PCB News - Analysis of the causes of PCB board deformation and how to improve

When printed circuit boards undergo reflow soldering, most of them are prone to board bending and warping. In severe cases, it may even cause components such as empty soldering and tombstones. How to overcome it?

1. The hazards of circuit board deformation
In the automated surface mount line, if the circuit board is not flat, it will cause inaccurate positioning, components can not be inserted or mounted on the holes and surface mount pads of the board, and even the automatic insertion machine will be damaged. 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. Therefore, the assembly factory encounters the board warping.It is very annoying. The current surface mount technology is developing in the direction of high precision, high speed, and intelligence, which puts forward higher flatness requirements for boards that are home to various components. In the IPC standard, it is specifically pointed out that the allowable deformation of the board with surface mount devices is 0.75%, and the allowable deformation of PCB boards without surface mount is 1.5%. In order to meet the requirements of high precision and high speed placement, some electronic assembly manufacturers have stricter requirements on the amount of deformation. The PCB board is composed of copper foil, resin, glass cloth and other materials. The physical and chemical properties of each material are different. After being pressed together, thermal stress will inevitably occur, which will cause deformation. At the same time, in the process of PCB processing, it will go through high temperature, mechanical cutting, wet treatment, etc.Various processes will also have an important effect on the deformation of the board. In short, the reasons for the deformation of the PCB board are complex and diverse. How to reduce or eliminate the deformation due to different material characteristics or processing has become one of the complex problems faced by PCB board manufacturers. one.

2. Analysis of the causes of deformation
The deformation of PCB board 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, and sometimes the Vcc layer is alsoThere will be large-area copper foils. When these large-area copper foils are not evenly distributed on the same circuit board, it will cause uneven heat absorption and heat dissipation. Of course, the circuit board will also expand. Cold shrinkage. If the expansion and contraction cannot be done at the same time, it will cause different stress and deformation. At this time, if the temperature of the board has reached the upper limit of the Tg value, the board will open.Begin to soften, causing 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 are connection points (vias) like rivets between the layers, and the connection points are again Divided into through holes, blind holes and buried holes, where there are connection points, the effect of the expansion and contraction of the board will be limited, and it will also indirectly cause the board to bend and warp. The weight of the board will cause the board to dent and deform. Generally, the reflow furnace will use a chain to drive the circuit board forward in the reflow furnace, that is, the two sides of the board are used as fulcrums to support the entire board. If there are heavy parts on the board, or If the size of the board is too large, it will show a depression in the middle due to the amount of the board itself, causing the board to bend. The depth of V-Cut and the video of the connection bar Basically, V-Cut is the main culprit to destroy the board structure. Because V-Cut cuts grooves in the original large sheet, so the place of V-Cut is prone to deformation.

2.1 Analysis of the influence of the material, structure, and graphics on the deformation of the plate
The PCB board is formed by pressing the core board and the prepreg and the outer copper foil. The core board and the copper foil are heated and deformed when they are pressed. The amount of deformation depends on the coefficient of thermal expansion (CTE) of the two materials. The coefficient of thermal expansion of the copper foil ( CTE) is left and right, while ordinary FR-4 substrate is at the Z-direction CTE below the Tg point; above the TG point is (250~350)X10-6, and the X-direction CTE is due to the presence of glass cloth, generally with copper foil is similar. Note on TG point: When the temperature of a high Tg printed board rises to a certain area, the substrate will change from "glassy" to "rubbery", and the temperature at this time is called the glass transition temperature (Tg) of the board. That is, Tg is the temperature (°C) at which the base material maintains rigidity. In other words, ordinary PCB board substrate materials will not only soften, deform, melt, etc., at high temperatures.At the same time, it is also manifested in a sharp decline in mechanical and electrical characteristics. 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 board, especially in the lead-free process, where 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 board substrate materials as an important guarantee.The emergence and development of the representative high-density mounting technology has made PCB boards more and more inseparable from the support of the high heat resistance of the
substrate in terms of small aperture, fine wiring, and thinning. 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 expansion. High Tg products are obviously better than ordinary PCB board 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. The asymmetry or uneven pattern distribution of the PCB board laminate structure will cause the CTE of different core boards to 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/ degree Celsius, and the core board length is 1000mm. The pressing process is used as the prepreg of the bonding sheet, and the two core boards are bonded together through three stages of softening, flowing and filling with graphics, and curing. At this time, the deformation of the two core plates are respectively △LA=(180 degree Celsius~30 degree Celsius)x1.5x10-5m/ degree CelsiusX1000mm=2.25mm; △LB=(180 degree Celsius~30 degree Celsius)X2.5X10-5M/ degree CelsiusX1000mm =3.75mm. In the free state, the two core plates are long and short, do not interfere with each other, and have not yet deformed. During pressing, it will be kept at high temperature for a period of time until the semi-cured is completely cured. At this time, the resin becomes a cured state and cannot flow at will. The two core boards are combined together. When the temperature drops, if there is no interlayer resin binding, the core The board will return to its original length without deformation. The upper two core boards are bonded by the cured resin at high temperature and cannot shrink at will during the cooling process. The A core board should shrink by 3.75mm. In fact, when the shrinkage is greater than 2.25mm, it
will be hindered by the A core board. The force between the two core plates is balanced, the B core plate cannot shrink to 3.75mm, and the A core plate shrinks more than 2.25mm, so that the entire plate faces the B core plate.

2.2 Deformation caused during PCB processing
The reason for the deformation of the PCB board processing process is very complicated and can be divided into two types 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 there is a temperature difference 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, and different temperature rises. There are also large differences in dynamic viscosity at different speeds, so local stresses due to differences in the curing process will also occur. Generally, this kind of stress will maintain balance after pressing, but will gradually release and deform in future processing.
Pressing: The PCB board 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, causing the board to deform.
Baking process of solder mask, characters, etc.: Since solder mask inks cannot be stacked on top of 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. The resin above the Tg point is highly elastic, and the plate is easily deformed 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 degree Celsius~265 degree Celsius, and the time is 3S-6S when the ordinary board hot-air solder leveling. The temperature of the hot air is 280 degree Celsius~300 degree Celsius. 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, the structure is uneven. Thermal stress will inevitably occur in the process, leading to microscopic strain and overall deformation warping zone.
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. Improvement measures
So how can we prevent the board from bending and warping when the PCB board passes through the reflow furnace?
1) Reduce the influence of temperature on the stress of the board: Since temperature is the main stress of the board, as long as the temperature of the reflow furnace is reduced or the rate of heating and cooling of the board in the reflow furnace is slowed down, the bending and the plate can be greatly reduced. The situation of warping happened. But there may be other side effects.
2) Using high Tg plates: Tg is the glass transition temperature, that is, the temperature at which the material changes from glass to rubber. The lower the Tg value, the faster the plate starts to soften after entering the reflow furnace. And the time
to become soft rubber state will be longer, of course the deformation of the board will be more serious. Using a higher Tg sheet can increase its stress and deformation, But the price of materials is relatively high.
3) Increase the thickness of the circuit board: In order to achieve the purpose of lighter and thinner for many electronic products, the thickness of the board has been left 1.0mm, 0.8mm, or even 0.6mm thickness. This thickness should keep the board after the reflow furnace It is not deformed, which is really difficult. It is recommended that if there is no requirement for lightness and thinness, the board can be used with a thickness of 1.6mm, which can be greatly reduced.Risk of bending and deformation of the plate.
4) Reduce the size of the circuit board and reduce the number of panels: Since most of the reflow furnaces use chains to drive the circuit board forward, the larger the size of the circuit board will be dented in the reflow furnace due to its own weight, So try to put the long side of the circuit board as the side of the board on the chain of the reflow furnace, so as to reduce the depression and deformation caused by the weight of the circuit board itself, and to put the board together. The reduction in the number is also based on this reason, that is, when passing the furnace, try to use the narrow side to be perpendicular to the direction of the furnace to achieve the amount of depression deformation.
5) Used furnace tray fixture: If the above methods are difficult to achieve, the furnace tray is used to reduce the amount of deformation. The reason why the furnace tray can reduce the bending of the plate is because whether it is thermal expansion or cold contraction, It is hoped that 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, so that the size of the garden can be maintained. If the single-layer support. The tray cannot reduce the deformation of the circuit board, so it is necessary to add a layer of cover to clamp the circuit board with the upper and lower trays, so as to greatly reduce the problem of the deformation of the
circuit board through the reflow furnace. However, the oven trays are quite expensive, and manual placement and recycling of the trays are required.
6) Use real connections and stamp holes instead of V-Cut's sub-board: Since V-Cut will destroy the structural strength of the board between the circuit boards, try not to use V-Cut's sub-board, or reduce V -Cut depth. Optimization in PCB board. production engineering: The influence of different materials on the deformation of the board will be counted for the rate of defects in the board of different materials. Deformation of low Tg materials. The sinking rate 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. At the same time, during the processing after pressing, the baking temperature is 150 degree Celsius. The impact will definitely be greater than that of medium and high Tg materials. Engineering design should try to avoid structural asymmetry, material asymmetry, and graphic asymmetry design to reduce deformation. At the same time, during the research process, it was also found that the core plate directly laminated structure is easier to deform than the copper foil laminated structure. 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 reason why the deformation of the two frame forms is different is because of the strength of the continuous copper frame High, the rigidity is relatively large in the process of pressing and splicing, so that the residual stress in the plate is not easy to release, and the release is concentrated after the shape is processed, resulting in more serious deformation. The discontinuous copper dot frame gradually releases the stress during the pressing and subsequent processing, and the veneer deforms less after the shape. The above are some possible influencing factors involved in the engineering design.Use it flexibly at the time. It can reduce the influence of deformation caused by the design.

3.3 Compression research
The effect of pressing on deformation is very important. Reasonable parameter settings, press selection and stacking methods can effectively reduce stress. For general panels with symmetrical structure, it is generally necessary to pay attention to the symmetrical stacking of panels when pressing, and to place auxiliary tools such as tool panels and cushioning materials symmetrically. At the same time, choosing a hot and cold integrated press to press is also obviously helpful to reduce thermal stress. In order for the hot and cold split press to transfer the plates to the cold press at high temperatures (above GT temperature), the loss of pressure and rapid cooling of the material above the Tg point will cause the rapid release of thermal stress and deformation, while the cold and hot integrated press can Realize the temperature reduction at the end of the hot pressing to avoid the pressure loss of the plate under high temperature. At the same time, for the special needs of customers, it is inevitable that there will be some plates with asymmetrical 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 solve this problem by using an asymmetric stacking method. The principle is to use the asymmetric placement of the buffer material to reach the PCB board. The double-sided heating speed is different, which affects the expansion and contraction of different CTE core cypress trees in the heating and cooling stages to solve the problem of inconsistent deformation. It is the test result on a certain structural asymmetric plate of our company. Through the asymmetric stacking method, and adding a post-curing process after pressing, and leveling before shipment, this board finally meets the customer's 2.0mm requirements.

3.4 Other production processes
In the PCB board production process, in addition to pressing, there are several high-temperature processing processes of solder mask, characterization and hot air leveling. Among them, the temperature of the solder mask and the baking board after the character is 150 degree Celsius. 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. Therefore, avoid stacking the plates to prevent the lower plate from being bent when drying the plates, and ensure the plates when drying the plates. The direction of the piece is parallel to the blowing direction. In the hot air leveling process, it is necessary to ensure that the plate is placed in the tin furnace for cooling for more than 30 seconds to avoid sudden cold deformation caused by cold water washing after post-processing at high temperature. In addition to the production process, the storage of PCB boards at each station also has a certain impact on the deformation. In some manufacturers, due to the large number of products to be produced and the small space, multiple boards are stacked together for storage. This will also cause the board to be deformed by external forces. Since the PCB board also has a certain degree of plasticity, these deformations will not be 100% restored in the subsequent leveling process.

3.5 Leveling before shipment
Most PCB board manufacturers will have a leveling process before shipment. This is because the deformation of the board caused by heat or mechanical force will inevitably occur during the processing process, and the board will be leveled by mechanical leveling or thermal baking before shipment. Can be effectively improved. Affected by the heat resistance of the solder mask and the surface coating layer, the temperature of the general baking sheet is below 140 degree Celsius~150 degree Celsius, which just exceeds that of ordinary materials Tg temperature, which is very beneficial to the leveling of ordinary boards, but the leveling effect for high Tg materials is not so obvious, so on individual high Tg boards with serious board warping, the baking board temperature can be appropriately increased, but the main Ink and coating quality. At the same time, the method of pressing the plate during drying and increasing the cooling time with the furnace also has a certain effect on the deformation. From the test results of the leveling effect of the PCB board, it can be seen that increasing the weight and extending the furnace cooling time have a significant effect on the leveling of the deformation.