What is PCB Surface Finishes?

PCB surface treatment refers to the exposed copper foil area of the printed circuit board (such as pads, conductive paths) covered with a layer of metal or alloy coating, as the copper surface of the “protective barrier” and “welding medium”.

PCB Surface Finishing Core Functions

Physical Protection: Insulates copper from contact with air and moisture, preventing oxidation, sulfidation, and other corrosive reactions;
Solderability optimization: Provide a flat and stable soldering interface to ensure a reliable connection between the solder (e.g., solder paste) and the copper layer;
Electrical performance guarantee: to maintain the stability of the circuit conduction, to avoid impedance abnormalities or short-circuit risk due to the deterioration of the copper surface.

The importance of PCB surface treatment

Core purpose: to solve the “oxidation problem” of the copper surface

Copper at room temperature with oxygen in the air, water vapor contact will generate copper oxide (CuO) or alkaline copper carbonate (copper green), these oxidized layers will significantly reduce the wettability of soldering — specifically manifested as solder “refuses to solder”, solder joints false or cracked. Surface preparation ensures that the copper surface is active during soldering by covering it with a coating that radically blocks the copper’s contact path with the oxidizer.

Industry Importance: a critical process throughout the PCB life cycle

1.Manufacturing
Ensure SMT (Surface Mount Technology) yields and reduce rework costs due to poor solderability;
Coating uniformity directly affects the mechanical strength of components after soldering (e.g., solder joint tension, shear force).

2.Storage and transportation
In long-term storage, the coating can resist humidity, salt spray, and other environmental factors erosion (such as coastal areas with equipment, PCBs need to pay special attention to the ability to prevent rust);
Avoid copper surface damage caused by friction and collision during transportation.

3.Adaptation to the use of scenes
High-temperature environments (such as automotive electronics, industrial control) require the coating to have aging resistance to prevent decomposition or oxidation of the coating at high temperatures;
In high-frequency circuits, the flatness of the coating affects the signal transmission loss (e.g., immersion gold process is commonly used in RF PCBs because of the good uniformity of the coating).

In-Depth Comparison of 7 PCB Surface Finishes

1. Hot Air Solder Leveling (HASL)

Process Principle:
PCB immersion in 260°C molten solder (Sn63Pb37 or SAC305), followed by high-pressure hot air (400°C) removal of excess solder, creates uneven “hilly” surfaces.

Ideal For:

• Consumer electronics (chargers, LED drivers)
• Cost-sensitive high-volume orders

Hard Lesson:
A router manufacturer experienced widespread BGA voids using lead-free HASL, ultimately adding a “pad pre-tinning” step that increased cost by $0.17/board.

Critical Controls:

2. Electroless Nickel Immersion Gold (ENIG)

Layer Structure:
“Sandwich” deposition: Electroless Ni (3-5μm) → Displacement Au (0.05-0.1μm). Ni acts as copper “firewall,” Au as a “soldering interface.”

High-Freq Case Study:
A mmWave radar board chose ENIG over OSP because Au’s skin effect loss was 23% lower (@77GHz).

Black Pad Analysis:
When Ni bath exceeds 91°C, phosphorus segregation forms brittle Ni3P phases (SEM shows “cracked” morphology). Prevention:

• Add citric acid buffer
• Implement pulse plating
• Include micro-etch before Au deposition

3. Organic Solderability Preservative (OSP)

Molecular Protection:
Benzimidazole-copper chelates form 0.2-0.5μm films resisting 6-month natural oxidation.

5G Preferred Choice:
A base station AAU board using OSP+LDI saved $4.2/m² vs ENIG with 0.3dB/cm lower insertion loss (@28GHz).

Storage Don’ts:

• RH>60% causes film hydrolysis
• Sulfur-containing packaging creates Cu2S black spots
• Must SMT within 24 hours after unpacking

4. Immersion Tin (ImSn)

Microstructure:
Cu6Sn5 intermetallic thickness (ideal:1.2-1.8μm via EDX) determines reliability.

Automotive Success:
An ECU module passed 3000x -40°C~125°C cycles with ImSn vs ENIG’s 2400x.

Process Risks:

• Tin whisker growth (suppressed by reflow pre-aging)
• Cross-contamination in double-sided boards
• Incompatible with Al wire bonding

5. Immersion Silver (ImAg)

Signal Integrity Edge:
10GHz insertion loss is 15% lower than ENIG (per IPC-6012B).

Migration Countermeasures:
“Nanoparticle doping” raises the migration threshold from 3.1V to 5.6V for 48V power modules.

Thickness Control:

• Sodium thiosulfate as an inhibitor
• Spray-type plating tank
• Chromate passivation post-treatment

6. Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

Layer Innovation:
0.1-0.2μm Pd between Ni (3-4μm) and Au (0.03-0.05μm) prevents Au diffusion.

SiP Application:
A 3D package achieved mixed Au wire/SnAgCu soldering using ENEPIG.

Cost Optimization:

• Gradient Pd thickness (0.15μm edge/0.08μm center)
• Pd-Co alloy instead of pure Pd

7. Electrolytic Hard Gold

Military-Grade Protection:
Co-doped Au (1-3μm) at 180HV hardness withstands 50x more wear than ENIG.

Connector Specs:

• Gold finger bevel: 30°±1°
• Ni thickness ≥5μm
• 3mm transition zones required

Cost Trap:
A backplane’s incorrect plating area increased finish cost from 8% to 34% of the total.

Selection Decision Tree

5 Common Failure Clinics

Q1: Black residues on ENIG pads after reflow?
→ “Gold embrittlement”! Immediately check:

1. Ni-P content (7-9% optimal)
2. Au thickness >0.08μm?
3. Solder paste Bi content

Q2: Tin whiskers on ImSn after 3-month storage?
→ Execute “rescue trio”:

1. 150°C baking for 2hrs
2. Apply anti-diffusion nano-coating
3. Switch to the matte tin process

Q3: OSP boards show poor wettability after multiple reflows?
→ The organic film degrades! Take these steps:

1. Verify reflow peak temperature didn’t exceed 245°C
2. Check storage time – OSP degrades after 6 months
3. Consider adding a nitrogen atmosphere during reflow

Q4: ENEPIG boards failing wire bond pull tests?
→ Usually, a palladium layer issue:

1. Measure Pd thickness (0.15-0.25μm ideal)
2. Check for Pd oxidation (XPS analysis recommended)
3. Adjust PD bath pH to the 8.2-8.6 range

Q5: Have HASL boards with uneven solder thickness?
→ Air knife calibration needed:

1. Verify air knife pressure (25-35 psi typical)
2. Check leveling time (3-5 seconds optimal)
3. Inspect board support fixtures for warpage

Pro Tips from PCB Manufacturers

1. HASL – For double-sided boards, request “dual dip” processing to prevent the shadow effect
2. ENIG – Always specify “mid-phosphorus” nickel (6-9% P) for best reliability
3. OSP – For high-reliability applications, choose “Type 3” OSP formulations
4. ImSn – Storage in nitrogen cabinets extends shelf life from 6 to 12 months
5. ImAg – Add anti-tarnish treatment if the boards will undergo multiple thermal cycles
6. ENEPIG – Specify “low-stress nickel” for flexible PCB applications
7. Hard Gold – Cobalt content should be 0.1-0.3% for optimal wear resistance

Cost-Performance Tradeoff Analysis

Future Trends in Surface Finishes

1. Nanocomposite OSP – Graphene-enhanced formulations showing 2x longer shelf life in trials
2. Low-Temperature ENIG – New chemistries allow processing at 65°C vs traditional 85°C
3. Selective Finishes – Combining different finishes on single boards (e.g., ENIG + OSP)
4. Self-Healing Films – Experimental OSP that repairs minor scratches during reflow
5. Halogen-Free Processes – Meeting upcoming EU environmental regulations

When evaluating surface finishes, remember: there’s no universal “best” option – only the most appropriate solution for your specific design requirements, budget constraints, and manufacturing capabilities. The most expensive finish isn’t necessarily the right choice, just as the most economical option could lead to field failures. Always conduct real-world testing with your actual PCB design and components before finalizing your selection.