PCB Ionic Contamination
Approximately 15% of PCBA failures occur due to contamination. Ionic contamination can cause multiple issues that lead to defective PCBs. Testing the bare board for ionic contamination before completing assembly reduces the risk of defects caused by contaminants. This guide will describe ionic contamination, the problems it causes and how manufacturers conduct ionic contamination testing.
What Is Ionic Contamination in PCBs?
Ionic contamination happens when ionic residues that interfere with reliability and functionality remain on a complete PCB. An ionic residue contains atoms or molecules that become conductive when in a solution. Exposure to moisture makes ionic residues disassociate into negatively or positively charged elements, changing the solution’s overall conductivity.
A PCB can also have nonionic contamination, which involves nonionic residues. Nonionic residues don’t have conductive properties, so they can usually remain on the PCB after production and assembly. Therefore, most manufacturers focus on ionic contamination when examining a board’s cleanliness. The ionic residues that affect PCB components during production include:
- Inorganic and organic acids
- Flux activators
- Plating chemistries
Ionic contamination has two common sources:
- Lack of bare board cleanliness: Many ionic contaminants come from the board itself. The board fabrication process, as well as environmental exposure, can leave residues such as particulate residues, oils, salts and dust. Before adding components to a bare board, manufacturers must ensure no contaminants were left over from previous steps in the production process.
- Use of aggressive chemistry: Copper etching liquid, water-soluble soldering chemistry and other kinds of aggressive chemistry can leave behind residues that change the board’s conductivity when not cleaned properly.
What Issues Do Ionic Residues Cause?
When manufacturers fail to remove excess ionic residues, the following issues can occur:
- Corrosion: Most PCBs will corrode eventually due to their metal materials. However, ionic contamination can result in a much shorter time until corrosion compared to the PCB’s expected lifespan. Corrosion refers to the process of oxygen bonding to metal and creating rust. When moisture comes in contact with ionic residue, the risk of short-circuiting increases. The corroding metal flakes off, losing the chemical properties needed for the PCB to operate correctly.
- Dendritic growth: During dendritic growth, conductive metal slivers, or dendrites, grow on a PCB through an electrolytic solution influenced by a DC voltage bias. Dendrites can appear quickly when the pores in a solder mask retain flux or another ionic residue. When dendrites come in contact with each other, defects such as short-circuiting and intermittent operation can arise.
- Electrochemical migration: Electrochemical migration also involves dendrites, but specifically when they grow across a dielectric material. Since dendrites form from conductive ions, they can direct currents differently from the PCB’s intended design. Dendritic growth causes electrochemical migration, which creates total or intermittent failures. Dendritic growth and electrochemical migration are closely related to each other and tend to happen simultaneously.
Cleanliness Measurements Used in Ionic Contamination Testing
To ensure that ionic residues don’t reduce the lifespan of the PCB, many manufacturers already clean the board as part of the manufacturing process. Ionic contamination testing lets manufacturers determine if they’re using sufficient cleaning techniques during production. Cleanliness testing methods used to detect ionic contamination include:
- Resistivity testing in batch-format aqueous cleaning systems: The systems used to clean PCBs often come from a built-in resistivity measurement tool. While these results cannot be used to meet IPC standards, they can provide insight into the cleaning system’s effectiveness.
- Resistivity of solvent extract (ROSE) test: ROSE testing detects bulk ionics that can lead to contamination. A zero-ion or similar type of ionic testing unit draws the ions found on the PCB into a solvent solution. The test measures results as bulk ions per square inch.
- Modified ROSE test: The modified ROSE test adds thermal extraction methods to the standard ROSE test. It still involves a solvent that draws out bulk ions. However, instead of drawing the ions out in standard conditions, the PCB and solvent solution are exposed to an elevated temperature. The solution then undergoes testing from ionograph-style equipment.
- Ion chromatography test: An ion chromatography test involves similar thermal extraction methods to the modified ROSE test. After extraction, the solution undergoes testing in an ion chromatograph test unit. The results from this test provide information about the specific ionic species in the sample and each species’ level per square inch.
Clean, Tested PCBs
Millennium Circuits Limited only supplies PCBs that are best-in-class quality assurance testing to ensure that their products have no contaminants. Contact us online to learn more about our quality standards today.