Comparing PCB Surface Finishes

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Aerospace Printed Circuit Board Production

When dealing with a printed circuit board (PCB) you must protect the copper finish, which will oxidize and depreciate if left untreated. You also need a surface to which components can be directly soldered. Superficially, this concept seems simple: add a surface treatment. Viola! You’re all set. Unfortunately, it is never that easy.

There are assorted options for PCB surface finishes and each one has advantages and disadvantages. Further, regulations such as RoHS and WEEE have changed industry standards.

This is a guide to the most popular types of PCB surface treatments including their pros and cons and is a great place to get started with your decision making.

HASL and Lead-Free HASL

“Hot Air Solder Leveling” is the least expensive type of PCB surface finish. It is widely available and very economical.

The board is dipped in molten solder and then leveled off with a hot air knife. If you are using through-hole or larger SMT components, HASL can work well; if your board will have fine pitch SMT components or smaller than 0402 components, it is not ideal. The surface is not completely level, so this can cause an issue with small components.

Lead-free HASL is similar to standard HASL, but with an obvious difference, it does not use tin-lead solder. Instead, tin-copper, tin-nickel or tin-copper-nickel germanium may be used. This makes lead-free HASL an economical and RoHS compliant choice. However, like standard HASL, it is not ideal for smaller and fine-pitch components.

Advantages Disadvantages
  • Low cost
  • Uneven surfaces which are not good for fine pitch components
  • Widely available
  • Solder bridging
  • Can be repaired/reworked
  • Plugged or reduced plated through holes
  • Long shelf life
 

Electroless Nickel Immersion Gold (ENIG)

Thanks to a double layer metallic coating of nickel and gold, where nickel both protects the copper and provides a surface to which components are soldered, and gold protects the nickel, ENIG is becoming the most popular surface finish in the industry. This treatment has also seen an increase in popularity due to the RoHS regulations and provides a level exterior for complex surface components such as BGAs and flip chips.

Of course, just having gold involved means it is not always cost-effective.

Advantages  Disadvantages
  •  Flat surface to solder to
  • Slightly more expensive than HASL
  •  Lead Free and RoHS compliant
  •  Signal loss for signal integrity applications
  • Longer shelf life
 
   

OSP or Entek

A water-based, organic surface finish, organic solderability preservative (OSP) is an environmentally friendly option that requires low equipment maintenance. The finish places selective bonds to the exposed copper, usually using a conveyorized process, and provides an organometallic layer that protects the copper before soldering.

The downside is OSP is not as robust and it can be sensitive to handling, so multiple soldering can destroy the film.

Advantages


Disadvantages

  • Flat surface for soldering
  • Short shelf life, high requirements for storage
  • RoHS compliant and Lead-Free
  • Handling the PCB can cause soldering issues
  • Can be reworked
  • Thickness isn’t measurable
  • Cost effective
 

Immersion Tin (ISn)

With immersion coatings, a chemical process is used. A flat layer of metal gets deposited on the copper traces. The flatness of coating makes it ideal for small components.

Although tin is an economical choice, it comes with some drawbacks. The main drawback is that after it is deposited onto copper it begins to tarnish. That means that if you want to avoid lower quality solder joints, you need to do your soldering within 30 days.

If you are expecting a high volume of production this may not be an issue. And if you are using large batches of boards quickly, you can avoid tarnishing.

Advantages  Disadvantages
  • Smooth, flat surface
  • Not good for multiple reflow/assembly steps
  • Can be reworked (although with more limitations than other options)
  •  Exposed tin on final assembly can corrode
  • Best for press fit pin insertion
  • Easy to cause handling damage
   

Immersion Silver

Immersion silver does not react with copper the way that tin does. However, it does tarnish when exposed to air. That means it needs to be stored in anti-tarnish packaging. When stored in proper packaging it will still be solderable for 6-12 months.

However, once the PCB is removed from its packaging, it will need to go through solder reflow within a day. A higher shelf life can be achieved with gold plating (ENIG).

Advantages  Disadvantages
  • Excellent flatness
  • Very sensitive to handling/tarnishing/cosmetic concerns.
  • Good for fine pitch/BGA components
  • Special packaging required.
  • Lead-free and RoHS compliant
  • Reduced supply chain options to support this finish
 
  • Short operating window between assembly stages

Hard Gold

Among the most expensive PCB surface finish types, hard gold applications are extremely durable and have a long shelf life. It is not often used for soldering points, due to poor solderability.

Hard gold is typically used for edge connectors, battery contacts, and some test pads.

Advantages  Disadvantages
  • Hard, durable surface
  • Very expensive
  • Long shelf life
  • Extra processing/Labor intensive
 
  • Difficult with other surface finishes
   

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG technology developed based on ENIG technology by adding a palladium layer to improve performance. The palladium layer totally covers the nickel layer to avoid degeneration of nickel, commonly referred to as black pad. Palladium has a higher hardness level than gold, which results in the improvement of solder reliability, wire bonding capacity and antifriction.

Advantages  Disadvantages
  • Ideal for gold wire bondable surface
  • Palladium does not form a strong intermetallic bond with lead in Sn/Pb solders
  • Excellent multiple reflow cycles
  • Many chemical steps and long process
  • Process costs substantially lower electronic nickel gold
 
   

There is no surface finish that is suitable for all applications and environments in the industry. Here are some of the considerations for surface finishes:

  • Lead- or lead-free assembly
  • Cost
  • Shelf life
  • Fine pitch components
  • End environment
  • Volume and throughput
  • RF applications

One advantage to working with a full-service electronics manufacturing services partner is that we can help you determine the best finish for your printed circuit board.  We can help you make the best decisions around even the smallest details such as PCB surface treatments. Contact ACDi today for your printed circuit board project.

 

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