A PCB (printed circuit board) surface finish is a protective layer over the bare copper pads that keeps the copper from oxidizing and solderable until assembly. No finish is good at everything: first write down your assembly conditions (fine pitch, number of reflow passes, press-fit, wire bonding, time until assembly), then choose among HASL, immersion tin, immersion silver, OSP and ENIG, and agree on storage, handling and incoming inspection together. This article covers how assemblers and designers choose a finish before ordering; hard and soft gold for gold fingers and joint strength comparisons of OSP and ENIG are covered elsewhere. It is divided into 5 stages:
- Stage ① Write down the assembly conditions: fine pitch and BGA, reflow passes, through-holes and press-fit, wire bonding, storage time.
- Stage ② Know the five common finishes: what each is, and the strengths and limits public sources give.
- Stage ③ Match to your conditions: flatness, multiple reflows, press-fit, wire bonding, electrical test and exposure.
- Stage ④ Storage and handling: shelf lives that differ by source, handling, and time limits after opening.
- Stage ⑤ Incoming inspection and wetting problems: whether thickness can be measured, black pad, tin whiskers, and what to look back at when solder won’t wet.
For whether expired boards should be baked, see our published article “PCB baking: should expired boards be baked? A 4-step check.” This describes the general case; individual cases still need their own assessment.
Figure 1. Five stages: assembly conditions (a conditions table) → five finishes (know each one’s limits) → match to conditions (two or three left) → storage and handling (shelf life written into the specification) → incoming checks and problems (a path to trace problems).
Stage ①: Write down the assembly conditions before choosing
- Put the board’s assembly conditions in a table: fine-pitch parts or BGAs (ball grid arrays), how many reflow passes, through-holes for wave soldering, press-fit connectors, wire bonding, keypad or mating contact surfaces, and whether copper will be exposed on the finished product (the table is this article’s compilation).
- Then the timing: how long from board fabrication to assembly, whether assembly will be in batches, and whether at different factories (this article’s compilation).
- Lead restrictions: if the product must meet regulations restricting lead, confirm the finish itself contains no lead (tin-lead HASL does) and that your lead-free solder wets it.
- Compare cost last: compare cost just among the finishes that meet the other conditions (this article’s compilation).
An EMS provider’s blog article (page date 27 December 2023; metadata shows modified June 2025) lists what to consider: solderability, how long it stays solderable in storage, resistance to repeated heating and cooling, resistance to wear from handling and mating, cost, and whether lead-free solder wets it, along with regulations.
Stage ②: Know the five common finishes
- HASL (hot air solder leveling): the board is dipped in molten solder and the surface leveled with hot-air knives; lead-free HASL uses tin-copper, tin-nickel or tin-copper-nickel solder instead of tin-lead.
- Immersion tin: a layer of tin chemically deposited on the copper.
- Immersion silver: a layer of silver immersion-plated on the copper, with a very flat surface.
- OSP (organic solderability preservative): a thin organic coating applied by dipping or spraying, which bonds to copper alone and protects it until soldering.
- ENIG (electroless nickel immersion gold): electroless nickel first, then a thin immersion gold layer that keeps the nickel from oxidizing; nickel, palladium and gold in sequence is called ENEPIG.
Figure 2. Cross-sections of the five finishes (illustration, thickness not to scale): all cover the copper pad; HASL’s solder layer is thick and uneven, while immersion tin, immersion silver, OSP and ENIG (thin gold over nickel) are thin layers with flatter surfaces.
Table 2 | Strengths and limits public sources give (Source 1; ENIG also Source 2)
| Finish | Strengths | Limits |
|---|---|---|
| HASL | Low cost, easy to source, reworkable, very good shelf life | Uneven surface, poor for fine pitch, tin-lead versions contain lead, thermal shock, bridging, holes may be plugged or narrowed |
| Immersion tin | Flat, lead-free, reworkable, the usual choice for press-fit pins | Easily damaged by handling, the process uses a carcinogen (thiourea), exposed tin may corrode, tin whiskers, poor for multiple reflows, thickness hard to measure |
| Immersion silver | More uniform than HASL, cheaper than ENIG, shelf life similar to HASL | Must be soldered the same day it leaves storage, discolors with poor handling, no nickel layer so less durable than ENIG |
| OSP | Flat, lead-free, simple process, reworkable, economical | No way to measure thickness, poor for through-holes, short shelf life, may cause in-circuit test (ICT) problems, copper exposed after assembly, handling-sensitive |
| ENIG | Flat, lead-free, good for through-holes, long shelf life; very flat surface matters for BGAs (Source 2) | Expensive, not reworkable, black pad, damaged by electrical test probing, signal loss at high frequency, complex process; early processes had poorer adhesion and wetting (Source 2) |
| ENEPIG | Very flat, multiple assembly, wire bondable, no black pad risk, shelf life 12 months or more | Still expensive, limited rework, process constraints |
Lead-free HASL also cannot be made fully flat (Source 1). Source 2 (an online encyclopedia’s ENIG entry) adds that ENIG may form a non-conductive layer containing nickel and phosphorus, called black pad, and that its quality is specified by IPC-4552A. An online encyclopedia’s OSP entry says OSP uses water-based organic compounds that selectively bond to copper, and the process is complex, with tight control of the coating’s composition and quality.
How JIAHON can help:
- Table 1 of our PCB baking article lists, by surface finish, whether baking is possible and shelf life references, for comparison.
Stage ③: Match to your conditions, and strike out what doesn’t fit
- Fine-pitch parts or BGAs: HASL’s uneven surface puts it at the back of the list; immersion tin, immersion silver, OSP and ENIG are all flatter.
- Multiple reflows or assemblies: immersion tin is listed as poor for multiple reflows; ENEPIG is listed as suitable for multiple assembly.
- Through-holes for wave soldering: OSP is listed as poor for through-holes; ENIG as good; HASL holes may be plugged or narrowed.
- Press-fit connectors: immersion tin is listed as the usual choice for press-fit pins.
- Wire bonding: ENEPIG is listed as wire bondable; some sources say ENIG and immersion silver can be wire bonded, so confirm for your bonding method (gold or aluminum wire).
- In-circuit test, exposure and high frequency: OSP may cause ICT problems and leaves copper exposed after assembly; exposed immersion tin may corrode; ENIG has signal loss at high frequency. Where these apply to your board, strike further.
- With two or three left, compare shelf life and cost (this article’s compilation).
These “listed as” points come from Source 1, whose conclusion says lead-free HASL may be considered for products restricted on lead, but just for designs without fine-pitch parts, as it cannot be made fully flat. Source 5 (the EMS provider) says HASL’s uneven surface may cause problems for parts smaller than 0805 or fine-pitch BGAs, with bridging risk on dense boards; immersion finishes may be a better choice for denser, finer-pitch boards despite somewhat higher cost; ENIG is used for fine-pitch parts, BGAs and small chip packages and can be wire bonded; immersion silver and tin are also used for fine pitch and BGAs; and ENEPIG can be wire bonded.
Table 3 | A board maker’s comparison table (Source 4)
| Finish | Fine pitch (paste printing) | Aluminum wire bonding | Cost |
|---|---|---|---|
| ENIG | Excellent | Yes | High |
| Immersion silver | Excellent | Yes | Medium |
| Immersion tin | Excellent | No | Medium |
| OSP | Excellent | No | Lower |
| Lead-free HASL | Good | No | Low |
| Tin-lead HASL | Good | No | Low |
How JIAHON can help:
- Solder paste and solder wire: the product page lists lead-free SAC305 solder paste and solder wire in various specifications. For validating a paste step by step through a small trial run, see “Choosing solder paste: 5 validation stages from data sheet to small-batch trial.”
- Box-type vacuum plasma cleaner: the product page lists desmear and surface cleaning for PCB/FPC and pre-bond treatment in semiconductor packaging; RF power 0 to 600 W, 13.56 MHz.
Stage ④: Storage and handling: shelf lives differ by source
- Write the shelf life into the purchase specification, and go by what your board maker commits to: the published figures disagree (below), so no single figure fits every maker (this article’s compilation).
- Record the storage environment: measure and log temperature and humidity, so you know the conditions the boards sat in (this article’s compilation).
- Immersion silver: solder the same day it leaves storage; it discolors with poor handling and needs protective packaging.
- OSP and immersion tin: handling-sensitive and easily damaged by handling.
- For batch assembly, check in advance that the last batch will still be within shelf life (this article’s compilation).
Figure 3. Four storage and handling reminders (illustration): shelf life follows the board maker’s commitment, as published figures differ; immersion silver must be soldered the same day it leaves storage and needs protective packaging; OSP has a short shelf life and is handling-sensitive; immersion tin is easily damaged by handling and poor for multiple reflows.
Table 4 | Shelf life and handling: two sources give different figures
| Finish | Source 4 (at 15 to 28 °C) / Source 5 |
|---|---|
| ENIG | 12 months / long (years) |
| ENEPIG | Not listed / 12 months or more |
| Immersion silver | 6 months / 6 to 12 months; solder within one day of opening, with protective packaging (Source 1 also says the same day it leaves storage) |
| Immersion tin | 6 months / 30 days (assemble within 30 days of plating) |
| OSP | 3 to 6 months / 3 to 6 months; thickness in the 0.05 to 0.2 µm range |
| HASL | 12 months (lead-free and tin-lead) / medium |
Source 5 also says immersion silver often discolors in air, so it should be protected in packaging and soldered within a day of opening, avoiding peelable masks; and OSP should be handled carefully so as not to damage the coating.
How JIAHON can help:
- Temperature and humidity data logger: ±0.3 °C (10 to 40 °C), ±2.5% RH (30 to 80% RH at 25 °C), range −25 to 70 °C and 0 to 99% RH, 8,000 readings × 2 channels, 1 second to 60 minute sampling, Wi-Fi (per the product page).
- Temperature and humidity transmitter: wall-mounted, LCD showing both, humidity 0 to 100% RH (non-condensing), accuracy ±2% or ±3%, output 4 to 20 mA or 0 to 10 V; temperature range depends on the model (the page says commonly 0 to 50 °C).
- Smart programmable oven: nitrogen purge and multi-segment ramp profiles, listed for prebaking and drying electronic components.
- Our PCB baking article notes that general baking is not recommended for immersion tin, immersion silver and OSP, while a board maker’s document says HASL and ENIG can be baked.
Stage ⑤: Incoming inspection and wetting problems: look back at the finish first
- At incoming inspection, confirm the finish type matches the specification, is within shelf life, and the packaging is intact (this article’s compilation).
- Thickness: OSP has no way to measure thickness and immersion tin’s is hard to measure; for these two, rely on the board maker’s process control and shipping data, and at incoming you can mostly inspect appearance and run a solder trial (this article’s compilation).
- ENIG: watch for black pad, a non-conductive layer of nickel and phosphorus.
- Immersion tin: watch for tin whiskers, conductive filaments that can grow from tin surfaces and may cause shorts.
- When solder won’t wet, look back first: whether shelf life has passed, whether immersion silver has discolored, whether OSP has handling damage, and whether immersion tin has already been through several reflows; then check the solder paste and reflow profile (this article’s compilation).
A NASA NEPP tin whisker page (updated 17 January 2019) says tin whiskers are conductive tin crystals that sometimes grow from tin surfaces; which platings are prone is still much debated, with most literature finding pure tin electroplate (especially with brighteners) prone, and whiskers also reported on tin-lead plating; in low-voltage, high-impedance circuits whiskers may cause stable shorts, and in high-current, high-voltage environments may trigger metal vapor arcs; mitigations include avoiding pure-tin-plated parts, re-dipping or replating with tin-lead solder, and conformal coating or foam encapsulation; the page covers parts and platings, not immersion tin on boards specifically.
How JIAHON can help:
- Damp boards may delaminate or crack in reflow, which is separate from the surface finish; our popcorn effect article covers what happens when damp boards go into reflow, so the two can be investigated together.
Related reading
- “PCB baking: should expired boards be baked? A 4-step check”
- “What is the popcorn effect? Why moisture in ICs and boards makes them crack in reflow”
- “Choosing solder paste: 5 validation stages from data sheet to small-batch trial”
Sources and limits
Sources: Source 1: a PCB company’s technical article (undated). Source 2: an online encyclopedia’s ENIG entry (multi-author, subject to change). Source 3: an online encyclopedia’s OSP entry (multi-author; short). Source 4: a board maker’s surface finish page (undated). Source 5: an EMS provider’s blog article (page date 27 December 2023; metadata shows modified June 2025). Source 6: NASA NEPP website tin whisker page (updated 17 January 2019). IPC-4552A (ENIG specification) is named by Source 2; its full text was not read, and no board maker’s written specification was read.
Limits: Sources 1, 4 and 5 are companies selling boards or assembly services, and Sources 2 and 3 are multi-author encyclopedias. Shelf lives differ a lot between Sources 4 and 5 (immersion tin 6 months vs. 30 days; immersion silver 6 months vs. 6 to 12 months), and both are given; go by your board maker’s commitment and your own validation. On OSP’s process, Source 1 calls it simple and Source 3 complex; both are given. Most Stage ③ strengths and limits come from one maker’s list (Source 1) without test data. Source 6 covers parts and platings; the claim that immersion tin grows whiskers is from Source 1. Cost is given in relative terms alone. Product details follow the product pages.
This is a general summary, not a restatement of any standard, an endorsement of any maker or service provider, or a promise about your product or process; follow your product specifications, the board maker’s written specifications, the relevant standards and your own validation.