Why are There so Many Hazards When PCBs are Deformed?
The Hazards of PCB Deformation
In the automated surface mount line, circuit boards that are not flat will cause inaccurate positioning, components can not be inserted or affixed to the holes of the board and surface mount pads, and even crash the automatic insertion machine.
Components mounted on the circuit board after welding bending, component feet are difficult to cut flat and neat. The board can not be installed into the chassis or machine socket, so the assembly plant encountered board warping is also very annoying.
The current surface mount technology is moving towards high precision, high speed, intelligent direction, which is as a home for a variety of components PCB board put forward a higher level of requirements.
In fact, in order to meet the needs of high precision and high speed mounting, some electronic assembly manufacturers on the deformation of the more stringent requirements, such as the requirements of the permissible amount of deformation of 0.5%, and even some individual requirements of 0.3%.
PCB board by the copper foil, resin, glass cloth and other materials, the physical and chemical properties of the materials are not the same, pressed together will inevitably produce thermal stress residual, resulting in deformation.
At the same time in the PCB processing, will go through a variety of processes such as high temperature, mechanical cutting, wet processing, will also have an important impact on the deformation of the board, in short, can lead to PCB board deformation of the reasons are complex and diverse, how to reduce or eliminate due to the material characteristics of the different or processing-induced deformation, become one of the complex problems faced by PCB manufacturers.
PCB Board Deformation Causes Analysis
PCB board deformation needs to be studied from the material, structure, graphic distribution, processing and other aspects of the process, the article will be possible to produce deformation of the various causes and improvement methods to analyze and elaborate.
Uneven copper surface area on the circuit board will deteriorate board bending and board warping.
Generally, circuit boards are designed with a large area of copper foil for grounding, and sometimes the Vcc layer is also designed with a large area of copper foil. When these large areas of copper foil are not evenly distributed on the same board, it will cause uneven heat absorption and dissipation.
Of course, the board will also be thermal expansion and contraction, if the expansion and contraction can not be at the same time will result in different stress and deformation, the board's temperature if the board has reached the upper limit of the Tg value, the board will begin to soften, resulting in deformation.
The connection points of each layer on the board will limit the board's expansion and contraction.
Most of today's circuit boards are multilayer boards, and there will be like rivets between the layers of the connection point (vias), the connection point is divided into through holes, blind holes and buried holes, there are connection points will limit the board up the effect of cold shrinkage, but also indirectly caused by the board bending and board warping.
PCB Board Deformation Causes:
A. The Weight of The Circuit Board Itself Will Cause the Board to Sink and Deform
General soldering furnace will use the chain to drive the circuit board in the soldering furnace in the forward, that is, to the board on both sides as a fulcrum to support the entire board.
If there are too heavy parts on top of the board, or the board size is too large, the board will be bent due to the weight of the board itself and show the phenomenon of depression in the center.
B. The Depth of The V-Cut and The Connecting Strip Will Affect The Amount of Deformation of The Board.
Basically, the V-Cut is the culprit that destroys the structure of the board, because the V-Cut is to cut out grooves on the original sheet, so the V-Cut is prone to deformation.
Influence of press-fit material, structure and graphics on board deformation:
PCB board from the core board and semi-cured sheet and the outer layer of copper foil pressed together, of which the core board and copper foil in the pressing of heat deformation, the amount of deformation depends on the coefficient of thermal expansion (CTE) of the two materials.
The coefficient of thermal expansion (CTE) of copper foil is about 17X10-6; while the Z-direction CTE of ordinary FR-4 substrate is (50~70)X10-6 under Tg point; above TG point is (250~350)X10-6, and the X-direction CTE is generally similar to that of copper foil due to the existence of glass cloth.
Deformation Caused By PCB Processing.
PCB board processing process deformation causes are very complex can be divided into thermal stress and mechanical stress caused by two kinds of stress.
Thermal stress is mainly generated in the press process, mechanical stress is mainly generated in the board stacking, handling, baking process. The following is a brief discussion of the process order.
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