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Circuit Board Manufacturing Process
Buried hole, is printed circuit board (PCB) internal arbitrary circuit layer connection, but not with the outer conduction, that is, no extension to the circuit board surface of the conduction hole. This manufacturing process cannot be achieved through the way of drilling after the bonding of circuit boards. Drilling must be carried out at the time of individual circuit layers. After partial bonding of the inner layer, electroplating is carried out, and finally all the bonding is carried out. Since the operation is more laborious than the original conduction and blind hole, the price is also the most expensive. This process is usually only used for high-density circuit boards, increasing the space utilization of other circuit layers.
After we finished PCB, send it to the PCB factory proofing or mass production, we in order to board factory, will enclose a copy of PCB process documentation, one is to indicate the choice of PCB surface treatment, surface treatment process and different PCB, it will be on the final PCB processing quotation is larger, the influence of different PCB surface treatment process have different charges, below let's science some terms about the PCB surface treatment process.
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Why special treatment on PCB surface? Because copper easily oxidized in the air, copper oxide layer on the welding has a great influence, it is easy to form false welding, welding, caused serious welding plate and components cannot be welded, as such, PCB in production, there will be a process, in the welding plate surface coating (plating) after a layer of material on, protect welding plate from oxidation.
At present, the PCB surface treatment technology in domestic board mills has been like HASL, hot air leveling, tin sinking, silver sinking, OSP(oxidation prevention), chemical gold sinking (ENIG), electric plating and so on. Of course, there are also special PCB surface treatment technology in special application occasions.
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Comparing different PCB surface treatment technology, their cost is different, of course, the situation is different, only don't choose the right, there is no perfect PCB surface treatment to suit all application scenarios (here is cost-effective, at the lowest price they can satisfy all of the PCB application scenario), so just can have so many process to let our choice, each process is different, of course, there is reasonable, the key is we want to know they use them well.
Next, compare the advantages and disadvantages of different PCB surface treatment technologies and applicable scenarios. Advantages of bare copper: low cost, flat surface, good weldability (without being oxidized). Disadvantages: easy to be affected by acid and humidity, cannot be stored for long. It should be used up within 2 hours after opening, because copper is easy to be oxidized when exposed to air. Cannot be used on double panels as the second side has oxidized after the first reflow. If there are test points, solder paste must be printed to prevent oxidation, otherwise subsequent contact with the probe will not be good.
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Advantages of HASL (Hot Air Solder Levelling) : lower price, better welding performance. Disadvantages: not suitable for welding small gap pins and components, because the surface flatness of spray tin plate is poor. Solder bead is easy to be generated in PCB processing, which is more likely to cause short-circuit in fine pitch components. When using two-side SMT process, because the second side has been through a high temperature reflow welding, it is very easy to produce tin beads or similar water droplets falling into spherical tin points affected by gravity, resulting in uneven surface and affecting the welding problem.
Tin spraying technology has been in the leading position in PCB surface treatment technology. In the 1980s, more than three-quarters of PCB's were tin-sprayed, but the industry has been cutting back over the past decade. Tin-spraying has never been a popular process because it is dirty, smelly and dangerous, but it is excellent for large components and wires with large spacing. In PCB with high density, the flatness of tin spraying process will affect the subsequent assembly. Therefore, HDI plate generally does not use spray tin technology.
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With the progress of technology, the industry has now appeared to be suitable for assembling smaller spacing of QFP and BGA tin spraying process, but the practical application is less. At present, some factories use OSP process and gold leaching process instead of tin spraying process; Technological development has also led some factories to adopt the process of tin and silver sinking. In addition to the trend of lead free in recent years, the use of tin spraying technology is further restricted. Although there have been so-called lead-free spraying of tin, this can involve equipment compatibility issues.
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