Causes and improvement of Rigid PCB deformation
Circuit board after Reflow soldering most prone to plate bending, serious words will even cause the components of air welding, monument and so on, how to overcome it?
1, Rigid PCB board deformation damage
On the automatic surface Mount line, if the circuit board is not flat, will cause positioning is not allowed, components can not be inserted or affixed to the board of the hole and surface mount solder plate, or even break the automatic insert machine. The circuit board with the components is bent after welding, and it is difficult to cut and flatten the component feet. The board can not be fitted to 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 of development, which is made for a variety of components home Rigid PCB board put forward a higher flatness requirements.
In the IPC standard, it is pointed out that the maximum allowable deformation of Rigid PCB board with Surface mount device is 0.75%, and the maximum deformation capacity of Rigid PCB board without surface mount is 1.5%. In fact, in order to meet the high precision and high speed mounting requirements, some of the electronic equipment manufacturers to the deformation of the requirements more stringent, such as my company has multiple customer requirements to allow the maximum distortion of 0.5%, or even individual customer requirements 0.3%.
Rigid PCB board consists of copper foil, resin, glass cloth and other materials, the physical and chemical properties of the materials are not the same, pressure together will inevitably produce thermal stress residue, resulting in deformation. At the same time in the Rigid PCB processing process, will be high temperature, mechanical cutting, wet treatment and other processes, will also have an important impact on the deformation of the plate, in short, can lead to the Rigid PCB deformation of the reasons for complex and diverse, how to reduce or eliminate the material characteristics of different or processing caused by deformation, Rigid PCB manufacturers are facing one of the most complex problems
2. Cause analysis of deformation
The deformation of Rigid PCB board needs to be studied from material, structure, graphic distribution, processing process and so on, this paper will analyze and expound various reasons and improvement methods which may produce deformation.
The copper surface area on the circuit board is uneven, which will worsen the plate bending and the plate warping.
General circuit board will be designed with a large area of copper foil to be used as grounding, sometimes the VCC layer will have a large area of copper foil design, when these large area of copper foil can not be evenly distributed on the same circuit board, it will cause heat absorption and cooling speed uneven problem, the circuit board of course will also expand and contraction, If the shrinkage can not be caused by different stress and deformation, at this time, if the board temperature has reached the upper limit of the TG value, the board will begin to soften, resulting in permanent deformation.
The link point (vias, over hole) of each layer on the circuit board will limit the board to shrink
Today's circuit boards are mostly multilayer boards, and the layer and the layer will have the same connection point to the Rivet (vias), the link point is divided into through hole, blind hole and buried hole, where there is a link point will limit the effect of the board to shrink the cold, will also indirectly lead to plate bending and warping.
The weight of the Rigid PCB itself will cause the deformation of the board dent.
The general welding furnace will use the chain to drive the circuit board in the back of the welding furnace forward, that is, both sides of the board to prop up the entire board, if the board above the heavy parts, or the size of the board is too large, it will be due to the amount of the middle concave phenomenon, resulting in plate bending.