PCB Surface Finish Selection: Four Requirements
Selecting a surface finish is a decision that is made once and affects the board for its whole life, because the finish determines how the board stores, how it solders and how its joints behave in service. The options are well characterised, and the choice follows from four requirements rather than from preference.
The Four Requirements
A finish has to be solderable after the storage period the product expects, planar enough for the finest component pitch on the board, compatible with the number of thermal cycles the assembly will see, and compatible with the reliability requirement of the joints it forms.
Those four requirements are usually in tension. The finish with the best shelf life is not the most planar, and the most planar is not the cheapest. Establishing which requirement dominates is what makes the choice straightforward, and it should happen before the stack-up is released. Our plating thickness notes describe how the deposit is specified.
Hot Air Solder Levelling
Hot air levelling coats the copper with solder, which makes it solderable for a long period and inexpensive to apply. The disadvantage is that the surface is not flat, because the air knife leaves a meniscus on every pad, and the uneven surface is difficult for a fine-pitch stencil to print against.
The planarity limit is usually expressed as the smallest pitch that can be printed reliably, and it falls in the region where a leaded package is comfortable and an area array is not. The finish also has a thermal history already, since the solder has been melted once. Our hot air levelling notes describe the process.
Immersion Gold Over Nickel
Electroless nickel with immersion gold gives a flat surface, a good shelf life and a wide process window, and it is the usual choice where fine pitch and long storage both matter. The two metals are there for different reasons: the nickel provides the barrier and the solderable surface, while the gold protects the nickel while it is in storage.
The gold dissolves into the solder joint during reflow, and a thick layer leaves brittle intermetallics behind. The coating thickness is therefore specified tightly, and a board that is over-plated is more of a risk than one that is thin. The process also requires care over the nickel bath, because a contaminated bath produces a joint that fails under thermal cycling.
Immersion Tin and Immersion Silver
Both give a flat surface and a short shelf life, and both are sensitive to storage conditions. Immersion tin forms an intermetallic layer that grows with time and temperature, and beyond a certain thickness the layer is no longer solderable, which puts a practical limit on the storage period.
Immersion silver is solderable and flat but tarnishes, and the tarnish is not always removable by the flux. It also requires careful packaging, since airborne sulphur compounds attack it. The two finishes are common on consumer products where the storage period is short and the cost matters.
Organic Solderability Preservatives
An organic coating is the cheapest planar option, and it works by forming a thin protective layer on the copper that decomposes at soldering temperature. The advantage is that the joint is formed directly with the copper, with no intermetallic barrier, which gives good joint strength.
The disadvantage is fragility. The coating is damaged by handling, by repeated thermal cycles and by heat, so it is limited to products that are assembled soon after fabrication and that go through one or two reflow passes at most.
Hard Gold and Edge Connectors
Where the board itself is a connector, the contact surface must survive repeated insertion, and that requires a hard gold deposit over nickel on the contact fingers. The gold is alloyed with a small amount of a hardening element, and the deposit is much thicker than on a solderable surface.
The contact area is usually kept free of solder mask and of any other coating, and the transition between the contact and the solderable area is defined in the fabrication drawing. Gold fingers that are soldered must be handled differently, since gold in a solder joint is the embrittlement risk described earlier.
Cost and Lead Time
The finishes rank in a consistent order of cost, with organic coatings cheapest and hard gold the most expensive, and lead time follows the same order because the more specialised processes are scheduled less frequently.
The cost difference is small per board and large per program, because a finish that reduces yield or requires a rework step costs more than the plating itself. Our cost reduction notes describe how the trade is evaluated.
Confirming the Choice
The finish should be stated on the fabrication drawing by name and by thickness, with the storage period the product expects. A drawing that says only that the board must be solderable leaves the choice to the shop, and the shop will choose what its process handles best rather than what the product needs.
Where the product has a long storage period before assembly, the finish and the packaging should be specified together, since a good finish in poor packaging performs worse than a moderate finish in a sealed bag with desiccant.
Process Control and Verification
On a design of this kind, surface finish is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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.
Process Control and Verification
On a design of this kind, surface finish is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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.
FAQ
Which finish is best for fine pitch? A flat finish, which means one of the immersion processes or an organic coating. Hot air levelling is excluded by its surface profile rather than by its solderability, and the limit appears at the pitch where the paste deposit becomes inconsistent.
How long can a board be stored? The answer depends on the finish, the packaging and the environment. A flat immersion finish in a sealed bag with desiccant will typically remain solderable for a year, while an organic coating may be marginal after a few months regardless of packaging.
What does gopcb recommend for a new design? We recommend choosing the finish from the pitch, the storage period and the thermal cycles, and stating all three on the drawing. Where a customer has no preference we propose the finish that best fits those three, together with the packaging that makes it work.
