How to Select PCB Pad Coating Materials for Laser Soldering

How to Select PCB Pad Coating Materials for Laser Soldering

It is well known that all metals exposed to air undergo oxidation. To prevent oxidation of PCB copper pads, the pad surfaces must be coated (or plated) with a protective layer. The material, process, and quality of PCB pad surface treatment directly affect soldering techniques and joint quality. Furthermore, the choice of PCB pad surface treatment varies depending on the electronic product, manufacturing process, and soldering materials. Below, Songsheng Optoelectronics briefly discusses the impact of PCB pad coatings on laser soldering.

Impact of PCB Pad Coating Materials on Laser Soldering

The influence of PCB pad coating materials on laser soldering is primarily reflected in the following aspects:

Oxidation and Contamination Prevention: To prevent oxidation and contamination of copper pads before soldering, protective coatings are typically applied. These coatings effectively safeguard the copper surface and prolong its solderability.

Coating Chemical Composition: Different coating materials significantly affect soldering quality. For instance, ENIG Ni(P)/Au plating is a commonly used solderable coating, where the nickel and gold components enhance corrosion resistance and solderability. Coatings with moderate phosphorus content (e.g., 7%–9%) exhibit optimal corrosion resistance and solderability.

Coating Density and Structure: The density and structure of coatings also impact soldering quality. Insufficiently dense coatings may lead to "black pad" phenomena, characterized by surface cracks or voids, adversely affecting soldering results.

Optical Properties of Coatings: The optical characteristics of coatings, such as light absorption and reflectivity, influence laser soldering effectiveness. The material's resistivity and surface finish (smoothness) affect beam absorption rates, thereby altering the soldering process.

Physical Properties of Coatings: Physical attributes like hardness and adhesion further determine soldering quality.

The impact of PCB pad coating materials on laser soldering is multifaceted, encompassing oxidation resistance, chemical composition, structural density, optical properties, and physical characteristics. Selecting appropriate coating materials and processes is critical to ensuring high-quality laser soldering results.

Illustration of Songsheng Optoelectronics' Laser Constant-Temperature Soldering with Real-Time Thermal Feedback System

1. ENIG Ni(P)/Au Plating

1) Coating Characteristics
The ENIG (Electroless Nickel Immersion Gold) Ni(P)/Au plating process is conducted after applying solder mask (green lacquer) on PCBs. The fundamental requirements for ENIG Ni/Au plating are solderability and solder joint reliability. The electroless nickel plating thickness ranges 3-5μm, while the immersion gold layer (also termed replacement gold) measures 0.025-0.1μm. For thick electroless gold plating (reduction gold), the thickness ranges 0.3-1μm, typically maintained at ~0.5μm.

The phosphorus (P) content in electroless nickel plating critically determines the coating's solderability and corrosion resistance. An optimal P content of 7-9% (medium phosphorus) is recommended. Insufficient P content compromises corrosion resistance, promoting oxidation. In corrosive environments, galvanic corrosion between Ni/Au degrades the Ni surface layer, forming black nickel oxide (NiₓOᵧ) that severely deteriorates solderability and joint reliability. Higher P content enhances both corrosion resistance and solderability.

2) Application Characteristics

● High cost;
● The black pad issue remains fundamentally unresolved, with consistently high rates of cold solder joint defects;
● The secondary interconnection reliability of ENIG Ni/Au surfaces is inferior to that of OSP, immersion silver (Im-Ag), immersion tin (Im-Sn), and hot-air solder leveling tin (HASL-Sn) coatings;
● Since ENIG Ni/Au utilizes nickel co-deposited with 5%–12% phosphorus, when PCBA operating frequencies exceed 5GHz, the skin effect becomes pronounced. The composite Ni-P coating exhibits poorer conductivity than copper for signal transmission, resulting in slower signal propagation speeds;
● The formation of AuSn₄ intermetallic compound fragments from dissolved gold in the solder prevents high-frequency impedance from "returning to zero";
● "Gold embrittlement" poses a latent risk to solder joint reliability. Typically, the soldering duration is extremely brief (completed within seconds), preventing uniform gold diffusion in the solder. This leads to localized high-concentration layers with reduced mechanical strength.

2.Im-Sn Plating Characteristics

Immersion Tin (Im-Sn) has emerged as a critically important lead-free solderable coating in recent years. The tin layer, deposited through chemical reactions using either stannous sulfate or stannous chloride, typically achieves a thickness ranging from 0.1 to 1.5μm, with a minimum requirement of 1.5μm thickness for reliable performance in multiple soldering processes. This thickness variation primarily depends on three key factors: the concentration of stannous ions in the plating solution, process temperature, and the resulting coating porosity. It should be noted that tin's inherent high contact resistance makes Im-Sn coatings perform less effectively in contact resistance testing compared to immersion silver alternatives. In conventional Im-Sn processes, the resulting coating exhibits a distinctive gray appearance due to its characteristic honeycomb-like surface structure. This unique morphology unfortunately leads to several technical challenges including excessive porosity, increased permeability, and consequently accelerated aging of the coating.

2)Application Characteristics:
● Costs lower than ENIG Ni/Au and Im-Ag, but higher than OSP;
● Prone to tin whisker formation, significantly affecting fine-pitch and long-life devices while having minimal impact on PCBs;
● Susceptible to tin pest phenomenon with Sn phase transition point at 13.2°C, below which it transforms into powdered gray tin (α-Sn), causing complete loss of strength;
● Under thermal conditions, Sn-Cu intermetallic compounds accelerate diffusion with the copper layer, leading to growth of Sn-Cu intermetallic compounds (IMCs) as shown in Table 1.

● New boards demonstrate excellent wettability, however this property degrades rapidly after storage or multiple reflow cycles, resulting in poor performance for backend application processes.

● As shown in Table 2, high-temperature processing accelerates tin layer thickness depletion, consequently reducing storage duration.

3.OSP Coating

OSP (Organic Solderability Preservative), emerging in the 1990s, refers to an organic protective film applied on copper surfaces. Certain nitrogen-containing heterocyclic compounds - including benzotriazole (BTA), imidazole, alkyl imidazole, and benzimidazole in aqueous solutions - readily react with clean copper surfaces. The nitrogen-containing heterocycles in these compounds form coordination complexes with copper, creating an anti-oxidation protective layer on the Cu surface.

2)Application Characteristics:

● Low cost with simple processing requirements;
● During soldering heating, the copper complex decomposes rapidly, leaving only bare copper. As OSP constitutes merely a molecular monolayer, it completely decomposes when exposed to dilute acids or flux without generating residual contaminants;
● Exhibits superior compatibility with both leaded and lead-free soldering processes;
● The OSP protective coating demonstrates compatibility with RMA (moderately active) fluxes, but proves incompatible with lower-activity rosin-based no-clean fluxes;
● The OSP thickness (typically 0.2-0.4μm in current applications) demands precise flux matching, with varying thicknesses requiring different flux formulations;
● Requires stringent storage environmental controls and has limited shop-floor lifespan, making it unsuitable for operations lacking rigorous production management protocols.

4.Im-Ag Plating

Silver (Ag) exhibits excellent thermal conductivity, electrical conductivity, and solderability at room temperature, along with strong reflectivity, low high-frequency loss, and superior surface conduction capability. However, silver demonstrates high chemical affinity for sulfur - trace atmospheric sulfur compounds (H₂S, SO₂, or other sulfides) will cause discoloration through formation of Ag₂S and Ag₂O, resulting in complete loss of solderability. Another significant limitation involves silver ion migration in humid environments, where Ag⁺ ions readily diffuse across both surfaces and through the bulk of insulating materials, progressively degrading dielectric properties and potentially causing electrical shorts.

The silver layer deposited on the copper substrate ranges from 0.075 to 0.225μm in thickness, featuring a smooth surface suitable for wire bonding applications. 

2) Application Characteristics:

● Cost-effective compared to gold (Au) or palladium (Pd) coatings;
● Excellent wire bond ability and compatibility with Sn-based solder alloys;
● Forms intermetallic compounds (Ag₃Sn) between silver and tin without significant brittleness;
● The high conductivity of silver becomes particularly advantageous in RF circuits due to skin effect;
● Reacts with atmospheric sulfur (S), chlorine (Cl), and oxygen (O) to form surface compounds (Ag₂S, AgCl, Ag₂O), causing tarnishing that affects both appearance and solderability.

Optimizing PCB Pad Coating Density and Structure to Improve Laser Soldering Efficiency

To optimize the density and structure of PCB pad coatings for improved laser soldering efficiency, the following aspects should be considered:

Coating Selection:

Choosing the right solderable coating is crucial. For example, ENIG Ni(P)/Au plating provides excellent solderability and joint reliability. This electroless nickel immersion gold process is performed after applying the solder mask (green oil), ensuring higher soldering quality and efficiency.

Pad Design:

Pad symmetry is critical for maintaining balanced surface tension of molten solder. Both end pads must be symmetrical to ensure uniform solder distribution during the process.

The shape and size of pads also require careful design. For instance, pads with apertures exceeding 1.2mm or diameters larger than 3.0mm should adopt diamond or plum blossom shapes. These special pad designs better accommodate larger components while reducing thermal stress during soldering.

Pad Spacing:

Ensure proper overlap between component terminals/pins and pads. Avoid excessive or insufficient spacing as it affects soldering quality. Appropriate pad spacing ensures uniform solder flow and good joint formation.

Laser Welding Process Optimization:

Optimizing laser welding parameters (laser power, welding speed, defocus distance, etc.) is key to improving soldering quality and efficiency. Adjusting these parameters makes the laser beam more focused, increases energy density, and consequently improves both welding speed and joint quality.

Minimum Pad Dimensions:

According to standard PCB packaging specifications, the minimum unilateral pad dimension should not be less than 0.25mm, with maximum pad diameter not exceeding three times the component's aperture size. This ensures sufficient solder filling during the process, resulting in reliable solder joints.

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The system integrates real-time temperature feedback, CCD coaxial positioning, and semiconductor laser technologies. Its innovative PID-based temperature regulation system enables precise temperature control during soldering, ensuring high yield and accuracy. Particularly suitable for temperature-sensitive high-precision soldering applications, interested clients may request free samples.