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With the continuous evolution and upgrading of electronic technology, the pcb manufacturing techniques and pcb surface finish processes have undergone comprehensive innovation, while the associated production processes have been continually optimised and refined. Currently, quality standards for PCB boards are rising across all industries. Taking circuit boards for core electronic devices such as mobile phones and computers as an example, their surface finishs incorporate various materials such as gold and copper, making it easier to distinguish between high-quality and inferior boards.
From an aesthetic perspective, the protective outer layers of PCB boards are primarily categorised into three mainstream colour schemes: gold, silver and light red. Different colours correspond to distinct manufacturing processes and cost structures. In terms of overall cost, gold-plated boards are the most expensive, followed by silver-plated boards, with light red-plated boards being the least expensive. By examining the external colour, one can intuitively determine whether hardware manufacturers have cut corners or skimped on materials. As for the internal conductive circuits of various PCBs, the core material is always pure copper substrate, commonly referred to in the industry as bare copper board.
Bare Copper Board
The performance characteristics of bare copper board are very distinct, with both its advantages and shortcomings being quite pronounced. On the positive side, this board has a low production cost, a flat and smooth surface, and, provided the copper has not oxidised, excellent solderability, capable of meeting the requirements of standard soldering operations.
However, the limitations of bare copper boards are also evident. Copper is highly susceptible to acidic environments and atmospheric humidity, with extremely poor resistance to oxidation. It cannot be stored for long periods; once the packaging is opened, processing must be completed within two hours, otherwise the exposed copper traces will rapidly oxidise and fail. Furthermore, this type of board is unsuitable for the production of double-layer PCBs. After the first high-temperature reflow soldering process, the exposed copper traces on the reverse side will oxidise completely, rendering them unsuitable for further processing. Additionally, test points on bare copper boards must be pre-coated with solder paste as an anti-oxidation measure; otherwise, during subsequent testing, the probes will be unable to make stable contact with the points, leading to failure of electrical testing.
Precisely because of the susceptibility of pure copper to oxidation, the outer layers of PCB boards must be coated with a protective layer. Many people mistakenly believe that the golden surface of the board is the natural colour of the copper; this is a misconception. The gold is merely a protective plating on the surface of the copper layer, and the process used for large-area gold plating is the immersion gold process commonly employed in PCB board production.
Gold-plated boards (electrolytic gold plating)
The gold plating on the surface of gold-plated boards is genuine gold; even when the plating is extremely thin, its cost can account for around 10% of the total production cost of the circuit board. Precisely because the boards contain gold, many businesses in Shenzhen and elsewhere specialise in recycling waste circuit boards, using specialised processes to extract the gold, thereby forming a mature recycling industry chain for waste circuit boards.
The primary purposes of applying a gold plating to the copper layer are twofold: to optimise soldering performance and to enhance corrosion resistance. The gold-plated contacts on standard memory modules are a prime example; even after prolonged use and repeated insertion and removal, the plating remains bright and intact. Were these replaced with ordinary metals such as copper, aluminium or iron, they would have long since corroded and failed due to prolonged exposure to the air.
Gold plating is primarily applied to critical contact points on PCB boards, such as component pads, gold fingers and connection springs, to ensure the stability and durability of device connections. Genuine mobile phone motherboards on the market generally utilise gold-plated or electroplated gold PCB boards, whereas motherboards for computers, audio equipment and small digital devices rarely employ gold plating. If a circuit board in a high-end device features a silver-coloured plating, it is highly likely that the manufacturer has cut corners on materials or substituted inferior components.
The advantages and disadvantages of the immersion gold process are quite clear. Its strengths lie in the plating’s excellent resistance to oxidation, allowing the boards to be stored for long periods; high surface flatness; and suitability for the soldering of precision components with fine-pitch pins and micro-solder joints. It is the preferred process for PCB boards with button functions (such as mobile phone motherboards). Furthermore, the boards can undergo multiple reflow soldering processes without a significant decline in soldering performance, and they can also serve as the base substrate for COB chip bonding processes.
The corresponding drawbacks are the relatively high production costs and weaker solder joint strength. As the process does not involve nickel plating, the boards are prone to ‘black board’ faults, and the nickel underlayer will gradually oxidise over time, which may affect the long-term reliability of the PCB board.
Tin-plated PCBs
The silver-coloured coating on the surface of a PCB board is not made of silver, but rather a layer of tin; boards produced using this process are known as tin-plated PCBs. This process involves spraying a protective layer of tin onto the outer surface of the pure copper traces, which effectively aids in the soldering of components and meets basic assembly requirements. However, unlike a gold layer, the tin coating cannot provide long-term stable contact performance. Whilst it has virtually no adverse effect on components that have already been soldered in place, areas such as exposed pads, ground points and pin headers that are left exposed to the air for extended periods lack sufficient resistance to oxidation and corrosion. Prolonged use can lead to oxidation and rusting, resulting in issues such as poor contact and equipment failure.
The circuit boards of various small digital products generally employ the tin-plating process, primarily because it is cost-effective and helps to keep production costs under control.
The advantages and disadvantages of tin-plated boards are clearly defined. The advantages include lower production costs, good basic soldering performance, and suitability for conventional component assembly. The disadvantages are that the board surface has poor flatness, making it unsuitable for soldering fine-pitch pins or ultra-small precision components. During the PCB manufacturing process, solder ball defects are prone to occur, which can very easily lead to short-circuit faults in fine-pitch pins. In double-sided SMT production processes, after the board has undergone the first high-temperature reflow soldering, the surface tin layer is prone to remelting during secondary processing. Under the influence of gravity, this forms spherical solder balls, further compromising the board’s surface flatness and triggering various soldering faults.
OSP Process Boards (Organic Solder Paste Film Boards)
Light red circuit boards are copper substrates manufactured using the OSP process, also known as mineral lamp thermoelectric separation copper substrates. This process involves coating the surface of the copper traces with a layer of organic solder paste film, which acts as a non-metallic protective layer. The overall production cost is lower than that of the tin-plating process, making it a highly cost-effective basic protective process.
The OSP process retains the excellent soldering performance of bare copper boards, and boards that have exceeded their shelf life can be restored to working condition through re-surface finish, resulting in higher resource utilisation. However, the process also has notable drawbacks: the organic film has poor resistance to acids and moisture, and is less adaptable to environmental conditions. When boards undergo secondary reflow soldering, the duration of the process must be strictly controlled, and the overall results of secondary soldering are generally poor. Standard storage duration must not exceed three months; beyond this period, the surface protection must be reapplied. Furthermore, once the packaging is opened, processing and use must be completed within 24 hours.
Additionally, as the OSP film is an insulating material, solder paste must be applied in advance to the test points on the board. Electrical contact testing can only be performed after the surface insulating film has been removed. The primary function of this organic film is to seal out air and prevent oxidation of the internal copper foil prior to soldering. Under the high temperatures of the soldering process, the film rapidly evaporates, allowing the solder to adhere directly to the copper traces and components, thereby achieving a stable solder joint.
The main drawback of this process lies in its poor corrosion resistance; if OSP boards are exposed to air for more than ten days, the protective effect of the surface layer will be completely lost, rendering them unsuitable for component soldering. Most computer motherboards commonly seen today utilise the OSP process. The primary reason for this is that motherboards are large in size and contain a significant amount of components; if a gold-plating process were used, production costs would rise substantially, resulting in extremely poor value for money.
Surface finish processes for PCB boards each have their own advantages and disadvantages; cost and performance are not entirely directly correlated. When making an actual selection, one should take into account the product’s positioning, soldering requirements and operating environment as a whole, rather than blindly pursuing a particular surface colour.