Applying the surface finish is one of the last steps of manufacturing a printed circuit board. The surface finish on a PCB is typically applied after the soldermask process and is designed to prevent oxidation of the remaining exposed copper, while the soldermask covers the majority of the circuitry. This is important because oxidized copper cannot be soldered.

In rare instances, when a printed circuit board requires “body gold,” this surface finish is applied to the copper prior to the soldermask process, unlike with other PCB finishes. Gold finishes tend to be more expensive, however, when it comes time for printed circuit board assembly, the consistent finish may be worth the extra cost, especially for designs that require a tight surface mount.

Below you will find some of the most common printed circuit board finishes and a brief description of their use and key measurements.

 

Leaded Solder (HASL)

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 and it is a shiny silver-colored finish.

Why is it used?

It is used as a default process and 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.

 

Lead Free Solder (HASL Lead-Free)

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 and 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 a similar 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.

 

Electroless Nickel Immersion Gold (ENIG)

What is it?

This is a very thin amount of Gold applied over Nickel, also called Electroless Nickel Immersion Gold, or ENIG. The nickel is plated onto the copper prior to the application of Gold and 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 and many contract assemblers prefer this finish.

What is the process used to apply the finish?

An electroless process is used to apply this finish which deposits 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 but in a few cases it is applied prior (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 and Gold is plated 3 to 10 micro inches.

 

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 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 and the typical thickness is 30 micro inches of Gold but can go up to 100 micro inches in increments of 10, plated over 100 – 200 micro inches of Nickel.

 

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 – 200 micro inches. The Gold is 30 – 50 micro inches thick.

 

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 metals available thus forming excellent interconnect surface and produces a really flat surface which is advantageous for SMD assembly. It has excellent solderability comparable to solder plating, is an environmentally friendly lead-free finish and is 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 and is applied after etch but before soldermask.

What are the critical measurements?

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

 

Immersion Tin

What is it?

Commonly referred to as White Tin due to its mostly white appearance, a very thin layer of Tin is applied over the copper.

Why is it used?

It is a lead-free alternative.  The surface is very flat which is good for SMD assembly and it has a shelf life of up to one year.  It is fairly low in cost 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 – 1.0 microns of Tin to the copper.

What are the critical measurements?

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

 

Organic Surface Protectant (OSP)

What is it?

Organic Surface Protectant is an organic chemical finish that is applied to the copper.   The shelf life is very short and the boards should be assembled soon after applying the coating.

Why is it used?

Organic Surface Protectant is very flat and is a lead-free alternative.

What is the process used to apply the finish?

It is a chemical bath process and 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.

 

 

General Gold Information

Flash Gold:  Hard Gold with a maximum thickness of 3 micro inches.

Body Gold: Indicates that the full body of the board is plated Gold.  This may occur during either electroplating 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.

 

 

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