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Why does flux residue on an assembled printed circuit board cause electrical failure?
2024-01-31
No matter what kind of flux is used, there will always be more or less residues left on the PCB and solder joints after soldering, which not only affect the appearance of assembled circuit board, but also pose a potential threat to the reliability of PCB, especially when electronic products work under high temperature and humidity conditions for a long time, the residue may lead to problems such as line insulation aging and corrosion, and then the insulation resistance (SIR) decreases and electrochemical migration (ECM) occurs. With the full implementation of lead-free requirements in the electronics industry, fluxes that accompany solder paste have also gone through the development process of rosin (resin) flux, water-soluble flux and today's widely used no-clean flux, but the impact of its residue has always been a particular concern.
There are four main methods of welding
Surface Mount Reflow Soldering (SMT)
Full plate wave soldering
Selective welding
Hand welding
Each method leaves a different degree of risk that can lead to the presence of flux residues that can lead to failure. The SMT soldering method has the lowest risk, while the use of liquid flux has the highest risk. Understanding the application process, the ingredients in the flux, and the recommendations of the flux manufacturer can greatly improve the reliability of electronic devices.
What is Flux?
Flux is an acidic mixture of chemicals that is used to remove metal oxides during the soldering process, resulting in a good solder bond. You may hear the terms "low activity" and "high activity" to describe whether flux residue after soldering poses a risk of causing cleaning-related failures. However, from a chemical perspective, these terms are not precisely defined, and there is no single standard analysis or chemical test to classify flux residue as "low activity" or "high activity". This is because failures caused by leakage currents not only depend on the chemistry of the flux and the amount of flux applied, but also the electrical sensitivity and the environment in which it will be used can have a significant impact on reliability. Most liquid flux ingredients used in wave soldering, selective soldering, and hand soldering include:
solvent
activator
Film formers
additives
Activators and film formers have a greater impact on the risk of failure than other factors.
activator
No-clean fluxes typically use organic weak acids (WOAs) as activators. Some of the weak acids are glutaric acid, succinic acid and adipic acid. The presence of activators makes flux residues risky because they are acidic but necessary to get good solder joints. They react with metal oxides to form metal salts, which promote wetting, and form metallurgical bonds when the salts are dissolved. The acid, which is gradually depleted during the soldering process, may be used in other reactions that have contamination or decomposition reactions, but these reactions are not consistent and depend on the chemistry of the flux and other factors that are not easily controlled. Most organic weak acids do not evaporate at welding temperatures. Therefore, it is important to adjust the amount of activator (and flux) to the minimum amount required for good soldering.
Film formers
Film formers are chemicals with a high melting point that are insoluble in water. After welding, they form most of the visible residue. They function as containing activators and prevent them from dissolving in water. The "low solids" flux formulation contains little to no film former and has almost no visible residue. Theoretically, more film formers can reduce the risk of failure, but at the same time, it can also make the component look dirty. The most common ingredients in film formers are rosin, chemically modified rosin, and synthetic resins.
Solvents
The main function of the solvent is to dissolve all the components in the flux, making it a homogeneous viscous liquid that is easy to use. Sometimes, several solvents with different boiling points can be used to ensure that the physical properties are maintained at different temperature stages of the welding curve, which must be completely evaporated during the welding process. If the solvent is present in flux residues, it increases the risk of failure. It is important to ensure that the flux is only applied to areas of the component that are exposed to peak soldering temperatures. During wave soldering, flux can flow through the hole to the top side of the component or under the protective layer so that it does not appear at high temperatures. Manually applied liquid fluxes can be particularly problematic, with the human factor being significant.
Additive
Additives usually make up only a small portion of the flux. They can be plasticizers, dyes, or antioxidants. While manufacturers may add chemicals to help improve reliability, the residual impact is minimal.
Flux application
There are several ways in which flux can be applied, the most common are:
Flux in solder paste for surface mounting
Liquid flux for wave soldering or selective soldering
Liquid flux for hand soldering
Flux in welding wire or electrode
Because the amount of flux used is important, these different application processes present different levels of risk of clean-related failures. Solder paste flux is the least risky because a stencil or printing press is used to control the amount of solder paste flux applied. There are very few failures caused by reflow residues in surface mount (which can be problematic with QFN).
The use of liquid fluxes carries greater risks. Jetting uses more flux than other processes. If not optimally controlled, the process may apply more flux than needed, leaving more acidic residues, creating more favorable conditions for potential chemical reactions, and liquid flux may also flow to parts that are not exposed to high temperatures. Controlling the amount of flux applied during manual soldering can also be difficult, excess flux can flow under nearby components, and manual proficiency can be significant.
Failure analysis techniques during or after assembly
Although there is no one tool that can perform a comprehensive risk assessment, there are a number of risk assessment methods that can successfully reduce the risk of failure. The cleanliness of flux residue ions is often indirectly determined by the resistivity of the solvent extract (ROSE) during the SMT cleaning operation, and the numerical results help to ensure a good solder and cleaning process.
Ion chromatography (IC) has become a commonly used technique to identify common ions on SMT surfaces and to provide a direct measurement of the amount of organic weak acid activator remaining after soldering. This is especially important for liquid fluxes, as the applied flux dose can be easily detected and different ion chromatography methods produce different results. A complete component immersion test is the average ion concentration detected on the entire component surface, while the local extraction technique measures the ion concentration over a small area.
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