What is Surface Finish in PCB Manufacturing?

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In the world of PCB manufacturing, applying the right surface finish is vital for ensuring reliability and extending the shelf life of the final assembly. This article explores the various types of surface finishes available for printed circuit boards, which enhance metallic bonding and longevity.

Current PCB assemblies predominantly feature SMT components where pad flatness is crucial. The common surface finishes include electroless nickel and immersion gold, among others such as soft gold, sterling silver, and OSP finishes. Sierra Circuits offers multiple surface finish options to cater to diverse customer needs, including hot air solder leveling (HASL), lead-free HASL, electroless nickel immersion gold (ENIG), and electroless nickel electroless palladium immersion gold (ENEPIG). Each surface finish offers different levels of shelf life and application suitability.

The most widely used finish is HASL, which traditionally employs a tin/lead alloy. However, RoHS compliance necessitates lead-free variants, utilizing alloys solely composed of tin and its composites.

The Essence of PCB Surface Finish

A PCB surface finish is essentially an intermetallic joint that connects the bare copper area of a PCB to its components. This finish prevents the base copper from oxidizing and prepares the board for efficient soldering while enhancing its overall shelf life.

Impact of Surface Finish

Choosing the right surface finish affects several factors:

• Quality of the inter-metallic joint
• Yield of the manufacturing process
• Rework and scrap rates
• Testing efficacy
• Overall manufacturing cost
• Performance reliability during actual use

Types of PCB Surface Finishes

PCB surface finishes are generally categorized into metallic and organic types.

Metallic Surface Finishes

Utilizes metallic layers to protect the copper traces on the PCB.

HASL Finish (Hot Air Solder Leveling)

HASL is a traditional finish where the board is dipped into molten solder, covering all exposed copper. This is followed by the removal of excess solder through hot-air knives. The solder mix is typically tin-lead, though lead-free variants are available due to RoHS standards.

Advantages:

• Ensures superior wetting during soldering
• Prevents copper corrosion
• Allows a larger processing window
• Common for non-RoHS boards

Disadvantages:

• Thickness inconsistency between large and small pads
• Not suitable for < 20mil pitch BGA and SMD
• Tends to bridge on fine pitches
• Not recommended for high-density interface (HDI) products

Lead-free HASL

This variant uses lead-free alloys like Sn/Ag/Cu (SAC), Sn/Cu/Co, or Sn/Cu/Ni/Ge.

Advantages:

• Allows for delamination checks at high temperatures
• Widely available
• Eco-friendly
• More durable and re-workable
• Cost-effective

Disadvantages:

• Potential for thermal shock
• May result in uneven surfaces
• Not ideal for fine-pitch components
• Potential for solder bridging

Immersion Gold Plating (ENIG)

ENIG includes a layer of electroless nickel covered by immersion gold, providing effective oxidation protection.

Advantages:

• Ideal for flip chips and BGAs
• Provides flat, even surfaces
• RoHS compliant
• Suitable for PTH applications
• Extended shelf life

Disadvantages:

• Risk of "black pad" defects
• Challenging to rework

ENIG is commonly used in castellated circuit boards, SMT, lead-free soldering, and BGA packages due to its reliability.

For further details, visit lead-free hasl.

ENEPIG Plating

ENEPIG, featuring an additional palladium layer to prevent nickel oxidation, offers excellent solderability despite its higher cost.

Advantages:

• Universal applicability across various boards
• Easy to process
• RoHS compliant
• Suitable for multiple reflow cycles
• Compatible with Sn-Ag-Cu solders
• Long shelf life

Disadvantages:

• Frequent occurrence of the "black pad"
• Reduced solder joint reliability
• Ineffective palladium layer for solderability
• Longer wetting times
• Sensitivity to plating conditions
• Higher cost

Hard Gold

This involves plating a gold alloy (with nickel, cobalt, or iron) over a nickel coat, typically for parts exposed to wear, like edge connectors.

Advantages:

• Durable for high-wear applications
• RoHS compliant
• Long shelf life

Disadvantages:

• Costly
• Labor-intensive
• Limited coverage on trace sidewalls
• Risk of etching defects

Immersion Silver (ImAg)

A lead-free silver layer protects copper traces from corrosion, ideal for fine pitch applications.

Advantages:

• Good for fine pitch applications
• Long shelf life (around 12 months)
• Stable compared to other finishes
• Affordable

Disadvantages:

• Potential silver whiskering
• Not suitable for compliant-pin interaction
• Unsuitable for micro vias with 1:1 aspect ratios

Immersion Tin (ImSn)

Lead-free and RoHS compliant, this finish ensures flat surfaces suitable for fine-pitch components and small geometries.

Advantages:

• Excellent flatness
• Good for press-fit applications
• Cost-effective
• Retains solderability after multiple thermal cycles

Disadvantages:

• Handling sensitivity
• Short shelf life
• Unsuitability with peelable masks
• Incompatibility with contact switches
• Special equipment setup required for electrical testing

Organic Surface Finishes

These finishes use organic compounds to protect the copper layer on a PCB.

OSP (Organic Solderability Preservative)

OSP uses a water-based organic layer to protect copper pads, ideal for flat surfaces and simple to rework.

Advantages:

• Lead-free
• Flat surface compatibility
• Simple process
• Easy to rework

Disadvantages:

• Unsuitable for PTH
• Handling challenges
• Short shelf life

Carbon Ink Surface Finish

Carbon ink coatings protect copper pads, useful in RF shielding, keypads, remote controllers, and welding equipment.

Advantages:

• Cost-effective alternative to hard gold
• More robust than other finishes

Disadvantages:

• Requires careful handling and cleaning

Choosing the Right PCB Surface Finish

The surface finish selection is crucial for the long-term reliability of a PCB, factoring in the operating environment and application demands. Important considerations include usage conditions, product aesthetics, shock and drop resilience, corrosion risk, reliability requirements, fine-pitch component presence, and SMT pad flatness for BGA applications.

Surface Finish Comparison

Use the comparison chart below to evaluate different surface finishes for reflow, aluminum wire bond, and gold wire bond assembly applications:

Key factors to consider:

• Operating environment
• Board aesthetics
• Shock and drop resilience
• Corrosion resistance
• Reliability necessities
• Presence of fine-pitch components
• Flatness requirements for SMT pads

Ensuring the appropriate surface finish is applied according to the board's intended application will greatly enhance its shelf life and reliability.

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