The surface finish of a printed circuit board affects the bill of materials cost, but it can have an even greater impact on labor cost and yields for PCB assemblies.
Most buyers, when sending a job to an electronics manufacturing service (EMS) provider, leave it up to the EMS to source the printed circuit boards and specify the surface finish. Some EMS providers, though, won't make that decision for you and require you to specify the finish. You will also have to specify the finish if you are providing a kit including bare boards to your EMS partner.
The surface finish protects the copper traces from oxidation until component placement and solder reflow. A cheaper finish may reduce the BOM cost, but if it's not appropriate to the design or if the EMS doesn't use the right handling and storage for that finish, those savings will be more than wiped out by lower yields, lower quality, and higher labor costs due to rework.
Here is a quick look at four of the most common RoHS finishes, in order from least expensive to most expensive, and their applicability.
1. Hot air surface leveling (HASL)
HASL is the least expensive finish. The board is dipped in molten solder, which is then blown off or “leveled” with an air knife, leaving a layer of solder on the copper traces.
HASL is a very good, economical choice for through-hole PCBs. It can be used for some SMT boards, but only if the design does not call for small or fine-pitch components because HASL coatings have very low surface planarity, which causes significant solderability issues with small components. We do not recommend HASL for SMT components smaller than 0805 or SOIC.
2. Immersion tin
Immersion coatings use a chemical process to create a layer of metal just a few molecules thick on the copper. It results in a very flat finish, suitable for fine-pitch SMT.
Immersion tin is the least expensive option using this process. However, the tin layer forms a battery with the copper traces and begins to tarnish immediately, regardless of storage conditions. Increased tarnish means decreased solderability, lower-quality solder joints and poor yields in PCB assembly. The shelf life of bare boards with immersion tin surface finish is about 30 days.
Immersion tin is a good choice for high-volume production, where the boards move quickly from plating to assembly and reflow and the limited shelf life is not an issue.
3. Immersion silver
Immersion silver finish is created in same manner as immersion tin. Silver is more expensive but does not react with the copper traces so its shelf life is longer. It does tarnish when exposed to air, and the boards must be stored in anti-tarnish packaging to slow oxidation. Properly stored, immersion tin finished boards have a shelf life of six to 12 months. When removed from the packaging, they should be placed and reflowed within about 24 hours depending on humidity and environmental air quality.
Immersion silver is preferable to tin for fine-pitch SMT production in low- to mid-volumes, where multiple board assembly runs may be scheduled over the course of a year. It allows more economical purchase quantities for a year's supply of boards. If using immersion silver finish, you should ask your EMS partner about their processes for storing and handling the boards.
4. Electroless nickel / immersion gold (ENIG)
ENIG is chemically deposited nickel followed by a layer of immersion gold. The nickel provides a non-reactive barrier layer between the gold and copper. ENIG finish has excellent surface planarity and a shelf life of several years. It also has better solderability than tin or silver, producing higher yields and consistently better solder joints.
At Z-AXIS we've been trending towards ENIG finish boards for almost all of our SMT work, which is mainly low- to mid-volume in a high-mix setting. ENIG is the most expensive of these options, adding up to 5 percent to the cost of bare boards, but contributes to our lower labor costs by reducing the need for rework due to solder issues. It also helps us ensure more reliable products with consistently high-quality solder joints.
We hope this helps you when it comes time for you to make a decision about surface finishes in your own operations.