1. The dangers 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 board’s holes and surface mount pads, or even crash the automatic insertion machine. Components mounted on the circuit board after welding bending, component feet are difficult to cut flat and neat. Boards 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 done as a home for a variety of components PCB put forward a higher level of requirements. In the IPC standard specifically pointed out with surface mount devices PCB board allows the maximum amount of deformation for 0.75 per cent, no surface mount PCB board allows the maximum amount of deformation for 1.5 per cent. PCB electronics
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 processing of PCB, will go through a variety of processes such as high temperature, mechanical cutting, wet processing, etc., will also have an important impact on the deformation of the board, in short, can lead to PCB deformation of the reasons for the complexity and variety of how to reduce or eliminate due to the different characteristics of the material or processing-induced deformation, become one of the most complex problems faced by the PCB manufacturer.
2. PCB board deformation cause analysis
(1) Uneven copper surface area on the board will worsen the board bending and board warping.
General circuit boards are designed with a large area of copper foil to be used as a grounding, sometimes Vcc layer will also be designed with a large area of copper foil, when these large areas of copper foil can not be evenly distributed on the same circuit board, it will result in the absorption of heat and heat dissipation rate is not uniform, of course, the circuit board will also be thermal expansion and contraction, if the expansion and contraction can not be caused at the same time will result in a different stress and deformation, the board’s temperature if it has reached the upper limit of the Tg value, the board will begin to soften, resulting in board bending and board warping. If the temperature has reached the upper limit of the Tg value, the board will begin to soften, resulting in permanent deformation.
(2) The connection points (vias, vias) of each layer on the board will limit the board’s expansion and contraction.
Most of today’s circuit boards are multilayer boards, and between layers and layers will be riveted to the same 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 will indirectly result in the board bending and board warping.
(3) The weight of the 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 the board above the overweight parts, or board size is too large, because of its own species of the amount of the phenomenon presented in the middle of the depression, resulting in the board bending.
(4) The depth of the V-Cut and the connecting strip will affect the amount of deformation of the board.
V-Cut is the culprit of destroying the structure of the board, because V-Cut is to cut grooves in the original large sheet, so the place of V-Cut is prone to deformation.
3. PCB deformation improvement measures
(1) Reduce the impact of temperature on board stress
Reduce the temperature of the soldering furnace or slow down the board in the soldering furnace heating and cooling speed, can greatly reduce the board bending and board warping occurs.
(2) Adopt high Tg boards.
Tg is the glass transition temperature, that is, the temperature of the material from the glass state into the rubber state, the lower the Tg value of the material, that is, the board into the soldering furnace began to soften the speed of the faster, and into a soft rubber state will become longer, of course, the board deformation will be more serious. The use of higher Tg boards can increase the ability to withstand the stress of deformation, but the price of the material is relatively high.
(3) Increase the thickness of the board
In order to achieve the purpose of thinner and lighter, the thickness of the board is designed for 1.0mm, 0.8mm, and even made to the thickness of 0.6mm, such a thickness in the soldering furnace after the PCB is easy to deformation.
(4) Reduce the size of the circuit board and reduce the number of boards
Most of the soldering furnace are used to drive the chain circuit board forward, the larger the size of the circuit board will be because of its own weight, in the soldering furnace in the depression deformation, so try to put the long side of the circuit board as a board edge on the chain of the soldering furnace, you can reduce the weight of the circuit board itself caused by the depression of the deformation of the board to reduce the number of boards is also based on this reason. Over the furnace, try to use the narrow side of the vertical over the furnace direction, you can achieve the lowest amount of depression deformation.
(5) Use of Reflow Tray Fixture
Reflow carrier/template can reduce the board bending and warping because no matter whether it is thermal expansion or cold contraction, it is hoped that the tray can hold the board until the temperature of the board is lower than the Tg value of the beginning of the re-hardening of the board, and then it can be maintained at the original size. If a single layer of trays is not enough to reduce the amount of deformation of the board, it is necessary to add another layer of cover, the board with the top and bottom of the two layers of trays clamped together.
(6) Switch to solid connection, stamp holes, instead of using V-Cut’s sub-panel use V-Cut will destroy the structural strength of the circuit board between the splicing board, try not to use V-Cut of the sub-panel, or reduce the depth of the V-Cut.
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