What is PCB surface finishes?

PCB surface finishes is also called PCB surface treatment, it is a technology used to apply a layer of metal or compound on the copper pads of printed circuit board by chemicals or by physical method. PCB surface finishes have 2 main functions. 1stly, to avoid copper pads/conductor oxidation, and 2ndly to make the PCBs are easy to solder electronic components on its.

What kinds of surface finishes do we do in Viasion?

In Viasion we do almost very kinds of surface finishes as following.

  • Immersion Gold (Au)
  • Immersion Silver(Ag)
  • Immersion Tin
  • ENEPIG
  • Pb-Free HAL
  • OSP
  • Hard Gold Plating
  • Soft Gold Plating
  • Selective Finishes
  • Carbon ink / Peelable blue mask

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    Detailed introduction of difference
    kinds of PCB surface finishes

    1
    Immersion Gold (ENIG)

    Immersion gold is short for ENIG, the full name is electroless nickel immersion gold, it means cover a thick layer of nickel and gold on the copper surface without solder mask to protect the PCB free from oxidation in normal atmosphere. It has very long shelf life (12 months) and can achieve very good electrical performance because gold has excellent conductivity. Because it can be applied in very high density PCBs, immersion gold is very popular for higher standard PCBs.

    2
    Immersion Silver

    A layer of sliver will be deposited on the copper pad of PCBs by chemical displacement reaction. Immersion silver can provide very good electrical performance since silver is a very good conductivity. At the same time, silver is also easy to solder with tin in PCB assembly process. The cost of immersion silver is cheaper than immersion gold, but the disadvantage is are easier get yellowish when exposed in normal environment.

    3
    Immersion Tin

    In immersion tin process, a layer of tin will be deposited on the copper pad of printed circuit boards by chemical displacement reaction. Firstly, tin protects the copper free from oxidation, and secondly it is very good in soldering components on the PCBs. Since both the surface treatment and the solder past/bar are tin, the solder-ability is very good. Comparing HAL, immersion tin is very smooth, but the cost is higher.

    4
    ENEPIG

    The full name of ENEPIG is Electroless Nickel Electroless Palladium Immersion Gold. It is develops above ENIG. Different from the traditional immersion nickel/gold (ENIG) surface treatment process, a layer of palladium is introduced between the nickel layer and the gold layer, which can effectively prevent the migration of nickel to the gold layer which may cause defects in PCB assembly process, so it has good wiring performance and aging resistance,and also avoid black pad defects in ENIG process.

    5
    HAL

    HAL means Hot Air Leveling. In the process, tin will be melted and then leveled (blow) on the surface of the PCB by hot air. Then the solder will be adhered on the copper pads which are free from solder mask, its thickness is about 5-40um. HAL is the most commonly used surface finish because of it’s excellent considerable and cost saving.

    6
    OSP

    A layer of organic compound is placed on the copper trace which are free from solder mask by chemical method. This thin organic film/compound can protect the copper trace free from oxidation in normal environment. And the same time, OSP surface is very good for PCB soldering, because the organic film will be vaporized during SMT soldering or waving soldering process because of high temperature.

    7
    Hard Gold Plating

    In gold plating process, a layer of nickel will be plated on the copper traces which are not covered by solder mask first, and then with a layer of gold around 2-5u’’. Nickel is used to prevent the diffusion between gold and copper which will lead to defects in PCB assembly process, and the gold plating is used to prevent oxidation and archive good solder-ability as well. Hard gold plating means the surface is hard because it contains cobalt and some other elements, and the surface looks brighter than soft gold. Hard gold plating is mainly used for at non-welding parts, the most typical one is edge connectors.

    8
    Soft Gold Plating

    In gold plating process, a layer of nickel will be plated on the copper traces which are not covered by solder mask first, and then with a layer of gold around 5-30u’’. Nickel plating is mainly used to prevent the diffusion between gold and copper, and gold plating is used to prevent oxidation and solder components on PCB. Pure gold (no other compound) is used in soft gold plating, the gold face does not look as bright as hard gold plating.Soft gold is mainly used for soldering electronic components on the copper pads of printed circuit boards.

    9
    Selective Finishes

    To utilize the advantages of difference surface finishes, two or more surface treatment methods may be selected on one printed circuit board, the achieve the best performance. The common selective finishes are: ENIG + OPS, ENIG+ hard gold plating on edge connectors, HAL+ hard gold plating, OSP + hard gold plating.

    The advantage and disadvantage of difference kinds of PCB surface finishes?

    Surface finishesCostShelf LifeSolder-ability Advantages and Disadvantages
    HAL with lead
    Normal
    12 months
    High
    Cost-effect and high efficient in production. Very good solderablity. It is NOT RoHS/Reach compliant Not suitable for solder pins distance less then 0.5mm. Not suitable for BGA because it is not smooth enough.
    PB free HAL
    Normal
    12 months
    High
    Cost-effect and high efficient in production. Very good soldrablity. Not suitable for solder pins distance less then 0.5mm. Not suitable for BGA because it is not smooth enough.
    OSP
    Low
    3 months
    Normal
    Cost-effect and high efficient in production. Not suitable for PTH & SMT mixed assembly. Poor thermal stability. After the first reflow soldering, the PCBs should be soldered within 14 hours generally. Not suitable for PCBs with EMI grounding area, mounting hole and test pad. It is also not suitable for press-fit holes.
    Plating gold
    High
    12 months
    High
    Good solderability and excellent conductivity Long shelf life and very durable and stable. Surface is very smooth which is good for fine-pitch. Best solution for edge connectors need multiple plugging Cost if very high. There is a risk of embrittlement in the solder joint/weld.
    Immersion gold
    High
    12 months
    High
    Good solderability and excellent conductivity Long shelf life and very durable and stable. Surface is very smooth which is good for fine-pitch. There is a risk of “black pad” issue.
    Immersion silver
    Normal
    12 months
    High
    Excellent solderablity and cost effective Surface is very smooth which is good for small ICs It is easy to vulcanize (sensitive to sulfur), and the non-welding area is easy to discolor at high temperature.
    Immersion tin
    Normal
    12 months
    High
    Excellent solderablity and cost effective Surface is very smooth which is good for small ICs High requirement for immersion tin chemicals
    ENGPIG
    High
    12 months
    High
    No “Black Pad” risk, and surface finish is very very durable and stable. It is perfect alternative for OSP or ENIG fractionally. But the cost is higher.

    What are the shelf life and workshop life for PCBs with difference surface finishes

    Surface finishes Storage Condition & Shelf Life (count from the production date) Storage Condition & Workshop Life after unpack
    Storage Condition
    Shelf Life
    Storage Condition
    Workshop Life
    Immersion Silver
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    6 months
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    ≤48 hours
    (OSP)Organic solderability preservative
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    6 months
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    ≤24 hours
    ENIG,ENEPIG, Plating Gold
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    6 months
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    ≤24 hours
    Immersion Tin
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    6 months
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    ≤24 hours
    (HASL) Hot Air Solder Level
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    6 months
    Temperature ≤ 27 ℃, humidity ≤ 60%, no acid, alkali and organic solution
    ≤24 hours

    Notes: For HDI boards, if storage time more than three months but less than six months , PCBs need to be baked before production, and PCB need to be assembled within 8 hours:

    • The baking temperature of silver-immersed PCB is 120 ± 5 ℃, 2 hours, and the stacking height is ≤ 2.5cm;
    • The baking temperature of the whole nickel gold PCB is 120 ± 5 ℃, 2-4 hours, and the stacking height is ≤ 2.5cm;
    • The baking temperature of OSP finish and selective OSP finish PCB is 120 ± 5 ℃, 2 hours, and the stacking height is ≤ 2.5cm.
    • The oven must be kept clean and free of volatile gases from corrosive chemicals such as acids, alkalis and organic solvents.
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