PCB surfaces / coating

Leaded

Soldering coats
Lead-free tin(Sn) coat with hot air leveling (Leadfree HAL)

Peculiar advantages:
Strong surfaces for soldering in the solder spot, good storage ability.

Peculiar disadvantages:
Unsmooth surfaces, due to which BGA and similar contact pads are critical, high thermal HAL pressure.

SET PCB Technology recommends the following solutions:

For all integrated circuits where protrusion does not lead to problems and the thermal HAL pressure is not critical.

Lead-free

Lead-free soldering coats
Lead-free tin(Sn) coat with hot air leveling (Leadfree HAL)

Peculiar advantages:
Strong surfaces for soldering in the solder spot; good storage ability

Peculiar disadvantages:
Unsmooth surfaces, due to which BGA and similar contact pads are critical; high thermal HAL pressure

SET PCB Technology recommends the following solutions:

For all integrated circuits where protrusion does not lead to problems and the thermal HAL pressure is not critical.

Chemical coating

Chemical Ni/Au

Peculiar advantages:
Universal, smooth surfaces, good storage ability

Peculiar disadvantages:
"Black pads" as risk, concerning loss, high costs

SET PCB Technology recommends the following solutions:

For particular SMD chips and/or with special functions like bonding

Chemical Sn

Peculiar advantages:
Universal, smooth surfaces

Peculiar disadvantages:
Formation of moustaches /"viscera"/; multiple solder-ability and very complicated circumstances

SET PCB Technology recommends the following solutions:

For particular SMD chips and/or with special functions like bonding

Main notes on soldering coats:



* The physically separated coats as is the case with liquid solder ones, the so called HAL-coats, are visually thicker but unsmooth.



** Chemically, which means metal coats deposited without electric current, for example the Ni/Au or Sn ones, cannot be deposited at random thickness due to technical reasons, as the depositing process “gets asleep” when the more precious metal completely over-covers the less precious Cu. By that method a smooth surface, identical with the topography of the previous Cu-surface, is obtained.



*** Despite of the deposit method as a whole, a so called diffusion layer is formed between the two metal coats. This border layer is formed by the atom exchange between the different precious metals. This leads to “mixing” of basic Cu and the metal of the precious metal improved soldering surface. The diffusion layer thickness depends on the type of the metals used, as well as the temperature conditions and the time of their action on the layer system. Higher temperatures and longer action time lead to a diffusion layer growth (for example during storage or soldering).