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In the wake of developments in science and technology, PCB manufacturing technology improves a lot. Every field in electronic technology urges PCB manufacturers to elevate the corresponding technology in PCB fabrication. When it comes to printed circuit boards which are in a computer or a mobile , materials like gold or silver are adopted, leading to a larger gap among different printed circuit boards.
What Is Surface Finish
PCB surface finish is an inter-metallic joint between the bare copper of the solderable area of the printed circuit board and the components.
How Does Surface Finish Affect PCB
There are three different colors in the printed circuit board surface finish: gold, silver, and light red. Gold is the most expensive, silver is second, and light red is the cheapest. It is easy to judge from the color of PCB whether manufacturers did shoddy work.
1. Bare Copper Board
The traces inside the board is mainly pure copper, that is, bare copper board.
Bare Copper Printed Circuit Board
The advantages and disadvantages are obvious:
Advantages:
1. Low cost
2. Smooth surface
3. Good solderability (Not oxidized)
Disadvantages:
1. Bare copper board is susceptible to acid and humidity.
It should not be stored for a long time and must be used up within 2 hours after unpacking because copper is easily oxidized when exposed to the air; it cannot be used for double-sided boards, because the second side is already oxidized after the first reflow soldering. If there is a test point, solder paste must be printed to prevent oxidation, otherwise, it will not be in good contact with the probe.
2. Copper is easily oxidized when exposing to the air and must have a protective layer. Some people think that copper is blonde. Actually, the blonde is the protective layer on the copper but not the copper itself
When it comes to plate a large area of gold on the circuit board, there is another surface finish - ENIG.
2. Gold-plated Board
Gold is real gold. Even if a very thin golden layer will account for nearly 10% of the cost of the circuit board. In Shenzhen, many merchants specialize in washing out gold through waste-printed circuit boards, which gains a good income.
Using gold as a plating layer is one for facilitating soldering and the other for preventing corrosion. Even the gold finger of the RAM that has been used for several years still shining as before. If the materials are copper, aluminum, and iron, they will have now been rusted into a pile of scraps.
The gold-plated layer is widely used in the component pads, gold fingers, connector shrapnel, and other positions of the printed circuit board. If you find that the color of surface is silver you should call the consumer rights hotline immediately, the manufacturer must cheat on workmanship and materials, who failing to use materials properly instead of other metals to fool customers.
The motherboards of the most widely used mobile circuit boards are mostly gold-plated boards, immersed gold boards. And computer motherboards, audio, and small digital circuit boards are generally not gold-plated boards.
Printed Circuit Board with Immersion Gold Surface Finish
The advantages and disadvantages of immersion gold technology are not difficult to draw:
Advantages:
1. Not easy to oxidize
2. Long shelf-life
3. The surface of the gold-plated board is flat, which is suitable for soldering fine-gap pins and components with small solder joints.
The first choice for console keyboard & display board (such as mobile PCB). Because reflow soldering can be repeated many times without reducing its solderability, the gold-plated board can be used as a substrate for COB (Chip On Board) wire bonding.
Disadvantages:
1. High cost
2. Poor solderability
Because of the electroless nickel plating process, it is easy to cause a black disk. The nickel layer will oxidize over time, and the long-term reliability appears as a problem.
Now we know that gold is gold. But does silver is silver? Of course not, it is tin!
3. HASL
The silver board is used HASL (hot air solder leveling), which sprays a layer of tin on the outer layer of the copper circuit. It cannot provide long-term contact reliability like gold. Though HASL does not affect the PCB whose components have been soldered, the solderability will be influenced by the pads that have been exposed to the air for a long time, such as grounding pads and pin sockets.
It is prone to oxidation and corrosion in the long term, resulting in poor contact between the board and components. Generally, for small digital products, the surface finish choice is HASL. The reason is HASL is cheap.
Lead Free HASL Surface Finish
Its advantages and disadvantages are summarized as:
Advantages:
1. Lower price
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2. Good solderability
Disadvantages:
1. Not suitable for soldering pins with fine gaps and components that are too small, because the surface flatness of the HASL board is poor.
2. Easy to generate solder bead in PCB processing, resulting in short circuit to fine-pitch components.
When it comes to HASL used in the double-sided SMT process, it is easy to re-melt and generate tin beads or similar droplets that are affected by gravity into spherical tin dots, which will reduce the flatness of the surface of PCB. It is important because the flatness affects soldering.
4. OSP
OSP is the abbreviation of organic soldering film. It used organic in surface finish but not metal, leading to a cheaper price than HASL.
The only function of this organic film is to ensure that the inner copper foil will not be oxidized before soldering. This layer of film volatilizes as soon as it is heated during soldering and solder the copper traces and the components together. But OSP is not resistant to corrosion. If an OSP circuit board is exposed to the air for ten days, the components cannot be soldered.
Many computer motherboards use OSP technology. Because the area of the circuit board is too large, it is expensive for using immersion gold.
Printed Circuit Board with OSP
The advantages and disadvantages are:
Advantages:
1. It has all the soldering advantages of bare copper plate, and even the expired board can also take a surface finish again.
Disadvantages:
2. Easily affected by acid and humidity.
When it is used in secondary reflow soldering, OSP needs to be completed within a certain period. Moreover, the second reflow soldering has poor solderability.
3. The shelf-life of the OSP board is short, which can not exceed three months. And it must be finished within 24 hours after opening the package.
Disadvantages:
OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer to contact the pinpoint for electrical testing.
PCBgogo has multiple choices for the surface finish in the PCB assembly service. They do a lot of low to mid-volume runs in a high-mix setting, and with ENIG, they can buy bare boards in a larger economical quantity and store them for several years while doing multiple smaller runs to customers' release schedules. It's one more way we ensure high quality and flexibility to meet the customers' needs.
A printed circuit board (PCB) surface finish has two main functions: To prevent the copper from oxidizing and to provide a solderable surface.
Some PCB surface finishes, however, are more popular and versatile than others. As you review your options, you'll likely consider durability, cost, and the materials used within the finish, such as tin-lead, lead-free, various gold finishes, and silver. Additionally, you may have concerns about the finish compatibility with the subsequent termination process ' such as wire bonding or welding. But how do you know which surface finish is best for your next PCB project?
We've compiled a comprehensive list of the nine different surface finishes. In this article, we'll explain the purpose and common uses of each surface ' plus the advantages and disadvantages ' so you can find the best option for your next project.
The 9 Different PCB Finishes
How to Choose the Best PCB Finish for Your Project
ENIG is the most popular option and is used on about 80% of all PWBs. This finish provides a thin, gold, solderable layer that protects the copper traces with a nickel barrier between it and the copper. ENIG is a good lead-free option that results in a durable, long-lasting finish.
Advantages:
With HASL, the printed circuit board is held vertically by clamps and dipped into a flux bath and then a molten solder hot vat. The finish, which contains tin/lead, is leveled off with hot air knives and the printed circuit board panel is raised out of the molten solder vat. This ensures a homogenous thickness over the entire board. It was once one of the most popular finishes but is now mainly used only on military and aerospace applications.
Advantages:
Disadvantages:
The properties and uses of this finish are the same as HASL ' except there is virtually no lead in the alloy. Instead, the alloy is a tin and copper, tin and nickel, or tin, copper, nickel and germanium alloy. This finish is not widely used.
Advantages:
Disadvantages:
With this finish, a layer of tin is deposited onto the copper surface using a chemical process. This is frequently used in high-speed circuits. Immersion tin works well to protect underlying copper from oxidization, but copper and tin in such close proximity can create diffusion from one metal into the other, resulting in challenges.
Advantages:
Disadvantages:
A thin layer of material is applied over the copper to protect it prior to soldering. Though this water-based organic surface finish was once very popular, it is not widely used today.
Advantages:
Disadvantages:
Hard gold is plated over the nickel, which holds up well and doesn't break down easily compared to other finishes. Because of its durability, this type of PCB plating is used in applications that involve mechanical contact, such as buttons and keypads.
Advantages:
Disadvantages:
Similar to tin, this PCB finish is used in high-speed signals that don't contain any nickel. Immersion silver, however, does require unique storage as it will tarnish if exposed to air.
Advantages:
Disadvantages:
This three-layer metallic finish is used for printed circuit boards that also require wire bonding. The electroless nickel protects the copper and prevents it from coming into contact with the other metals. Palladium provides another anti-oxidation insurance layer and prevents the nickel from corroding, and the final gold layers adds more of a barrier against oxidation ' plus it completely covers the PCB.
Advantages:
Disadvantages:
This PCB finish has been used for a long time and has the same wire bonding process as ENEPIG. The pad is a crowned surface, which means there is less area to land wires.
Advantages:
Disadvantages:
It's important to pick the right PCB plating for your product. While there are a lot of available options, you want to consider the size and scope of your project ' and budget.
For a solid overall option, ENIG offers a high quality finish and low-cost option with good solderability. You may, however, have to choose a finish based on a particular technology or requirement ' such as HASL, which is military, aerospace, and space approved.
Selecting the right PCB surface finish can be a challenge, but we are here to help. Cirexx is your best-in-class solution for PCB layout, fabrication, and assembly. With more than four decades of experience, we have manufactured PCBs for almost every leading technology company ' and we've worked with industries all over the world. Contact us today to learn more about what PCB surface finish you should choose for your next project or to connect with our experienced sales team.
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