Sometimes, on PCBs (which are used to make electronic devices), there can be a rare process called dendritic growth. It happens because of electricity when certain conditions are present, including high humidity, voltage bias & ionic contamination. The increased temperatures can cause a reduction in relative humidity sufficient to dry surface impurities that would otherwise lead to dendritic growth under sticky situations approaching condensation.
Of the metals commonly used in PCBs, silver is the most susceptible to dendritic crystallization, followed by tin and copper. Nickel dendritic crystallization/growth is rare, and this paper presents a nickel dendrite formation analysis in specific circumstances of moisture condensation on freshly manufactured PCB contaminated with H2SO4 etching. The H2SO4 contamination on the PCBs produced during manufacturing was confined to the solder resist(mask) crevice. Still, under humidity conditions, it spread across the gap between the nickel-gold plating connector bonding pads.
Dendrite Growth PCB
Contamination is responsible for around 15% of failures in printed circuit board assemblies (PCBAs). Ionic contamination, in particular, can give rise to several issues that result in faulty PCBs. To mitigate the hazard of flaws caused by impurities, it is advisable to test the bare board for ionic contamination before completing the assembly. This guide aims to provide an overview of ionic species, the problems they can cause, and the methods manufacturers use to conduct ionic impurities testing.
- Lack of bare board cleanliness: Numerous ionic contaminants can originate from the PCB. During the board production process and through exposure to the environment, various residues like particulate matter, oils, salts, & dust can accumulate. Before installing components onto a bare circuit board, manufacturers must verify that no contaminants remain from prior stages in the fabrication procedure.
- Use of aggressive chemistry: Residues from copper etchant solution, aqueous soldering flux, and other aggressive chemicals can alter the conductivity of a PCB if they are not cleaned adequately.
Issues Due to Dendrite Growth or Ionic Residues
If manufacturers do not eliminate excess ionic remnants, the following problems may arise:
- Corrosion: Most PCBs are prone to corrosion over time due to the metallic elements. However, if ionic contamination is present, it can accelerate the corrosion process, leading to a shorter lifespan for the PCB. Corrosion occurs when oxygen bonds with the metal, causing rust to form. When moisture interacts with ionic remnants, the likelihood of a short-circuit occurring increases. The rusting metal may chip away, which results in the loss of the chemical characteristics necessary for the printed circuit board to function correctly.
- Dendritic growth: Dendritic growth is a process whereby conductive metal fragments and dendrites produce on the printed circuit board through the conductive solution affected by a Direct current voltage bias. So the dendrites can emerge rapidly when the pinholes in the PCB mask preserve fluctuation. When dendrites contact one another, problems like short circuits can occur.
- Electrochemical migration: It also incorporates dendrites, particularly when extending transversely a dielectric material. As dendrites originate from ionized particles, they can shift currents in ways that are not aligned with the intended design of the PCB. Dendritic growth can trigger electrochemical migration, leading to either complete or sporadic failures. Electrochemical migration and Dendritic growth are closely interrelated and often co-occur.
Cleanliness Measurements For Dendrite Growth PCB Or Contamination Testing
To guarantee that ionic remnants do not decrease the life of the printed circuit board, numerous manufacturers already incorporate board cleaning as a fabrication process. Ionic purity testing enables manufacturers to verify if their cleaning techniques are sufficient during production. Techniques used to check ionic contamination in cleanliness testing include:
- Resistivity testing: The PCB cleaning systems frequently incorporate a built-in impedance measurement tool. Although these outcomes are inadequate to fulfill IPC specifications, they can furnish valuable information regarding the efficiency of the cleaning process.
- ROSE test: ROSE (Resistivity of Solvent Extract) testing checks bulk ionic contaminants on a PCB. A zero-ion or similar kind of ionic unit extracts the ions present on the printed circuit board into the solvent solution.
- Modified test: The customized resistor open short equipment test is an enhanced version of the standard available temporary equipment test that includes thermal extraction methods. Instead of extracting bulk ions in ordinary situations, the printed circuit board & solvent are subjected to high temperatures. After exposure, the solution is tested using iconography-style equipment. This modified test provides additional information about ionic contamination on the PCB.
- Chromatography test: A thermal extraction method similar to the modified resistor open short equipment test is used in chromatography testing. Once extracted, the resolution undergoes testing using ion chromatography equipment. The test results provide data about the particular electrolytes in a sample and their concentration PSI.
Why Complete Dendrite Growth PCB or PCB Ionic Contamination Analysis
It may surprise you that over 25 percent of printed circuit board failures are attributed to ionic impurity. When a PCB becomes contaminated, it is typically returned to a producer for rework. In extreme cases, the contamination can cause catastrophic faults, resulting in significant financial losses for the manufacturer. As a result, testing and analyzing PCBs for ionic adulteration is a topic of widespread discussion in the electronics fabrication industry.
How to Keep Your PCB Clean?
Exposure to environmental factors, including temperature changes, humidity, and chemicals, can be harsh for PCBs in the outside world. The use of printed circuit board casing, solder(PCB) mask & isolation equipment can help reduce the impact of these factors. However, collecting ionic residues during fabrication & assembly, like salt, flux activators, inorganic & organic acids & other chemicals, can accelerate the contamination process. A lack of board cleanliness can also contribute to ionic contamination.
IPC standard 650-2.3.25 provides guidelines for conducting ionic testing on circuit boards. This test method outlines how to measure the levels of ionic pollutants on a PCB. It also outlines the acceptable limits for these impurities in board assemblies & components.
History of Ionic Contamination Testing
The ionic impurities test, ROSE testing, was initially created for space and military electronic systems. Its early use in the 1960s focused on detecting ionic residues. However, with the advent of “unleaded” solder in the 1990s, PCB manufacturers faced new challenges in the cleaning process. This makes it more difficult to test for ionic contamination. As electronic circuits continue to become smaller & denser in the coming decades, electronic assembly cleaning & inspection will become even more challenging.
How Does Ionic Contamination Testing Work?
Two commonly used types of ionic contamination tests are the Resistivity of Solvent Extract (ROSE) test and Ion Chromatography (IC) testing.
The ROSE test
The Resistivity of Solvent Extract test is a simpler and faster method of testing for ionic contamination. It provides less detailed information. This test measures the entire ionic content of a product. So it allows for a quantitative determination of the level of contamination present.
· How the ROSE test works
In the ROSE test, a conductive sample is extracted using a solution of isopropyl alcohol & deionized water. This extracted sample is then subjected to resistivity measurement by passing it through the ionic testing unit. This unit uses a liquid conductivity and conductivity bridge to compare the sample’s impedance to a NaCl reference standard.
· What to use the ROSE test for
Although the Resistivity of the Solvent Extract test effectively determines the total amount of ionic contamination present, it cannot identify the particular ions present. It helps assess the cleanliness of things for QA & safety purposes. However, it does not provide information on specific contaminants or their sources.
IC Testing
Compared to the ROSE test, Integrated circuit testing is more comprehensive. So It can evaluate the total amount of ionic contamination present. It also considers an ionic charge, the particular ionic species in a sample.
· How IC testing works
Like the Resistivity of Solvent Extract test, IC testing involves extracting a product sample using a solution of isopropyl alcohol & deionized water. The model is then passed through the high-performance chromatography system. This system separates ions based on their size, species, and charge.
Liquid chromatography equipment used in IC testing typically contains a cation column or anion column. Additionally, the sample is introduced into the system like the solvent system. It travels via column and interacts differently with the queue based on its charge. It causes the ions to separate from one another. They either speed up or slow down in their movement through the column.
As the different kinds of ions move through the column at various speeds, they are detected by the conductivity detector immediately after leaving the column casing. This detector generates a chromatogram that plots conductivity against time. We can identify the type & concentration of each ion present in the sample by analyzing changes in conductance.
· What to use IC testing for
Unlike the Resistivity of Solvent Extract test, IC testing can be used to identify particular contaminants & their concentrations. It allows us to identify the contaminants present. It also understands how they may be presented to a sample during fabrication.
By providing insights into the specific contaminants and their sources, IC testing helps clients identify vulnerabilities in their production process and address any issues affecting product performance.
