Solderable finishes

When a printed circuit board is manufactured a solderable finish is applied to the exposed copper pads. A number of finishes can be used. Traditionally one of the most popular is a tin-lead coating in what is known as a hot air solder levelled (HASL) finish. The move to Lead-Free assembly means that this will no longer be possible. Fortunately, a number of planar finishes have been developed as alternatives and have been in widespread use for number of years and are production proven. The main alternatives to tin-lead solderable finish are:

• Organic Solderability Preservative (OSP)
• Electroless Nickel - Immersion Gold (ENIG)
• Electrolytic Nickel Gold
• Immersion Tin
• Immersion Silver
• Lead-Free HASL
• Palladium based finishes

They all offer benefits although problems can occur under specific conditions.

Organic Solderability Preservative (OSP)

OSP coatings offer a very low cost finish, which may be as little as half of the cost of HASL. Soldering with OSP coated copper involves a direct bond with the copper; it is, therefore, possible to achieve very strong solder joints. It is also able to withstand multiple solder cycles. However it has reduced wettability with Lead-Free solders. OSPs are susceptible to failure from handling or environmental conditions and since they are clear, it can be difficult to identify coating faults.

Electroless Nickel - Immersion Gold (ENIG)

This uses an electroless nickel process which deposits up to 5 microns of nickel, followed by immersion gold. The resulting gold layer is typically less than 0.1 microns thick and is generally considered to be unsuitable for wire bonding applications. The electroless nickels typically require deposition temperatures of up to 90C and thus there can be issues with certain solder masks. Nickel-gold finishes are also relatively expensive and can exhibit a failure mechanism known as black pad (see later sections).

Electrolytic nickel gold

Electrolytic nickel gold is an alternative to the ENIG process in which a thicker gold layer is deposited. This makes it more suitable for wire bonding applications but also more expensive. Large quantities of gold can lead to brittle solder joints. The gold can be either hard or soft depending on the application. A common application for hard gold is as a contact point for press pads or slider switches and edge connectors. Soft gold is applied specifically for wire bonding applications. As such, it is often applied in conjunction with other surface finishes and only in certain areas.

Immersion tin

Immersion tin is likely to be one of the main finish choices for Lead-Free assembly. It is relatively low cost, easy to use and compatible with many Lead-Free solders. In the past its use has been limited by fears of tin whiskering, but these issues have been addressed.

Immersion silver

Its relatively low cost means that immersion silver is another solderable finish that is predicted to be a key choice for Lead-Free assembly. There have been concerns about silver finishes tarnishing but newer formulations co-deposit an organic material that enhances the performance. Immersion silver provides a single surface finish that is wire bondable and has low contact resistance for key contacts as well as being easy to inspect.

Lead-Free HASL

For relatively simple, coarse pitched applications it is possible to convert the conventional tin-lead HASL process to Lead-Free by using a Lead-Free alloy such as tin-copper. Many smaller PCB makers are following this route because they already have HASL equipment in place and wish to avoid additional investment in new process lines. However, Lead-Free HASL will only represent a very small percentage of the board finishes used globally.

Palladium based finishes

There are various palladium based finishes available and these are relatively expensive. Examples include electroless palladium with an immersion gold top coat and electroless palladium with immersion gold i.e. no nickel. The electroless nickel, electroless palladium, immersion gold combination is aluminium wire bondable, whereas the electroless palladium, immersion gold finish is not. Palladium is harder than gold and thus these finishes offer better wire bonding and connector application performance.

Summary

• It is unlikely that there will be a single universal preferred alternative Lead-Free solderable finish.
• Choice of alternatives will be governed by a number of important considerations including functionality, ease-of-use and cost.
• Immersion tin, immersion silver and OSP are predicted to become the predominant solderable board finishes.

Summary of solderable finish properties