Printed circuit board (PCB) 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. According to the quality grade, PCB can be divided into three quality grades: 1, 2 and 3, of which 3 is the highest requirement. Differences in PCB quality levels lead to complexity and differences in testing and inspection methods.

To date, rigid double-sided and multi-layer PCBS have occupied a relatively large range of applications in electronics, with flexible PCBS sometimes used in some cases. Therefore, this paper will focus on the quality inspection of rigid double-sided and multi-layer PCB. After PCB manufacturing, inspection must be carried out to determine whether the quality meets the design requirements. It can be said that quality inspection is an important guarantee to ensure the quality of products and the smooth implementation of subsequent procedures.

Inspection standard

PCB inspection standards mainly include the following aspects:

A. Standards set by each country;

B. Military standards for each country;

C. Industrial standards, such as SJ/T10309;

D. PCB inspection instructions formulated by the equipment supplier;

E. Technical requirements marked on PCB design drawing.

For PCBS that have been identified as critical to the equipment, these critical characteristic parameters and indicators must be examined from head to toe in addition to regular inspection.

Inspection items

Regardless of the type of PCB, they must go through similar quality inspection methods and programs. According to the inspection method, the quality inspection items usually include appearance inspection, general electrical performance inspection, general technical performance inspection and metal coating inspection.

• Appearance inspection

Visual inspection is easy with the aid of a ruler, vernier caliper, or magnifying glass. The items checked include:

A. Thickness, surface roughness and warpage of the plate.

B. Appearance and assembly dimensions, especially assembly dimensions compatible with electrical connectors and guide rails.

C. Integrity and clarity of the conductive pattern, and whether there are bridge short circuits, open circuits, burrs or gaps.

D. Surface quality, whether there are pits, scratches or pinholes on printed wires or pads.

E. Location of pad holes and other holes. Check whether the through holes are missing or drilled incorrectly, whether the diameter of the through holes meets the design requirements and whether there are nodules and gaps.

F. Quality and firmness of pad coating, roughness, brightness and voidage of raised defects.

G. Coating quality. Electroplating flux is uniform, firm, position is correct, flux is uniform, its color is in line with relevant requirements.

H. Character quality, such as whether they are firm, clear and clean, without scratches, punctures or breaks.

• Routine electrical performance inspection

There are two types of tests under this type of check:

A. Connection performance test. During this test, a multimeter is usually used to check the connectivity of the conductive pattern, with emphasis on the metallized perforations of double-sided PCBS and the connectivity of multi-layer PCBS. For this test, the PCB manufacturer will provide a routine inspection of each prefabricated PCB before it leaves the warehouse to ensure that its basic functions are fulfilled.

B. Insulation performance test. This test is designed to check the insulation resistance on the same plane or between different planes to ensure the insulation performance of the PCB.

• General technical inspection

General technical inspection covers solderability and coating adhesion inspection. For the former, check the wettability of the solder to the conductive pattern. For the latter, inspection can be carried out by qualified tips that are first glued to the plating surface to be examined and can then be quickly removed even after pressing. Next, the plating plane should be observed to ensure that peeling occurs. In addition, some inspection methods can be selected according to the actual situation, such as the fall strength of copper foil and metallization through tensile strength.

• Metallization through inspection

The quality of metallized through holes is very important for double-sided PCB and multi-layer PCB. Many failures of electronic modules and even the whole equipment are due to the quality of the metallized holes. Therefore, it is necessary to pay more attention to the inspection of metallized through holes. A. The metal plane of the through hole wall shall be complete, smooth and free of cavities or small nodules by checking metallization covering the following aspects.

B. Electrical properties should be checked according to the short and open circuit of pad and metallized through hole coating, and the resistance between the through hole and the lead.

C. After environmental testing, the resistance change rate of the through-hole should not exceed 5% to 10%.

D. Mechanical strength refers to the bonding strength between the metallized hole and pad.

E. Metallographic analysis tests check coating quality, coating thickness and uniformity, and adhesion strength between coating and copper foil.

Metallization through inspection is usually a combination of visual inspection and mechanical inspection. Visual inspection involves exposing the PCB to light and seeing if the intact, smooth through-hole wall reflects light evenly. However, walls containing nodules or voids will not be too bright. For volume production, an in-line inspection device (e.g., a flying needle tester) should be used.

Due to the complex structure of multi-layer PCBS, it is difficult to locate faults quickly once problems are found during subsequent unit module assembly tests. As a result, inspections of its quality and reliability must be very rigorous. In addition to the above routine inspection items, other inspection items also include the following parameters: conductor resistance, metallization through hole resistance, inner short circuit and open circuit, insulation resistance between the lines, coating adhesion strength, adhesion, thermal impact resistance, mechanical impact impact strength, current strength, etc. Each indicator must be obtained through the use of specialized equipment and methods.