PCB Surface Finish - A Comparison of Several PCB Surface Finish Types

Copper oxidizes and enough oxidation will cause the solder to not reflow nicely during the reflow process and/or the connection will deteriorate. To extend the life of the bare board, PCB manufacturers will add a surface finish.

Hot Air Solder Leveling (HASL) used to be the standard method until component pitches got to be too small. Under a microscope, HASL looks like a bumpy surface with peaks and valleys across exposed pads. When those bumps can protrude up into the stencil and the solder paste will be significantly inconsistent. Pads 0.5mm and smaller will suffer from this inconvenience. Nonetheless, it is cheaper in volume production so it will stick around for a while.

The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using 'air knives', which blow hot air across the surface of the board.

Advantages:

• Low cost

• Standard

• Re-workable

  
  

Disadvantages:

• Uneven Surfaces (not good for fine pitch)

• Occasional solder bridging

Immersion Tin

Immersion tin is not common and has few advantages. You make it by diffusing tin and copper together and it’s popular only for press fit pin insertion designs.

Advantages:

• Flat Surface

• No lead

Disadvantages:

• The process uses a carcinogen (Thiourea)

• The exposed tin can corrode

• Forms tin whiskers

• Not good for multiple reflow/assembly processes

• Difficult to measure thickness

OSP / Entek

OSP (Organic Solderability Preservative) used to be very common before 1999. It’s lower cost than HASL and has good protection and solder wettability. These days it’s not so common because the chemicals are harder to come by for the PCB manufacturer, it scratches easily, and other surfaces finishes are as effective or more and more economical. 

Advantages:

• Flat Surface

• No Pb

• Simple Process

• Re-workable

• Cost Effective

Disadvantages:

• No Way to Measure Thickness

• Shorter Shelf Life

• Handling Sensitive

Electroless Nickel Immersion Gold (ENIG)

ENIG (Electroless Nickel Immersion Gold) is a two layer metallic coating of 2-8 μin Au over 120-240 μin Ni and protects the copper from corrosion and other abnormalities. ENIG is now arguably the most used finish in the PCB industry due the growth and implementation of the RoHs regulation.

Advantages:

• Flat Surface

• No Pb

• Good for PTH (Plated Through Holes)

• Long Shelf Life

Disadvantages:

• Black Pad / Black Nickel

• Signal Loss (RF)

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is a three-layer metallic coating of nickel, palladium, and gold. ENEPIG outperforms ENIG, Lead Free-HASL, immersion silver and OSP when it comes to after-assembly corrosion resistance. It’s good for those who need to perform wire bonding. ENEPIG is higher cost due to the layer of palladium and low demand of use.

Advantages:

• Flat Surface

• No Lead Content

• Excellent Solder Joints

• Wire Bondable

• No Corrosion Risks

• 12 Month or Greater Shelf Life

• No Black Pad Risk

Disadvantages:

• Expensive

• Processing Limits

Gold – Hard Gold

Hard Electrolytic Gold consists of a layer of gold over a nickel. Hard gold is durable and scratch resistant, and therefore is most commonly applied to edge connectors and keypads. Solder has a hard time sticking to hard gold surfaces so it’s not recommended to apply this finish to solderable areas. 

Its thickness is set by controlling the duration of the plating cycle. Typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1 and Class 2, 50 μin gold over 100 μin nickel for Class 3.

Advantages:

• Robust 

• No Pb

• Long shelf life

Disadvantages:

• Expensive

• Extra processing / labor Intensive

• Difficulty with other surface finishes

• Etching undercut can lead to slivering / flaking

• Not solderable above 17 μin

• Finish does not fully encapsulate trace sidewalls, except in finger areas

Most people that do not have special requirements will select HASL or ENIG. Choose ENIG if you have fine pitch SMT components.

Unsure of what you will need? Consult with Henway Technologies prior to you making a selection. This will ensure that the combination of the surface finish and material will result in a high-yielding, cost-effective design that will perform as expected.