PCBA Tech - Causes and hazards of PCB Assembly distortion

Causes of PCB Assembly Deformation

Copper clad sheet from PCB manufacturer is deformed: In general, the clad sheets from PCB manufacturer are double-panel, symmetrical in structure, no graphics. The difference between CTE of copper foil and glass cloth is not very big, so PCB manufacturer operation will hardly produce PCB sheet deformed due to the difference of CTE. However, because of the large size of the copper clad laminate press and the temperature difference in different areas of the hot plate, the curing speed and degree of the resin in different areas will be slightly different during the bonding process. At the same time, the dynamic viscosity under different heating rates will also be quite different, so the local stress due to the difference in the curing process will also be generated. Usually this stress will maintain balance after PCB board is pressed together, but it will gradually release during PCB subsequent PCBA manufacturing, resulting in PCB board distortion.

PCB board bonding in PCB manufacturer: PCB board bonding process is the main process that produces thermal stress. It is similar to copper clad plate bonding, but also produces local stress caused by curing process differences. PCB board bonding process thermal stress will be more difficult to eliminate than copper clad plate due to thicker thickness, diverse graphics distribution, more semi-cured sheets and other factors. The stress in the PCB board is released in the following processes, such as drilling holes, shape or baking PCB, which also easily leads to the distortion of the PCB board.

PCB circuit boards are deformed by resistance welding and character baking process: Because the resistance ink cannot be stacked with each other when curing, the PCB board will be cured vertically in the rack. The resistance temperature is about 150 C, just above the Tg point of medium and low Tg material, the resin above the Tg point is high elastic, and the PCB board is easy to deform under the action of self-weight or strong wind in the oven.

Hot-air solder leveling distortion: ordinary PCB board hot-air solder leveling tin furnace temperature is 225 ~265 C, time is 3S-6S, hot-air temperature is 280 ~300 C, solder leveling PCB board from room temperature into tin furnace, after two minutes after the furnace out, after treatment and washing at room temperature, the whole hot-air solder leveling process is a process of sudden heat and sudden cooling of PCB board. Due to the uneven structure of PCB printed circuit board materials, thermal stress will inevitably occur during the cooling and heating process, resulting in micro-strain and overall distortion of PCB board.

Storage Deformation: PCB Assembly board from PCBA board manufacturer is generally stuck in shelves during the semi-finished stage. The improper tightness adjustment of shelves or stacking of PCB Assembly boards during storage can cause mechanical deformations of PCB Assembly boards, especially for PCB Assembly board under 2.0 mm. In addition to the above factors, there are many factors that affect the deformation of PCB Assembly board.

Harm of PCB Assembly Deformation

On the surface mounting line of PCB automatic SMT, if the PCB board is not flat, it will lead to improper positioning, the PCB Assembly components cannot be mounted or mounted to the holes of the PCB Assembly board and the PCB Assembly surface mounting pad, or even the PCB Assembly auto-plugger will be damaged.

PCB Assembly with components is bent after welding, the foot of components on PCB Assembly is difficult to cut flat and even, and PCB Assembly cannot be mounted on the chassis or the socket in the machine. Therefore, PCB Assembly factory is also very troubled when it encounters PCBA board warping. Now SMT mounting technology is developing towards high precision, high speed and intelligence. This requires higher smoothness for the circuit board Assembly as a variety of components. The IPC standard specifically indicates that the allowable distortion of PCB Assembly with surface mounted devices is 0.75% and that of PCB boards without surface mounting is 1.5%.

In fact, in order to meet the requirements of high-precision and high-speed SMT mounting, some electronic mounting PCB Assembly manufacturer have more stringent requirements on the amount of distortion, such as 0.5% allowable and even 0.3% individual requirements.

PCB boards are made of copper foil, resin, glass cloth and other materials. The physical and chemical properties of each material are different. When pressed together, thermal stress will be residual, resulting in distortion and warping of PCB boards.

At the same time, PCB circuit board manufacturing process, will go through a variety of processes such as high temperature, mechanical cutting, wet treatment, and will also have an important impact on PCB circuit board components distortion. In a word, the causes of PCB circuit board distortion can be complex and diverse, how to reduce or eliminate the distortion caused by different material characteristics or PCB Assembly manufacturing. It has become one of the complex problems faced by PCB Assembly manufacturers.

The deformation of PCB Assembly needs to be studied from the aspects of material, structure, graphic distribution, PCBA manufacturing process, etc. The uneven area of copper paving on PCB circuit board will deteriorate the bending and warping of PCB board.

In general, a large area of copper foil is designed on PCB circuit board for grounding purposes. Sometimes there is also a large area of copper foil in Vcc layer. When these large areas of copper foil are not evenly distributed on the same PCB circuit board, it will cause uneven heat absorption and heat dissipation. PCB printed circuit boards will certainly expand and shrink. If the expansion and shrinkage cannot cause different stress at the same time, the PCB Assembly will deform. At this time, if the temperature of the board has reached the upper limit of Tg value, the PCB board will start to soften, resulting in the PCB board distortion. The connection points (vias) of the layers on the PCB board will limit the expansion and shrinkage of the PCB board.

Today's PCB board is mostly multilayer PCB board, and there will be rivet-like connection points (vias) between the layers, which are divided into through holes, blind holes and buried holes. Where there are connections, the expansion and cooling effect of PCB Assembly will be limited, and PCB board will be warped indirectly when PCB Assembly is used.