Contaminants in Manufacturing and Their Impact on PCBs

Contamination, especially the harmful presence of certain metals in solder, significantly affects the quality and reliability of soldered joints. This contamination markedly influences the microstructure of the joint, its strength, brittleness, wetting characteristics, and electrical properties such as resistance. Problems associated with soldering, such as solder balls, dull surfaces, cracks in joints, or unfilled upper sides of plated holes, may also stem from increased contamination levels in the solder.

Sources of contaminants in solder and their dangers:

• Aluminium (Al): A small amount of Al dissolves in tin at elevated temperatures, leading to decreased adhesion, coarseness, higher solder viscosity, and potential surface oxidation.
• Antimony (Sb): Sb additives significantly improve wetting, limit whisker* growth, and eliminate tin whiskers. Simultaneously, they enhance solder strength.
• Arsenic (As): Harmful additive causing long needles in the microstructure and inducing non-wettability.
• Bismuth (Bi): Considered an alloying element, Bi improves wetting characteristics and results in positive changes in the lattice.

*Whisker: A thin, hair-like growth that can appear on the surface of metallic materials. Whiskers often form on solder surfaces, especially on soldered joints. These microscopic structures resemble fine hairs or needles and can gradually elongate.

Contaminants from Substrate and Components

• Iron (Fe): Iron contamination causes solder brittleness and coarseness.
• Other Metals: Many other metals (Mg, Ni, Au, Cu, Ag, Zn) form intermetallic compounds that can affect solder properties.

Contaminants from the Production Process

Contamination can arise, especially from metallic layers on the substrate, dissolution of components, and impurities during processing and dissolution of the PCB. Additional sources, such as metals, can contribute to impurities in the solder and affect the joint's properties.

Assessing and Controlling Contaminant Levels in Joints

Evaluating and controlling the level of contaminants in joints are crucial steps in ensuring the quality and reliability of soldered joints on PCBs. High contamination levels can lead to serious issues with electronic components, emphasizing the need to choose solder carefully and monitor the soldering process to minimize impurities.

Causes of Contamination

There are two main types of contamination: ionic and non-ionic. Ionic contamination is often caused by residues of flux and mostly contains inorganic salts or acids. These residues can lead to solution conductivity, creating an environment for corrosion and dendrite growth, ultimately causing board failure. Non-ionic contamination includes non-conductive substances such as oils, inert flux residues, or greases.

Identifying the Source of Contamination

White spots, known as white residue, are a common manifestation of ionic contamination. These metal salts, acting as activators in fluxes, can cause issues, especially if sufficient cleaning is not performed. Identifying the source of contamination is a necessary step in addressing the problem. Changes in processes, materials used, or cleaning solutions can lead to unexpected residues and cleaning challenges.

Choosing the Right Cleaning Fluid

Selecting the right cleaning fluid for printed circuit boards (PCBs) depends on several factors, including the type of contamination, PCB material, solder used, and the desired cleanliness level.

• Non-ionic Fluids

For removing non-ionic contamination, such as oils, inert flux residues, resins, and greases, non-ionic cleaning fluids are often recommended. These fluids are designed not to leave ionic residues that could affect conductivity on the PCB.

• Avoiding White Spots

To prevent white spots, often associated with ionic contamination, it is crucial to choose a cleaning fluid that is effective in removing flux residues while minimizing residues.

Choosing the appropriate cleaning fluid is crucial for effective cleaning. Modern cleaning agents with low viscosity and surface tension, capable of efficiently removing flux residues beneath densely populated components, are an ideal solution.

Verification of Cleaning Process through Inspection

Visual inspection is one of the simplest ways to assess board cleanliness. However, reliance solely on visual inspection is not recommended, as some ionic contaminations are not optically visible. Methods such as Surface Insulation Resistance (SIR) testing or Ion Chromatography (IC) provide quantitative results and allow for a more efficient identification of contamination.

Surface Insulation Resistance (SIR) Testing and Ion Chromatography (IC)

SIR testing measures the reliability of electronic components by monitoring signal strength and quality after cleaning. IC is a common classification for evaluating PCB cleanliness, detecting weak organic acids, and individual ions. While ion chromatography may be more expensive, it provides the most reliable results for assessing board cleanliness.  More about the need and reasons for testing.

Cleaning even with No-Clean Solder Paste

No-clean fluxes can hinder the proper adhesion of protective coatings on the board surface. This is a particular issue for boards exposed to harsh environmental conditions. Residues after soldering can absorb moisture, which will be released during curing, causing the coating to separate from the board. Ensuring the removal of all residues after soldering can effectively prevent these problems.

Innovations in cleaning agents and testing methods bring significant improvements in the reliability and lifespan of electronic components. Modern cleaning processes not only offer higher-quality cleaning but also economic benefits for electronics developers. With advanced cleaning capabilities, manufacturers achieve higher PCB quality, ensuring the reliable operation of modern electronic devices.

At Gatema PCB, we place a strong emphasis on ensuring optimal reliability of electronic devices through innovative PCB cleaning processes. Our company prefers an approach focused on reliability testing, a key element in our manufacturing processes. In contrast to traditional cleaning methods, our methods emphasize efficiency and effectiveness in eliminating contamination without compromising the integrity of the board or the environment.