Causes and hazards of PCB Assembly distortion

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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, many factors affect the deformation of PCB Assembly board.

Causes and hazards of PCB Assembly distortion

Possible Harm of PCB Assembly Deformation

During the surface mounting process of PCB automatic SMT, if the PCB board is not flat, it can result in incorrect positioning, preventing components from being mounted to the holes and surface mounting pads. In some cases, the auto-plugger may also get damaged.

When a PCB assembly with components is bent after welding, it becomes difficult to cut the component foot flat and even, making it challenging to mount the PCB assembly on the chassis or the socket in the machine. This can cause significant trouble for PCB assembly factories. As SMT mounting technology progresses towards higher precision, speed, and intelligence, the smoothness of the circuit board assembly becomes more critical. The IPC standard specifies that the allowable distortion for PCB assembly with surface-mounted devices is 0.75%, and for PCB boards without surface mounting, it is 1.5%.

To meet the requirements of high-precision and high-speed SMT mounting, some electronic PCB assembly manufacturers have stricter distortion requirements, such as 0.5% allowable distortion or even 0.3% in some cases.

PCB boards are composed of copper foil, resin, glass cloth, and other materials, each with different physical and chemical properties. When these materials are pressed together, residual thermal stress can result in distortion and warping of PCB boards.

The PCB manufacturing process involves various stages such as high temperature, mechanical cutting, and wet treatment, all of which can significantly impact the distortion of PCB components. Consequently, reducing or eliminating distortion caused by different material characteristics or PCB assembly manufacturing has become a complex problem for PCB assembly manufacturers.

The study of PCB assembly deformation needs to consider various factors including material, structure, graphic distribution, and the manufacturing process. Uneven copper paving on the PCB circuit board can exacerbate bending and warping.

Large areas of copper foil on PCB circuit boards are often designed for grounding or Vcc layer purposes. Uneven distribution of these large areas of copper foil on the same PCB circuit board can cause uneven heat absorption and dissipation, leading to expansion and shrinkage. If the expansion and shrinkage do not occur simultaneously, PCB assembly deformation may result. When the temperature of the board reaches the upper limit of the Tg value, the PCB board will soften, leading to distortion. The connection points (vias) of the layers on the PCB board can also limit expansion and shrinkage.

Modern PCB boards are mostly multilayered, with rivet-like connection points (vias) between the layers, including through holes, blind holes, and buried holes. These connection points can limit the expansion and cooling effect of the PCB assembly, indirectly causing warping.

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