Surface Finish and Solderability Compared
The surface finish is the interface between the board and the solder, and it decides how well a joint forms and how long the board can be stored before it is assembled. The finishes differ in cost, in flatness, in shelf life and in the defects they produce.
The choice is therefore made against the assembly process and the storage interval rather than against the finish alone. A finish that suits a hand soldered prototype may be the wrong one for a fine pitch reflow assembly.
What the Finish Has to Do
The finish protects the copper from oxidation and provides a surface that the flux and the alloy can wet. It has to remain wettable for the storage period and to survive the number of thermal cycles the board will see.
It also affects the surface it presents to the paste, since a flat finish allows a consistent deposit while a rough one disturbs the stencil seal. The requirement is therefore both chemical and mechanical.
Hot Air Levelled Tin Lead and Lead Free
A hot air levelled finish is inexpensive and has a long record, and it produces a surface that is not flat. The unevenness is a limitation for fine pitch parts, where the paste deposit depends on the stencil sealing.
The finish is also prone to a thick intermetallic if it is reflowed more than once. It remains a reasonable choice for a coarse board with generous pads.
Immersion Silver and Immersion Tin
An immersion finish deposits a thin layer over the copper, and it is flat, which suits fine pitch work. The layer is thin and can be consumed by a single reflow or affected by handling, so the shelf life and the handling rules matter.
Immersion silver is sensitive to contamination and to humidity, and the storage packaging has to protect it. Immersion tin has its own ageing behaviour, particularly at elevated temperature.
Electroless Nickel Immersion Gold
ENIG gives a flat, durable surface with a long shelf life and good performance on fine pitch parts. Its weaknesses are cost and the possibility of a black pad condition at the nickel surface if the process is not controlled.
The black pad problem is a fabrication control issue rather than a design one, and the supplier should be able to demonstrate process control on the immersion step.
Organic Solderability Preservative
An organic coating is the cheapest flat finish, and it protects the copper for a limited period. It is consumed by the first reflow, so a board that requires a second pass has a reduced surface on the second side.
The storage life is short and the finish is sensitive to handling, which makes it suitable for a product that is assembled promptly rather than one that is stored.
Solderability and Ageing
Solderability degrades with time and with exposure to humidity and contamination. The rate depends on the finish and on the storage, and it is measured by a wetting balance or a solder globule test on a sample.
Where a product has a long storage interval, the finish should be selected for the interval rather than for the assembly process alone. The two requirements are both part of the decision.
Cost and Process Effects
The finishes are priced differently, and the difference is small per board compared with the cost of a yield problem. The process effect is often the deciding factor, particularly on a fine pitch board where flatness matters.
The finish also affects the flux choice and the profile, since the activation temperature and the reaction with the surface differ. Changing the finish should trigger a review of the flux and the profile, as described for flux application.
Edge and Hole Coverage
Some finishes cover the inside of a barrel well and some do not, which matters for a through hole joint and for a press fit pin. The coverage should be verified on a section rather than assumed.
The same applies to an edge or a castellation, where a vertical surface is more difficult to coat evenly. This is part of the requirement described for edge plating.
Documentation
The drawing should state the finish, the thickness where it is specified, and the storage interval that the choice assumes. Where a second source is used, the finish should be reviewed as part of the qualification.
A finish substitution that appears equivalent can change the wetting behaviour and the shelf life, so it should be treated as a design change rather than a purchasing decision.
Process Control and Verification
Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.
FAQ
Which finish suits fine pitch work? A flat finish such as ENIG or an immersion layer, because the stencil seal depends on the surface being level.
Is the cheapest finish a false economy? It can be, since a short shelf life or a poor wetting performance creates a cost that is larger than the saving.
Does the finish affect the profile? It does, because the reaction between the flux and the surface depends on the chemistry of the finish.
How is shelf life verified? By a solderability test on a sample stored under the intended conditions for the intended interval.
