Dec 4, 2020|General
The PCB ionic contamination test calculates ionic cleanliness of PCBs to determine potential electrical or physical failures that happen at various points in the manufacturing process. Contaminant tests can also help establish cleaning procedures. Some common contaminants, or residues, are sodium, dendrite growth, and flux activators.
Types of Contaminants: Ionic and Nonionic
Table of Contents
- 1 Types of Contaminants: Ionic and Nonionic
- 2 What is Ionic Contamination in PCBs?
- 3 What Issues Do Ionic Residues Cause?
- 4 Ionic Cleanliness Testing
- 5 Cleanliness Measurement Techniques Used in Testing
- 6 Final Thoughts
PCB contaminants include both ionic and nonionic compounds.
Positively charged ions and negatively charged ions assemble into ionic compounds when non-metal, and metal atoms react to each other.
Solid ionic compounds are too bound together to become conductive. They make excellent insulators, though.
Ionic compounds become capable of conducting electricity when they are dissolved in water. Therefore, soldering has to take place in circuit manufacturing.
Overall, nonionic contaminants are harmless to PCBs because they are non-conductive. Manufacturers don’t typically get rid of nonionics unless they need to refine the board’s reliability.
Note: PCB contamination testing and cleaning processes may wipe out nonionics anyway.
Nonionics are mostly organic species and acids:
What is Ionic Contamination in PCBs?
Ionic compounds are conductive and greatly affect the PCB’s reliability and functionality.
Photo by Michael Dziedzic Licensed under Unsplash
Common Causes of Contamination:
- Soldering with (RMA) fluxes
- Dwell time
- Prolonged material storage
- Human byproducts
- Use of impure cleaning products
- Water-soluble soldering
- Damaged or malfunctioning parts: i.e., nozzles, hose vacuums, and convey belts
Common Ionic Compounds
Chloride, fluoride, potassium, and sodium are ion culprits that frequently form parts of these common ionic compounds:
- Flux activators
- Ionic surfactants
What Issues Do Ionic Residues Cause?
Manufacturers need to ensure flux residues are removed. If moisture and contaminants are trapped anywhere on the board, the PCB’s adhesion will fade off, and closely spaced conductors can easily short out.
The Characteristics of Excessive Ionic Residues
Today, there are three flux types:
Residues can be either mildly corrosive or non-corrosive.
Residues can be weakly corrosive or non-corrosive.
Residues are strongly corrosive.
Corrosion and Flux
Solder flux helps prevent oxidation during the soldering process. Not too long ago, manufacturers used flux that contained corrosive chemicals like chlorine. The material would flake off and slowly change the PCB’s chemical properties.
Manufacturers are now using organic flux at high temperatures and commonly utilizing a process known as reflow soldering. The leftover material from the flux is mostly ignored, as with other nonionic compounds.
Tests are still conducted to monitor and control harmful contamination levels to avoid the possibility of corrosion.
Conductive metal silvers are called dendritic growth. If a solder mask retains flex and electrolytic solution and DC voltage bias interacts with it, dendrites may form. Dendrites that touch each other may short-circuit and cause intermittent operations, among other defects.
When flux isn’t removed correctly, bubbles form, become trapped, and rise above the solder joint. The bubbles contain ionic compounds or contaminants.
The contamination ultimately causes local stress and cracks when exposed to temperature fluctuations and vibrations. Notably, contamination can be found in or topside a hole by the component body.
PCB Dewetting and non-wetting
PCB Dewetting is a formation of irregularly shaped mounds of solder. When the solder is applied and receded from the area, dewetting occurs. The solder recedes for numerous reasons: inappropriate oxidation levels in the paste defected oven temperatures, and, the most common problem, contamination at the PCB pads’ surface.
Ionic Cleanliness Testing
To prevent issues with conformal coatings and assembly functions, manufacturers measure ionic cleanliness.
The dynamic method measures a sample’s conductivity continuously as it is passed through a conductivity cell.
The Static Method
The resistivity is measured once at the end of the PCB testing extraction.
The necessary cleaning method is determined by the ionic contamination testing results. Manufacturers can then plan their assembly lines to ensure the quickest assembly runtime.
Cleanliness Measurement Techniques Used in Testing
Ionic Cleanliness testing samples the type and amount of contaminants present on PCBs.
The Resistivity of Solvent Extract (ROSE)
ROSE measures the resistance drop and determines the amount of salt content. Too much salt is formed when solder and chemical residues are left behind during the manufacturing processes.
The leftover residue causes electrical shorts and poor performance. The test cannot pinpoint a specific contaminated location.
Ion Chromatography (IC)
ICs are usually used when dendrites are found on the board. The goal is to analyze the ions that make up the contaminates and measure the overall cleanliness.
Cleanliness should be viewed as a sliding risk scale. Most manufacturers will not define a standard unless the board’s reliability is of the utmost importance, like in military-grade devices.
Fourier Transform Infrared Spectroscopy (FTIR)
Infrared radiation and algorithms identify suspected organic components. Moreover, this is used for Infrared radiation, and algorithms identify suspected organic components. This data is used to understand the scope of the damage. A lab worker can view the damage in its entity, select a region of interest (ROI), and examine it.
Each spot is measured in an infrared (IR) spectrum. The scientists analyze the data.
Surface Insulation Resistance (SIR)
SIR determines the electrical resistance. High resistance means a clean board, and a low resistance means the board is contaminated. This information is used to compare, classify, and qualify cleaning processes and soldering fluxes.
Line widths and track spacing represent the data for SIR. The data almost looks like a sequence of barcodes. The manufacturer will let the customer decide what an acceptable flux performance is.
The basic premise is to see whether or not the water stays on the board or not. If the waterfalls off, then the board isn’t conductive enough. If the water spreads out evenly, then the board is passable.
Note: solder voids and dewetting caused by contaminants won’t pass this test.
Several shortcomings are easy to see. Look for signs of residues, corrosion, and discoloration on the top and bottom of the surfaces.
PCB contaminants and ionic cleanliness tests are extensive topics that are relevant to PCB manufacturers. Circuit board manufacturing is a precise art, and so is testing PCBs for contaminants to ensure high quality and performance.