Causes and Improvements of PCB Deformation

Most of the circuit boards are prone to board warpage when they are reflowed. In severe cases, they may even result in empty soldering of components, erection of monuments, etc. How should they be overcome?

1, PCB circuit board deformation hazard

In an automated surface mount line, if the circuit board is not flat, it can cause misalignment. Components cannot be inserted or mounted on the board's holes and surface mount pads, and they can even damage the automatic insertion machine. The circuit board on which the components are mounted is bent after soldering, and the component legs are hardly trimmed and trimmed. The board can not be mounted on the chassis or the socket in the machine, so the assembly plant is also very annoying to encounter board tilt. The current surface mount technology is moving toward high precision, high speed, and intelligent development. This puts higher flatness requirements on the PCB boards used as various components and homes.

In the IPC standard, it was specifically pointed out that PCB boards with surface mount devices allow for a maximum deformation of 0.75%, and PCB boards without surface mount allow a maximum deformation of 1.5%. In fact, in order to meet the demand for high-precision and high-speed placement, some manufacturers of electronic assembly have more stringent requirements on the amount of deformation. For example, if our company has multiple customer requirements, the maximum deformation 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 the materials are not the same. After being pressed together, thermal stress residuals are inevitable, resulting in deformation. At the same time, in the processing of PCB, various processes such as high temperature, mechanical cutting, and wet processing will also have an important influence on the deformation of the plate. In short, the causes of deformation of the PCB can be complicated and various, and how to reduce or eliminate the material properties Different or processing-induced deformations have become one of the most complex problems faced by PCB manufacturers.

2, analysis of the causes of deformation

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 the various causes and improvement methods that may cause distortion.

The uneven area of the copper surface on the circuit board will deteriorate the board bend and the board warpage.

A large area of copper foil is commonly used on the circuit board as grounding. Sometimes the Vcc layer also has a large area of copper foil. When these large-area copper foils are not evenly distributed on the same circuit board, On the time, it will cause the problem of non-uniform heat absorption and cooling speed. Of course, the circuit board will also expand and shrink. If the expansion and contraction can not be caused at the same time will cause different stress and deformation, when the temperature of the board has reached With the upper limit of Tg, the board will begin to soften, causing permanent deformation.

The vias (vias) on each layer of the board limit the board's expansion

Today's circuit boards are mostly multi-layer boards, and there are rivets like vias between layers. The connection points are divided into through holes, blind holes and buried holes, where the connection points restrict the board. The effect of rising cold contraction will also indirectly lead to board bending and board warping.

The weight of the board itself can cause the board to sag

Generally, the reflow oven will use the chain to drive the circuit board to advance in the reflow furnace, that is, when the support points hold the whole board on both sides of the board, if the board has excessive parts or the board is oversized, It will show the phenomenon of depression in the middle due to its own species, resulting in a board bend.

The depth of the V-Cut and the connection bars affect the deformation of the panel

Basically, V-Cut is the culprit of the destruction of the board structure, because V-Cut is to cut grooves in the original large sheet, so the V-Cut place is easily deformed. (Related reading: Board to board - V-Cut board machine)

2.1 The analysis of the deformation of the plate by pressing material, structure and pattern

The PCB board is formed by laminating the core board and the prepreg and the outer layer copper foil, wherein the core board and the copper foil are heated and deformed when they are pressed together, and the deformation amount depends on the coefficient of thermal expansion (CTE) of the two materials.

The coefficient of thermal expansion (CTE) of the copper foil is about

The normal FR-4 substrate at the Tg point Z to CTE;

Above the TG point is (250~350)X10-6, X-direction CTE is generally similar to copper foil due to the presence of glass cloth.

*Note about TG points:

High Tg printed boards When the temperature rises to a certain area, the substrate will change from "glassy" to "rubbery", and the temperature at that time is called the glass transition temperature (Tg) of the board. That is, Tg is the maximum temperature (° C.) at which the substrate maintains rigidity. In other words, ordinary PCB substrate materials not only produce softening, deformation, melting, etc. at high temperatures, but also show a sharp drop in mechanical and electrical properties.

 General Tg plate is above 130 degrees, high Tg is generally greater than 170 degrees, medium Tg is greater than about 150 degrees.

PCB printed boards with Tg≥170°C are usually called high Tg printed boards.

The Tg of the substrate is improved, and the characteristics such as heat resistance, moisture resistance, chemical resistance, and stability resistance 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.

High Tg refers to high heat resistance. With the rapid development of the electronics industry, electronic products represented by computers, in particular, are moving toward higher functionality and higher levels of multi-layered development. The need for higher heat resistance of PCB substrates is an important guarantee. The emergence and development of high-density mounting technologies represented by SMT and CMT have made PCBs increasingly inseparable from the substrate's high heat resistance in terms of small apertures, fine lines, and thinness.

Therefore, the difference between the general FR-4 and the high Tg FR-4 is: the mechanical strength, dimensional stability, adhesiveness, water absorption, and thermal decomposition of the material in the hot state, especially after heat absorption under 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 in which the inner layer pattern is made is different because the pattern distribution is different from 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, the pattern distribution is uniform and the material type is the same, and It will be deformed. When there is asymmetry in the laminated structure of the PCB board or the uneven distribution of the patterns, the CTEs of different core boards will be greatly different, resulting in deformation during the lamination process. The deformation mechanism can be explained by the following principles.(Fig.1 Dynamic viscosity curve of common prepreg)

Assume that there are two kinds of core boards with large differences in CTEs that are pressed together by a prepreg, in which the core board ATE has a CTE of 1.5x10-5/°C and the core board length is 1000 mm. In the prepreg as a bonding sheet during the lamination process, the two core plates are bonded together in three stages of softening, flowing and filling the pattern and curing.

Fig. 1 shows the dynamic adhesion bottom curve of ordinary FR-4 resin at different heating rates. Under normal circumstances, the material begins to flow from about 90°C, and begins to crosslink and solidify above the TG point. The prepreg is free before curing. At this moment, the core plate and the copper foil are in a state of free expansion after being heated, and the deformation amount can be obtained through the respective CTE and the temperature change value.

Analog compression conditions, temperature rise from 30°C to 180°C,

At this point, 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 point, since the semi-curing is still in the free state, the two core plates are one long and one short, and do not interfere with each other and have not yet undergone deformation.

As shown in Fig. 2, when pressed, it will remain at a high temperature for a period of time until the semi-curing is completely cured. At this time, the resin becomes a solidified state and cannot flow freely. The two kinds of core plates are combined together. When the temperature drops, there is no layer between layers. Resin bound, the core board will return to its original length, and will not be deformed, but in fact the two core boards are already bonded by the cured resin at high temperature, and they cannot shrink freely during the cooling process, in which the A core board should shrink 3.75 Mm, in fact, when the contraction is greater than 2.25mm, it will be hindered by the A core board. To achieve the balance between the two cores, the B core board cannot shrink to 3.75mm, and the A core board shrinks more than 2.25mm, which makes the whole The board buckles toward the B core board, as shown in Figure 2.(Fig.2 Deformation of different CTE core plates during press-fitting)

Based on the above analysis, it can be seen whether the laminated structure and material type of the PCB have been evenly distributed, directly affecting the CTE difference between different cores and copper foils, and the difference in shrinkage during the lamination process will pass through the prepreg. The process is preserved and eventually forms a deformation of the PCB board.

2.2 Deformation caused during PCB board processing

The cause of the deformation of the PCB board process is very complex and can be divided into two kinds of thermal stress and mechanical stress. The thermal stress is mainly generated during the lamination process, and the mechanical stress is mainly generated during stacking, handling, and baking of the panel. The following is a brief discussion in the order of processes.

Copper clad materials: Double-sided copper clad plates, symmetrical structure, no graphics, copper foil and glass cloth CTE almost the same, so in the compression process almost no deformation caused by the CTE. However, the size of the copper clad laminate press is large, and there are temperature differences in different regions of the hot plate, which may lead to slight differences in the speed and degree of curing of the resin in the different regions during lamination. At the same time, the dynamic viscosity at different heating rates also differs greatly. Local stress due to differences in the curing process. Generally this kind of stress will maintain equilibrium after pressing, but will gradually release deformation in the future processing.

Pressing: The PCB lamination process is the main process for generating thermal stress. The deformation due to different materials or structures is discussed in the previous section. Similar to the lamination of copper clad laminates, local stress caused by differences in the curing process may also occur. Due to the thicker PCBs, various pattern distributions, and more prepreg, the thermal stress of PCBs may be more difficult to eliminate than clad laminates. The stress in the PCB board is released in the process of subsequent drilling, shape, or grilling, resulting in deformation of the board.

Soldering process such as solder resist, characters, etc.: Since the solder resist ink cannot be stacked on each other when cured, the PCB board will be cured in a rack by baking. The solder resist temperature is about 150°C, just beyond the Tg point of the low-to-medium Tg material. Tg The resin above the point is highly elastic, and the plate is easily deformed under its own weight or the strong wind of the oven.

Hot-air solder leveling: The temperature of the tin furnace is 225°C~265°C and the time is 3S-6S. The hot air temperature is 280°C~300°C. When the solder is leveled, the board goes from the room temperature to the tin furnace, and after two minutes from the furnace, the room temperature water treatment is performed. The entire hot-air solder leveling process is a sudden heat quenching process. Due to the different materials of the circuit board and the uneven structure, thermal stress will inevitably occur in the cold and hot processes, leading to microscopic strain and the overall deformable area.

Storage: The PCB board is usually stored in the shelf during the semi-finished stage. If the shelf is not properly adjusted, or the stacking plate is placed during storage, the board will be mechanically deformed. In particular, the impact of thin sheets below 2.0 mm is more severe.

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

3, improvement measures

How can we prevent the board from overturning in the furnace and cause board bending and board warping?

1. Reduce the effect of temperature on the stress of the board

Since "Temperature" is the main source of board stress, as long as the temperature of the reflow furnace is lowered or the temperature of the board is raised and cooled in the reflow oven, the situation of board bending and board warping can be greatly reduced. However, there may be other side effects.

2. Using high Tg plates

Tg is the glass transition temperature, which is the temperature at which the material changes from a glassy state to a rubbery state. The lower the Tg value, the faster the board starts to soften when it enters the reflow furnace and the time it becomes a soft rubbery state. It will also grow longer, and the more serious the deformation of the board will be. The use of a higher Tg plate can increase its ability to withstand stress deformation, but the relative cost of the material is also relatively high.

3. Increase the thickness of the circuit board

For many electronic products, the thickness of the board has been left to 1.0mm, 0.8mm, or even 0.6mm in order to achieve a thinner and lighter purpose. This kind of thickness is very difficult to keep the board from being deformed in the reflow furnace. It is recommended that if there is no light and thin requirement, the board can use a thickness of 1.6mm, which can greatly reduce the risk of bending and deformation of the board.

4. Reduce the size of the circuit board and reduce the number of panels

Since most of the reflow ovens use chains to drive the board forward, the larger board will be concave and deformed in the reflow oven due to its own weight, so try to put the long side of the board as a board edge. Reflow furnace chain, you can reduce the circuit board itself caused by the weight of the depression deformation, the number of puzzles is also reduced for this reason, that is, when passing the furnace, try to use the narrow side of the vertical furnace direction, can reach the lowest The amount of sag deformation.

5. Using the oven tray fixture

If all of the above methods are difficult to achieve, the final step is to use the reflow carrier/template to reduce the amount of deformation. The reason that the trays can reduce the plate bending is because it is desirable whether it is thermal expansion or shrinkage. The tray can hold the circuit board until the temperature of the circuit board is lower than the Tg value and then it can be hardened again. It can also maintain the size of the residence.

If a single-layer tray cannot reduce the amount of deformation of the circuit board, it is necessary to add another layer of cover to sandwich the circuit board with the upper and lower trays. This can greatly reduce the problem of deformation of the circuit board through the reflow oven. However, this tray is quite expensive, and it has to be manually placed and recycled.

6. Use real links and stamp holes instead of V-Cut's sub-boards

Since V-Cut will destroy the structural strength of the board-to-board puzzle, try not to use the V-Cut board or reduce the depth of the V-Cut.

Real connection: adopts a knife-type splitter

Stamp hole

Optimization in PCB production engineering:

Effect of Different Materials on Plate Deformation

The rate of deformation exceeding the standard for different material plates was counted. The results are shown in Table 1.

It can be seen from the table that the 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 materials, and the CTEs are all lower than those of low Tg materials. At the same time, during the processing after lamination, The maximum baking temperature is 150°C, and the effect on low Tg materials is definitely greater than that of medium-to-high Tg materials.

Engineering Design Research

Engineering design should try to avoid structural asymmetry, material asymmetry, graphic asymmetry design, in order to reduce the generation of deformation. At the same time, it was found in the research process that the core directly pressed structure is more easily deformed than the copper foil laminated structure. Table 2 shows that Test results for two structural panels.

From Table 2, it can be seen that there are obvious differences in the defect rate of the two structural deformation failures. It can be understood that the press-fitting structure of the core plate is composed of three core plates. The increase and decrease of stress between different core plates and the stress change are more complicated and difficult to eliminate. .

In the engineering design, the shape of the puzzle frame also has a great influence on the deformation. Generally, the PCB factory will have a continuous large copper skin frame and a non-continuous copper or copper frame border, and there are also different differences.

Table 3 shows the comparative test results of two types of frame design panels. The reason why the deformation of the two kinds of frame forms is different is that the continuous copper frame has high strength, and the rigidity is relatively large in the process of press-fitting and jigging, so that the residual stress in the plate is not easily released, and is concentrated after the profile processing is released. Lead to more serious deformation. The non-continuous copper point frame gradually releases the stress during press-fitting and subsequent processing, and the veneer deforms less after the appearance.

The above are some of the possible influencing factors involved in engineering design, such as flexible use in design. It can reduce the effect of deformation caused by the design.

3.3 Pressing Study

The effect of press-fitting on the deformation is very important, and the generation of stress can be effectively reduced through reasonable parameter setting, press selection, and stacking modes. For general structurally symmetrical plates, it is generally necessary to pay attention to the symmetrical stacking of the plates during press-fitting, and symmetrically place auxiliary tools such as tool plates and buffer materials. At the same time, the choice of hot and cold integrated press lamination also helps to reduce the thermal stress. The reason is that the hot and cold split press presses the plate to the cold press at high temperature (above the GT temperature) and the material loses pressure above the Tg point. Rapid cooling will cause rapid release of thermal stress and deformation, while the integrated cold and hot press can achieve temperature reduction at the end of hot-pressing to avoid pressure loss at high temperatures.

At the same time, for the special needs of customers, there will inevitably be some materials or structures with asymmetrical plates. At this time, the distortions brought by the previous analysis due to different CTEs will be very obvious. We can try to use asymmetrical for this problem. The method of stacking board to solve, the principle is that the asymmetric placement of the buffer material to achieve double-sided PCB board heating speed is not the same, thereby affecting the CTE core cypress in the heating and cooling stage of the increase and decrease to solve the problem of inconsistent deformation. Table 4 shows the test results on a structurally asymmetrical plate of our company.

Through asymmetric stacking, post-cure post-curing, and leveling operations prior to shipping, this board meets the customer's 2.0mm requirements.

3.4 Other production processes

In the PCB production process, in addition to lamination, there are several high-temperature processing processes such as solder resist, characterization, and hot air leveling. The maximum temperature of the solder mask after the solder mask and character is 150°C. This temperature is mentioned above in the common Tg material. Above the Tg point, the material is highly elastic and easily deformed under external forces. Therefore, it is necessary to avoid stacking of the laminate to prevent bending of the lower laminate during baking, and to ensure that the orientation of the panel is parallel to the blowing direction when the panel is baked. In the hot air leveling process, it is necessary to ensure that the plate out of the tin furnace cooling more than 30s, to avoid the high temperature after the cold water after the treatment caused by rapid deformation.

In addition to the production process, the storage of the PCB board at each station also has a certain influence on the deformation. In some manufacturers due to the need for more production and narrow space, multiple boards will be stacked and stored together, which will also lead to board failure. The pieces are deformed by external forces. Because the PCB board also has certain plasticity, these deformations will not be recovered 100% in the following leveling process.

3.5 leveling before shipment

Most PCB manufacturers have a leveling process before shipment. This is because the deformation of the plate due to thermal or mechanical forces is unavoidable in the machining process. It can be leveled by mechanical or heat baking before shipping. Effectively improved. Resistance to soldering and the heat resistance of the surface coating layer, the general baking temperature is below 140 °C ~ 150 °C, just beyond the ordinary material Tg temperature, which has a great advantage for the leveling of ordinary plates, and for high Tg materials The leveling effect is not so obvious, so the temperature of the baking plate can be properly increased on the high Tg plate with individual plate warping, but the quality of the main ink and coating layer is required. At the same time baking plate pressure, increase the furnace cooling time with the practice also has some improvement in the deformation, Table 5 for the different pressure and furnace cooling time on the plate leveling test results, from which you can see increased pressure Extended furnace cooling time has a significant effect on the leveling of deformation.(Different pressure and furnace cooling time leveling effect diagram)