Anion Analysis

A PCB assembly passes visual inspection. Automated optical inspection finds nothing. Functional test at room temperature is clean. The board ships, gets installed in a vehicle, and three months later starts producing intermittent faults that are difficult to reproduce and expensive to trace. The cause, when it is eventually found, is ionic contamination. Residual flux activator left on the board surface after soldering, invisible to optical methods and undetectable by functional test under dry conditions, has begun to drive electrochemical corrosion and leakage current in the presence of operating temperature cycling and humidity. The contamination was there from the start. The damage accumulated over time. Anion and cation testing by ion chromatography is the analytical method that would have found it. This article explains how the test works, what it detects, why it matters specifically for automotive electronics, and how to interpret the results it produces. What Is Ionic Contamination and Why Does It Matter? Ionic contamination refers to charged chemical species, either positive ions (cations) or negative ions (anions), present on the surface of an electronic assembly or component. In the context of PCB manufacturing and assembly, ionic contamination originates primarily from flux residues left after soldering. All soldering fluxes contain activators, which are acidic or ionic compounds that break down the metal oxides on solder pads and component leads to allow good solder wetting. After soldering, these activators and their reaction products remain on the board surface as ionic residues. Under dry conditions, ionic residues are typically benign. They sit on the surface, insoluble in dry air, causing no immediate problem. The failure mode is activated by moisture. When humidity rises or when condensation occurs on the board surface, ionic residues dissolve into a thin electrolytic film. That film becomes a conductive path between adjacent conductors. Current flows where it should not. In the presence of an applied voltage, the ionic species migrate: cations move toward the cathode and anions toward the anode, depositing metal at the cathode in a process called dendritic growth or electromigration. Dendrites are metallic crystalline growths that bridge the gap between conductors, causing intermittent or permanent short circuits. The ionic species of greatest concern are those that are most mobile, most soluble, and most corrosive: chloride, fluoride, bromide, sulfate, acetate, and formate on the anion side; sodium, potassium, and ammonium on the cation side. Ammonium and organic amines from no-clean flux formulations are particularly significant because they indicate the presence of insufficiently activated or partially decomposed flux residues that retain moisture-absorbing and corrosion-promoting properties. Ionic contamination is a latent failure mechanism. It is present from the moment of manufacture but causes damage only when activated by humidity and temperature. Testing before shipment is the only reliable way to detect it before it reaches field conditions.   How Ion Chromatography Works Ion chromatography (IC) is an analytical technique that separates and quantifies ionic species in a liquid sample. The technique was developed in the 1970s and has become the standard method for trace ionic analysis across water quality, food safety, pharmaceutical, and electronics testing applications. The operating principle is ion exchange chromatography. A liquid sample is injected onto a column packed with a charged stationary phase. Ionic species in the sample are attracted to and retained by the stationary phase, then released sequentially as the mobile phase gradient changes. Because different ions have different affinities for the stationary phase, they travel through the column at different speeds and emerge at the detector at different times, producing a chromatogram with a distinct peak for each ionic species. Detection in modern IC systems uses a suppressed conductivity detector. As each ionic species elutes from the column, it passes through a suppressor device that converts the mobile phase background ions to low-conductivity water, leaving the analyte ions as the dominant conductivity signal. This suppression step dramatically improves sensitivity and selectivity, allowing detection of ionic species at concentrations of parts per billion (micrograms per litre) in solution, which translates to nanograms per square centimetre on a PCB surface. Quantification is achieved by comparison with external calibration standards: solutions of known ionic concentration that are run alongside the samples and used to construct a calibration curve for each ionic species. The result for each ion is expressed as the measured concentration in the extract solution, which is then converted to surface density (typically micrograms per square centimetre) using the board surface area extracted. Anion Analysis vs Cation Analysis Anion and cation analysis require separate analytical conditions because anions and cations have opposite charges and require stationary phases and mobile phases of opposite polarity for effective separation. In practice, most IC instruments configured for anion analysis use an anion exchange column with a carbonate or hydroxide mobile phase, while cation analysis uses a cation exchange column with a dilute acid mobile phase. The two analyses can be conducted sequentially on the same instrument platform or simultaneously on a dual-channel instrument. For a comprehensive ionic contamination assessment, both anion and cation analysis should be conducted from the same extraction solution, providing a complete profile of all ionic species present. The Standard Method IPC-TM-650 2.3.28 The primary standard governing ionic contamination testing of PCBs and electronic assemblies by ion chromatography is IPC-TM-650 Method 2.3.28, published by IPC (the Association Connecting Electronics Industries). This method defines the extraction procedure, the IC analytical conditions, and the reporting requirements for ionic contamination testing. The extraction procedure in IPC-TM-650 2.3.28 uses a mixture of 75 percent isopropyl alcohol and 25 percent deionised water (the IPA-water extract). The board or assembly is placed in a clean vessel and covered with a defined volume of the extraction solvent. The extraction is conducted for one hour at 40 degrees Celsius under agitation. The extract is then filtered and injected into the IC system for anion and cation analysis. The extraction is designed to dissolve ionic species from the board surface into the solvent, including flux residues that are not fully soluble in water alone. The IPA component improves dissolution of organic flux residues while the water component provides the ionic medium for dissolution of inorganic ionic contaminants. The result is an extract that captures the full range of ionic species relevant to electronics reliability assessment.   Ionic Species Ion Type Primary Source on PCB Failure Risk Chloride (Cl-) Anion Flux activator residue, environmental deposition, halogenated materials High – aggressive corrosion initiator, highly mobile Fluoride (F-) Anion Some flux formulations, etching process residues Moderate to high – corrosive to aluminium and some metals Bromide (Br-) Anion Flame retardant materials, some flux systems Moderate – corrosive at higher concentrations Sulfate (SO4 2-) Anion Environmental deposition, some flux chemistry Moderate – sulfate-induced corrosion Nitrate (NO3-) Anion Environmental, some cleaning chemistry Low to moderate – less corrosive than chloride Acetate (CH3COO-) Anion No-clean flux activator decomposition products Moderate – hygroscopic, promotes leakage current Formate (HCOO-) Anion No-clean flux activator decomposition products Moderate – indicates flux residue activity Sodium (Na+) Cation Environmental, handling contamination, process water Moderate – hygroscopic, promotes corrosion Potassium (K+) Cation Environmental contamination Moderate Ammonium (NH4+) Cation No-clean flux amine activators, flux decomposition High – hygroscopic, indicates active flux residues Methylamine / TEA Cation (amine) No-clean flux amine-based activators High – indicates incompletely deactivated flux   Chloride and ammonium are the two most diagnostically significant species in PCB ionic contamination testing. Elevated chloride indicates aggressive corrosion risk. Elevated ammonium or organic amines indicates the presence of active no-clean flux residues that retain corrosion-promoting properties. Interpreting Anion and Cation Test Results IC results for PCB ionic contamination are expressed as micrograms of each ionic species per square centimetre of board surface area. These surface density values are compared against the acceptance limits defined in the applicable cleanliness specification. Acceptance Limits & Where Do the Numbers Come From? Acceptance limits for ionic contamination in PCB assemblies originate from a combination of IPC standards, OEM-specific cleanliness specifications, and the results of reliability studies correlating ionic contamination levels with field failure rates. The most widely referenced historical limit is 1.56 micrograms sodium chloride equivalent per square centimetre, which was the original threshold defined for cleaned assemblies in earlier revisions of IPC standards. Modern automotive electronics specifications typically apply tighter limits, particularly for chloride, which is often limited to 0.2 to 0.5 micrograms per square centimetre for safety-critical assemblies. The specific limit applicable to your assembly is defined by your customer’s specification, the relevant IPC document (IPC-7711, IPC-7721, or the cleanliness section of J-STD-001), or the OEM supplier quality requirement. What High Chloride Tells You Elevated chloride concentration is the most common and most diagnostically significant finding in PCB ionic contamination testing. The sources of chloride contamination on a PCB include residual flux activator (particularly from rosin and organic acid flux systems), environmental deposition of chloride aerosols in manufacturing or storage environments, and halogenated materials in the board laminate or component packaging that have been mobilised during processing. When chloride is elevated above specification, the corrective action depends on identifying the source. If chloride tracks with the presence of specific component types or board regions near specific assembly operations, the source is likely process-related. If chloride is uniformly distributed across the board, environmental contamination during storage or handling is more likely. IC results alone identify that chloride is elevated; source investigation may require additional analytical steps including surface mapping by point extraction from specific board areas. What Ammonium and Organic Amines Tell You Ammonium and organic amine cations are characteristic markers of no-clean flux residue activity. Modern no-clean flux formulations use amine-based activators that are designed to fully decompose and become electrochemically inert during the soldering thermal profile. When ammonium or methylamine is detected at elevated levels in IC analysis, it indicates that the flux activator has not been fully deactivated, either because the soldering thermal profile was inadequate, the flux loading was excessive, or the specific flux chemistry is not compatible with the soldering process conditions. This finding is significant because it means the residue retains hygroscopic and corrosion-promoting properties even though the board may have been manufactured under a no-clean process that is not expected to require cleaning. The corrective action is typically thermal profile optimisation, flux type review, or in some cases, a move to a cleaning process to remove the residue entirely. Ion Chromatography vs ROSE Testing & Understanding the Difference Ion chromatography is not the only method for assessing ionic contamination on PCBs. An older technique, ROSE testing (Resistivity of Solvent Extract), is still used in some applications and is worth understanding in the context of IC analysis. ROSE testing measures the total ionic content of a board extract by its electrical conductivity, expressed as equivalent sodium chloride contamination in micrograms per square centimetre. It is a rapid, low-cost method that provides a single aggregate number representing all ionic contamination on the board. It does not identify which ionic species are present or in what proportions. Ion chromatography supersedes ROSE testing in technical information value. IC identifies each ionic species individually, enabling diagnosis of the contamination source and targeted corrective action. ROSE testing tells you that contamination is present above a threshold. IC tells you what it is and, by inference, where it came from. For automotive electronics qualification, where the identity of contaminating species is increasingly required by OEM specifications and where root cause investigation of any failures is mandatory, IC is the appropriate method.   Dimension ROSE Testing Ion Chromatography (IC) Output Single conductivity number (NaCl equivalent) Individual concentration of each anion and cation Species identification None Full identification of all ionic species present Sensitivity Moderate High – parts per billion detection in extract Diagnostic value Low – pass/fail only High – identifies species and enables source tracing OEM acceptance Declining – many specs now require IC Accepted by all major automotive OEM specifications Standard reference IPC-TM-650 2.3.25 IPC-TM-650 2.3.28 Cost Lower Higher – more information per test Typical application Production line screening where IC is used for qualification OEM qualification, failure investigation, process validation Ionic Contamination in Automotive Electronics & Why the Stakes Are Higher Ionic contamination matters in all electronics applications, but the consequences in automotive electronics are more severe than in most other sectors. Automotive electronics operate in conditions that maximise the risk of ionic contamination driven failure: wide temperature cycling that promotes condensation, vibration that can crack conformal coatings and expose underlying surfaces, extended service lives measured in decades rather than years, and safety-critical functions where intermittent faults have direct consequences for driver safety. An engine control unit that develops an intermittent fault from ionic contamination-driven leakage current is not a product return. It is potentially a safety incident, a warranty campaign, and a significant engineering investigation. The cost difference between finding ionic contamination before shipment by IC testing, and finding it after installation in vehicles through field failures, is several orders of magnitude. For automotive electronics manufacturers in Malaysia and Southeast Asia, the ionic contamination testing requirement typically enters the supply chain through OEM qualification requirements, customer cleanliness specifications, or process qualification programmes. Where no specific limit has been defined by the customer, the IPC standards provide a framework for establishing appropriate internal cleanliness limits based on the application criticality. For the full range of chemical and electronics testing services including ionic contamination analysis: https://www.alstesting.co.th/anion-test-specialist-malaysia/ Ionic Contamination and Component Cleanliness & The Connection Ionic contamination testing on PCBs and technical cleanliness testing on precision mechanical components address the same fundamental problem from different perspectives: contamination that is invisible to standard inspection methods but causes field failures in service. The analytical techniques differ but the quality management principle is identical. For manufacturers who produce both precision mechanical components and automotive electronics, or who supply into supply chains that require both types of testing, understanding the connection between the two disciplines helps in establishing a coherent quality testing programme. In both cases, the contamination is measured at a level of precision that only accredited laboratory analysis can provide, the results are compared against defined limits, and the findings drive corrective action in the manufacturing process. For technical cleanliness testing of precision mechanical components to ISO 16232 and VDA 19:  https://www.alstesting.co.th/technical-cleanliness-testing/ Ion Chromatography at ALS Testing ALS Testing provides anion and cation analysis by ion chromatography to IPC-TM-650 2.3.28 for PCB assemblies, individual components, and process solution analysis. Our IC capability covers the full range of ionic species relevant to electronics reliability assessment: the primary anions including fluoride, chloride, bromide, nitrate, phosphate, sulfate, acetate, and formate; and the primary cations including sodium, potassium, ammonium, and the amine species associated with no-clean flux residues. All ionic contamination testing at ALS is conducted within our ISO/IEC 17025:2017 accredited quality management system. Results are reported with individual species concentrations in micrograms per square centimetre, compared against the limits specified in your cleanliness specification or OEM requirement, with clear pass/fail designation for each species and for the total ionic contamination level. Our reports include the full IC chromatogram data alongside the tabulated results, enabling your engineering team to review the species profile and make informed decisions about corrective action priorities. For investigations where elevated ionic contamination has been detected and source tracing is required, we can design follow-up sampling strategies including area-specific extractions to localise contamination to specific board regions or process steps. Summary Anion and cation testing by ion chromatography is the definitive analytical method for ionic contamination assessment of PCBs and automotive electronics assemblies. It identifies individual ionic species at concentrations that are analytically significant but invisible to all other inspection methods, enabling both pass/fail qualification and diagnostically useful information about contamination sources and corrective actions. The primary standard for the test is IPC-TM-650 2.3.28. The most diagnostically significant species are chloride on the anion side, which indicates aggressive corrosion risk, and ammonium and organic amines on the cation side, which indicate active no-clean flux residues that retain corrosion-promoting properties. IC supersedes ROSE testing in diagnostic value and is the method required by automotive OEM cleanliness specifications. For automotive electronics manufacturers, the cost of finding ionic contamination before shipment through IC testing is a small fraction of the cost of the field failures it prevents. For production-line application, IC provides the species resolution that allows root cause investigation and targeted process improvement rather than pass/fail screening alone. Next Steps See our full Chemical and Electronics Testing services including ionic contamination analysis:  https://www.alstesting.co.th/anion-test-specialist-malaysia/ Learn about technical cleanliness testing for precision mechanical components: https://www.alstesting.co.th/technical-cleanliness-testing/ Contact our team for an IC testing quotation or technical consultation: https://www.alstesting.co.th/contact-us/