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How to choose PCB surface finish?

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Published by Pinsheng Electronics Co., Ltd February 20,2020

What is PCB surface finish?

 

PCB surface finish is between the metal face of the components and the pads on PCB, for protecting the copper foil from oxidation and providing a solderable surface for PCB assemblyEvery PCB surface finish has its pros and cons, could be customized as the requirements.
 
Although, currently conventional choices of PCB surface finishes are nothing less than HASL(Lead-free) and ENIGPS Electronics covers all the types of PCB surface finishes, like OSP, Hard Gold, ENEPIG, ISn, IAg, etc. 
 
This article was edited by PS electronics to guide you know every of the mianstream PCB surface finish, as below.

 

Comparison Points of PCB Surface Finishes

  • Planarity
  • RoHS (Lead or Lead-free)
  • Environment friendly
  • Cost
  • Shelf life
  • Reworkability

 

Measurement Units of PCB Surface Finish

  • Micro-inch (1μ”) = 0.000001” (inch)
  • Micron (1μm) = 39.37μ” (μinch)
  • 1mil = 25.4μm = 0.001” (inch)

 

 Types of PCB Surface Finish

1. Hasl/Hasl Lead-free

"The PCB is dipped into molten solder so that all exposed copper surfaces are covered, excess solder is removed by 'hot air knives'". 
The composition of hasl treatment commonly is Tin (63% ) and Lead (37%), typical thicknesses range from 30-1500 μ'', of which the melts point is about 183 degrees C.
For hasl lead-free, the common alloys refer to Sn/Ag/Cu (SAC), Sn/Cu/Co, Sn/Cu/Ni/Ge, etc., of which the thicknesses range from 300-1000 μ''. The hasl PCBs (lead-free) even can bear a high temperature of 260 degrees C. Hasl lead-free is the predominant PCB surface finish for circuit boards for it is RoHS&WEEE compliant.
 
Hasl PCB finish
Hasl PCB finish

 Pros:

  • Low cost
  • Widely applicable
  • Easy processing
  • Reworkability
  • Excellent shelf life (12+ months)
  • Visual inspection & testing

 Cons:

  • Uneven surface (solder bead)
  • Not good for PTH (high thermal stress)
  • Not good for thick copper boards
  • HASL is unfriendly to envirronment

2. ENIG (Immersion Gold)

Two layer metallic coating - ENIG cover the copper using thick, well-electrically nickel-gold, commonly 2-8 μ'' Au over 120-240 μ'' Ni, in which nickel is deposited on palladium-catalyzed copper surfaces, gold adheres to the nickel-plated area by molecular exchange, which protects the nickel (actual solder surface) until the soldering process, also Au provides low contact resistance and good wetting
ENIG technology answered the request for the flat surface used in fine pitch assembly and lead-free, even though it is a little bit expensive than others, but no doubt ENIG has already become the most commonly-used PCB surface finish for many categories, such as cellphone or power supply.
 
ENIG PCB finish
ENIG PCB finish

 Pros:

  • Flat surface excellent for fine-pitch (BGA, QFP...)
  • Lead-free
  • Good for PTH
  • Long shelf life (about 12 months)
  • Good contact resistance
  • Excellent for thick copper PCBs

 Cons:

  • Expensive
  • Black phosphorus pad
  • Signal loss at high-frequency
  • Electromagnetic interference
  • Not re-workable

3. Electrolytic Ni/Au (Hard Gold)

The same two-layer metal coating, what differs from ENIG is that this PCB surface finish is electroplated rather than chemical immersion, in which nickel layer gets plated before gold plating. Since the PCB must act as an electrode, the electroplating must be done before the solder mask process.
Because of unstable solderability, it is currently only used in IC carrier boards (PBGA/FCBGA/FCCSP...), or in the gold finger that is explicitly required.

 Pros:

  • Flat surface
  • Lead-free
  • Excellent shelf life (12 months+)
  • Especially suitable for contact switch or "gold finger"
  • Good for IC carrier PCBs

 Cons:

  • Gold surface easy oxidizing
  • Gold whiskers might cause short-circuit
  • Skin effect

4. ENEPIG (Electroless Nickel/Palladium Immersion Gold)

A solution to "Black phosphorus pad" of ENIG. By introducing a layer of palladium (4 to 20 μ''), prevent Ni layer (120 – 240 μ'') from corroding overly by Au (2 to 8 μ''), ENEPIG got come up in the 1990s, also brought in a better option to wire bonding.
Although ENEPIG it is not a commonly-used PCB surface finish at present, the development grows rapidly.

 Pros:

  • Flat surface
  • Lead-free
  • Excellent shelf life (12 months+)
  • Good for IC carrier PCBs
  • Excellently suitable for gold/aluminum wire bonding
  • Forms reliable Ni/Sn solder joints

 Cons:

  • Expensive
  • Complicated in plating processing
  • Immature technology

5. OSP (Organic Solderability Preservative)

On the clean bare copper surface, a layer of organic film is chemically applied. This film is a water-based organic compound that selectively combines with copper and provides an organic metal layer to protect the copper surface from rust in the normal environment. In the subsequent soldering, it will be quickly removed.
 
OSP PCB finish 
OSP PCB finish 

 Pros:

  • Lead-free
  • Flat surface, good for fine-pitch pads (BGA, QFP...)
  • Very thin coating
  • Can be applied together with other finishes (e.g. OSP+ENIG)
  • Low cost
  • Reworkability

 Cons:

  • Not good for PTH
  • Handling Sensitive
  • Short Shelf life (<6 months)
  • Not suitable for crimping technology
  • Not Good for multiple reflow
  • Copper will get exposed at assembly, requires relatively aggressive flux
  • Difficult to inspect, might cause issues in ICT testing

6. Immersion Tin (ISn)

Tin will be applied directly on copper for protection. It does not bring in any new elements to the soldering surface, especially workable for communication backplanes. But copper and tin have a strong affinity with each other, the diffusion of one metal to another will inevitably occur, which will directly affect the shelf life. ISn requires good storage, while it is restricted due to the presence of carcinogens.

 Pros:

  • Lead-free
  • Flat surface
  • Uniform coating with superior hole wall lubricity
  • Low cost
  • Suitable for crimping technology
  • Re-workable

 Cons:

  • Not good for PTH 
  • Presence of carcinogens
  • Risk of Handling damage
  • Tin Whiskers (Better to use within 6 months)
  • Not Good for multiple reflow
  • Difficult to inspect

7. Immersion Silver (IAg)

A good alternative to ENIG, with a moderate shelf life (about 12 months) and good electrical properties unmatched by other PCB surface finishes, but it is susceptible to sulfur dioxide, which tarnishes the surface and forms an AgS2 layer. This layer adversely affects solderability. To avoid this contamination, we package the PCB in silver-plated paper and sealed packaging.

 Pros:

  • Lead-free
  • Excellent solderability
  • Flat surface for fine pitch
  • Sample processing
  • Aluminum wire bondable
  • Cost effective (than ENIG)
  • Low high frequency signal loss (skin effect)
  • Great Shelf life (6-12 months)

 Cons:

  • Tarnishes 
  • Silver Whiskering
  • Sensitive to handling, electrical test, packaging
  • Difficult to inspect
  • Not good for micro-vias of ratio > 1:1
  • Not suited for compliant-pin intertion (Ni-Au Pins)

 

Not sure which type of PCB surface finish is suitable for your PCB boards? Now is the time to contact us for a further manufacturability suggestion.

 
 

 
 

 
 

 
 

 
 

 
 

 
 
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