Two boards can leave the same fabrication line on the same day with the same laminate and the same copper, and only one of them assembles cleanly three months later. The variable is often the pad finish: a layer measured in micro-inches that decides how the paste wets, how flat the land stays, and how long the panel can sit in a store before joint quality drops.
This article compares the finishes used on most RF, medical, telecom and defense boards, shows where each runs out of room, and lists what the drawing has to state.
Copper oxidises in air, so every pad carries a protective layer until reflow, and that layer then has to get out of the way and let the solder bond. Three properties matter at once: solderability after storage, flatness across the pad, and compatibility with the metals in the joint.
The finishes that cover most orders
| Finish | What it is | Where it fits | Where it runs out |
|---|---|---|---|
| OSP | Organic film over bare copper | Cost driven boards assembled soon after fabrication | One heat cycle, then no protection |
| Lead-free HASL | Molten solder levelled by hot air | Coarse pitch, connectors, through hole, power terminals | Uneven pad thickness, weak coplanarity |
| Immersion tin | Thin tin deposit on copper | Press fit connectors | Short shelf life, whisker control |
| Immersion silver | Thin silver, no nickel barrier | Fine pitch, high frequency lines, fast wetting | Sulphur staining in storage |
| ENIG | Electroless nickel with gold cap | Fine pitch BGA and QFN, wire bonding, long shelf life | Black pad if the nickel bath drifts |
| ENEPIG | Nickel, palladium, gold | Gold and aluminium bonding on one board | More steps, higher cost |
Flatness: where HASL stops working
HASL leaves a solder layer levelled by air knives, so thickness across a pad varies with the copper around it and with the direction the knife passes. At 0.65 mm pitch that is tolerable. At 0.5 mm and finer, and under a BGA or QFN where the joint self centres, the same variation becomes bridges, lifted leads and joints that pass inspection and fail test.
Fine pitch work therefore moves to a plated finish with a flat surface. Flatness does not fix a warped panel or a starved paste deposit, so the stencil still has to match the land.
Storage: shelf life is a schedule item
On a low volume programme panels are fabricated once and assembled in batches across a year or more, so the finish has to survive in a store.
| Finish | Shelf life, sealed and controlled | What changes with time |
|---|---|---|
| OSP | 6 months | Copper oxidises once the film is disturbed by reflow |
| Immersion tin | 6 months | Intermetallic growth consumes the tin, whisker risk rises |
| Immersion silver | 6 to 12 months | Sulphur staining |
| ENIG and ENEPIG | 12 months and longer | Stable while the nickel barrier is intact |
RF and microwave: the nickel question
Above roughly 1 GHz the current crowds into the surface, and every layer the signal crosses adds loss. Nickel under gold is the awkward case: it protects the copper and gives a flat bondable pad, but it is resistive and magnetic next to copper, so a microstrip over plated nickel shows more insertion loss than the same line on silver.
The usual answer on an RF board is to keep transmission lines on a finish with no nickel in the path, such as immersion silver or OSP, and reserve ENIG for component pads and aluminium wire bonding. Selective plating makes that split possible on one panel. On an impedance controlled board the finish also belongs on the coupon, because a change of finish shifts the measured impedance and has to be re-measured.
SUPERB PCB builds RF work from 500 MHz to 77 GHz on Rogers, Taconic and PTFE laminates, with impedance to plus or minus 5 percent, tighter to plus or minus 2 percent on critical nets, lines down to 0.05 mm, and a TDR coupon on each panel so etch and finish are measured together.
Medical and long-life equipment
Two failure modes dominate here. The first is black pad, a hyper-corroded nickel surface created during immersion gold. It appears as dewetting or as a brittle joint that cracks in thermal cycling, and no visual check will catch it, so it is controlled by bath chemistry, nickel thickness and a microsection on the first panel of a lot.
The second is corrosion after cleaning, where flux or finish residues stay under a component or under coating. A nickel barrier tolerates repeated cleaning and sterilisation better than a thin organic film, which is why ENIG stays common on instrument boards even where the pitch allows something cheaper.
What the drawing should state
- The finish by name, standard and thickness, for example ENIG to IPC-4552 with nickel at 100 to 200 micro-inches.
- Areas that must stay unplated: press fit holes, test points, an RF line that has to remain on bare copper.
- Whether wire bonding, press fit or a wear surface is involved, and how long the panels will be stored before assembly.
- The workmanship class per IPC-A-610, because the sample size and report scope follow it.
How SUPERB PCB handles finish control
The board shop runs OSP, ENIG with nickel at 100 to 200 micro-inches and gold at 2 to 3 micro-inches, selective ENIG, ENEPIG, immersion tin, immersion silver, lead-free HASL, and hard or flash gold for contacts. Layer counts run from 1 to 68, copper to 6 oz inner and 10 oz outer, warpage held to 0.75 percent for surface mount work.
Orders can ship with a microsection for plating and dielectric thickness, an impedance report where controlled impedance is called out, and X-ray data for thermal pads. The quality system is ISO 9001, with ISO 13485 applied to medical builds and workmanship to IPC-A-610 Class 2 or 3 as the drawing states. SUPERB Assembly runs twelve zone reflow on SAC305 with nitrogen below 500 ppm and 3D SPI before placement, so a finish that wets late shows as a paste volume trend instead of a defect at final test.
- Process parameters: finish list, thickness ranges and tolerances.
- PCB capability: layer counts, copper weights, minimum pad and pitch.
- RF boards: laminate choice, impedance control and mixed finish panels.
- PCB testing: microsection, impedance and the reports that ship with the order.
Frequently asked questions
Is ENIG always better than HASL?
No. ENIG buys flatness and shelf life and adds plating steps. A board with coarse pitch parts, connectors and through hole power pins often assembles better and cheaper on lead-free HASL. The choice follows the finest pitch and the storage time.
Can one panel carry two finishes?
Yes, with selective plating or masking. It is common on RF boards where the lines stay on silver or bare copper and only the component pads take ENIG. The areas have to be drawn, because the shop cannot infer which line is which.
Next step
Send the Gerber data, the stack up, the finish on the drawing and the assembly plan, including the build date and the storage time. The reply confirms which finish matches the finest pitch and the storage window, flags any area that needs selective plating, and quotes the build with the inspection scope attached.
