How to Implement Quality Inspection of PCBs
PCB (printed circuit board) can be divided into rigid PCB and flexible PCB. The former can be divided into three types: single-sided PCB, double-sided PCB and multi-layer PCB. Depending on the quality level, the PCB can be divided into three quality levels: Class 1, Class 2 and Class 3, with Class 3 having the highest requirements. Differences in PCB quality levels lead to differences in complexity, testing and inspection methods. Rigid double-sided PCBs and multilayer PCBs have so far accounted for relatively large applications in electronic products, sometimes in some cases using flexible PCBs. Therefore, this article will discuss quality inspection issues with a focus on rigid double-sided and multi-layer PCBs. After the PCB is manufactured, a check must be made to determine if the quality is compatible with the design requirements. It can be considered that quality inspection is an important guarantee for product quality and a smooth implementation of subsequent procedures.
PCB testing standards mainly have the following aspects:
One. Standards developed by countries;
Military standards of the Gulf countries;
C. Industry standards such as SJ / T10309;
d. PCB inspection operation manual formulated by the equipment supplier;
That is, the technical requirements marked on the PCB design drawings.
For PCBs that have been identified as keyboards in the device, in addition to periodic inspections, these key characteristic parameters and metrics must be concentrated and checked from head to toe.
Regardless of the type of PCB, they must undergo similar quality inspection methods and projects. According to the inspection method, quality inspection items usually include visual inspection, general electrical performance inspection, general technical performance inspection and metallization inspection.
• Vision check
With the help of a ruler, vernier caliper or magnifying glass, visual inspection is easy. The inspection contents include:
One. Plate thickness, surface roughness and warpage.
Bay exterior and assembly dimensions, especially for assembly sizes compatible with electrical connectors and rails.
C. The integrity and clarity of the conductive pattern and the presence of bridging shorts, open circuits, burrs or voids.
d. The presence of printed marks or surface quality on the mat, pits, scratches or pinholes.
That is, the location of the pad vias and other vias. The via should be inspected for missing or inaccurate punching, whether the through hole diameter meets the design requirements, and whether there are nodules and voids.
F. Pad quality and robustness, roughness, brightness and voids in raised defects.
G. Coating quality. Whether the plating flux is even and firm, the position is correct, the flux is uniform, and the color meets the requirements.
H. Personality qualities, such as whether they are firm, clean and clean, without scratches, penetration or disconnection.
• General electrical performance check
There are two tests under this check:
One. Connection performance test. In this test, the multimeter is usually used to check the connectivity of the conductive patterns, focusing on the metallized vias of the double-sided PCB and the connection performance of the multilayer PCB. For this test, PCBCart provides a general check before each manufactured PCB leaves its warehouse to ensure the implementation of its basic functions.
b. Insulation performance test. This test is designed to check the insulation resistance between the same plane or different planes to ensure the insulation properties of the PCB.
• General technical inspection
General technical inspections include solderability and plating adhesion checks. For the former, the solder is tested for the wetting properties of the conductive pattern. For the latter, inspection can be carried out by means of qualified tips which are first glued onto the plating plane to be inspected and then quickly pulled out after a uniform press. Next, the plating plane should be observed to ensure that shedding occurs. In addition, some inspections can be selected according to the actual situation, such as the anti-drop strength of copper foil and the tensile strength of metallization.
• Metalized via inspection
The quality of metallized vias is extremely important for double-sided PCBs and multilayer PCBs. The cause of a large number of failures in electrical modules or even entire equipment is due to the quality problems of metallized vias. Therefore, it is very necessary to pay more attention to the inspection of metallized vias. Metallization inspections include the following:
One. The metal plane of the through-hole wall should be complete, smooth, free of voids or nodules.
Bay electrical performance checks should be based on shorts and opens of the pads and electrical resistance through the plating plane, the resistance between the vias and the leads.
C. After the environmental test, the resistance change rate of the via should not exceed 5% to 10%.
d. Mechanical strength refers to the bond strength between the metallized through hole and the pad.
That is, the metallographic analysis test is responsible for checking the plating plane quality, the thickness and uniformity of the plating plane, and the bonding strength between the plating plane and the copper foil.
Metallization by inspection usually incorporates visual inspection and mechanical inspection. The visual inspection is to observe that the PCB is placed under light, and the complete and smooth through-hole walls are able to reflect light evenly. However, walls that contain nodules or voids are not as bright. For mass production, inspections should be performed by online testing equipment such as flying probe testers.
Due to the complicated structure of the multilayer PCB, it is difficult to quickly locate the fault once there is a problem in the subsequent unit module assembly test. Therefore, the quality and reliability of inspection must be very strict. In addition to the above-mentioned conventional inspection items, other inspection items include the following parameters: conductor resistance, metallized through-hole resistance, inner layer short circuit and open circuit, insulation resistance between wires, plating plane bonding strength, adhesion, thermal shock resistance, impact resistance Mechanical shock, current intensity, etc. Each indicator must be obtained through the application of specialized equipment and methods.