PCBA Tech - Introduction to the cleanliness control of PCB circuit boards

Cleanliness has received more and more attention in the machinery industry and microelectronics industry. This article mainly introduces the concept of cleanliness, particulate matter cleanliness and ionic pollutants detection methods, and combined with the situation in this laboratory, introduces the related equipment for cleanliness detection.

1 Introduction

1.1 Cleanliness concept

Cleanliness indicates the amount of dirt remaining on a certain surface of a part or product after cleaning. Generally speaking, the amount of dirt includes metrics such as type, shape, size, quantity, and weight.

1.2 The necessity of cleanliness testing

1.2.1 Control of car cleanliness

Cleanliness control is an important link to ensure that the product meets the quality requirements during the production process. Its impact on the car is mainly manifested in two aspects:

On the one hand, the impact on product performance. For example, the blockage of the fuel line causes difficulty in starting the engine or insufficient power, the blockage of the lubricating oil line leads to insufficient or no pressure of the oil, which increases the friction between the moving pairs, the engine cannot exert effective power, and even causes the cylinder to be blocked. The brake line is blocked., Will reduce the braking effect, or cause braking failure.

On the other hand, the impact on the life of the vehicle. A domestic factory conducted a test on the impact of cleanliness on engine life. Statistics show that after cleanliness treatment of diesel engine parts and the whole machine, the life of the diesel engine has more than doubled.

1.2.2 Control of circuit board cleanliness

The performance of the circuit board is directly related to the specific amount of visible and invisible residues between the pads and holes on all areas of the active circuit. In a humid environment, ion pollution can cause many problems, such as short circuits between conductors caused by crystal growth, or direct corrosion of conductors to reduce the surface impedance of the product. For the circuit board, it is particularly important to monitor the degree of ion pollution to ensure the reliability of the product.

The main pollutants that may be exposed in the production of integrated circuits are: particulate impurities, inorganic ions, organic substances, microorganisms, and gas impurities. Broadly speaking, unsuitable temperature, humidity, illuminance, excessive electrostatic and electromagnetic noise, air noise, and micro-vibration are also special pollutants. Among them, when the amount of particulate impurities, inorganic ions, organic substances, microorganisms, and gas impurities exceeds a certain limit, it will cause surface scratches, pattern disconnections, short circuits, pinholes, and peeling of integrated circuit products, resulting in circuit leakage, The electrical characteristics are abnormal. The lighter ones will affect the circuit performance and service life, and the more serious ones can cause the circuit to be scrapped. Unsuitable temperature, humidity, illuminance and excessive static electricity, vibration and noise not only affect the processing accuracy and service life of process equipment, but also affect the emotions of operators, which in turn affects the quality, production efficiency and yield of integrated circuits. The proportion of product failures due to pollution problems may be as high as 60%.

2 Cleanliness testing method

The cleanliness test method is very important for process control, quality assurance and failure analysis. The test method is to summarize the detailed procedures used to obtain the cleanliness data of the test subject such as various mechanical and electronic parts.

Some contaminants on the surface of the workpiece can be detected directly, while others need to be detected by indirect methods. For example, the cleanliness of components and system cavities in machinery is usually determined by indirect methods, using the pollution degree of the working fluid or process fluid (cleaning fluid) used, such as the cleanliness of hydraulic system by testing the cleanliness of hydraulic oil Degree to determine. Indirect detection mainly involves the collection of oil samples, the purification and weighing of oil samples. CTI Cleanliness Laboratory is also developing indirect cleaning methods based on a large number of direct cleaning methods.

The degree of ion contamination is also measured by indirect methods. First, rinse the circuit board surface with a test solution, dissolve the ion contamination in the rinse solution, and then detect the resistivity or conductivity of the extract. The main detection method is NaCl equivalent method. And ion chromatography.

2.1 Method overview:

For the cleanliness of particulate matter, there are 5 commonly used detection methods: visual inspection method, fluorescence luminescence method, weighing method, contact angle method, particle size quantity method.

For the test of circuit board cleanliness, there are mainly NaCl equivalent method and ion chromatography.

Visual inspection method: The visual inspection method is a relatively simple inspection method. The outer surface or the inner cavity surface of the part that can be observed under a magnifying glass or a microscope is directly inspected manually. And then identify whether the pollution particles are metal, non-metal or fiber, and its size. The visual inspection method is mainly used to inspect the relatively large and obvious particles, spots, rust spots and other pollution remaining on the surface of the parts. The main disadvantage of this method is that the inspection results are easily interfered by human factors.

Fluorescence method: mainly uses ultraviolet light to detect the cleanliness of the surface of the part. Under the irradiation of ultraviolet light, the pollutant particles on the surface will fluoresce. According to the fluorescence, the position of the dirt on the surface of the part can be visually detected, and the fluorescence intensity can also be tested with a signal detector to indicate the degree of contamination of the surface. But if you want to identify the composition and other characteristics of the pollutants, you must rely on other analysis methods.

Contact angle method: The so-called contact angle is the angle held by the liquid when it forms a thermodynamic equilibrium on the solid surface. The measurement of the contact angle formed between solid and liquid is an analysis technique that is widely known in many similar fields such as adhesion, surface treatment and polymer surface analysis. It is a surface analysis that is very sensitive to single-layer changes of multiple units. technology. The contact angle method is mainly realized by using a surface cleanliness analyzer. It evaluates the difference between the surface contact angle (water drop angle) of glass, chip, PCB board and other materials contaminated by oil and the angle of glass and chip. How to clean the chip and PCB board.

Weighing method: Weighing method is a cleanliness test method commonly used in industrial production and testing. It is also the most commonly used test method in this laboratory. The test principle is to use the selected cleaning fluid to clean a certain number of samples under certain conditions. The cleaned liquid is filtered through a filter membrane with a certain pore size (commonly used filter membranes have 5μm, 10μm, 20μm, 30μm, etc.), and the dirt is collected on the surface of the filter membrane. The filter membrane before and after filtration is weighed, and the difference between the two weighings is the weight of the contaminant.

Particle size quantity method: It is a new method to test the cleanliness of parts. The basic principle is that the surface to be detected and the pollutant particles have different light absorption or scattering rates. The test method is the same as the gravimetric method. After the filter paper is dried, use a microscope (the best device is an image recognition and analysis device with photographing function) to detect under light irradiation, and the particle size and area can be obtained. The result of measuring the solid particle contamination of the part. It is suitable for precise cleaning quantitative cleanliness detection method, especially suitable for detecting tiny particles and colored impurity particles. However, if the filter membrane is white, the recognition of white dirt and bubbles may cause misjudgment.

NaCl equivalent method: The amount of conductive ions in the solution can be simply expressed by the conductivity of the solution. According to the conductivity of the known amount of NaCl in the extract, the unknown conductive ion content can be simply given. The specific operation method is as follows: each 250mm2 printed board is rinsed with 100ml of solution in a trickle mode until the solution is collected in the beaker (this process takes at least 1min). Use a conductivity tester to test the resistivity of the solution. The result output method is expressed by NaCl equivalent per square centimeter area.

Ion chromatography: According to the different degrees of adsorption and desorption of different ions in the mobile phase and the stationary phase, the time for the ions to pass through the adsorption column is different to achieve ion separation. Then according to the size of the ion peak area, the concentration of the detected ion is calculated. Use isopropanol as the extractant to extract the ions to be tested on the PCB circuit board and then use ion chromatography for detection, which is mainly achieved by relying on ion chromatography. Unlike the NaCl equivalent method, ion chromatography can characterize the content of individual ions.