Ionic contamination may occur during PCB fabrication and assembly processes. During fabrication, the creation of PTHs, etching process, and drilling are responsible for the contamination. On the other hand, during assembly, flux residue in soldering, component biocides by pick and place machine, and ionic surfactants from solder preparation are responsible for PCB ionic contamination. Read the post on component placement guidelines for PCB design and assembly.
- Moisture trapped in PCB layers: Ionic contaminants in contact with moisture can increase short-circuit risk.
- Etching chemicals: Chemicals used for PCB etching purposes are highly conductive and corrosive. It is essential to neutralize/rinse them since they can be a potential cause of leakage current.
- Flux residues: Common conductive flux residues from the soldering process include various unreacted activators, binders, rheology components, and saponifiers. These residues must be removed by using solvent cleaning, say vapor degreasing, or by aqueous chemistries in the common batch.
- Inter-layer residues during drilling and plating (PTHs): Inter-layer residues generated during drilling and plating are also responsible for ionic contamination.
- Improper PCB surface cleaning: The majority of ionic contaminants come from the board itself. Before placing components over the bare board, you need to ensure that no contaminant is left from the previous production step.
What are the effects of ionic contamination in PCB?
The occurrence of ionic contamination on PCB assembly can inhibit its performance. In the production house, bare boards can carry ionic contaminants in the form of left-over flux, etching chemical, and solder material. If these residues are left on the board, major issues, like Electrochemical Migration (ECM), corroded traces, parasitic leakages, and dendritic growth, can hamper the PCB’s life-cycle. Read 8 soldering tips to avoid possible ionic contamination issues.
Electrochemical migration (ECM): Electrochemical migration is caused by the electric field in which metal gets dissolved, resulting in ion formation between two copper features. If the electric field is applied, the metal dissolves at the Anode and gets deposited to the Cathode resulting in dendrite growth. Because dendrites grow from conductive ions, they can direct PCB currents differently from what is intended, leading to short circuits. Dendrites create alternative current paths that can affect the accuracy of current measurements in a PCB.
Corrosion: Usually, PCBs corrode due to their material. Ionic contamination can corrode the entire PCB in a much shorter time. When ionic residue comes in contact with moisture, the short-circuit risk increases. The corroding metal flakes off, overlooking the chemical properties needed for the PCB to operate correctly. Selecting apt PCB material and laminates helping with ionic contamination issues.
Ionic contamination alone is not responsible for the above consequences; moisture and environmental exposure are also. That is why PCBs are baked to eliminate moisture before starting any new process. The ionic contamination level needs to be controlled during the manufacturing of circuit boards by adopting a process control tool. This tool ensures that the individual production process steps (e.g., surface-mount or through-hole) are performed in a controlled manner to achieve a manageable ionic contamination level.
What is text of ionic contamination in PCB?
Bad quality control during PCB fabrication, component placement, and a sub-standard final cleaning are some of the major issues responsible for ionic contamination. It is crucial to identify, control, and mitigate such issues. This can be done with proper ionic contamination testing equipment and analysis. ROSE testing, ion extraction, and ion chromatography are some of the well-known methods. Sometimes, humidity validation testing is also performed at the beginning of the production.
To ensure the life-span of the PCB, testing of the bare board for ionic contamination prior to assembly reduces the risk of defects caused by contaminants. Ionic contamination testing is performed to detect ionic residues resulting from the fabrication and soldering steps. Let’s have a detailed analysis of cleanliness testing methods for the determination of ionic contamination.
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