Did you know that more than 25% of PCB failures occur due to ionic contamination? These contaminated PCBs are sent back to the manufacturer for revision. Additionally, ionic contamination may cause a catastrophic failure, leading to a substantial financial loss for a fabricator. PCB testing and analysis for ionic contamination is a widely discussed topic in the electronics manufacturing industry.
The Why’s and The How’s to Keep Your PCB Clean
A PCB in the outside world is exposed to harsh environmental conditions such as humidity, temperature changes, chemicals, etc. The use of PCB casing, solder mask and isolation equipment may reduce the effect of environmental factors. However, the accumulation of ionic residues such as salts, inorganic and organic acids, flux activators and other chemicals, during fabrication and PCB assembly can accelerate contamination processes. Ionic contamination may also be caused due to the lack of board cleanliness.
IPC TM 650-2.3.25 describes how to perform ionic contamination testing for circuit boards. The test method described in these documents will help determine the amounts of ionic contaminants present on a printed circuit board. These tests also determine the types of impurities on board assemblies and components and their acceptable limits.
History of Ionic Contamination Testing
The ionic contamination test, sometimes also referred to as the resistivity of solvent extract (ROSE) test, was developed for electronic systems used in space and military applications. Early developments for ionic contamination testing or ROSE testing was, in the 1960s, particularly used to detect ionic residues. However, in the 1990s, several PCB manufacturers started using a higher temperature “lead-free” solder. This made the board cleaning process more challenging while testing PCBs. Over the next two decades, electronics technologies will advance further, as the electronic circuits will get smaller and denser. This will lead to more challenges while cleaning and inspecting electronic assemblies.
The ionic contamination test was first incorporated into the military-grade standard MIL-P-28809, which later on changed to MIL-STD-2000. Several changes were made to an original procedure. For instance, extraction should not be less than 2 Mohm-cm when the starting solution had a resistivity greater than 6 Mohm-cm. The process mentioned in MIL-STD-2000 compares the conductivity of test solutions with the conductivity of sodium chloride solutions of known concentrations. The values are measured in mg/cm2 NaCl equivalents. The acceptable criteria were revealed as 0.00156 mg/cm2 NaCl equivalents (0.01 mg/in2) maximum as per the standard.
Cleanliness Measurement Techniques Used in Ionic Contamination Testing
The ionic residues reduce the lifespan of the PCB. For this purpose, PCB manufacturers perform board cleanliness as part of their manufacturing process. Ionic contamination testing will likely provide information on the requirement for excessive cleaning during production. Let’s have a detailed analysis of cleanliness testing methods for the determination of ionic contamination.
The Resistivity of Solvent Extract (ROSE) Test
The widely used ROSE test determines bulk ionic, which is responsible for the contamination. The ROSE test measures the ions found on the PCB that is dipped into a solvent solution. The test results are measured in bulk ions per square inch.
The following diagram shows a ROSE test:
As per the IPC 650-2.3.25 standard, the ionic contamination test specimen is provided by the equipment manufacturer’s recommendations. Calculations for the bare-board and assembled boards are also determined in the standard.
Length x Width x 2 = Unpopulated Board Surface Area
(Length x Width x 2) + up to 50% = Assembled Board Surface Area
The selection of the solution caries as per specification is given by the equipment manufacturer. The industry has established two different standard test solutions that are used worldwide:
75 % v/v 2-propanol/DI water
50 % v/v 2-propanol/DI water
Several changes have been made to ROSE testing over the years in terms of equipment, testing procedure, types of chemicals, and more. The ROSE test was reiterated with the addition of thermal extraction methods, and called a modified ROSE test. Similar to the standard ROSE test, it involves an exchange of bulk ions between a PCB and solvent. A modified ROSE test is carried out at an elevated temperature to accelerate ion transfer. The modified ROSE test is performed in ionograph-style equipment.
Results with Example:
Ionic cleanliness test or ROSE test determines the conductivity or resistivity of a PCB sample. The ionic contamination tester is used along with the combination of isopropyl alcohol (IPA) and deionized water. The ionic tester collects a sample of possible residues present on the board. The reside is deployed for resistivity (conductivity) testing, which indicates the board’s soil levels.
The test results provided by OMEGA 600 SMD of the final contamination level are predetermined as per IPC TM 650 and MIL-P 28809 (for military applications). The Omega Meter register higher values compared with the Contaminometer. For bare boards or non-assembled boards, the permitted contamination level is less than 0.0001 mg/cm2 NaCI equivalents, while for the hand-soldered assemblies, the permitted contamination level is about 0.0003 mg/cm2 NaCl equivalents. For PCB assemblies with a dispensed rosin-based solder paste, the permitted contamination level is 0.0003 mg/cm2 NaCl equivalents.
Here are a few interesting facts about ROSE cleanliness testing:
Fact 1: Ionic contamination test or ROSE test is one of the most trusted and proven analytical process adopted in the electronics industry. These processes are also sometimes referred to as six sigma processes.
Fact 2: The ROSE test involves the immersion of a PCB sample in isopropanol and deionized water at room temperature.
Fact 3: The process is performed at multiple intervals with less than 10 minutes in duration.
Fact 4: The primary purpose of the ROSE test includes the measurement of the conductive materials present on the PCB that is already subjected to chemical baths and rosin-based fluxes.
Ion Chromatography Test
Another method for evaluating the board’s cleanliness is ion chromatography, which is based on the solvent extraction principle. The effectiveness of ion chromatography is enhanced by ultrasonic agitation. The test involves solvent extraction, after which the solution is tested in an ion chromatography test unit. The test data highlights the specific ionic impurity content in the sample, the impurity measured in per square inch.
The conductivity of a sample is dependent on the amounts of ionic materials present. The conductivity of the solution increases as the number of ionic contamination increases. Certainly, there are two different types of impurities present on the PCB sample i.e. ionic and non-ionic. Ionic impurities are typically flux residues or chemicals that are accumulated during the manufacturing processes.
Water-soluble organic and inorganic salts may also be present on the PCB sample. These water-soluble organic and inorganic salts consist of charged atoms that are conductive in nature. The charged (positively or negatively) molecules increases the overall conductivity of the solution. These ionic impurity present on the sample degrades the reliability of the PCB assemblies. The impurity can cause current leakage, thus leading to dendrite growth. This will further lead to corrosion of the PCB. Different types of impurities include formate, chloride, bromide, nitrate, and sulphate. The impurities are calculated in mg/em2.
As per the IPC 650 2.3.28, the surface area of the sample is calculated by the formula:
(Length x Width x 2) + 10% for a populated PCB with 75/25 IPA/H20 solutions
The addition of 100-250 ml of the extraction solution to the extraction bag is recommended.
Benefits of Cleanliness Measurement for Ionic Contamination
The PCB sample dipped in an extraction solution releases ions within an extraction liquid, which is determined by a conductance detector. The final result is calculated by ionic contamination meter with the help of a software. The measurement value is analyzed in reference to sodium chloride. The resistivity/conductivity readings can be used to
convert the µg NaCl equivalent as follows:
- The cleanliness measurement is ideal for post soldering evaluation as per official regulation. Also, it verifies surface quality prior to PCB coating and other chemical processes.
- The testing also performs quantitative determination of the ion contaminates for PCBs in µg/cm² equivalent NaCl.
- The cleanliness measurement continuously monitors of contamination via quality through regular sample-based measurement.
PCB Cleaning Process
PCB cleaning is a delicate process, similar to manufacturing. A number of issues may arise leading to ionic contamination including, damaged connections, loosened connections and washed away materials if the cleaning process is not performed properly. Here are a few PCB cleaning processes worth mentioning:
- Aqueous cleaning process
- Semi-aqueous process
- Vacuum cleaning process
- Vapour phase degreaser
The use of ultrasonic cleaning machines has increased for PCB cleaning over the past few years. The ultrasonic cleaning offers cavitation at high frequencies with the implosion of billions of bubbles in the cleaning solution. The process blasts away contaminants on the surface of PCB parts placed in the ultrasonic tank.
Sierra Circuits loves cleanliness, whether it’s in our office, our manufacturing floor, and even our PCBs. Learn more about our PCB assembly steps.