Quality inspection of any design after manufacturing is important. With appropriate technology of quality inspection one can detect the flaws in the original design. In the quality inspection technique first few products are being examined on various parameter and then if found satisfactory one can go for mass production of the product.
Every electronic product nowadays is equipped with printed circuit boards. In last few decades designers have put their effort to make the boards thin with high conductivity and attach series of integrated circuit to perform the required task with precision. Flaws in defining parameters like size , power consumptions not only directly impact the performance but it can also ruin the entire process of manufacturing of printed circuit boards. Printed circuit boards are of two types one is rigid PCBs and another is flex PCB. The former is further classified in three types Single: single sided PCB, double sided PCB and multi-layer PCB. On quality basis there are class 1, class 2 and class 3 and amongst these three classes 3 is of high requirements. These three classes based on complexity and technology of the inspection method. After fabrication of printed circuit boards’ compatibility with the design requirements are checked with the inspection technique. Quality inspection is important to ensure the quality during operation of printed circuit boards. In order to perform the quality inspection of printed circuit board the key parameters and index are required to be identified and have to be in focus.
In spite of being different types of printed circuit boards, same quality checks are required to be performed. PCB Quality inspection process includes visual inspection, electric performance and general technological inspection.
All processes are discussed in detail here:
Visual Inspection:
With the help of basic instruments like ruler, gauge, vernier caliper and glass etc are used to perform the visual inspection. The inspection process is such:
- 1- Surface roughness, board thickness and warpage can be checked.
- 2- Appearance, assembly dimensions, electrical connectors are also checked.
- 3- Clearness of conductive patterns, and also the checks of short circuit and open circuit.
- 4- Surface quality like pin holes burr and pits are required to be inspected.
- 5- Coating quality is also part of visual inspection. Like plating of flux, its position and color requirements.
Electrical Inspection:
Two kind of tests are performed under this method.
1- It is known as connecting performance test. In this test conductive pattern’s conductivity is inspected using multimeter. Generally double-sided PCB and multi-layer PCBs are tested under this inspection method.
2- The second one is insulating performance test. Insulation performance of insulating resistor, which are on same plane or different plane, is being tested under this technique of quality inspection.
General technological Inspection:
This inspection method covers solderability and adhesion of plates. In the first part wetting performance of solder to conductive patterns is inspected. In the second process plates are used. Further, to inspect in greater details other procedures can be adopted according to requirements.
Metalized via Inspection:
This process is usually utilized for double sided PCBs and multi-layer PCBs. Most often electric breakdowns of module in printed circuit board or even failure of whole device is undesirable and it lies in the quality of metalized vias. Therefore, this inspection is paramount as far as quality of the board is concerned. This method covers the following:
- 1- Electric performance is checked with open circuit and short circuit methods by inspecting resistance between via and leads.
- 2- The change in resistance of vias should be in limit of 5% to 10%.
- 3- The capacity of pad to adhere with metalized via.
The procedure adopted in Metalized via inspection method involves both visual inspection and mechanical inspection. Visual inspection involves inspect the printed circuit board. For large scale production the inspection is carried out on-line by means of different type of probe tester.
Complicated structure of printed circuit boards especially in multi-layer printed circuit boards, the problems occurs with positioning of breakdown, it causes severe problems at the time of operation. Because of these problems the quality and reliability of printed circuit board is required to be maintained strictly. There are other parameters of inspection which are required to be involved in quality inspection for example resistance of conductor, resistance of metalized vias, short and open circuit of inner layers, insulating resistance between conducting lines, adhesion between elements, resistance to heat , mechanical properties like impact resistance, ductility and malleability, current intensity etc. Each of these parameters are required to be analyzed with professional equipments. A preferred tool for Board quality assurance inspection cameras allows the quality operator to view the printed circuit board with respect to its ergonomics. These images taken by camera are monitored over computer and allowed to be manipulated to get details related to microscopic view of the board.
Thickness and surface finish most of the industries are using x-ray technique to test the thickness and its purity. Because of impurity presence in the printed circuit board materials matrix hampers the mechanical properties of requirement. In some lower quality commercial grades automated optical inspection method is used. This method utilizes webcams in conjunction with open source computer to monitor to find out the defect present.
There are two quality standards the first is defects per unit, which counts defects of each board and another is defects per million opportunities, it is a normalizing factor that measures defects of large number of boards. With these two indexes manufacturer can identify the quality of product and process.
With so many techniques available for quality inspection of printed circuit boards it is imperative the quality assurance should make paramount from designing to the manufacturing and PCB assembly process. It boosts efficiency of the process which ultimately result saving of cost of production of printed circuit boards.
