How To Select The Best Surface Finish For Your Electronics Assemblies

Surface Finish - There are many types to choose from when designing a PCB (Printed Circuit Board). How do you know which one is best for your electronics assemblies? Here are some popular finishes with pros and cons for each.

HASL (Hot Air Solder Leveling)

HASL (pronounced "Ha-Sol") has been around for many years. A thin layer of solder is mechanically applied to copper surfaces. A hot air knife is used to sheet the excess solder away to create a flat finish.

Advantages of HASL:

• Low cost.
• Available from all PCB suppliers.
• Very good resistance against oxidation.
• Very good resistance against handling.
• Very good shelf life. Finish remains solderable after prolonged exposure to ambient.

Disadvantages of HASL:

• Surface flatness is difficult to control. This could be an issue for components that are sensitive to surface flatness (ie, BGA, fine pitch components, etc).
• Hole size may not be uniform. This could be an issue where pressfit connectors are used.
• It adds an extra heat cycle to the substrate.
• Boards may have some contamination from flux if not cleaned properly.
• Very fine exposed features (ie, copper traces, soldermask, fiducials, etc) could be damaged during the process.
• Not suitable for RoHS application.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a chemical plating process where a thin layer of nickel and gold is deposited onto the copper surfaces. The gold preserves the copper below and it remains solderable and conductive with very low contact resistance.

Advantages of ENIG:

• Surface flatness is excellent, suitable for BGA or other fine pitch components.
• Available from all PCB suppliers.
• Good resistance against oxidation.
• Good resistance against handling.
• Very good shelf life.

Disadvantages of ENIG:

• Higher cost due to price of gold in plating solution.
• Process control before plating is critical or it could lead to defects called "black pads", where the underlying surface is contaminated or oxidized prior to the deposition of gold.
• It requires a tightly controlled plating process to prevent excessive gold being plated.
• It may not be suitable for some high frequency or high-powered RF application.

Immersion Silver

Immersion silver is similar to immersion gold, where silver and other different chemistries are used instead. This finish gained some popularity during the RoHS transition with OEMs pushing for immersion silver instead of gold for concerns over "black pads" and cost.

Advantages of Immersion Silver

• Surface flatness is excellent, similar to ENIG.
• Less expensive than ENIG.
• It does not suffer from "black pads".

Disadvantages of Immersion Silver

• Low resistance to handling and contamination. Gloves must be worn when handling unsoldered surfaces.
• Shorter shelf life compared to HASL and ENIG.
• It reacts to sulphur and will result in degradation of the circuit board when exposed. It is not suitable for application where the board may be exposed to air pollution or environment where sulphur is present.
• Contact resistance is higher thus it may cause issues during ICT (In-Circuit Test).

Organic Solderability Preservative (OSP)

OSP is a water-based organic compound that is applied to the copper surfaces. It is an environmentally friendly process and does not require usage of hazardous chemicals.

Advantages of OSP

• It is a self-limiting process thus the plating thickness can be easily controlled.
• Surface flatness is excellent, similar to ENIG or immersion silver.
• Low cost.

Disadvantages of OSP

• Low resistance against oxidation.
• Low resistance against handling and contamination. Gloves must be worn when handling unsoldered surfaces.
• Short shelf life.
• Not suitable for critical through-hole application
• Surface becomes less conductive and harder to penetrate after it has been heated. This may become an issue during ICT.

Which Surface Finish Is Best?

Although the surface finish may depend on your design, budget and application, etc, I would highly recommended ENIG as the finish of choice. It offers very good flatness, good solderability and good resistance against handling and oxidation. It is also widely available. It is true that cost could be a bit higher; but it is more worthwhile than dealing with potential problems with other finishes.

This post was originally posted at www.EMSEngineer.com - http://wp.me/p8hPPt-46