Modern PCBA electronic assembly is a relatively complex process. In the process of production, PCB defects are caused by factors of PCB substrate and PCB board processing. Next, we will introduce some common PCB defects and their causes.
In the PCB manufacturing process, common PCB defects are: white spots, micro-cracks, blistering, delamination, wet weave, exposed fabric, halo and solder mask defects.
1. Vitiligo
At the intersection of glass fibers on the surface of the sheet, the resin and fibers are separated, and white spots or "cross patterns" appear under the surface of the substrate.
a. The impact of improper mechanical external force on the plate will cause partial separation of resin and glass fiber to form white spots.
b. The weave point of the glass fiber cloth is eroded by the penetration of fluorine-containing chemicals in the local pull material, forming regular white spots (squares can be seen when it is more serious).
c. Improper thermal stress on the plate will also cause white spots and white spots.
2. Micro cracks
Microcracks are defined as continuous white spots or "cross streaks" that occur within the laminate substrate.
Reason: Mainly affected by mechanical stress, microcracks are generated inside the laminated substrate.
3. Foaming
The imagination of localized separations caused by local expansion between layers of substrates or between substrates and conductive foils, between substrates and protective coatings.
a. Board surface pollution (oxidation, oil stains, glue stains, other alkaline pollution)
b. The post-curing time is insufficient, most of which are manifested as bubbling and oil loss on both sides of a corner, which is found after spraying tin.
c. The tin removal is not clean. There is a thin layer of tin on the board surface. After the tin is sprayed, the tin on the board surface will melt at high temperature and the ink will be lifted up to form a bubble.
d. The ink is printed before the water vapor in the hole is dried. After spraying tin, circle-shaped bubbles will be formed around the hole where the water is hidden.
e. During the welding process of the PCB board, there is water vapor in the board, and it is easy to imagine that bubbles are prone to occur during reflow soldering.
4. Layering
Visualization of separation between substrate layers, insulating substrate and conductive or any layer within a multilayer board.
a. Lamination parameters are not set according to the specifications
b. The cleaning is not in place, and there are sundries on the board surface, resulting in delamination after welding.
5. Wet weaving
The braided fibers that are completely covered with resin and are not broken in the base material have a braided pattern on the surface.
6, exposed fabric
The phenomenon of braid fibers that are not completely covered by resin or are not broken on the surface of the substrate.
7. Halo
Destruction or delamination on or under the surface of the substrate is usually manifested in whitish areas around holes or around other machined parts.
a. The machine or the bakelite is not flat, and there is a gap between the board and the bakelite.
b. The board is warped and deformed, and there is a gap between the boards
c, milling cutter wear
d. The inspector does not know that the second diamond halo standard is missed.
8. Solder mask defects
Solder mask is a heat-resistant coating material, and solder mask defects can easily lead to solder deposition in non-soldering areas during PCB soldering.
a. Excessive spacing or air gap around pad features
b. After solder mask printing, the baking time and temperature are not enough, resulting in incomplete curing of solder mask.
These PCB defects are an important factor affecting the defect rate of electronic products. By understanding these defects and their causes, the process can be improved in the electronic assembly process and the yield of products can be improved.
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