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

When the PCB board is reflowed, it is easy to cause the plate to bend. If it is serious, it will even cause the components to be welded, tombstone, etc. How to overcome it? 1. The damage of PCB board deformation On the automatic surface mount line, if the circuit board is not flat, it will cause the positioning to be inaccurate. The components cannot be inserted or mounted on the hole of the board and the surface mount pad, and even hit. Bad automatic inserter. The board on which the components are mounted is bent after soldering, and the component legs are difficult to trim. The board can't be installed in the chassis or the socket inside the machine, so it is very troublesome for the assembly factory to encounter the board. The current surface mount technology is moving toward high precision, high speed, and intelligent direction, which puts higher flatness requirements on PCB boards as homes for various components. In the IPC standard, it is specifically pointed out that the maximum allowable deformation of a PCB board with a surface mount device is 0.75%, and the maximum allowable deformation of a PCB without a surface mount 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 on the amount of deformation. For example, the maximum deformation allowed by our company is 0.5%, and even individual customer requirements. 0.3%. 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 pressing together, thermal stress remains inevitably, resulting in deformation. At the same time, in the processing of PCB, it will go through various processes such as high temperature, mechanical cutting and wet processing, which will also have an important impact on the deformation of the board. In short, the causes of PCB board deformation are complicated and diverse, how to reduce or eliminate the material characteristics. Different or processing-induced deformations have become one of the most complex problems faced by PCB manufacturers. 2. Analysis of the causes of PCB board deformation The deformation of PCB board needs to be studied from several aspects such as material, structure, pattern distribution and processing process. This paper will analyze and explain the various causes and improvement methods that may cause deformation. The uneven copper surface area on the board will deteriorate the board bend and the board curl. Generally, a large area of copper foil is designed for grounding. Sometimes the Vcc layer is also designed with a large area of copper foil. When these large areas of copper foil are not evenly distributed on the same board. When it is on, it will cause the problem of uneven heat absorption and heat dissipation. The circuit board will of course also expand and contract. If the expansion and contraction cannot cause different stresses and deformation at the same time, the temperature of the board can be reached. At the upper end of the Tg value, the board begins to soften, causing permanent deformation. The junctions (vias) of the various layers on the board limit the board's expansion and contraction. Most of today's boards are multi-layer boards, and there are joints (vias) to the rivets between the layers. The joints are divided into through holes, blind holes and buried holes. Where there are joints, the board is limited. The effect of rising and contracting will indirectly cause the plate to bend and the plate to be warped. Reasons for PCB board deformation: (1) The weight of the circuit board itself will cause the board to be deformed. Generally, the reflow furnace will use the chain to drive the circuit board forward in the reflow furnace, that is, the two sides of the board are used as the fulcrum to support the whole piece. The board, if there are heavy parts on the board, or if the size of the board is too large, it will show the phenomenon of the middle depression due to its own quantity, causing the board to bend. (2) The depth of V-Cut and the connecting strip will affect the deformation of the board. Basically, V-Cut is the culprit of destroying the structure of the board, because V-Cut is to cut the groove on the original sheet, so V -Cut is prone to deformation. 2.1

Analysis of the deformation of the plate by the pressing material, structure and pattern. The PCB board is made up of the core board and the prepreg and the outer layer of copper foil. The core board and the copper foil are thermally deformed when pressed, and the deformation depends on two types. The coefficient of thermal expansion (CTE) of the material; the coefficient of thermal expansion (CTE) of the copper foil is about 17×10-6; while the ZF of the ordinary FR-4 substrate is (50~70)×10-6 at the Tg point; (250~350) X10-6, X to CTE is similar to copper foil due to the presence of glass cloth. Note on TG point: High Tg printed board When the temperature rises to a certain area, the substrate will change from glass state to "rubber state", the temperature at this time

It is called the glass transition temperature (Tg) of the board. That is, Tg is the highest temperature (°C) at which the substrate remains rigid. That is to say, the ordinary PCB substrate material not only softens, deforms, melts, etc. at high temperatures, but also exhibits a sharp drop in mechanical and electrical properties. Generally, the Tg plate is 130 degrees or more, the high Tg is generally greater than 170 degrees, and the medium Tg is greater than about 150 degrees. A PCB printed board with a typical Tg ≥ 170 ° C is called a high Tg printed board. The Tg of the substrate is improved, and the characteristics such as heat resistance, moisture resistance, chemical resistance, and stability of the printed board are improved and improved. The higher the TG value, the better the temperature resistance of the sheet.

Especially in the lead-free process, high Tg applications are more. 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 multi-layer requires the higher heat resistance of PCB substrate materials as an important guarantee. The emergence and development of high-density mounting technology represented by SMT and CMT makes PCBs more and more inseparable from the high heat resistance of substrates in terms of small aperture, fine wiring and thinning. Therefore, the difference between general FR-4 and high Tg FR-4 is that it is in the hot state, especially under heat absorption, the mechanical strength, dimensional stability, adhesion, water absorption and thermal decomposition of the material. 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 plate in which the inner layer pattern is made is different due to the difference in the pattern distribution and the thickness of the core sheet or the material characteristics. When the pattern distribution is different from the thickness or material characteristics of the core sheet, when the pattern distribution is relatively uniform, the material types are uniform, and Will produce deformation. When the PCB board laminate structure is asymmetrical or the pattern distribution is uneven, the CTE difference of different core boards will be large, and deformation will occur during the pressing process. The deformation mechanism can be explained by the following principle. It is assumed that there are two kinds of core plates with different CTE differences, which are pressed together by a prepreg, wherein the C core of the A core plate is 1.5×10-5/° C., and the core plate length is 1000 mm. In the press-forming process as a prepreg of the bonding sheet, the two core sheets are bonded together by softening, flowing and filling the pattern, and curing in three stages. Figure 1 shows the dynamic adhesion bottom curve of ordinary FR-4 resin at different heating rates. Under normal circumstances, the material starts to flow from about 90 °C, and cross-linking cure starts at the point above TG. The prepreg is in a free state before curing. At this time, the core plate and the copper foil are in a state of being freely expanded after being heated, and the amount of deformation thereof can be obtained by the respective CTE and temperature change values. Under simulated compression conditions, the temperature is raised from 30 °C to 180 °C. At this time, the deformation of the two core plates is △LA=(180°C~30°C)x1.5x10-5m/°CX1000mm=2.25mm△LB=(180 °C~30°C)X2.5X10-5M/°CX1000mm=3.75mm At this time, since the semi-curing is still in a free state, the two core plates are long and short, do not interfere with each other, and have not been deformed. See Figure 2, when pressed, it will be kept at high temperature for a period of time until the semi-curing is completely cured. At this time, the resin becomes solidified and cannot flow freely. The two core plates are combined. When the temperature drops, such as no interlayer When the resin is restrained, the core plate will return to the original length without deformation, but in fact, the two core plates are bonded by the cured resin at high temperature, and cannot be freely contracted during the cooling process, wherein the A core plate should shrink 3.75. Mm, in fact, when the shrinkage is greater than 2.25mm, it will be hindered by the A core plate. In order to achieve the balance between the two core plates, the B core plate cannot shrink to 3.75mm, and the A core plate shrinks more than 2.25mm, thus making the whole The plate is bent toward the B core plate as shown in Fig. 2. According to the above analysis, the laminate structure and material type of the PCB board have been uniformly distributed, which directly affects the CTE difference between different core sheets and copper foil during the pressing process. The difference in shrinkage and contraction is retained by the solidification process of the prepreg and eventually forms a deformation of the PCB. 2.2

Deformation caused by PCB board processing The cause of deformation of PCB board processing is very complicated and can be divided into two stresses: thermal stress and mechanical stress. The thermal stress is mainly generated during the pressing process, and the mechanical stress mainly occurs during the stacking, handling and baking of the plates. The following is a brief discussion in the order of the process. CCL material: CCL is double-sided, symmetrical structure, no pattern, copper foil and glass cloth CTE are almost the same, so there is almost no deformation caused by different CTE during the pressing process. However, the size of the CCL press is large, and there is a temperature difference in different areas of the hot plate, which may cause slight differences in the curing speed and degree of the resin in different regions during the pressing process, and the dynamic viscosity at different heating rates also has a large difference, so it also occurs. Local stress due to differences in the curing process. Generally, this stress will maintain equilibrium after pressing, but will gradually release deformation during future processing. Press-fit: The PCB press-fitting process is the main process for generating thermal stress. The deformation due to different materials or structures is shown in the previous section. Similar to the clad copper clad, it also produces local stress caused by the difference in the curing process. Due to thicker thickness, various pattern distribution, and more prepreg, the thermal stress is more difficult to eliminate than the clad laminate. The stress existing in the PCB is released in subsequent processes such as drilling, contouring or grilling, resulting in deformation of the panel. Soldering, character and other baking processes: Since the solder resist inks cannot be stacked on each other when cured, the PCB boards are placed on the shelf and cured by a baking sheet. The soldering temperature is about 150 °C, just above the Tg point of the medium and low Tg materials, Tg Above the point, the resin is in a high elastic state, and the plate is easily deformed under the action of its own weight or strong wind of the oven. Hot air solder leveling: The normal temperature of the hot plate solder is 225 ° C ~ 265 ° C, the time is 3S-6S. The hot air temperature is 280 ° C ~ 300 ° C. When the solder is leveled, the plate is fed into the tin furnace from room temperature, and then subjected to room temperature post-treatment water washing within two minutes after the furnace is discharged. The entire hot air solder leveling process is a quenching and quenching process. Due to the different material of the circuit board and the uneven structure, thermal stress will inevitably occur during the hot and cold process, resulting in microscopic strain and overall deformation. Storage: The storage of the PCB in the semi-finished stage is generally inserted in the shelf. The loose adjustment of the shelf is not suitable, or the stacking of the board during storage will cause mechanical deformation of the panel. Especially for thin plates below 2.0mm, the impact is even more serious. In addition to the above factors, there are many factors that affect the deformation of the PCB. 3, PCB board warpage deformation prevention circuit board warpage has a great impact on the production of printed circuit boards, warpage is also one of the important issues in the circuit board manufacturing process, the board mounted with components is bent after welding The component feet are difficult to tidy. The board can't be installed in the chassis or the socket inside the machine, so the board warpage 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 circuit board warpage are getting higher and higher. So we have to find the reason for the halfway to help warp. 1. Engineering design: Precautions for printed board design: A. The arrangement of inter-layer prepreg should be symmetrical, for example, six-layer board, the thickness between 1~2 and 5~6 layers and the number of sheets of prepreg should be the same, otherwise the layer It is easy to warp after pressing.

B. Multilayer core sheets and prepregs should use the same supplier's products. C.

The area of the line pattern of the A and B sides of the outer layer should be as close as possible. If the A side is a large copper surface and the B side is only a few lines, the printed board is easily warped after etching. If the line areas on both sides are too different, add a separate grid on the thin side to balance. 2. Drying before the blanking: The baking sheet before the copper clad is cut off (150 ° C, time 8 ± 2 hours) is to remove the moisture in the plate, and at the same time completely cure the resin in the plate, further eliminating the residual stress in the plate. This is helpful in preventing plate warpage. At present, many double-sided, multi-layer boards still adhere to the step of before or after baking. However, there are some exceptions for plate companies, currently PCBs.

The time regulations for the factory baking sheet are also inconsistent, from 4-10 hours. It is recommended to decide according to the grade of the printed board produced by the customer and the customer's requirements for warpage. After cutting into a jigsaw and drying it, the whole piece of material is baked and then unloaded. Both methods are feasible. It is recommended to dry the plate after cutting. The inner panel should also be baked. 3. Warp and weft of prepreg: The warp and weft shrinkage rates of the prepreg are different after lamination, and the warp and weft directions must be distinguished when cutting and laminating. Otherwise, it is easy to cause warpage of the finished board after lamination, even if it is pressed, it is difficult to correct. The reason for the warpage of the multi-layer board is that the latitude and longitude of the prepreg is not separated when it is laminated, and it is caused by the disorder. How to distinguish the warp and latitude? The rolled prepreg is wound in the warp direction and the width direction is the weft direction; for the copper foil sheet, the long side is in the weft direction and the short side is the warp direction. If it is not certain, it can be inquired to the manufacturer or supplier. 4.

Stress removal after lamination

: The multi-layer board is taken out after hot pressing and cold pressing, sheared or milled off the burrs, and then baked in the oven at 150 ° C for 4 hours to gradually release the stress in the board and completely cure the resin. This step cannot be omitted. . 5. Straightening of thin plate requires straightening: 0.4~0.6mm ultra-thin multi-layer board for plate surface plating and pattern plating. Special nip rolls should be made. After the thin plate is clamped on the flying plate of the automatic plating line, use a circle. The sticks string the nip rolls on the entire flying bar to straighten all the plates on the roll so that the plate after plating does not deform. Without this measure, after plating a copper layer of twenty or thirty microns, the sheet would bend and be difficult to remedy. 6. Cooling of the board after hot air leveling: When the hot plate of the printed board is leveled, it will be impacted by the high temperature of the solder bath (about 250 degrees Celsius). After being taken out, it should be placed on the flat marble or steel plate for natural cooling, and sent to the post-processing machine for cleaning. . This is good for the board to prevent warpage. In some factories, in order to enhance the brightness of the lead-tin surface, the hot air of the board is put into the cold water immediately after being leveled. After a few seconds, it is taken out for post-treatment. This kind of hot and cold impact is likely to cause warping on some types of boards. Curved, layered or blistering. In addition, an air floating bed can be installed on the equipment for cooling. 7. Treatment of warped plates: Manage the orderly factory, the printed board will be 100% flatness inspection during the final inspection. All unqualified boards will be picked out, placed in an oven, baked at 150 degrees Celsius and under heavy pressure for 3 to 6 hours, and naturally cooled under heavy pressure. Then, the plate is taken out by pressure relief, and the flatness is checked. This saves some of the boards, and some of the boards need to be pressed two to three times to level. The Shanghai Huabao agent's pneumatic plate warping straightening machine has a very good effect in remediating the warpage of the circuit board by the use of Shanghai Bell. If the above-mentioned anti-warping process measures are not implemented, some of the board drying pressure is useless and can only be scrapped. 4, PCB board warping standard PCB board tilt standard please refer to IPC-A-600G

The 2.11 flatness standard:

For printed boards with surface mount components (such as SMT mount), the distortion and bowing standard is no more than 0.75%, and other types of boards are