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What are the reasons for the deformation of the PCB board and how to prevent it?

Updating:2019-11-21 17:44:24  Hits:

PCB boards are mostly prone to board bending after reflow soldering Warping, if it is serious, it may even cause components such as empty soldering, tombstones, etc. How to overcome it? 1. The hazard of PCB board deformation. On the automated surface mount line, if the circuit board is not flat, it will cause inaccurate positioning. Bad automatic insertion machine. The circuit board on which the components are installed is bent after soldering, and the component feet are difficult to cut neatly. The board cannot be installed on the chassis or the socket inside the machine, so it is also very annoying for the assembly plant to encounter the board warping. The current surface mount technology is developing in the direction of high precision, high speed, and intelligence, which puts forward higher flatness requirements for PCB boards that are home to various components. In the IPC standard, it is specifically pointed out that the maximum allowable deformation of PCB boards with surface mount devices is 0.75%, and the maximum allowable deformation of PCB boards without surface mounting is 1.5%. In fact, in order to meet the needs of high-precision and high-speed placement, some electronic assembly manufacturers have stricter requirements for deformation. For example, our company has multiple customers who require a maximum deformation of 0.5%, and even some customers require it. 0.3%. The PCB board is composed of copper foil, resin, glass cloth and other materials, and the physical and chemical properties of each material are different. After being pressed together, thermal stress will inevitably occur and cause deformation. At the same time, in the PCB processing process, 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 reasons that can lead to the deformation of the PCB board are complex and diverse. Distortion or deformation caused by processing has become one of the most complicated problems faced by PCB manufacturers. 2. Analysis of the causes of PCB deformation The deformation of the PCB needs to be studied from several aspects such as material, structure, pattern distribution, processing process, etc. This article will analyze and explain various reasons and improvement methods that may cause deformation. The uneven copper surface area on the circuit board will worsen the bending and warping of the board. Generally, a large area of ​​copper foil is designed on the circuit board for grounding purposes. Sometimes there is also a large area of ​​copper foil designed on the Vcc layer. When these large area copper foils cannot be evenly distributed on the same circuit board When it is installed, it will cause the problem of uneven heat absorption and heat dissipation. Of course, the circuit board will also expand and contract. If the expansion and contraction cannot be performed at the same time, it will cause different stress and deformation. At this time, if the temperature of the board has reached At the upper limit of the Tg value, the board will begin to soften, causing permanent deformation. The connection points (vias, vias) of each layer on the circuit board will limit the expansion and contraction of the board. Today's circuit boards are mostly multi-layer boards, and there will be rivet-like connection points (vias) between the layers. The connection points are divided into through holes, blind holes and buried holes. Where there are connection points, the board will be restricted. The effect of expansion and contraction will also indirectly cause plate bending and plate warping. Reasons for PCB board deformation: (1) The weight of the circuit board itself will cause the board to dent and deform. Generally, reflow furnaces use chains to drive the circuit board forward in the reflow furnace, which means that the two sides of the board are used as fulcrums to prop up the whole piece. The board, if there are heavy parts on the board, or the size of the board is too large, it will show a depression in the middle due to the amount of the board, causing the board to bend. (2) The depth of the V-Cut and the connecting strips will affect the deformation of the panel. Basically, V-Cut is the culprit that destroys the structure of the board. Because V-Cut cuts grooves in the original large sheet, so V-Cut -Cut the place is prone to deformation. 2.1

Analysis of the influence of laminated materials, structure, and graphics on board deformation The foil is deformed by heat during pressing, and 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 ordinary FR-4 substrate is Z-direction CTE at the Tg point It is (50~70)X10-6; above the TG point is (250~350)X10-6, the X-direction CTE is generally similar to copper foil due to the presence of glass cloth. Note on TG point: When the temperature of the high Tg printed board rises to a certain area, the substrate will change from the glassy state to the rubbery state, and the temperature at this time

It is called the glass transition temperature (Tg) of the board. In other words, Tg is the highest temperature (°C) at which the substrate maintains rigidity. That is to say, ordinary PCB substrate materials not only produce softening, deformation, melting and other phenomena at high temperatures, but also exhibit a sharp decline in mechanical and electrical properties. Generally, the Tg of the plate is more than 130 degrees, the high Tg is generally greater than 170 degrees, and the medium Tg is about greater than 150 degrees. Usually PCB printed boards with Tg≥170°C are called high Tg printed boards. As the Tg of the substrate increases, the heat resistance, moisture resistance, chemical resistance, stability and other characteristics of the printed board will be improved and improved. The higher the TG value, the better the temperature resistance of the plate

, especially in the lead-free process, high Tg applications are more common. High Tg refers to high heat resistance. With the rapid development of the electronics industry, especially the electronic products represented by computers, the development of high functionality and high multilayers requires higher heat resistance of PCB substrate materials as an important guarantee. The emergence and development of high-density mounting technologies represented by SMT and CMT have made PCBs more and more inseparable from the support of high heat resistance of substrates in terms of small aperture, fine circuit and thinner. Therefore, the difference between general FR-4 and high Tg FR-4 is the mechanical strength, dimensional stability, adhesion, water absorption, and thermal decomposition of the material in the hot state, especially when heated after moisture absorption. There are differences in various conditions, such as thermal expansion, and high Tg products are obviously better than ordinary PCB substrate materials. The expansion of the core board with the inner layer pattern is different due to the difference between the pattern distribution and the thickness of the core board or the material characteristics. When the pattern distribution is different from the thickness of the core board or the material characteristics, it will be different. When the pattern distribution is relatively uniform, the material type is the same. It will be deformed. When the PCB laminate structure has asymmetry or uneven pattern distribution, the CTE of different core boards will vary greatly, resulting in deformation during the lamination process. The deformation mechanism can be explained by the following principles. Suppose there are two core boards with a large difference in CTE that are pressed together by a prepreg, where the A core board CTE is 1.5x10-5/℃, and the core board length is 1000mm. In the pressing process, the prepreg, which is used as the bonding sheet, will bond the two core boards together through three stages of softening, flowing and filling with graphics, and curing. Figure 1 shows the dynamic adhesion curve of ordinary FR-4 resin at different heating rates. Under normal circumstances, the material starts to flow from about 90°C, and cross-links and cures above the TG point. The prepreg is in a free state before curing. At this time, the core plate and the copper foil are in a state of free expansion after being heated, and their deformation can be obtained by their respective CTE and temperature changes. Simulate the pressing conditions, the temperature rises from 30℃ to 180℃, and the deformation of the two core plates are △LA=(180℃~30℃)x1.5x10-5m/℃X1000mm=2.25mm△LB=(180 ℃~30℃)X2.5X10-5M/℃X1000mm=3.75mm At this time, because the semi-cured is still in a free state, the two core plates are long and short, without interference with each other, and have not yet been deformed. As shown in Figure 2, it will be kept at a high temperature for a period of time during pressing until the semi-cured is completely cured. At this time, the resin becomes a cured state and cannot flow at will, and the two core boards are combined. When the temperature drops, such as no interlayer Resin bound, the core board will return to the original length without deformation, but in fact, the two core boards have been bonded by the cured resin at high temperature, and they cannot shrink at will during the cooling process. Among them, the A core board should shrink 3.75 mm, in fact, when the shrinkage is greater than 2.25mm, it will be hindered by the A core board. In order to achieve the force balance between the two core boards, the B core board cannot shrink to 3.75mm, and the A core board shrinks more than 2.25mm, so that the whole The board is bent in the direction of the B core board, as shown in Figure 2. Deformation of different CTE core boards during the pressing process. According to the above analysis, it can be seen that the laminated structure and material type of the PCB board have been patterned uniformly, which directly affects the difference in CTE between different core boards and copper foils. During the pressing process The difference in expansion and contraction of the prepreg will be retained through the solidification process of the prepreg and finally form the deformation of the PCB board. 2.2

Deformation caused during PCB processing. The causes of deformation during PCB processing are very complex and can be divided into 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 there are temperature differences in different areas of the hot plate, which will cause slight differences in the curing speed and degree of the resin 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 Local stress due to differences in curing process. Generally, this kind of stress will maintain balance after pressing, but will gradually release and deform in 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 because of thicker thickness, diverse pattern distribution, and more prepregs. The stress in the PCB board is released during subsequent drilling, shape, or grilling processes, resulting in deformation of the board. Baking process of solder mask, characters, etc.: Since solder mask inks cannot be stacked on each other when they are cured, PCB boards will be placed in a rack for curing. The solder mask temperature is about 150°C, which just exceeds the Tg point of medium and low Tg materials, Tg The resin above the point is highly elastic, and the plate is easy to deform under the action of its own weight or the strong wind of the oven. Hot-air solder leveling: The temperature of the tin furnace is 225℃~265℃, and the time is 3S-6S when the ordinary board hot-air solder is leveled. The temperature of the hot air is 280℃~300℃. When the solder is leveled, the board enters the tin furnace from room temperature, and the post-treatment water washing at room temperature will be carried out within two minutes after being out of the furnace. The entire hot-air solder leveling process is a sudden heating and cooling process. Due to the different materials of the circuit board and the uneven structure, thermal stress will inevitably appear during the cooling and heating process, leading to microscopic strain and overall deformation and warping area. Storage: The storage of PCB boards in the semi-finished product stage is generally firmly inserted in the shelf, and the shelf tightness 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 2.0mm, the impact is more serious. In addition to the above factors, there are many factors that affect PCB board deformation. 3. Prevention of PCB board warpage and deformation. Circuit board warping has a great influence on the production of printed circuit boards. Warpage is also one of the important problems in the production process of circuit boards. The board with components is bent after welding. , The component feet are difficult to be neat. The board cannot be installed in the case or the socket inside the machine, so the warpage of the circuit board will affect the normal operation of the entire subsequent process. At this stage, the printed circuit board has entered the era of surface mounting and chip mounting, and the process requirements for the warpage of the circuit board can be said to be getting higher and higher. So we have to find the reason for the warping of the halfway help. 1. Engineering design: Matters needing attention when designing printed boards: A. The arrangement of interlayer prepregs should be symmetrical, such as six-layer boards, the thickness between 1-2 and 5-6 layers and the number of prepregs should be the same, otherwise the layers It is easy to warp after pressing.

B. Multilayer core board and prepreg should use the same supplier's products. C.

The area of ​​the circuit pattern of the outer layer A side and B side should be as close as possible. If side A is a large copper side, and

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