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This is used for tracking visitors and form submissions. B 1 year This Cookie is used by Yahoo to provide ads, contents or analytics. The deformation mechanism can be explained by the following principles. The reason for the deformation of PCB board processing is very complicated and can be divided into two kinds of stress: thermal stress and mechanical stress. Among them, the thermal stress is mainly generated during the pressing process, and the mechanical stress is mainly generated during the stacking, handling and baking of the plates.
The following is a brief discussion in the order of the process. Incoming copper clad laminate: The copper clad laminates are all double-sided, with symmetrical structure and no graphics. The CTE of copper foil and glass cloth is almost the same, so there is almost no deformation caused by the difference in CTE during the pressing process.
However, the size of the copper clad laminate press is large, and the temperature difference in different areas of the hot plate will cause slight differences in the resin curing speed and degree in different areas during the pressing process. At the same time, the dynamic viscosity at different heating rates is also quite different, so it will also produce Local stress due to differences in curing process.
Generally, this kind of stress will maintain balance after pressing, but will gradually release and deform during future processing. Pressing: The PCB pressing process is the main process that generates thermal stress. The deformation due to different materials or structures is shown in the analysis in the previous section. Similar to the pressing of copper clad laminates, local stresses caused by differences in the curing process will also occur.
PCB boards have more thermal stress than copper clad laminates due to thicker thickness, diverse pattern distribution, and more prepregs. The stress in the PCB board is released during subsequent drilling, shape, or grilling processes, causing the board to deform.
Baking process of solder mask, characters, etc. Storage: The storage of PCB boards at the stage of semi-finished products is generally firmly inserted in the shelf, and the tightness of the shelf is not adjusted properly, or the stacking of the boards during the storage process will cause mechanical deformation of the boards.
Especially for thin plates below 2. In addition to the above factors, there are many factors that affect PCB deformation. Improvement measures. So how can we prevent the board from bending and warping when the board passes through the reflow furnace?
Reduce the effect of temperature on the stress of the board. But there may be other side effects. Tg is the glass transition temperature, that is, the temperature at which the material changes from the glass state to the rubber state. The lower the Tg value of the material, the faster the board starts to soften after entering the reflow oven, and the time it takes to become soft rubber state It will also become longer, and the deformation of the board will of course be more serious.
The use of a higher Tg sheet can increase its ability to withstand stress and deformation, but the price of the material is relatively high. In order to achieve the purpose of lighter and thinner for many electronic products, the thickness of the board has left 1. Such a thickness must keep the board from deforming after the reflow furnace, which is really difficult.
It is recommended that if there is no requirement for lightness and thinness, the thickness of the board should be 1. Reduce the size of the circuit board and reduce the number of puzzles. Since most of the reflow furnaces use chains to drive the circuit board forward, the larger the size of the circuit board will be due to its own weight, dent and deformation in the reflow furnace, so try to put the long side of the circuit board as the edge of the board.
On the chain of the reflow furnace, the depression and deformation caused by the weight of the circuit board can be reduced. The reduction in the number of panels is also based on this reason.
That is to say, when passing the furnace, try to use the narrow edge to pass the furnace direction as far as possible. The amount of depression deformation.
The tray can hold the circuit board and wait until the temperature of the circuit board is lower than the Tg value and start to harden again, and can also maintain the size of the garden.
If the single-layer pallet cannot reduce the deformation of the circuit board, a cover must be added to clamp the circuit board with the upper and lower pallets. This can greatly reduce the problem of circuit board deformation through the reflow furnace. However, this furnace tray is quite expensive, and manual labor is required to place and recycle the trays. Use real connections and stamp holes instead of V-Cut sub-boards. Since V-Cut will destroy the structural strength of the panel between the circuit boards, try not to use the V-Cut sub-board or reduce the depth of the V-Cut.
Optimization in PCB production engineering: The deformation defect rate of low Tg materials is higher than that of high Tg materials. The high Tg materials listed in the above table are all filler-shaped materials, and the CTE is less than that of low Tg materials. The impact on low Tg materials will definitely be greater than that on medium and high Tg materials.
Engineering Design Research Engineering design should try to avoid structural asymmetry, material asymmetry, and graphic asymmetry design to reduce deformation. At the same time, it was found in the research process that the core plate directly laminated structure is easier to deform than the copper foil laminated structure. Table 2 shows Test results of two structural panels. In engineering design, the frame form of the jigsaw panel also has a greater impact on the deformation.
Generally, PCB factories will have a continuous large copper frame and a non-continuous copper dot or copper block frame, and there are also different differences. The effect of pressing on deformation is very important, and the generation of stress can be effectively reduced through reasonable parameter settings, press selection and stacking methods.
For general panels with symmetrical structures, it is generally necessary to pay attention to symmetrical stacking during pressing, and symmetrical placement of auxiliary tools such as tool panels and cushioning materials.
At the same time, choosing the hot and cold integrated press to press is also obviously helpful to reduce the thermal stress. The reason is that the cold and hot split press transfers the plate to the cold press at high temperature above GT temperature , and the material loses pressure above the Tg point.
Rapid cooling will lead to rapid release of thermal stress and deformation, while the integrated cold and heat press can cool down the final stage of hot pressing to prevent the plate from losing pressure at high temperatures.
At the same time, for the special needs of customers, it is inevitable that there will be some plates with asymmetric materials or structures. At this time, the deformation caused by the different CTEs analyzed in the previous article will be very obvious. For this problem, we can try to use asymmetric The principle is to use the asymmetrical placement of the buffer material to achieve different heating speeds on both sides of the PCB, which affects the expansion and contraction of different CTE core cypresses during the heating and cooling stages to solve the problem of inconsistent deformation.
Table 4 is the test result on a certain structural asymmetric plate of our company. In the PCB production process, in addition to pressing, there are several high-temperature treatment processes for solder mask, characterization and hot air leveling. As mentioned above, this temperature is in the ordinary Tg material. Above the Tg point, the material is in a highly elastic state and is easily deformed under external force.
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