Analysis of Optical Inspection of Soldering in PCB Assembly

An acute shortage of equipment for automatic control of electronic assemblies currently has led to a curbing of the development of the electronic industry. In this article, we will analyze the structure and functioning of modern optical inspections in Printed Circuit Boards. It discusses ways of improving the functional scheme of optical inspection are considered, and its main components and their interaction are described. Similarly, we shall also represent coordinate systems for equipment control and how to calculate them.

Analysis of existing foreign optical control systems showed that in these typical technological sets the following main ones are used pieces of equipment:

Cameras: In the basic configuration 2 blocks are used, including 4 orthogonal cameras with a total resolution of 57.6 × 43.5 mm (2592 × 1944 pixels per camera) and 4 corner color cameras. Rectangular Block Camera Resolution allows you to recognize large components. Corner cameras in the block are located along circles in 45° increments. Permissive the capability of orthogonal cameras is 23.4 µm per pixel; block - 16.1 microns per pixel. Camera Blocks are used in conjunction with a four-color backlight.

The Computer of the Verification Station

A computer connected to the AOI local network allows remote analysis of printed circuit board defects with subsequent correction of the inspection programs if necessary.

Manipulation device:  An antistatic belt conveyor is used to manipulate the printed circuit board. Linear drives are used in the camera block manipulation system. The conveyor is installed on built-in line AOI, which, when necessary, can also work in stand-alone mode.

For the operation of such technology, the complex uses specialized software (SW). In recent years, the main focus on the development of equipment for AOI is based on ease of operation, intuitive menu, the ability to quickly create inspection programs, and graphic interfaces. 

For these purposes, the most make effective use of work programs that are created on the basis of Gerber format - a file representing is a way of describing a print project board for its manufacture. Import file allows you to reproduce a 3D model PCB assembly.

Visual Assessment of Quality Inspection in PCB Assembly

Next, according to this model, there is a visual assessment of the quality inspection program and, if necessary, its adjustment. The software allows you to work with a two-dimensional image when debugging a program. 

When debugging, the menu is also used, which allows you to set the boundary between false positives and existing defects. By the end of debugging, control is carried out with the help of the program, by means which analyzes the defects received via the local network. 

Next on the monitor for comparison, the obtained images, images of the reference component, and examples of defects from the library are displayed. The software allows you to evaluate images from angle cameras for a more accurate defect. The program makes it possible to replenish the library of defects in accordance with the requirements of production.

Analysis of the Operation of Existing AOI Systems

An analysis of the operation of existing AOI systems made it possible to identify the following features of using equipment:

• When importing from a Gerber file, you can but find out only the coordinates of the location components and their labeling. The library stores 3D models of components; a visual representation of the board profile is built for the user.
• Algorithms for inspection, selection of light, methods of bypassing the board, etc. are not available in the public domain and require individual development for specific tasks. If necessary, you can only correct the confidence limits, due to the specifics of production.

The system uses 4-color illumination (red, blue, green, and white). The algorithm uses the first three colors, for a visual representation of the defect, the operator is given the opportunity to use white color. Based on this, it is impossible to understand exactly how the recognition and analysis algorithm works.

In the process of mounting electronic components on printed circuit boards, visual quality control of the assembly is carried out. At the same time, the quality of soldering is evaluated in accordance with the IPC-A-610 standard.

 During visual inspection, the following is assessed:

• Solder joint quality
• The presence of a component
• No displacement of the component relative to the pads on the printed circuit board
• Compliance with the position, type, and rating of the installed component of the design documentation, etc.
• Visual control in the process of mounting electronic components on printed circuit boards allows you to detect deviations from the specified parameters (for example, the occurrence of component displacement) and promptly make corrections

Solder Joint Quality Evaluation:

• The amount of solder in the area of the solder joint;
• Fillet/meniscus shape (compliance with technological standards);
• Condition of the surface of the leads (texture, uniformity, smoothness, color, gloss);
• Anomalies (for example, flux residues).

All signs are important for soldering quality control, but it is the state of the lead surface that provides the most information about the mechanical strength of the joint because it helps to conclude the conditions for the formation of an intermetallic diffusion layer during the soldering process.

The PCB quality control system is developed following the internal rules of the enterprise. All stages of the production process are subject to mandatory control.

Applied Quality Control Methods

Input quality control is required for the following elements used in the production of printed circuit boards:

• Components: for pre-prepared components, incoming inspection is desirable.
• Consumables: these are solder pastes, adhesive mixtures, and fluxes, which must be checked for compliance with the specifications for a particular printed circuit board.

Output quality control of printed circuit boards is carried out for finished products and is also necessary for printed circuit boards that are received for warranty repair. A prerequisite is the quality control of equipment at all stages of production.

Advantages of Modern Quality Control Methods

Comparison of quality control methods is inappropriate. It is recommended to combine them since each method has its own key advantages.

Optical Control of Solder Paste Application:

The optical quality control system for solder paste application, built into the production line, solves several problems:

• Reduction of possible defects in the application of solder paste;
• Prevention of defects of surface mounting;
• Reducing the time and cost of repairing the printed circuit board.

An important advantage of 3D optical PCB inspection is the accurate calculation of the volume and shape of the solder paste. This is a method of deep automatic control of products.

Automatic Control of Solder Joints

Automatic control of solder joints using optical equipment on the production line helps to prevent errors during installation and reduce the likelihood of rejects.

X-Ray Control

Equipment using X-rays allows you to detect hidden defects in printed circuit boards:

• Microcracks;
• Air bubbles;
• Unreliable contacts;
• Circuits inside the board;
• Soldering defects.

This is an informative inspection method that allows you to examine the printed circuit board without disassembly, damage, and decommissioning. X-ray control is characterized by high reliability; therefore it is used for all types of connections.

Visual Control

Thanks to powerful microscope-like lens technology, modern equipment does not create strain on the operator's eyes and shows a high efficiency of visual control.

Visual quality control of printed circuit boards has several areas of application:

• Assembly of printed circuit boards;
• Development of printed wiring drawing;
• Microchip assembly;
• PCB repair.

For surface mount PCB components, visual inspection is one of the most effective quality assessment methods. It allows you to fully assess the condition of the surface of the product and its main elements.

Author Bio:

PCB Trace Technologies is a one-stop PCB manufacturing service, where you can get everything from prototypes, PCB production and PCB assembly services all in one place.