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PCB Tech

PCB Tech

PCB Tech

PCB Tech

PCB board deformation has so much harm, why PCB board warpage?

Harm of PCB board deformation

In the automatic surface mount line, if the circuit board is not flat, it will cause inaccurate positioning, components can not be inserted or mounted to the board hole and surface mount pad, and even crash the automatic insertion machine. The circuit board with the components is bent after welding, so it is difficult to trim the component pins. The board can not be installed on the chassis or the socket in the machine, so the assembly plant is also very worried about board warping. At present, the surface mount technology is developing towards high speed and intelligence, which puts forward higher flatness requirements for PCB board which is the home of various components.

It is specially pointed out in IPC standard that the allowable deformation of PCB board with SMD is 0.75%, and that of PCB board without SMD is 1.5%. In fact, in order to meet the needs of high-speed and high-speed mounting, some electronic assembly manufacturers have more stringent requirements on deformation. For example, our company has several customers who require that the allowable deformation is 0.5%, and even some individual customers require 0.3%.

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, it will inevitably produce residual thermal stress and lead to deformation. At the same time, in the process of PCB processing, high temperature, mechanical cutting, wet treatment and other processes will also have an important impact on the deformation of PCB. In short, the causes of PCB deformation are complex and diverse. How to reduce or eliminate the deformation caused by different material properties or processing has become one of the complex problems faced by PCB manufacturers.

Cause analysis of PCB board deformation

The deformation of PCB board needs to be studied from the aspects of material, structure, graphics distribution, processing process, etc. This paper will analyze and elaborate the various causes of deformation and improvement methods.

The uneven copper surface area on the circuit board will worsen the bending and warping.

The general circuit board will be designed with a large area of copper foil for grounding, sometimes VCC When these large areas of copper foil can not be evenly distributed on the same circuit board, the problem of uneven heat absorption and heat dissipation will be caused. Of course, the circuit board will also expand and contract by heat. If the expansion and contraction can not be achieved at the same time, it will cause different stress and deformation. At this time, if the temperature of the board has reached TG Value, the board will begin to soften, resulting in deformation.

The vias (vias) of each layer on the circuit board will limit the board's expansion and contraction.

Nowadays, most circuit boards are multi-layer boards, and there are rivet like connection points (vias) between layers. The connection points are divided into through holes, blind holes and buried holes. Where there are connection points, the effect of board expansion and contraction will be limited, and the bending and warping of boards will be indirectly caused

The causes of PCB board  deformation are as follows

(1) The weight of the circuit board itself can cause the board to sag and deform

Generally, the soldering furnace will use the chain to drive the circuit board to move forward in the soldering furnace, that is, the two sides of the board are used as fulcrum to support the whole board. If there are heavy parts on the board, or the size of the board is too large, it will show the phenomenon of depression in the middle due to its own seed quantity, resulting in plate bending.

(2) The depth of V-CUT and connecting strip will affect the deformation of panel

Basically, V-CUT is the main culprit of destroying the board structure, because V-CUT is to cut grooves on the original large sheet of plate, so the place of V-CUT is easy to deform.

2.1 analysis on the influence of laminate material, structure and figure on plate deformation

PCB board is made of core board, semi cured sheet and outer copper foil. The deformation of core board and copper foil depends on CTE of the two materials;

The CTE of copper foil is about 17x10-6;

The CTE of FR-4 substrate in Z direction is (50 ~ 70) x10-6 at TG point;

It is (250 ~ 350) x10-6 above Tg point, and CTE in X direction is similar to copper foil due to the existence of glass cloth.

Deformation caused by PCB board processing

The deformation causes of PCB are very complex, which can be divided into thermal stress and mechanical stress. The thermal stress is mainly produced in the process of pressing, and the mechanical stress is mainly produced in the process of stacking, handling and baking. The following is a brief discussion in the order of the process.

Copper clad laminate: the copper clad laminates are all double-sided plates with symmetrical structure and no figure. The copper foil is almost the same as the CTE of glass cloth, so the deformation caused by different CTE will not be produced in the pressing process. However, due to the large size of the copper clad laminate press and the temperature difference in different regions of the hot plate, there will be slight differences in resin curing speed and degree in different regions during the pressing process. At the same time, the dynamic viscosity under different heating rates also has a large difference, so the local stress caused by the difference in curing process will also be produced. Generally, this kind of stress will maintain balance after pressing, but it will gradually release and produce deformation in the future processing.

Lamination: PCB lamination process is the main process to generate thermal stress. The deformation due to different materials or structures is analyzed in the previous section. Similar to the copper clad laminate, the local stress caused by the difference in curing process will also be produced. Due to the thicker thickness, diversified pattern distribution and more semi cured chips, the thermal stress of PCB is more and more difficult to eliminate than that of CCL. The stress in the PCB board is released in the following drilling, shape or barbecue process, resulting in the deformation of the board.

Solder mask, character baking process: because solder resist inks cannot stack with each other during curing, PCB boards will be placed vertically in the shelf for curing. The solder resistance temperature is about 150 ℃, which just exceeds the TG point of medium and low Tg materials. The resin above Tg point is in high elastic state, and the board is easy to deform under the action of self weight or strong wind in the oven.

Hot air solder leveling: the tin furnace temperature is 225 ℃ ~ 265 ℃ and the time is 3s-6s. The temperature of hot air is 280 ℃ ~ 300 ℃. When the solder is leveled, the plate is fed into the tin furnace from room temperature, and the post-treatment water washing at room temperature is carried out within two minutes after discharge. The whole hot-air solder leveling process is a sudden heating and cooling process. Due to the different materials and uneven structure of the circuit board, the thermal stress will inevitably appear in the cold and hot process, resulting in micro strain and overall deformation warping zone.

Storage: PCB board in the semi-finished product stage is generally firmly inserted in the shelf. Improper adjustment of shelf tightness or stacking of boards during storage will cause mechanical deformation of the board. Especially for the sheet less than 2.0 mm, the influence is more serious.

In addition to the above factors, there are many factors affecting PCB deformation.

Prevention of PCB board warpage

The warpage of printed circuit board has a great influence on the production of printed circuit board. Warpage is also one of the important problems in the process of circuit board production. The board with components is bent after welding, and the component feet are difficult to be neat. The board can not be installed on the chassis or the socket in the machine, so the warpage of the circuit board will affect the normal operation of the whole subsequent process. At present, the printed circuit board has entered the era of surface mounting and chip installation, and the requirements of PCB warpage are higher and higher. So we need to find out why the half way Gang is warping.

1. Engineering design: matters needing attention in PCB Design: a. the arrangement of interlayer prepreg should be symmetrical, for example, the thickness of 1-2 and 5-6 layers and the number of sheets of prepreg should be consistent, otherwise it is easy to warp after lamination. B. The products of the same supplier shall be used for multilayer core board and prepreg. C. The area of the circuit figure of the outer A and B surfaces should be as close as possible. If a side is a large copper side, and B side only a few wires, this printed board after etching is easy to warp. If the line area difference between the two sides is too large, some independent grids can be added on the sparse side to balance.

2. Drying plate before cutting: the purpose of drying the copper clad laminate before blanking (150 ℃ for 8 ± 2 hours) is to remove the moisture in the plate, and at the same time make the resin in the plate completely solidified, further eliminate the remaining stress in the plate, which is helpful to prevent the board from warping. At present, many double-sided and multilayer boards still adhere to the step of baking before or after blanking. However, there are also some exceptions in some board factories. At present, the drying time of each PCB Factory is inconsistent, ranging from 4 to 10 hours. It is suggested to determine the drying time according to the grade of printed circuit board produced and the warpage requirements of customers. The two methods are feasible, and it is suggested to cut and dry the board. The inner layer should also be dried.

3. Longitude and weft direction of prepreg: after lamination, the shrinkage rate in warp and weft is different, so the warp and weft must be distinguished when blanking and laminating. Otherwise, it is easy to cause the warpage of the finished board after lamination, and it is difficult to correct even if the pressure is added to the drying board. Many of the reasons for the warpage of the multilayer board are that the warp and weft direction of the prepreg are not clearly distinguished during lamination, which is caused by random stacking. How to distinguish longitude and latitude? For copper foil, the long side is latitudinal while the short side is longitude. If it is uncertain, please contact the manufacturer or supplier.

After milling the resin in the oven for 4 hours, remove the stress from the laminate after 4 degrees centigrade.

5. It is necessary to straighten the thin plate during electroplating: when 0.4 ~ 0.6 mm ultra-thin multilayer board is used for plate surface electroplating and graphic electroplating, special clamping roller shall be made. After the thin plate is clamped on the flying bus on the automatic electroplating line, the clamping roller on the whole flying bus shall be connected with a round rod to straighten all the plates on the roller, so that the plate after electroplating will not be deformed. If there is no such measure, the thin plate will bend after electroplating 20-30 micron copper layer, and it is difficult to remedy.

6. Board cooling after hot air leveling: PCB is impacted by high temperature of solder bath (about 250 ℃) during hot air leveling. After being taken out, it should be placed on flat marble or steel plate for natural cooling, and then sent to the post processor for cleaning. This is good for anti warping of the board. In order to enhance the brightness of the surface of lead and tin, some factories put the boards into cold water immediately after hot air leveling, and take them out after a few seconds for post-treatment. The impact of one hot and one cold may cause warping, delamination or blistering on some types of boards. In addition, air floating bed can be installed on the equipment for cooling.

7. Warping board treatment: in the factory with orderly management, 100% flatness inspection will be conducted for printed board in the final inspection. All unqualified boards will be picked out and put into the oven, dried at 150 ℃ and under heavy pressure for 3-6 hours, and naturally cooled under heavy pressure. Then take out the boards by pressure relief and check the flatness. In this way, some boards can be saved. Some boards need to be dried and pressed twice or three times to level them. The pneumatic board warping straightening machine of Shanghai Huabao agent has been used by Shanghai Bell, which has a very good effect in repairing PCB warpage. If the above anti warping process measures are not implemented, some boards are useless and can only be scrapped.