PCB News - Substrate surface defects of PCB quality control

See the real chapter below the surface

Earlier, we invited the experts from the PCB quality control department of Benqiang Circuits to explain how to identify some defects on the PCB surface.

Of course, when doing PCB quality control work, you can't "see the leopard", just look at the surface, not what it is. Only by taking both the inside and the outside into consideration, can we not give the "magic sprites" space to breed and improve the quality and yield of products.

So what are the defects under the surface of the PCB substrate?

White spots, microcracks, delamination, blistering and foreign inclusions are common defects in the lower part of the substrate surface. These defects are usually found in the process of PCB manufacturing or the quality inspection of the circuit board. E.g:

•In the incoming material inspection link;

• During the production process of the "inner layer" pattern of the multilayer printed circuit board after the metal foil is etched by the PCB manufacturer;

• In order to form the required conductive patterns and marks, after etching the "outer" layer of the printed board;

• After drying operation (such as solder mask or component characters);

• After thermal shock, during solder hot melt/coating or solderability testing.

1. Vitiligo

White spots are an inherent phenomenon that occurs in the woven fiber-reinforced laminated substrate, where the fiber yarn bundles in the substrate are separated at the intersection. It appears as discontinuous white squares or "cross" patterns under the surface of the substrate, and its formation is usually related to thermal stress.

White spots inside the PCB substrate

White spots under the surface of the PCB substrate

PCB substrate without white spots

Note: White spots are an internal phenomenon in PCB laminates and will not be expanded by the subsequent thermal testing of multiple assembly processes. At the same time, there is no clear conclusion that it is an inducement for the growth of anode conductive filaments (CAF). Aerospace PCB boards have strict requirements for white spots.

Note: White spots can be observed from the surface.

2. Micro cracks

Microcrack: An internal condition in which the fibers in the PCB laminate substrate are separated. Microcracks can appear in fiber interweaving or along the length of fiber filaments. The microcrack condition is manifested as white dots or "cross patterns" connected under the surface of the substrate, which is usually related to mechanical stress.

When the cross patterns are connected to each other, the micro-crack condition is evaluated as follows:

Microcrack

Note: The affected area is determined by dividing the total area of each defect by the total area of the printed board. To determine the percentage of the affected area on each side separately.

4. Foreign inclusions

Foreign inclusions: Refers to metallic or non-metallic particles entrapped or buried in insulating materials.

Foreign inclusions can be detected in substrate raw materials, prepreg materials (B-stage), or manufactured multilayer printed boards. The foreign object may be a conductor or a non-conductor. In both cases, it is determined whether it is not qualified according to its size and location.

PCB under ideal conditions

The above is an introduction to the substrate surface defects of PCB quality control. Ipcb also provides PCB manufacturers and PCB manufacturing technology.