected by acid and humidity. When used in secondary reflow soldering, it needs to be completed within a certain time, and the effect of the second reflow soldering is usually worse. If the storage time exceeds three months, it must be reprocessed. After opening the package, it must be used within 24 hours. OSP is an insulating layer, so solder paste must be added to the test points to remove the original OSP layer before contacting the pin for electrical testing.
OSP technology can be used in low-tech PCBs, as well as high-tech PCBs, such as single-sided TV PCBs and high-density chip packaging boards. OSP is also widely used in BGA applications. If the PCB does not have surface connection functional requirements or storage period limitations, OSP technology will be the most ideal surface treatment process. However, OSP is not suitable for use in small quantities of diverse products or products with uncertain demand forecasts. If the inventory of circuit boards in the company often exceeds six months, it is not recommended to use OSP surface-treated boards.
4. ENIG (Electroless Nickel Immersion Gold).
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Advantages: not easy to oxidize, can be stored for a long time, surface is flat, suitable for welding fine-pitch pins and small components. It is the preferred choice for PCBs with buttons. It can be repeatedly reflow soldered without significantly reducing its solderability. It can be used as a substrate for COB (Chip On Board) wire bonding.
Disadvantages: higher cost, weaker soldering strength, and the use of electroless nickel plating process can easily cause black pad problems. The nickel layer will oxidize over time, and long-term reliability is a problem.
The gold plating process is different from the OSP process. It is mainly used on boards with surface connection functional requirements and longer storage periods, such as button contact areas, edge connection areas of router shells, and electrical contact areas of chip processor elastic connections. Due to the flatness problem of the tin spraying process and the flux removal problem of the OSP process, gold plating was widely used in the 1990s. Later, due to the appearance of black pads and brittle nickel-phosphorus alloys, the application of gold plating process has decreased, but almost every high-tech PCB factory now has a gold plating line.
Considering that the solder joints will become brittle when the copper-tin intermetallic compound is removed, many problems will occur at the relatively brittle nickel-tin intermetallic compound. Therefore, portable electronic products (such as mobile phones) almost all use copper-tin intermetallic compound solder joints formed by OSP, immersion silver, or immersion tin, and use gold plating to form button areas, contact areas, and EMI shielding areas, which is called selective gold plating process.
5. ENIS (Electroless Nickel Immersion Silver).
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Immersion silver is cheaper than immersion gold. If the PCB has surface connection functional requirements and cost reduction is needed, immersion silver is a good choice. In addition to its good flatness and contact properties, immersion silver technology should be chosen.
Immersion silver is widely used in communication products, automobiles, and computer peripherals, and is also used in high-speed signal design. Due to its excellent electrical properties that other surface treatments cannot match, it can also be used in high-frequency signals. EMS recommends the use of immersion silver technology because it is easy to assemble and has good inspectability. However, due to defects such as loss of luster and solder voids, the growth of immersion silver is slow (but not declining).
6. Immersion Nickel
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The introduction of immersion tin as a surface treatment process is a result of the requirement for production automation in the past decade. Immersion tin does not introduce any new elements at the solder joint and is particularly suitable for communication backplanes. Tin will lose its solderability outside the storage period of the board, so immersion tin requires better storage conditions. In addition, the use of immersion tin technology is restricted due to the presence of carcinogenic substances in the process.
The differences and applicable scenarios between gold immersion and gold plating processes.
Many novice PCB engineers may be confused about the differences between gold plating and gold immersion processes. Below, we will compare the differences and applicable scenarios between gold immersion and gold plating processes.
Items | Appearance | Soldering | Signal Transmission | Quality |
Gold Plated Circuit Board | Whitish Gold Color | Occasionally there will be poor welding. | The skin effect is not conducive to the transmission of high-frequency signals. | 1. Gold surface is easy to oxidize. 2. It is easy to cause the gold wire to be slightly short. 3. The bonding force of the solder mask is not strong. |
Immersion Gold Circuit Coard | Yellow Gold Color | Good. | Skin effect has no effect on signal transmission. | 1. Gold surface is easy to oxidize. 2. It is easy to cause the gold wire to be slightly short. 3. The bonding force of the solder mask is not strong. |
Now, you might have found your answer about how to choose the right surface finish for you PCB boards. But if you still have any questions, please click the button below to contact our engineers! We SuperPCBA provide customers with one-stop PCBA services, offering low-cost, high-quality, and fast engineering solutions.