PCB deformation(bending) analysis, how to improve ?

After the circuit board are mostly prone to bending plate bending after reflow, serious cases even cause components such as air welding, erection and so on, how to overcome it?

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1, PCB circuit board deformation hazards

In the automated surface mount line, if the circuit board is not flat, will cause the positioning is not allowed, the components can not be inserted or mounted on the board hole and surface mount pads, or even crash automatic insertion machine. Circuit boards fitted with components bent after welding, component legs is difficult to cut neatly. Board can not be installed into the chassis or the machine's socket, so the assembly plant hit the board Alice is also very troublesome. The current surface mount technology is moving in the direction of high-precision, high-speed, intelligent direction, which on the PCB as a variety of components and home made a higher degree of flatness requirements.

The IPC standard specifies that the maximum allowable deformation of a PCB with a surface mount device is 0.75% and the maximum allowable amount of deformation of a PCB without a surface mount is 1.5%. In fact, to meet the needs of high-precision and high-speed placement, some manufacturers of electronic assembly deformation requirements more stringent.

PCB board by the copper foil, resin, glass cloth and other materials, the physical and chemical properties of the material are not the same, together with the pressure will inevitably generate thermal stress residues, resulting in deformation. At the same time in the PCB processing, high temperature, mechanical cutting, wet processing and other processes, will have a significant impact on the deformation of the board, in short, can lead to complex and diverse reasons for PCB deformation, how to reduce or eliminate due to material properties Distortion caused by different or processing becomes one of the most complex problems PCB manufacturers face.

2, analysis of the causes of deformation

The deformation of PCB board needs to be studied from the aspects of material, structure, pattern distribution and machining process. This article will analyze and elaborate various reasons that may cause deformation and improvement methods.

Copper surface area on the circuit board uneven, will deteriorate the bending and board Alice.

The general circuit board will be designed with a large area of copper foil as a grounding, and sometimes the Vcc layer will have a large area of the design of copper foil, when these large copper foil can not be evenly distributed in the same circuit board On the occasion, it will cause uneven heat absorption and cooling issues, of course, the circuit board will be thermal expansion and contraction, if the expansion can not be caused at the same time different stress and deformation, this time the temperature of the board if it has reached Tg value of the ceiling, the board will begin to soften, resulting in 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 the rivets will have the same connection point (vias) between the layers. The connection points are further divided into vias, blind holes and buried holes. Where there is a connection point, the board will be limited The effect of rising and shrinking, will also indirectly cause the plate bend and board Alice.

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

General reflow oven will use the chain to drive the circuit board in the reflow furnace forward, that is, when both sides of the board when the fulcrum propped up the entire board, if the board has overloaded parts, or the size of the board is too large, It will because of their own species and showing the phenomenon of the middle of the depression, resulting in bending.

V-Cut depth and connection bar will affect the amount of puzzle deformation

Basically V-Cut is the culprit of the destruction of the structure of the board, because V-Cut is the original cut a large sheet of the groove, so the V-Cut where the deformation is easy.

2.1 Laminate material, structure, graphics on the plate deformation analysis

PCB board by the core and the prepreg and the outer layer of copper laminated together, wherein the core plate and the copper foil under heat deformation, the amount of deformation depends on the thermal expansion coefficient of two materials (CTE)

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

The general FR-4 substrate in the Tg point under the Z direction CTE;

TG point above (250 ~ 350) X10-6, X to the CTE due to the existence of glass cloth, usually similar to the copper foil.

Notes on the TG point:

High Tg PCB When the temperature rises to a certain area, the substrate will change from "glassy state" to "rubbery state". The temperature at this time is called the glass transition temperature (Tg) of the board. That is, Tg is the maximum temperature (° C) at which the substrate remains rigid. In other words, ordinary PCB substrate materials at high temperatures, not only produce softening, deformation, melting and other phenomena, but also in the mechanical and electrical properties of the sharp decline.

The general Tg plate is above 130 degrees, high Tg generally greater than 170 degrees, medium Tg is greater than 150 degrees.

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

The Tg of the substrate is increased, the heat resistance, moisture resistance, chemical resistance, and stability characteristics 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 means high heat resistance. With the rapid development of the electronics industry, especially the electronic products represented by computers, the high heat resistance of the PCB substrate material is an important guarantee toward the development of high functionality and high multilayered technology. With SMT, CMT as the representative of the high-density installation of the emergence and development of the PCB in the small aperture, fine lines, thinner, more and more inseparable from the substrate with high heat resistance.

Therefore, the difference between FR-4 and FR-4 having a high Tg is that the mechanical strength, dimensional stability, adhesiveness, water absorption and thermal decomposability of the material under heat, especially after being absorbed by moisture, , Thermal expansion and other conditions are different, high Tg products were significantly better than ordinary PCB substrate materials.

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Wherein the expansion of the core pattern of the inner pattern is different due to the distribution of the pattern and the thickness of the core or the characteristics of the material. When the pattern distribution is different from the thickness of the core or the material characteristics, when the patterns are distributed uniformly and the types of the materials are the same, Will have deformation. When the PCB laminate structure asymmetry or uneven distribution of the graphics will lead to different core plate CTE larger differences in the process of compression deformation. The deformation mechanism can be explained by the following principle.

Figure 1 ordinary prepreg dynamic viscosity curve

Suppose there are two kinds of core plates with large difference in CTE. The CTE of A core plate is 1.5x10-5 / ℃ and the core plate length is 1000mm. The prepreg as the prepreg for the bonding sheet is softened, flowed, and filled with the pattern, and the three cores are cured to bond the two core sheets together.

Figure 1 shows the dynamic viscous bottom curve of ordinary FR-4 resin at different heating rates. Under normal circumstances, the material starts to flow from about 90 ℃ and begins to cross-link and cure above the TG point. The prepreg is free before curing, In this case, the core and the copper foil are in free expansion after being heated, and the deformation amount can be obtained through the respective CTE and the temperature change value.

Simulated pressure conditions, the temperature rose from 30 ℃ to 180 ℃,

At this point two kinds of core plate deformation are

Δ LA = (180 ° C. to 30 ° C.) × 1.5 × 10 -5 m / ° C. × 1000 mm = 2.25 mm

△ LB = (180 ℃ ~ 30 ℃) X2.5X10-5M / ℃ X1000mm = 3.75mm

At this point due to semi-curing is still free, the two core a long one short, non-interfering, has not yet deformed.

See Figure 2, the pressure will be maintained at high temperatures for some time, until the semi-cured fully cured, this time the resin becomes solidified state, can not flow freely, the two core plate together .When the temperature drops, Resin bound, the core plate will return to its original length, and does not produce deformation, but in fact the two core plate has been cured at high temperature resin bonding, in the cooling process can not arbitrarily shrink, which A core plate should be shrunk 3.75 mm, in fact, when the shrinkage is greater than 2.25mm A core will be hindered, in order to achieve the balance of forces between the two core, B core can not shrink to 3.75mm, and A core contraction will be greater than 2.25mm, so that the whole Plate to the direction of the B core distortion, shown in Figure 2.

Figure 2 Different CTE core lamination process deformation diagram

According to the above analysis shows that the PCB laminate structure, the type of material has a graphical distribution is uniform, a direct impact on the different core and copper foil between the CTE differences in the compression process will rise and fall through the prepreg sheet The process is retained and eventually the deformation of the PCB board.

2.2 PCB board deformation caused during processing

PCB deformation causes of the process is very complex and can be divided into two kinds of stress caused by thermal stress and mechanical stress. Among them, the thermal stress is mainly generated during the lamination process. The mechanical stress mainly causes the pallets to be stacked, transported and baked. The following process order to do a simple discussion.

CCL incoming: CCL are double-sided, symmetrical structure, no graphics, copper foil and glass cloth CTE is almost the same, so in the lamination process will hardly result in CTE caused by different deformation. However, the large size of CCL presses and the temperature difference in different areas of the hot plate will result in slight differences in the curing speed and degree of resin in different areas during the lamination process, and the dynamic viscosity at different heating rates will also vary greatly Local stress due to differences in curing process. Generally, the stress will maintain the balance after press-fitting, but will be gradually released and deformed in the future processing.

Compression: PCB lamination process is the main process of thermal stress, which due to the deformation of the material or structure, see the previous section analysis. Similar to CCL, local stress caused by the difference of curing process will also occur. Due to the thicker PCB, more diverse patterns and more prepreg, the thermal stress will be more difficult to eliminate than the CCL. However, the stress existing in the PCB board is released during subsequent drilling, contouring, or barbecuing, resulting in deformation of the board.

Solder Mask, Characters, etc. Baking Process: As the solder mask ink can not be stacked with each other during solidification, the PCB board will be erected in the shelf to be cured. The solder mask temperature is about 150 ℃, which just exceeds the Tg point of the medium- Point above the resin is a high-elastic state, the plate easily deformed under the weight or oven strong wind.

Hot air solder leveling: common board hot air solder leveling furnace temperature is 225 ℃ ~ 265 ℃, time is 3S-6S. Hot air temperature is 280 ℃ ~ 300 ℃. When the solder flat into the tin furnace from room temperature, baked within two minutes after the room temperature for post-treatment washing. The entire hot-air solder leveling process for the rapid thermal quenching process. Due to the different circuit board materials, the structure is not uniform, thermal stress will inevitably occur in the process of hot and cold, resulting in micro-strain and the overall distortion Alice area.

Storage: PCB board storage in the semi-finished products are generally stuck in the rack, the shelf is not properly adjusted, or stacked during the storage plate will cause the plate mechanical deformation. Especially for the 2.0mm below the impact of more serious sheet.

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

3, to improve the response

Then how can we prevent the board from going over the furnace to bend and bend?

1. Reduce the impact of temperature on the board stress

Since "temperature" is the main source of board stress, it is possible to greatly reduce the occurrence of slab and board warpage simply by reducing the temperature of the reflow oven or by slowing down the speed of warming and cooling of the board in the reflow oven. But there may be other side effects on the job.

2. The use of high Tg plate

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The Tg is the glass transition temperature, which is the temperature at which the material changes from the glassy state to the rubbery state. The lower the Tg, the faster the board begins to soften as it enters the reflow oven and becomes soft rubbery Will become longer, the deformation of the board will of course be more serious. The use of higher Tg sheet can increase its ability to withstand stress and deformation, but the relative price of materials is relatively high.

3. Increase the thickness of the circuit board

Many electronic products in order to achieve a more thin purpose, the thickness of the board has left 1.0mm, 0.8mm, and even made a thickness of 0.6mm, this thickness to keep the board after the reflow oven is not deformed, really a bit hard to find, It is recommended that if there is no frivolous requirement, it is best to use the 1.6mm thickness of the board, which can greatly reduce the risk of bending and deformation.

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

Since most of the reflow ovens use chains to drive the board forward, larger size boards deform in the reflow oven due to their own weight, so try to place the long side of the board as a board edge The reflow oven chain, you can reduce the weight of the circuit board caused by the depression deformation, the number of puzzle is reduced for this reason, that is to say when the furnace, as far as possible with the narrow side vertical furnace direction, you can reach the minimum The amount of depression deformation.

5. Used furnace tray fixture

If all of the above is hard to come by, the last step is to use a reflow carrier / template to reduce the amount of deformation. The reason that the furnace tray can reduce the warp of the slab is that both the warp and the warp shrink The tray can hold the circuit board until the circuit board temperature is below Tg began to re-harden, but also to maintain the size of the park.

If a single tray can not reduce the deformation of the circuit board, you must add a layer of cover, the circuit board with two upper and lower pallet clamp, which can greatly reduce the board over the deformation of the furnace problem. However, the oven tray expensive, but also have to add labor to set aside and recovery tray.

6. Use real connection, stamp hole, instead of V-Cut's sub-board use

Since V-Cut destroys the structural strength of boards, try not to use V-Cut boards or reduce the depth of V-Cut.

PCB production engineering optimization:

Effect of different materials on plate deformation

The deformation of different materials exceeding the standard defect rate statistics, the results in Table 1.

It can be seen from the table that the deformation defect rate of the low Tg material is higher than that of the high Tg material. All the high Tg materials listed in the above table are filler materials, and the CTEs are smaller than the low Tg materials. Meanwhile, during the post- Baking temperature up to 150 ℃, the impact of low-Tg materials will certainly be greater than the high-Tg materials.

Engineering Design Research

Engineering design should try to avoid structural asymmetry, material asymmetry, asymmetric graphics design, in order to reduce the generation of deformation, at the same time in the course of the study also found that the core direct compression structure is easier to deform than copper foil compression structure, Table 2 Test results of two kinds of structural plates.

It can be seen from Table 2 that the defect rates of the two structures failed to conform to each other obviously. It can be understood that the sandwich structure consists of three core plates, and the expansion and contraction between different core plates and the stress changes are more complicated and more difficult to eliminate .

In engineering design, the puzzle frame form also has a greater impact on the deformation, the general PCB factory there will be continuous large copper frame and non-continuous copper or copper border, but also different.

Table 3 for the two border design plate comparison test results. The reason why the two forms of border deformation is different is that the continuous shaped copper frame has high strength and is relatively rigid in the process of laminating and assembling so that the residual stress in the panel can not be easily released and concentrated in the shape after processing to be released. Lead to more serious deformation. The non-continuous copper border is in the process of lamination and subsequent release of stress gradually, in the shape of veneer less deformation.

The above is a small engineering design may involve some of the possible influencing factors, such as flexibility in the design. Can reduce the deformation caused by the design impact.

3.3 compression research

Pressing on the deformation is crucial, through a reasonable set of parameters, press selection and stacking methods can effectively reduce the generation of stress. For the general structure of the symmetrical plate, the general need to pay attention to the time of symmetry laminated plate, and placed symmetrically on the tool plate, cushioning materials and other auxiliary tools. At the same time select the integration of hot and cold press to reduce thermal stress also has obvious help, because the hot and cold splitter at high temperatures (GT temperature above) the plate to the cold press, the material below the Tg point and the pressure Rapid cooling will lead to the rapid release of thermal stress deformation, and integrated hot and cold press can be achieved at the end of the cooling temperature, to avoid the plate under high temperature loss of pressure.

At the same time, for the special needs of customers, there will inevitably be some material or structural asymmetry of the plate, then the analysis of the CTE caused by the deformation will be very obvious, in response to this problem we can try to use asymmetric The principle of which is to solve the problem of inconsistent deformation due to the asymmetric placement of buffer material to achieve different speeds of double-sided heating of the PCB, thereby affecting the rise and fall of different CTE core cypress trees during the heating and cooling phases. Table 4 is the result of the test on an asymmetric plate of a section of our company.

Through the asymmetric stacking method, as well as after the lamination increases the post-curing process and leveling before shipment, the board finally meets the customer 2.0mm requirement.

3.4 Other production processes

PCB production process, in addition to pressure welding there are solder mask, characterization and hot air leveling several high temperature treatment process, including solder mask, the character after the baking temperature of 150 ℃ above mentioned in this temperature in the ordinary Tg materials Tg point above, this time the material is high-elastic state, easy to deform under external force, so to avoid drying the stack plate to prevent the lower plate is bent, while drying plate to ensure that the plate direction and hair direction parallel. In the hot air leveling process, we must ensure that the plate out of the tin furnace flat cooling 30s or more, to avoid cold water after treatment under high temperature caused by quench deformation.

In addition to the production process, the PCB board pieces stored in the various positions also have a certain impact on the deformation, in some manufacturers due to more to be produced, the reasons for the small space, will be stacked together multi-board storage, which will lead to board Pieces by external deformation, due to the PCB board also has some plastic, so these deformation in the back of the leveling process will not get 100% recovery.

3.5 before school leveled

Most PCB manufacturers will have a flat flow process before shipment, which is due to the processing process will inevitably produce heat or mechanical deformation of the plate, before shipment by mechanical leveling or thermal baking level can be Be effectively improved. By the solder mask and the surface coating of the heat resistance, the general baking temperature of 140 ℃ ~ 150 ℃ below, just beyond the ordinary material Tg temperature, which is a great benefit to ordinary plate leveling, and for high Tg materials The leveling effect is less pronounced, so the temperature of the baking sheet can be appropriately increased on the high Tg sheets with individual sheets, but the quality of the main ink and coating is essential. At the same time when drying plate pressure, increase with the furnace cooling time practices also have some improvement on the deformation effect, Table 5 for different pressure and furnace cooling time plate leveling effect test results, from which you can see increased pressure And to extend the furnace cooling time on the deformation of leveling have a significant role.