PCB Printing Process of Several Special Surface Treatment Instructions

A. Why Should the PCB Surface Special Treatment?

Copper is easily oxidized in the air, the copper oxide layer has a great impact on the welding, it is easy to form a false soldering, false soldering, and in serious cases will cause the pad and components can not be welded. Therefore,PCB in the production of manufacturing, there will be a surface treatment process, in the pad surface coating (plating) coated with a layer of material to protect the pad from oxidation.

At present, the domestic board factory PCB surface treatment process: spray tin (HASL, hot air solder leveling hot air leveling), sinking tin, sinking silver, OSP (antioxidant), chemical immersion gold (ENIG), gold plating, etc., of course, there will be a number of special applications of special PCB surface treatment process.

Comparison of different PCB surface treatment processes, their costs are different, of course, the occasions used are also different, only choose the right not to choose expensive, there is no perfect PCB surface treatment process can be suitable for all application scenarios (here is the cost-effective, that is, to meet the lowest price of all the PCB application scenarios), so there will be so many processes to allow us to choose, of course, each process has its own unique characteristics. Each has a thousand years, the existence of both reasonable, the key is that we have to recognize them and use them well.

B. PCB Surface Treatment - bare Copper Plate.

Advantages: low cost, flat surface, good solderability (in the absence of oxidation).

Disadvantages: susceptible to acid and humidity, can not be placed for a long time, unpacked need to be used up within 2 hours, because the copper is exposed to the air is easy to oxidize; can not be used in double-sided panels, because after the first reflow soldering on the second side has been oxidized. If there is a test point, you must add printed solder paste to prevent oxidation, otherwise the follow-up will not be able to contact with the probe well.

C. PCB Surface Treatment - spray Tin Plate (HASL, Hot Air Solder Levelling).

Advantages: lower price, good welding performance.

Disadvantages: not suitable for welding fine gap pins and too small components, because the surface flatness of the tin spray board is poor. Easy to produce tin beads in PCB processing (solder bead), the fine pitch (fine pitch) components are more likely to cause a short circuit. Used in double-sided SMT process, because the second side has been a high-temperature reflow soldering, it is very easy to spray tin re-melting and produce tin beads or similar to the effect of gravity into a drop of spherical tin point, resulting in the surface is more uneven and thus affect the welding problem.

Tin spraying process used to be in the PCB surface treatment process in a dominant position. In the 1980s, more than three-quarters of PCBs using the tin spray process, but over the past decade the industry has been reducing the use of tin spray process. The tin spray process has never been a favorite because it is dirty, smelly and dangerous, but it is an excellent process for larger components and widely spaced wires. In the higher density PCB, the flatness of the tin spray process will affect the subsequent assembly; therefore, HDI boards generally do not use the tin spray process.

With technological advances, the industry has now appeared suitable for the assembly of smaller pitch QFP and BGA tin spraying process, but the actual application is less. At present, some factories use the OSP process and immersion gold process to replace the tin spray process; the development of technology also makes some factories use immersion tin, immersion silver process. Coupled with the trend of lead-free in recent years, the use of spray tin process is further restricted. Although the so-called lead-free tin spray has emerged, but this can be related to the compatibility of equipment.

D. PCB Surface Treatment-OSP (Organic Soldering Preservative, anti-oxidation).

Advantages:all the advantages of bare copper soldering, expired (three months) of the board can also be redone surface treatment, but usually to a limit.

Disadvantages:susceptible to acid and humidity. When used for secondary reflow soldering, it needs to be completed within a certain period of time, and usually the effect of the second reflow soldering will be worse. If the storage time is more than three months, it must be re-surfaced. OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer before contacting the needle point for electrical testing.

OSP process can be used in low-tech PCB, can also be used in high-tech PCB, such as single-sided TV with PCB, high-density chip packaging board. For BGA, OSP application is also more. PCB if there is no surface connection functional requirements or storage period of the limit, OSP process will be the most ideal surface treatment process. But OSP is not suitable for use in a small number of diverse products, also not suitable for use in the demand prediction is not allowed on the product, if the company's circuit board inventory is often more than six months, really do not recommend the use of OSP surface treatment of the board.

E. PCB Surface Treatment - immersion Gold (ENIG, Electroless Nickel Immersion Gold).

Advantages: not easy to oxidize, can be stored for a long time, the surface is flat, suitable for soldering fine gap pins as well as solder joints smaller components. It is the first choice for PCB boards with buttons. Can be repeated many times over the reflow soldering is not likely to reduce its solderability. Can be used as a COB (Chip On Board) to play the line of the base material. Disadvantages: Higher cost, poorer solder strength, easy to have black disk problems because of the use of electroless nickel plating process. The nickel layer oxidizes over time, making long-term reliability an issue.

The immersion gold process is different from the OSP process in that it is mainly used on boards with connection functionality requirements on the surface and longer storage periods, such as key contact areas, edge connection areas of router housings, and electrical contact areas of chip processor elastic connections. Sinking gold was widely used in the 1990s due to the flatness problems of the tin spray process and the flux removal problems of the OSP process; the use of the sinking gold process was later reduced due to the emergence of black disks and brittle nickel-phosphorus alloys, although at present almost every high-tech PCB factory has a sinking gold line.

Considering the removal of copper-tin intermetallic compounds when the solder joints will become brittle, relatively brittle nickel-tin intermetallic compounds will appear at a lot of problems. Therefore, portable electronic products (such as cell phones) are almost always used OSP, immersed silver or immersed tin to form the copper-tin intermetallic compound solder joints, and the use of immersed gold to form the key area, the contact area and the shielding area of the EMI, which is known as the selective immersion gold process.

F. PCB Surface Treatment - immersion Silver (ENIG, Electroless Nickel Immersion Gold).

Immersion silver is cheaper than immersion gold, if the PCB has a connection functional requirements and the need to reduce costs, immersion silver is a good choice; plus immersion silver good flatness and contact, it should choose immersion silver process. In communications products, automobiles, computer peripherals, immersed silver applied to a lot of high-speed signal design in the immersed silver has also been applied. It is also used in high frequency signaling because it has good electrical properties that are unmatched by other finishes, and is recommended by EMS because it is easy to assemble and has good inspectability. However, due to the existence of immersion silver such as loss of luster, solder joints and other defects such as voids, making its slow growth (but no decline).

G. PCB Surface Treatment - immersion Tin (ENIG, Electroless Nickel Immersion Gold).

Sinking tin was introduced into the surface treatment process is the last decade, the emergence of the process is the result of the requirements of production automation. Immersion tin does not introduce any new elements at the soldering point and is particularly suitable for backplanes for telecommunications. Outside the storage period of the boards tin loses its solderability and thus immersion tin requires good storage conditions. In addition, the use of immersion tin process is restricted due to the presence of carcinogenic substances. VIII. Difference between immersion gold process and gold-plating process and applicable scenarios:

1. a, immersed gold and gold-plated formation of the crystal structure is not the same, immersed gold plate is easier to weld than gold-plated boards, does not cause poor welding;
2. b, immersed gold plate only pads on the nickel gold, skin effect in the transmission of the signal is in the copper layer will not have an impact on the signal;
3. c, immersed gold is more dense than gold-plated crystal structure, not easy to oxidize;
4. d, immersed gold plate only pads on the nickel gold, does not produce gold wire caused by micro-short;
5. e, immersed gold plate only pads on the nickel gold, line solder resistors and copper layer more firmly combined;
6. f, Immersed gold gold color, more yellow than gold-plated also look better;
7. g, Immersed gold is softer than gold-plated, so in the abrasion resistance is not as good as gold-plated, gold-plated for the gold finger plate will be better.

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