Friday, December 09th
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Printed Circuit Board FinishesOne of the last steps of manufacturing a circuit board is applying the surface finish. While the soldermask covers the majority of the circuitry, the surface finish is designed to prevent oxidation of the remaining exposed copper. This is important because oxidized copper cannot be soldered.

In rare instances, when a circuit board requires “body gold,” the finish is applied to the copper prior to the soldermask process.

Gold finishes tend to be more expensive. However, when it comes time to send your board to assembly, the consistent finish, especially for designs that require a very tight surface mount, may be worth the extra cost.

Below you will find some of the most common finish types and a brief description of their use and key measurements.

 

Finish Type: Leaded Solder

What is it?

Leaded Solder is the most common finish type.  It is a mix of tin and lead that is applied to the board.  The mix is approximately a 63/37 mix of tin/lead.  Sometimes referred to as a 60/40 split.  Can be referred to as SN67 or SN66

The temp at which it flows (is liquid) is 485 degrees.

It is a shiny silver colored finish.

Why is it used?

It is used as a default process.  It is ideal for boards with thru-hole technology.  The surface finish is not very flat for SMDs or BGAs.

What is the process to apply the finish?

The typical process is HAL (Hot Air Leveler).  The boards are dipped into a tank of molten solder and then air knives blow off the excess solder.  It is applied after the soldermask has been applied.

What are the critical measurements?

Typically the thickness of the Leaded Solder is note measured, although that question does come up at times.  The IPC standard is visual coverage of the copper.

The cost is also very low compared to other types of finishes.

 

Finish Type: Lead Free Solder

What is it?

Similar in appearance and use as Leaded Solder.  Contains a mix of 99.3% Tin and 0.6% Copper.  It may be referred to as SN100CL.

The temp at which it flows is around 515 degrees as compared to around 485 for Leaded Solder.

It is a shiny silver colored finish.

Surface mount pads are very uniform and flat with this finish.

Why is it used?

It is used as a replacement for Leaded Solder where a lead free application is called for.

CAUTION: The customer may need to use a higher temp laminate to accommodate the higher temperature of the finish.  They could use 370HR or another high temp material

What is the process to apply the finish?

The typical process is HAL (Hot Air Leveler).  The boards are dipped into a tank of molten solder and then air knives blow off the excess solder.  It is applied after the soldermask has been applied.

What are the critical measurements?

Again the same as leaded solder the thickness is not measured.  The IPC standard is visual coverage of the copper.

 

Finish Type:  Electroless Nickel Immersion Gold

What is it?

Also called ENIG.  This is a very thin amount of Gold applied over Nickel.  The nickel is plated onto the copper prior to the application of Gold.  The nickel acts as a barrier to prevent the Gold and the Copper from migrating into one another.

The gold will disappear when soldered to as you are actually soldering to the nickel with the gold absorbed in the process.

Why is it used?

It is a very flat surface that is ideal for Surface Mount Devices (SMDs) and Ball Grid Arrays (BGAs).  Gold is a great conductor of electricity.  Many contract assemblers prefer this finish.

What is the process used to apply the finish?

An Electroless process is used to apply this finish.  It is the depositing of a metallic coating onto a surface without the use of an electric current, relying instead upon a controlled chemical reaction.

It is typically applied after soldermask has been applied.  In some cases it is applied before but that is not typical.  Obviously the cost is much higher if all copper is plated with gold and not just what is exposed after soldermask.

What are the critical measurements?

Nickel is plated 150 to 200 micro inches

Gold is plated 3 to 10 micro inches

 

Finish Type: Soft Gold

What is it?

Soft gold is commonly referred to as “wire bondable gold”.  It is softer than other gold finishes which allows it to be bonded to more easily for stronger and more conductive connections.

The gold does not disappear at the point of soldering or wire bond which produces a stronger welded joint.

Why is it used?

Primarily is necessary because of the need to do gold wire bonding.

What is the process used to apply the finish?

The process is electrolytic, that is it uses a current to apply the finish.  (similar to hard gold, but does not have the hardeners and brighteners added.)  It is typically applied before soldermask.

What are the critical measurements?

It is 99.99% pure – 24 carat gold

The typical thickness is 30 micro inches of gold.  We can go up to 100 micro inches in increments of 10, but the most common use is at 30 micro inches.

Plated over 100 to 200 micro inches of nickel.


Finish Type: Hard Gold

What is it?

Hard Gold is an electrolytic process and has hardeners in it for durability.  It is plated over a nickel finish.

It may be referred to as Deep Gold.  (the term Deep Gold is used to indicate that full panels are placed in the plating tank.)

Why is it used?

Due to its hardness, it can stand repeated use and is used most commonly for gold fingers.

What is the process used to apply the finish?

It is an electrolytic process and has hardeners added to it.  A very active flux should be used when soldering to Hard Gold.

What are the critical measurements?

Typical is 98% pure 24-carat gold.

Nickel is 150 to 200 micro inches.

The gold is 30 to 50 micro inches thick.

 

General Gold Information:

Flash Gold:  Is hard gold with a maximum thickness of 3 micro inches.  We can do this here but would require special handling.  Check with production on a case by case basis.

Body Gold: Indicates that the full body of the board is plated gold.  This may occur during either electro plating or immersion depending on the design.

Selective Gold: Indicates that a specific area in the interior of the board would be plated.  This does not include gold fingers.

 

Finish Type:  Immersion Silver

What is it?

Purpose of this silver plating is to protect the copper from corroding as well maintaining its solderability.  It does have a shorter shelf life than some other finishes and must be shipped with separator sheets to prevent tarnishing.

Why is it used?

  • Silver is the most electrically conductive metal available thus forming excellent interconnect surface.
  • Immersion Silver process produces a really flat surface.  This is advantageous for SMD assembly.
  • Excellent solderability, comparable to solder plating.
  • Environmentally friendly lead free finishing.
  • Ideal for high speed signals.

What is the process used to apply the finish?

Immersion silver plating is used to apply a 1 micron thick layer of silver on the copper surface.  It is an electroless plating process.  It is applied after etch and before soldermask.

What are the critical measurements?

1 Micron thick layer of silver over the copper.  That is 30 to 50 micro-inches.

 

Finish Type:  Immersion Tin

What is it?

Commonly referred to as White Tin.  It is a chemical process of applying a very thin layer of tin to the copper.  The appearance is mostly white.

Why is it used?

It is a lead free alternative.  The surface is very flat which is good for SMD assembly.  It should have a shelf life of up to one year.  It is fairly low cost compared especially compared to Immersion Gold.

What is the process used to apply the finish?

Applied using an electroless chemical bath that will apply approximately 0.7 to 1.0 microns (or about 44 micro inches) of tin to the copper.

What are the critical measurements?

The thickness is about 0.7 to 1.0 microns (44 micro inches)

 

Finish Type:  OSP (Organic Surface Protectant)

What is it?

It is an organic chemical finish that is applied to the copper.   The shelf life is very short with OSP and the boards should be used very shortly after applying the coating.

Why is it used?

It is very flat and is a lead free alternative.

What is the process used to apply the finish?

It is a chemical bath process.  It can only be applied after all other processes have been done.  Including Electrical Test and Inspection.

What are the critical measurements?

It will apply between .4 and .6 micro inches to the copper.

If you have questions or comments please feel free to post them below.

 

 

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7 Responses

  • Peter Brissette (bayareacircuits.com) June 17th, 2014

    @disqus_MilorenQbk:disqus That typically is not done and here is why. Both gold and copper are fairly porus metals. If you did not have the nickel as a barrier the copper and the gold would start to mix in with one another fairly quickly. Also the nickel helps makes sure that when you solder the components that there is a good electrical connection.

    If for some reason though you really want to have gold directly on copper I am sure we could do it. Just so you know the problems that it could cause.

  • Nathan_S June 17th, 2014

    Hello, do you have any gold finish options which are not plated over nickel?

  • Peter Brissette (bayareacircuits.com) January 16th, 2014

    Hey Tom, thanks for your comments. I am working on reworking this post to expand on those items.

  • Tom Taylor December 3rd, 2012

    How about Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) ?
    Also, you should make “Finish Type: Soft Gold” a heading above.



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