PCB Surface Coating – What And Why - Miracle Electronics

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Copper is the main conductive material used in
PCBs, due to its excellent conductivity and
physical performance. But, despite these
advantages, copper also tends to oxidize when
exposed to air, causing a solid thin oxide layer
to be formed on its surface, which affects the
solder joints, reducing the shelf life and
reliability of the product.

• The copper thus needs to be prevented against
  oxidization, which can be achieved by having a
  good surface coating. There are a number of
  surface coatings that have rapidly been
  developed with numerous classifications
  generated. So, how do you select a proper type of
  surface coating?
• What is surface coating?
• 70 of defects on PCBs occur due to incomplete
  solder joints, which are formed due to the
  oxidization on the surface. A surface coating
  prevents this and eases any possibilities of
  defects. However, the surface coating to be
  applied should be compatible with the following
  requirements
• Heat resistance This refers to the performance
  of melting point and thermal decomposition
  temperature the melting point of which should be
  close to or lower than that of tin, and the
  thermal decomposition temperature of which

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• should be higher than the melting point and
  soldering temperature of the solder.
• Coverability The entire surface of the PCB
  should be thoroughly covered to make sure that
  the melting solder can be completely soldered
  onto the pad. The surface tension of the melting
  surface coating should thus be small, and the
  decomposition temperature high so that high
  coverability can be guaranteed.
• Anti-corrosion This refers to the corrosion
  caused by residual solder to circuit board
  surface after soldering, such as PCB substrate
  material or metallic layer.
• Zero residue This refers to the residue left on
  the surface of the pad or solder joints after
  the implementation of soldering, which is
  generally hazardous, and thus should be
  eliminated through cleaning measures taken up
  after soldering.
• Classification of surface coating
• Surface coating can be classified into different
  categories depending upon the manufacturing
  technology, and application effect.
• Based on manufacturing technology, the PCB
  surface finish can be classified into surface
  and metal coating, where surface coating meaning
  to add a thin coating of heat resistant and
  solderable material like HASL or OSP physically
  onto the copper surface, and metal coating
  meaning to add an ENIG, ENEPIG, or immersion
  gold coating onto the surface through
  electroplating or electroless plating.
• Based on the application effect, the PCB surface
  finish can be classified into soldering flux
  soldered on the coating on the isolating layer,
  and soldering flux soldered on the isolating
  layer of the metal coating. Soldering flux
  soldered on the coating on the isolating layer
  is when the melting solder will drift on the
  surface of the solder after leaving copper during
  high-temperature soldering. Soldering flux
  soldered on the isolating layer of the metal
  coating is when there is no direct contact with
  the copper surface, thus eliminating IMC and
  diffusion between metals.

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• Which surface coating is the best?
• The surface coating plays a huge role in
  determining the solderability, reliability, and
  shelf life of the PCB. You must thus choose the
  most appropriate surface coating for your PCB,
  depending upon your PCB application .
• When you are looking for PCBs to be used in
  electronic products, the surface coating should
  be one whose soldering flux is soldered without
  isolating layer, such as HASL, OSP etc.
• When you are looking for high reliability and
  long shelf life for products to be used in the
  medical, military, and aerospace industries, the
  surface coating should be one whose soldering
  flux is soldered on the isolating layer, such as
  ENIG and ENEPIG.
• So now, you should be in a better position to
  choose your surface coated PCBs. If not, you may
  get in touch with professionals like Miracle
  Electronics, who can design a PCB assembly from
  India, specific to your individual requirements
  so that you can reap the best outcomes.