Ensuring Top-Quality Solder Joints on ENIG PCBs

Ensuring Top-Quality Solder Joints on ENIG PCBs

The field of printed circuit board (PCB) finishing and bonding methods has been growing and diversifying for decades. PCB design depends heavily on the use and environment of electronic packages, influencing design decisions including the types of Surface Mount Technology (SMT) employed. SMT encompasses the finish, bonding and soldering methods, flux cleaning, reflow and curing, wire bonding, and any operations used to attach components to the surface of circuit boards.

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For high-reliability applications, such as electronics used in military and defense aerospace, Electroless Nickel Immersion Gold (ENIG) is the most common surface finish utilized. This robust finish is ideal for maintaining the lifespan and performance of high-reliability electronics in systems like long-range surface-to-air missile systems and other critical applications.

In this article, we will explore some issues that can arise with ENIG and how to prevent them in your PCB assembly process.

Building PCBs for Electrical Performance

Most PCBs are designed with copper finishes on raw, unpopulated surfaces. Since copper is highly reactive, its exposed surface will oxidize and deteriorate quickly, compromising the circuit board. The surface finish acts as a protective barrier over the copper circuitry, providing a solderable surface for component assembly.

Selecting the appropriate surface finish and ensuring it is clean and well-prepared to bond or solder provides a critical interface between components and the PCB. Factors considered in choosing the best finish include:

• Versatility
• Cost of materials
• Lifespan
• Reusability
• Presence of harmful materials (carcinogens, lead content, etc.)
• Surface flatness
• Application difficulty

ENIG is a flat, lead-free finish known for its durability, making it one of the most widely utilized PCB surface finishes. Ensuring that the ENIG surface is genuinely ready for soldering, bonding, or coating is critical.

What is ENIG (Electroless Nickel Immersion Gold)?

ENIG is a two-layer plating process that involves depositing a thin layer of electroless nickel (approximately 120-240 µin thick) followed by immersion gold plating (2-8 µin thick). The nickel safeguards the copper on the board, while the gold plating extends the shelf life of the package, ensuring the nickel remains corrosion-resistant before assembly.

Common types of surface finishes include:

• HASL (Hot Air Solder Leveling)
• Immersion Tin
• Immersion Silver
• OSP (Organic Solderability Preservative)
• Hard Gold

While each of these finishes addresses the basic vulnerabilities of copper wiring to maintain high-performance conductivity, they all require proper handling, surface treatment, and cleanliness verification for successful bonding. ENIG, however, has been established as the preferred method for high-reliability electronics with complex circuitry design.

Download our checklist to diagnose the root causes of adhesion failures in your PCB manufacturing process: Checklist: Adhesion Failure Root-Cause Analysis for Manufacturers

Problems with ENIG Surface Finishing

PCB manufacturers take significant care in handling boards and components to prevent damage. Using gloves and sealed packaging contributes to preserving the shelf life and usability of electronic components, protecting them from corrosion due to moisture, oils, and other contaminants. Such factors can lead to contamination and subsequent issues.

ENIG surfaces are also susceptible to a specific kind of corrosion known as "black pad," resulting from excessive breakdown of nickel (some oxidation is necessary for gold plating). Over-oxidation or degradation during solder reflow can lead to darker, brittle nickel surfaces.

However, there are critical areas in the PCB assembly process that may be overlooked, potentially leading to adhesion failures. These include dewetting, non-uniform coatings, corrosion, dendrite growth, and shorts within the board.

Ways to Control ENIG Surface Finishes

Once PCBs receive the ENIG finish, they are typically stored in batches until ready to be populated with small components. These components will be soldered, adhered, wire-bonded, or sintered to the surface mount pads on ENIG boards.

One effective method is plasma treatment or cleaning, which activates a surface by engineering a compatible chemical arrangement at the top molecular layers of the materials being bonded.

When the ENIG substrate is in storage, the top layers may not readily bond with materials like silver, epoxy, or wires. Altering or removing these layers allows interested molecules to bond effectively.

The plasma treatment process involves bombarding the ENIG surface with ionized gas, which cleans organic impurities and alters the surface chemistry. It's essential to recognize the difference between plasma cleaning and ultrasonic cleaning; the former modifies surface chemistry while the latter removes physical contaminants.

Understanding plasma treatment and its effects on ENIG adhesion is vital for controlling outcomes. Prior to treatment, it is crucial to measure surface cleanliness, establishing a quality baseline before placing the boards in the treatment chamber and obtaining post-treatment readings.

Using surface quality measurement as a process control method allows for monitoring the effectiveness of treatment processes. Developing specifications and standards for surface quality before and after treatment optimizes surface preparation equipment, ultimately enhancing product quality.

Other Non-Obvious Variables Impacting Adhesion Issues

Once components are applied to the ENIG with epoxy during assembly, the package is cured in an autoclave or oven. To ensure even curing and heat transfer, a silicone rubber press pad is often used. While this process is gentle, silicone may become mobile during curing, contaminating the circuit board.

Silicone contaminants can significantly undermine adhesion. Even though many manufacturers design resilient pads for high temperatures, measuring chemical cleanliness of the boards before and after curing is imperative to avoid introducing unwanted substances.

The cleanliness of the components used in the boards is another often-overlooked factor. Components must be chemically compatible with adhesion requirements in the same manner as the ENIG surface. The sides of components must also be clean to allow for successful conformal Parylene coating.

To maximize the benefits of your manufacturing processes, it's critical to ensure that all incoming parts maintain high cleanliness standards right from suppliers to precede any bonding or treatment operations.

A Measured Process is an Effective Process

Manufacturing electronics involves multiple bonding processes, with each interfacial bond vulnerable to contamination. Proper measurement of the chemical cleanliness of every surface during the adhesion process is essential, beginning before cleaning and continuing after assembly. Each aspect affects the overall reliability of the final product.

To learn more about diagnosing the root causes of adhesion failures in PCB manufacturing and how to improve bonding outcomes with ENIG, download our eBook: Electronics Manufacturing: The Complete Guide to Implementing a New Approach to Increase Quality.

Thank you to EPEC: Engineered Technologies for the use of their images; all image rights reserved to EPEC.