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Knowledge For PCB Does PCB boards have a shelf life? & Introduction of PCB surface treatment process

Does PCB boards have a shelf life? & Introduction of PCB surface treatment process

August 6, 2022     Knowledge For PCB

PCB surface treatment

OSP surface treatment

OSP surface treatmentSince the copper surface of PCB is prone to oxidation in the environment, it is necessary to process the exposed area that is not covered with solder resist ink and cover it with a layer of coating again to protect the area from oxidation. In order to meet the needs of various post-processing processes, various surface treatment methods have been developed with different materials, prices, and different levels of protection.

PCB shelf life were defined based on PCB surface finishes, IPC define PCB shelf life as below:

HASL - 12 months
HASL lead free - 12 months
ENIG - 12months
ENEPIG - 12months
Immersion Silver - 6 months
Immersion Tin - 6 months
OSP - 6 months

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The common PCB surface treatments are as follows.

- Bare Copper: As the name implies, the exposed copper foil is not covered by any treatment at all. The advantage is the lowest cost, good solderability and flatness. The disadvantage is that it is easy to oxidize when exposed to air because there is no protection.

- Tin spraying (HASL/HAL): A layer of tin is applied to the surface of the copper foil and the surface is flattened by blowing with a hot air knife. There are two types of equipment: horizontal tin spraying and vertical tin spraying. Tin spraying is the most widely used surface treatment for PCBs, which has the advantages of good solderability, long storage time and low cost. However, its drawback is that the surface flatness is poor, and the SMT process is prone to poor soldering due to inconsistency in the amount of tin, and the short circuit will be easily caused by tin balls if the joints are too close to each other. Since the tin used for spraying is a tin-lead alloy, the demand for lead-free processes is increasing in recent years as countries promote environmental protection, so there is another lead-free spraying process available.

- Lead-Free HASL: The same as the tin spraying process, the only difference is that there is no lead in the alloy, and there are two common alloys: tin-copper-nickel and tin-silver-copper.

- Electroless Nickel Immersion Gold (ENIG): (Electroless Nickel Immersion Gold), here refers to the electroless nickel immersion gold process for PCB without external current. A layer of chemical nickel is chemically deposited on the bare copper surface, and the gold layer is gradually deposited on the nickel layer through the effect of chemical replacement in the bath, until the nickel surface is completely covered. Due to the chemical replacement method, the thickness of gold cannot be as thick as electroplated gold, and the thickness of gold layer is generally about 1μ"~5μ" and the thickness of nickel layer is about 100μ"~200μ". The advantage is that the PCB does not need to pull the lead wire electroplating - the design is easier, and because the area to be soldered is more flat and even, the parts are more dense area (such as BGA) is less likely to have bad soldering or short circuit due to tin bridge. The disadvantage is higher cost and deviation of soldering strength after SMT.

- Electroplated gold: electroplated nickel gold, mostly using electroplated hard gold. The nickel layer and gold layer are plated on the exposed copper surface of the PCB by connecting the lead wire to the electricity. Commonly, the gold is partially plated on the edge of the PCB to be repeatedly plugged in the contact area, such as the location of the commonly known as the gold finger, but there are a few products choose the whole board electroplated gold, or the local area within the board electroplated gold. Generally, the thickness of gold plating layer is about 5~30μ". The advantage is that the surface of electroplated gold is hard enough and wear-resistant, can be repeatedly rubbed without easy damage to oxidation. The disadvantage is that the plating needs to be connected to the wire and the surface flatness is poor, and the price is extremely expensive.

- Immersion Silver: Immersion Silver(IS),PCB silver plate is the use of immersion plating, the use of copper and silver oxidation potential of different characteristics, so that the copper atoms on the surface of the copper by the replacement of silver, in order to achieve the purpose of surface modification. The general thickness is about 6μ"~10μ". The advantage is the flat surface and good soldering effect. The disadvantage is that the price is slightly higher and it is easy to oxidize/sulfurize after contacting with air, so the PCB should be used as soon as possible after unpacking and the storage time is short.

- OSP (Organic Solderability Preservative): OSP is chemically coated with a layer of organic protective film on the bare copper surface of the PCB, this film can protect the bare copper PCB from oxidation due to air contact. The advantages are lower cost, faster processing, and good soldering and leveling advantages of bare copper. The disadvantage is that it is not possible to test directly after processing and it is difficult to inspect the appearance.

 

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