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. PCBS can be divided into three quality grades based on quality grade: Class 1, Class 2, and Class 3, with 3 of these having the highest requirements. Differences in PCB quality levels lead to differences in complexity and testing and inspection methods. So far, rigid double-sided and multi-layer PCBS account for a relatively large number of applications in electronic products, and sometimes flexible PCBS are used in certain situations. Therefore, this paper will focus on the quality inspection of rigid double-sided and multi-layer PCBs. After the PCB is manufactured, it must be inspected to determine whether the quality is compatible with the design requirements. It can be considered that quality inspection is an important guarantee of product quality 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 standard such as SJ/T10309;

D. PCB inspection instructions formulated by equipment supplier;

E. Technical requirements marked on PCB design drawings.

For PCBS that have been identified as keyboards in equipment, these key characteristic parameters and indicators must be centralized and checked from the head, in addition to regular inspection. To the toes.

Inspection items

Regardless of the type of PCB, they must undergo similar quality inspection methods and items. According to the inspection method, quality inspection items usually include visual inspection, general electrical performance inspection, general technical performance inspection, and metalization inspection.

• Visual inspection

Visual inspection is simple with the help of a ruler, vernier caliper, or magnifying glass. The inspection includes:

A. plate thickness, surface roughness, and warpage.

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

C. Integrity and clarity of conductive patterns and the presence of bridging short, open burr, or void.

D. Surface quality, presence of pits, scratches, or pinholes on a printed trace or pad. Location of pad holes and other holes. Holes should be checked for missing or incorrect punching, hole diameter meets design requirements, and nodules and voids.

F. Pad quality and firmness, roughness, brightness, and clearance of raised defects.

G. Coating quality. The coating flux is uniform and firm, the position is correct, the flux is uniform, and the color meets the requirements.

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

• General electrical performance inspection

There are two tests under this type of examination:

A. Connection performance test. In this test, a multimeter is typically used to check the connectivity of conductive patterns through the focused metalized through holes of double-sided PCBS and the connectivity of multi-layer PCBS. For this test, the PCBCart provides general checks on each manufactured PCB before it leaves its warehouse to ensure that its basic functions are fulfilled.

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

• General technical inspection

General technical inspection includes weldability and electroplating adhesion inspection. For the former, check the wettability of the solder to the conductive pattern. For the latter, it can be checked by qualified tips that are first glued to the plating surface to be examined and then quickly pulled out after being pressed evenly. Next, the plating plane should be observed to ensure that peeling occurs. In addition, some checks can be selected according to the actual situation, such as copper foil anti-fall strength and metalized anti-tensile strength.

• Metallization through inspection

The quality of metalized holes plays an important role in double-sided PCB and multi-layer PCB. A large number of failures of electrical modules and even the whole equipment are due to the quality of the metalized holes. Therefore, it is necessary to pay more attention to the inspection of metalized holes. Metallization inspection includes the following aspects:

A. The metal plane of the through-hole wall should be complete and smooth, with no void or nodule.

B. The electrical properties shall be checked according to the short and open circuit of the pad and the resistance between the through-hole and the lead through the metallization of the plating plane. After environmental testing, the resistance change rate of the through-hole should not exceed 5% to 10%. Mechanical strength refers to the bonding strength between the metalized through-hole and pad. Metallographic analysis tests are responsible for checking the quality of the plating surface, the thickness and uniformity of the plating surface, and the bonding strength between the plating surface and the copper foil.

Metallization inspection is usually combined with visual inspection and mechanical inspection. Visual inspection is to observe that the PCB is placed under light and that the complete smooth through-hole wall reflects light evenly. However, passing through walls containing nodules or voids will not be as bright. For mass production, inspection should be carried out by online testing equipment such as a flying needle tester.

Due to the complex structure of multi-layer PCB, it is difficult to locate faults quickly once problems occur in subsequent unit module assembly tests. Therefore, checking its quality and reliability must be very strict. In addition to the above routine inspection items, other inspection items include the following parameters: conductor resistance, metalized through-hole resistance, inner short circuit, and open circuit, insulation resistance between wires, electroplating plane bonding strength, adhesion, thermal shock resistance, impact resistance, mechanical impact, current strength, etc. Each indicator must be obtained through the application of specialized equipment and methods.