Common Surface Finishes in PCB Manufacturing
Surface finish is an important factor to consider in the PCB design and manufacturing process. Different surface finishes influence the manufacturing cost. In this article, we will discuss the common surface finishes in China, their pros and cons, and applicable scenarios.
Why Need to Do Special Surface Finish on PCB?
As copper is very easy to oxidize in the air, and the anodizing layer of copper has a very serious influence on soldering, it is easy to form false soldering and pseudo soldering, and in serious cases, the pads and components can not be soldered. So, in the PCB manufacturing process, there is a procedure to plate another layer on the pad with some special substance to prevent it from anodizing.
Now, there are surface finishes like HASL(hot air solder leveling), OSP, ENIG, Immersion Tin, Immersion Silver, and plating gold which are generally used in PCB manufacturing. In some special applicable scenarios, there are some special PCB surface finishes.
Comparing the surface finishes, they have different costs and suitably applicable scenarios. Till now, there is not a perfect PCB surface finish that can meet all the requirements, here we are talking about cost-effectiveness, that is to say, to meet all the PCB applicable scenarios with the lowest price. So there are so many surface finishes to choose and every finish has its advantages, we need to know them better and make good use of them.
Now, let’s compare the pros and cons of the surface finishes and their applicable scenarios;
SMOBC(Solder Mask on Bare Copper)
Pros: low cost with good surface flatness and good soldering performance(under the condition not oxide)
Cons: easily influenced by acid and humidity, can not store for a long time, must be used up within 2 hours after opening the vacuum package, since the copper easily oxidizes with air; can not used for double-sided or more layer PCB, as after the first reflow soldering, the second layer was oxidized. If there is any test point, must use solder paste to prevent it from oxidizing, or it can not contact well with test probes.
HASL(Hot Air Solder Leveling)
Pros: cost-effective with good soldering performance
Cons: Not suitable for soldering pins with fine pitches or too tiny components, as the flatness is not good. During the process, it is easy to generate solder beads, which can easily cause short circuits for components with fine pitch. When used in a double-sided SMT process, because the second side has been through a high-temperature reflow soldering, it is very easy to happen that the tin to re-melting and produce solder beads or similar to the water beads by gravity into a low-falling spherical tin point, resulting in the surface is more uneven and lead to soldering problems.
HASL was in the leading position once among all the PCB surface finishes. In the 1980s, more than three-quarters of PCBs used the HASL process, but in the past decade, the whole industry has tried not to use the HASL process, now about 25%-40% of PCBs are still using HASL. The process of HASL is relatively dirty and dangerous with a bad smell, which was not a fond process all the time, but for the PCB with large size components or circuits with big spacing, it is a nice process. For PCBs with high density, like HDI PCBs, the HASL process will influence the assembly, so generally HDI PCBs will use other finishes.
OSP(Organic Soldering Preservative)
Pros: With all the advantages of bare copper board, expired (three months) boards can also be resurfaced, but usually on a one-time basis.
Cons: Easily influenced by acid and humidity. ?When used for secondary reflow soldering, it needs to be completed within a certain period, and usually, the results of the second reflow soldering will be poorer. If the storage time is longer than three months, it must be re-surfaced. OSP is an insulating layer, so the test point must be covered with solder paste to remove the original OSP layer before doing the electrical test.
Now about 25% of PCB is using the OSP process, and the rate is going up these years. For BGAs, OSP is also used in wide applications. OSP is an ideal surface finish process for PCBs if there is no functional requirement for surface connection or limited storage life.
ENIG(Electroless?Nickel Immersion Gold)
Pros: Not easy to oxidize, can be stored for a longer time with nice flatness, suitable for soldering fine-pitch pins and components with smaller solder points. First choice for pushbutton PCB, like mobile phone boards. PCB with ENIG process can be reflow soldered repeatedly and the soldering performance won’t be lower. Can be used as the base material for COB(Chip On Board)
Cons: cost is high, poor soldering strength, because of the use of electroless nickel plating process, it is easy to have the problem of black pads. The nickel layer oxidizes over time, making long-term reliability a problem.
ENIG is a different process from OSP, it is mainly used on boards with connection functionality requirements and longer storage time, such as cell phone key areas, edge connection areas of router housings, and canonical electrical contact areas for chip processor elastic connections. Due to the flatness issue of HASL and the clarity of the flux of OSP, ENIG had wide use in the 1990s. Later due to the black pads and the emergence of brittle nickel-phosphorus alloys, the use of ENIG became fewer, but now almost every high-tech PCB factory has its own ENIG line.?
ENEPIG(Electroless Nickel Electroless Palladium Immersion Gold)
Pros: Extreme flatness, no lead, no risk for black pads with longer storage time
Cons: High cost than ENIG, lower solderability due to too thick a palladium layer, not stable in bonding compared with ENIG
The difference between ENEPIG and ENIG is the added palladium layer. The palladium helps protect the nickel layer from corrosion, which helps prevent the appearance of a "black pad." This is one of the significant advantages of ENEPIG over ENIG surface finish. The immersion gold layer on top of the palladium provides nearly complete board protection by protecting and preserving the palladium underneath.
Immersion Silver
Pros: Nice flatness, reasonable cost without lead(meets RoHS standard), high solderability
Cons: high storage requirement, easy to pollute, hard to do E-test
Immersion silver is cheaper than ENIG, if PCB has connection functionality requirements and needs to lower the cost, immersion silver is a good choice. In the meantime, the immersion silver process has nice flatness and contactability, which makes it a nice choice as well. In some telecommunication products, automotive, peripheral devices for computers, immersion silver process has very wide use, as well as a high-speed design aspect. Because the immersion silver process has good electrical properties that other surface finishes cannot match, it can be used in PCB which holds high-frequency signals. Most EMS companies recommend the immersion silver process because it is much easier to assemble with good checkability. However, the immersion silver process has defects such as loss of gloss and voids in solder joints, which slows down its growth.
Immersion Tin
Pros: Nice flatness and suitable for SMT process and components with fine pitch, BGA or tiny size, no lead during the surface finish process, and good solderability.
Cons: Short storage time, easy to pollute, special requirement for E-test
Immersion tin has been introduced in the past decades, and the emergence of this process is the result of the requirement for production automation.?Immersion tin does not bring any new elements into the soldering place, especially suitable for communication backboards. Outside the storage period of the board, tin will lose its solderability, so immersion tin requires better storage conditions. In addition, the immersion tin process contains carcinogens and is restricted for use.
Hard Gold Plating
Pros: Hard and stable surface, no lead, high Wear-resistant performance, and long storage time.
Cons: Expensive, not available for the soldering area, needs extra procedures during the process.
Hard gold, technically known as hard electrolytic gold, consists of a layer of gold plated over a nickel coating. The purity of the gold plating categorizes this finish as either hard gold (99.6% pure) or soft gold (99.9% pure) and is typically used in high-wear areas such as edge connector fingers, normally called golden finger. Hard gold plating or electrolytic gold plating uses a thin gold overlay on a nickel layer on a copper surface. The process produces a very durable gold layer, which makes hard gold plating very common in the PCB industry. Although the presence of gold makes the process expensive, it provides a perfect surface for soldering.
How to Choose the Suitable Surface Finish for PCB?
As mentioned previously, some surface finishes can result in an uneven surface, which can affect performance, solderability, and other factors. If flatness is an important factor, consider a surface finish with a thin, uniform layer. In this case, suitable options include ENIG, ENEPIG, and OSP.
Solderability is always a key factor when working with PCBs. Certain surface finishes such as OSP and ENEPIG have been shown to hinder solderability, while other surface finishes such as HASL are well suited.
If your PCB requires gold or aluminum wire bonding, you may be limited to ENIG and ENEPIG.
As mentioned earlier, some surface finishes such as OSP make the PCB fragile when handled, while other surface finishes improve durability. Storage and handling requirements should be considered in advance, and surface finishes that make the PCB fragile should only be used when risk-free storage and handling requirements can be met.
How many times will the PCB be soldered and reworked? Many surface finishes are ideal for rework. However, other methods such as immersion tin are not suitable for rework.
RoHS compliance is critical when determining the surface finish to use. In general, all surface finishes using lead are not suitable for RoHS compliance and should be avoided.
Jerico PCB has surface finish lines in our factories and also a stable supply chain for some special finishes. In this case, we are able to offer various options for surface finishes and support different requirements from customers.
We are always happy to talk about anything about PCB fabrication and it has always been our mission to help our customers resolve their concerns. Jerico PCB is your reliable PCB partner as always.