【PCB failure】What is the reason for poor flux performance?

In-depth analysis and countermeasures for poor flux performance

In the electronics production chain, the reliability of printed circuit boards (PCBs) is like a cornerstone, directly determining the service life of the end product. As a key auxiliary material in the welding process, once there are performance defects, flux may cause a series of problems such as false welding of solder joints, corrosion of components, and insulation failure, which seriously affects product quality. Next, we will start with common quality problems, deeply analyze their causes, and propose targeted solutions to comprehensively sort out the logic of flux performance control.

1. Four common quality hazards of flux

After the soldering operation is completed, if there is too much organic or inorganic substances left, these residues can easily absorb moisture in a humid environment, leading to a decrease in the insulation performance of the PCB. Not only that, under high temperature conditions, some residues will also carbonize, which is like planting a "short circuit bomb" in the circuit, which can cause circuit failure at any time. Some flux residues contain inadequately reacted acidic components that act like latent "corrosives" that erode the pads and component pins over time. Over time, it can cause serious consequences such as pad detachment and pin damage, directly affecting the normal connection of the circuit. When the flux is poorly wetted, the solder cannot be spread evenly during the soldering process, resulting in defects such as unevenness and pinholes on the surface of the solder joint. Such solder joints not only do not meet the standard in appearance, but also greatly reduce their mechanical strength and conductivity, making it difficult to ensure the stable operation of the circuit. In the preheating stage, if the flux is overvolatilized, a large amount of active ingredients will be lost, thus losing the solder effect. If the volatilization is insufficient, it will affect the stability of the welding environment, which may lead to problems such as bubbles and inclusions in the welding joint, affecting the welding quality.

2. Six key factors that lead to formula defects

1. Imbalance of active ingredients

Concentration control of active agents (e.g., organic acids, amines, etc.) is crucial. When the concentration is too low, it cannot effectively remove the oxide film on the welding surface, resulting in poor welding effect. Excessive concentrations can exacerbate the corrosiveness of residues and cause damage to PCBs and components. At the same time, the compatibility of the type of active ingredient with the substrate and solder also needs to be precisely considered, otherwise it will affect the overall performance of the flux.

2. There are loopholes in the design of solvent systems

The volatility of solvents is an important factor affecting flux performance. If the fast volatile solvent is depleted prematurely, it will cause the active agent to fail in advance and cannot play a role. Slow volatile solvents may remain around the solder joint, forming undesirable residues. In addition, if the ratio of mixed solvents is not properly matched, it is easy to cause delamination phenomenon or lead to sudden changes in local concentration, affecting the stability and uniformity of flux.

3. The resin matrix is not suitable

The heat resistance of the resin type (e.g., rosin, modified resin, etc.) matches the compatibility of the solvent is critical. If the two do not match, it may cause uneven film formation and affect the insulation performance and appearance of the solder joint. In high-temperature environments, decomposition may also occur, producing harmful gases or residues, affecting welding quality.

4. The synergy of additives is out of control

Auxiliary components such as corrosion inhibitors and surfactants need to form a good synergy to improve the performance of flux. If the synergy gets out of control, it will have an antagonistic effect. For example, if excessive amount of defoamer is added, it will cause the wettability of the flux to decrease and affect the spread of the solder.

5. Lack of dynamic stability of acid value

During storage or use, the pH of the flux may drift due to hydrolysis or oxidation of the ingredients, or side reactions with solvents. The instability of pH can affect the activity of the flux, which in turn can lead to fluctuations in soldering quality.

6. Residue dissolution capacity decreases

Components in flux used to dissolve residues (such as alcohol ethers) will not be able to effectively remove by-products generated during the soldering process if their molecular structure is not properly designed. These residual byproducts can affect the quality and reliability of the solder joints, potentially leading to subsequent failures.

3. Comprehensive and systematic solutions

1. Custom-designed active agents

According to the different soldering objects (such as lead-free solder, leaded solder, copper substrate, nickel substrate, etc.), choose the appropriate active agent type, such as carboxylic acid type, phosphate type, etc. Through dynamic thermogravimetric experiments, the optimal concentration of the active agent is accurately determined to ensure that it can effectively remove the oxide film without causing excessive corrosion problems.

2. Optimize the solvent compounding scheme

Gas-liquid phase equilibrium simulations are used to simulate the volatilization rate of the solvent to the process temperature profile at all stages of welding. This ensures that the flux can play a stable role at different temperature stages, reducing soldering defects caused by solvent volatilization problems.

3. Functional modification of the resin matrix

The introduction of fluorinated resin can improve the density of flux film-forming, enhance the insulation performance and corrosion resistance of solder joints; The use of branched chain resin can enhance its thermal stability and avoid decomposition during high-temperature welding. At the same time, the rheological properties of the resin are adjusted through molecular weight grading control, making it more suitable for the needs of the welding process.

4. Design additive chain reactions

Construct corrosion-inhibiting-surface-active bifunctional molecules that enable additives to perform multiple functions simultaneously, enhancing the overall performance of fluxes. Or load a variety of auxiliary agents on nanoscale carriers to achieve a controlled release of these auxiliaries, ensuring that they can synergize and achieve the best results during the welding process.

5. Establish a dynamic compensation mechanism for acid value

The addition of a pH buffer system, such as an amine and organic acid compounding system, allows the flux to maintain the effective ionic state of the active ingredient during heating. This ensures that the flux can maintain stable activity under different temperature conditions and improve the consistency of soldering quality.

6. Strengthen the design of residual cleaning

Introducing β-diketones with chelating in flux formulations to enhance the removal of residues such as metal salts; At the same time, the polar gradient of the solvent is optimized, and metal salts and organic residues are dissolved in stages to ensure the cleanliness of the surface of the solder joints and improve the reliability of welding.

4. The important supporting value of material analysis technology

1. Gas Chromatography - Mass Spectrometry (GC-MS)

This technology can accurately analyze the volatilization curve of solvents and the distribution of thermal decomposition products, providing strong data support for optimizing the solvent system and helping us better control the volatilization behavior of solvents.

2. Fourier Transform Infrared Spectroscopy (FTIR)

By identifying changes in active functional groups and the degree of cross-linking of resins, it is possible to gain insights into the chemical changes of the flux during storage and use, providing a basis for improved formulations.

3. Thermogravimetric - Differential Scanning Calorimetry (TG-DSC)

The ability to characterize the thermal stability and phase change behavior of each component of the flux helps us select the appropriate components to ensure the stability of the flux in high-temperature soldering environments.

4. Ion Chromatography (IC)

It can accurately determine the content of free corrosive ions such as Cl⁻ and SO₄²⁻ in the residue, providing accurate data for evaluating the corrosiveness of fluxes and ensuring the safety of PCBs and components.

5. Scanning Electron Microscopy - Spectroscopy (SEM-EDS)

By observing the morphology and elemental diffusion of the IMC layer at the interface of the solder joint, the interfacial reaction during the soldering process can be intuitively understood, which provides an important reference for optimizing the soldering process and fluxformulation.

6. Dynamic wetting angle test

Quantitatively evaluate the effect of flux on the spread ability of the solder, which helps us to screen out flux formulations with good wettability and improve the quality of soldering.

5. Conclusion: Accurate formula design determines the upper limit of performance

The performance of flux is essentially the result of the multi-parameter coupling of its chemical composition and process conditions. By systematically analyzing the molecular behavior of components such as solvents, active agents, and resins, combined with dynamic process analysis technology, we can break through the limitations of traditional "trial and error" methods and realize the forward design of formulations. With the trend of miniaturization and high-density development in electronic manufacturing, only by deeply understanding the intrinsic properties and interface reaction mechanisms of materials can we build a precise regulation system from microscopic components to macroscopic performance, providing a solid underlying guarantee for the reliability of PCBs.