PCB board deformation harm
On the automatic surface mounting line, if the circuit board is uneven, it will cause inaccurate positioning, and the components cannot be inserted or affixed to the holes of the board and the surface mounting pad, or even crash the automatic inserting machine.
The circuit board on which the components are installed is bent after welding, and it is difficult to cut the feet of the components neatly. The board can not be installed into the chassis or machine socket, so the assembly plant encountered the board is also very troublesome.
At present, the surface mounting technology is developing towards the direction of high precision, high speed and intelligence, which puts forward higher requirements for the smoothness of PCB board as the carrier of various components.
It is specified in the IPC standard that the allowable deformation of PCB board with surface mount is 0.75%, and the allowable deformation of PCB board without surface mount is 1.5%.
In fact, in order to meet the needs of high precision and high speed mounting, some electronic assembly manufacturers have stricter requirements on the amount of deformation. If there are requirements, the allowable amount of deformation is 0.5%, and even some individual requirements are 0.3%.
PCB board is composed of copper foil, resin, glass cloth and other materials, the physical and chemical properties of each material are different, after pressing together will inevitably produce thermal stress residual, resulting in deformation.
At the same time, in the process of PCB processing, it will go through various processes such as high temperature, mechanical cutting and wet treatment, which will also have an important impact on the deformation of the board. In short, the causes of PCB board deformation are complex and diverse. How to reduce or eliminate the deformation caused by different material characteristics or processing has become one of the complex problems faced by PCB manufacturers.
Cause analysis of PCB board deformation
The deformation of PCB board needs to be studied from several aspects, such as material, structure, graphic distribution, processing process, etc. This paper will analyze and elaborate various causes of deformation and improvement methods.
The copper surface area on the circuit board is uneven, which will worsen the bending and warping of the board.
Generally, a large area of copper foil will be designed on the PCB circuit board for grounding. Sometimes, Vcc layer will also be designed with a large area of copper foil. When these large areas of copper foil are not evenly distributed on the same circuit board, it will cause the problem of uneven heat absorption and heat dissipation speed.
Of course, the circuit board will also expand and contract with heat and cold. If the expansion and contraction cannot be achieved at the same time, it will cause deformation due to different stresses. At this time, if the temperature of the board has reached the upper limit of Tg value, the board will begin to soften, resulting in deformation.
Connecting points (vias) between the layers of the board limit board growth and shrinkage.
Most of today's circuit boards are multilayer boards, and there will be a connection point (vias) like rivets between layers, and the connecting point is divided into through holes, blind holes and buried holes, where the connecting point will limit the effect of the board, but also indirectly cause the bending and bending of the board.
Reasons for PCB board deformation:
(1) The weight of the circuit board itself will cause the board to sag
Generally, the back welding furnace will use a chain to drive the circuit board forward in the back welding furnace, that is, to both sides of the board when the fulcrum to hold up the whole board.
If there are too heavy parts on the board, or the size of the board is too large, it will show a depression in the middle because of its weight, resulting in the bending of the board.
(2) The depth of V-Cut and the connection strip will affect the deformation of the plate
Basically, V-Cut is the culprit of damaging the structure of the board, because V-Cut is in the original a large sheet of the board to cut out grooves, so the V-Cut place is prone to deformation.
Influence of pressing material, structure and figure on plate deformation:printed circuit boardis made of core board, semi-cured sheet and outer copper foil pressed together. The core board and copper foil are deformed by heat when pressed together. The deformation depends on the coefficient of thermal expansion (CTE) of the two materials.
The thermal expansion coefficient (CTE) of copper foil is about 17X10-6. The z-direction CTE of common FR-4 substrate is (50~70) X10-6 at Tg point. Above TG point is (250~350) X10-6, and x-direction CTE is generally similar to copper foil due to the existence of glass cloth.