Using a process-matched surface finish to improve board performance and durability is critical in today’s complex electronics environment, both to protect exposed copper pads and to provide a solderable surface during assembly. There are various technical means that can be used in this process, and experts still face difficulties in carrying out this process or comparing it with other types of finishes, for which reason we will discuss immersion silver among them in this article.
What is Immersion Silver Finish
“Impression Silver,” also referred to as “IAg Plating,” represents a prevalent surface treatment in conventional PCB production. This environmentally friendly process aligns with the Restriction of Hazardous Substances Directive (RoHS). Approximately one in ten circuit boards employ this type of PCB surface finish due to its advantages in promoting surface flatness, particularly advantageous for high-density circuits and fine-pitch surface-mount technology.
Pure silver emerges as an exemplary material for surface treatment, boasting desirable attributes like ductility, thermal conductivity, a soft surface, proficient electrical conduction, and resistance to oxidation and water. These properties render immersion silver plating a favored option within PCB manufacturing. In the immersion silver process, a delicate silver layer, ranging from 0.1 to 0.5 micrometers in thickness, is chemically deposited onto the copper surface. This layer serves as a protective barrier against oxidation while augmenting solderability.
The immersion silver process capitalizes on the Galvanic effect, stemming from disparities in electromotive force (emf) and redox potential between copper and silver. Silver ions acquire electrons from the copper, prompting copper dissolution and silver precipitation. This mechanism entails a displacement reaction between silver ions and copper atoms within the solution.
The chemical reaction finds expression in the equation:
2Ag+ + Cu → 2Ag + Cu2+
This equation elucidates the partial dissolution of copper at the surface, releasing a pair of electrons into the silver solution. Dissolved silver acquires two electrons, undergoing reduction into a metallic state, subsequently depositing onto the copper surface of the PCB.
Immersion Silver Chemical Equation
- Prior to initiating immersion silver technology, it is imperative to grasp and preempt potential challenges that might arise. These include aspects like cleaning standards, solution temperature, and nitrate concentration.
- Before commencing the official process, a preliminary treatment of the board becomes essential. This preparatory phase encompasses pickling, scrubbing, and cleaning procedures.
- To establish interaction with the silver deposit, execute a microetch procedure utilizing 80 g of H2SO4, 200 ml/L of 35% hydrogen peroxide, and 250 ml/L of WCD-126 at a temperature of 30°C for approximately one to two minutes.
- Immerse the board in a solution of 300 ml/L of MT-IMG 500A and 30 ml/L of MT-IMG 500B, maintaining a temperature of 40°C, for a duration of one to two minutes. This step facilitates the formation of a delicate silver layer.
- Following the silver immersion, it is advisable to utilize deionized water for the purpose of eliminating any residual deposits or chemical agents.
- Concluding the process involves thoroughly drying the board using hot air, ensuring the complete absence of moisture to preempt potential corrosion or related issues.
Problems for Immersion Silver Plating
Especially when employing an immersion silver finish on circuit boards characterized by high density and fine line width, the emergence of minuscule silver crystals on the board surface or pads raises concerns. These crystals can potentially give rise to complications such as short circuits or signal interference. To proactively avert the occurrence of silver whiskers, the subsequent measures can be taken:
- Applying a conformal coating onto the silver layer serves as a preventive measure against the growth of dendritic crystals.
- Precise adjustment of process parameters for silver deposition becomes imperative. Factors like temperature, current density, and deposition time should be meticulously calibrated to discourage the development of silver whiskers.
- Employing a suitable pretreatment on the board’s surface can significantly contribute to addressing this issue. This may involve chemical treatments or electroplating with an appropriate layer of metal.
Upon exposure to the atmosphere, immersion silver boards can undergo reactions with chlorine and sulfur, causing the immersion silver to tarnish. This reaction not only impacts aesthetics but also detrimentally affects soldering performance. Irregular deposition, the presence of air bubbles or particles, and surface contamination are potential contributors to this soldering issue. To effectively address this concern, consider implementing the following measures:
- Store boards in packaging that is devoid of sulfur content and ensure meticulous resealing when removing boards from their packaging.
- Achieving consistent current density across the entire electrode surface requires adjustments to the electrolyte’s composition, temperature, and flow rate.
- Opt for a pure electrolyte and prevent the formation of air bubbles through thorough stirring or other suitable methods.
During the process of silver immersion plating, concurrent silver oxidation and silver ion reduction reactions occur. To ensure an even distribution of the coating across the board, the copper layer serves a dual role as both an anode and a cathode.
However, under certain circumstances, such as corrosion, small gaps can form between copper layers, impacting the supply of silver. This process demands precise control to achieve a uniform and complete silver layer deposition. Uneven deposition could potentially result in galvanic effects, complications from electrochemical reactions, or interactions among different materials. The reduction of the galvanic effect is discussed as follows:
- Introducing the silver replenishment mechanism, the existing copper surface acts as an anode, providing electrons for the reduction of silver.
- Implementing an immersion silver process with a lower pH, which effectively reduces corrosion while increasing silver thickness.
- Exercising strict control over the amount of solution used for microetching.
- Modifying circuit designs to prevent situations where the copper surface is not connected to smaller copper components.
- Optimizing the pre-treatment, curing, and imaging processes to ensure the solder mask achieves necessary chemical resistance.
Characteristics of Immersion Silver PCB Finish
- Offers a flat surface area that facilitates the attachment of SMD components.
- Serves as an environmentally friendly finish, aligning with RoHS industrial standards, and poses no hazards to users.
- Demonstrates excellent solderability even after undergoing multiple thermal excursions, ensuring seamless solder spreading.
- Delivers robust electrical conductivity, rendering it highly advantageous for high-speed signal circuit designs.
- Not compatible with peel-off masks, rendering them unsuitable for this finish.
- Its limited operational scope constrains supply chain parameters.
- Challenging to manage, necessitating utilization within twenty-four hours of manufacture.
- Prone to susceptibility to inorganic acids, chlorides, and sulfuric acid.
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