We perform dozens of failure analyses every month for our clients in various industries and identify many different root causes of failure. One that can be difficult to identify and prove is soldering flux residue. As circuit designs shrink and become more complex, flux residues are more likely to cause failure from leakage current.
There are four main ways of soldering:
- Surface mount reflow (SMT)
- Full board or masked wave
Each poses a different level of risk of leaving flux residues that can cause failure. SMT is the least risky while liquid fluxes are the riskiest. Understanding the application processes, constituents in your flux, and the flux manufacturer’s recommendations can greatly improve the reliability of your electronics.
What is Flux?
Flux is an acidic mixture of chemicals used to remove metal oxide during soldering, allowing good metallurgical bonds. You might hear the terms “benign” and “active” to describe whether the flux residue after soldering presents risk of causing cleanliness-related failures. However, there is no definition of these terms from a chemistry perspective, and there is no single, standard analytical or chemical test to classify flux residue as “benign” or “active”. This is because failure from leakage current depends on more than just the flux chemistry and the volume of flux applied. The electrical sensitivity and use environment significantly affect reliability as well.
Most liquid fluxes used in wave, select, and hand soldering contain:
The activators and vehicles/binders affect risk of failure more than the others.
No clean fluxes typically use weak organic acids (WOAs) as an activator; some examples are glutaric acid, succinic acid, and adipic acid. The activators make the flux residues risky because they are acidic but are required for a good solder joint. They react with metal oxides to form metal salts, facilitating wetting and after the salt dissolution, a metallurgical bond. The acid is “used” in this reaction, increasing the pH of the flux (making it less acidic). The acid may be used in other reactions with contamination or from decomposition, but these are not consistent and depend on the flux chemistry and other factors that can’t easily be controlled. Most WOAs will not substantially evaporate at soldering temperatures. Therefore, it is important to minimize the volume of activators (and hence flux) to the least amount needed for good soldering. Excess activators that do not react with oxides or contamination or decompose are still acidic and increase the potential for problems in the field.
Vehicles and binders are high melting point chemicals that are insoluble in water. After soldering, they form the bulk of the visible residue. They function to contain activators and prevent them from being dissolved in water. “Low solids” flux formulations contain very little vehicles/binders, leaving less visible residue. Theoretically, more vehicles and binders reduce the risk of failure, but also makes the assembly look dirty. Examples of vehicles and binders are rosin, chemically modified rosin, and synthetic resins. The exact chemistry is less important than the amount.
The solvents dissolve the constituents to make the flux liquid for easier application. Occasionally, several solvents with different boiling points may be used to maintain physical properties through different temperature stages of a soldering profile. They must be evaporated completely during soldering. If solvents remain in flux residue, they increase the risk of failure. It is important to ensure that flux is only applied to areas of the assembly that will be exposed to peak soldering temperature. In wave processes, flux can flow to the top side an assembly through holes, or flow under a mask, where they will not be exposed to maximum temperature. Liquid flux applied by hand can be particularly problematic because of the difficulty in consistent application from one employee to another, and over time. The localized heat of the soldering iron increases the risk.
Additives are typically a small percentage of the flux. They can be plasticizers, dyes, or antioxidants. While manufacturers may add chemistry to help increase reliability, there is little to no insight and control over these constituents.
There are different ways that flux is introduced for soldering, the most common being:
- Flux in solder paste for surface mount
- Sprayed or foamed liquid flux for wave or select
- Liquid flux applied for hand soldering
- Flux core solder wire for hand soldering
Because the volume of flux applied is important, these different application processes pose different levels of risk of cleanliness-related failure. Solder paste fluxes pose the least risk because stencils or printers are used to control the applied volume of solder paste with flux. Failures from surface mount reflow residues are rare (QFNs can be problematic). Liquid fluxes pose a greater risk. Spray systems can deliver more flux than other processes. When not controlled optimally, the process can apply far more flux than necessary, leaving more acidic residues and creating greater potential for unwanted chemical reactions. Liquid fluxes can also flow in areas that are not exposed to peak temperature. Controlling the volume of flux applied during hand soldering can also be difficult. Excess flux can flow under nearby components. It can vary significantly from operator to operator or shift to shift. Use of flux core solder wire and dispensing equipment can help with consistent application.
Analytical Techniques During or After the Assembly Process
Although there is no one analytical method that provides a full assessment of risk, several have been used with success in reducing failures.
Ionic cleanliness, substantially affected by flux residue, is usually monitored during assembly cleaning operations with resistivity of solvent extract (ROSE). The data helps maintain a qualified solder and wash process. Ion chromatography has become a popular technique to identify common ions on assembly surfaces and provide a direct measurement of the amount of WOA activator left after soldering. It is especially relevant for liquid fluxes because of the amount of flux applied can easily be detected. Different ion chromatography methods produce different results. A full assembly soak will average the ions detected across the surface of the assembly, while localized extraction techniques measure ions in a small area. A downside to any ion chromatography method is the lack of any standard pass/fail criteria; each soldering process, design, and environment will affect the acceptable amounts. Our experience provides us with deep knowledge of average and problematic amounts of ions as well as problems with different ion chromatography systems and techniques.
Functional testing in elevated humidity is sometimes performed to evaluate the susceptibility of a design to residues in expected worst-case use environments. If failures occur, they are usually related to leakage current or shorting. Wherever possible, current limiting can minimize any damage from a shorting event. Thermal damage can destroy the evidence needed to show whether residues were the actual root cause of failure.
Electrical spacing is an important risk factor. The higher V/mil locations on an assembly are more likely to see electrochemistry in residue and see it in shorter times. The sensitivity of the design also influences risk of failure; some high frequency designs are sensitive to any residue (all residue is considered “active” and assemblies must be washed). When utilizing potting and coating, it is important to consider whether the potting or coating properly adheres to residues and if there are cavities where moisture can accumulate. Finally, the use environment is critical: use in high humidity is much riskier because the water in the atmosphere adsorbs to surfaces, dissolves ions, and drives electrochemistry.
The risk of failure caused by flux residue is based on the chemistry of the residue, application of flux, design, and coating. Understanding all these factors will allow you to minimize your risk.
To learn more about flux residue and its impact on electronics reliability, register for a webinar on Dec.12 "Flux Residue: Key Factors Causing Electronics Failures."