Most of the PCB boards are prone to board bending and warping after reflow soldering. In severe cases, it may even cause empty soldering of components, tombstones, etc. How to overcome it?
1. The hazards of PCB board deformation
If the circuit board is not flat on the automatic surface mount line, it will cause inaccurate positioning and the components cannot 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 loaded with components is bent after soldering, and it is difficult to cut the component feet neatly. The board also cannot be installed on the chassis or the socket inside the machine, so it is also very troublesome for the assembly plant to encounter board warping. The current surface mount technology is developing in the direction of high precision, high speed, and intelligence, which puts forward higher flatness requirements for PCB boards as the home of various components.
In the IPC standard, it is specifically pointed out that the maximum deformation allowed for PCB boards with surface mount devices is 0.75%, and the maximum deformation allowed for PCB boards without surface mount devices 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, many customers in our company require that the maximum allowable deformation amount is 0.5%, and some customers even require 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 being pressed together, thermal stress residue will inevitably occur, resulting in deformation. At the same time, in the process of PCB processing, various processes such as high temperature, mechanical cutting, and wet treatment will also have an important impact on the deformation of the board. In short, the reasons for the deformation of the PCB board are complex and diverse. Distortion caused by processing has become one of the most complex problems faced by PCB manufacturers.
2. Analysis of the causes of PCB board deformation
The deformation of the PCB board needs to be studied from several aspects such as material, structure, pattern distribution, and processing process. This article will analyze and explain the various reasons and improvement methods that may cause deformation.
The area of the copper surface on the circuit board is uneven, which will worsen the bending and warping of the board.
Generally, a large-area copper foil is designed on the circuit board for grounding, and sometimes the Vcc layer is also designed with a large-area copper foil. When these large-area copper foils cannot be evenly distributed on the same circuit board When it is on, it will cause the problem of uneven heat absorption and heat dissipation speed. Of course, the circuit board will also expand with heat and contract with cold. If the expansion and contraction cannot be performed at the same time, it will cause different stresses and deformation. At the upper limit of the value, the board will start to soften, causing permanent 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 will be rivet-like connection points (vias) between the layers. The connection points are divided into through holes, blind holes and buried holes. The places with connection points will limit the board from cooling The effect of shrinkage will also indirectly cause board bending and board warping.
Reasons for PCB board deformation:
(1) The weight of the circuit board itself will cause the board to sag and deform
Generally, the reflow furnace will use the chain to drive the circuit board forward in the reflow furnace, that is, use the two sides of the board as the 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 a phenomenon of depression in the middle due to the amount of its own species, causing the plate to bend.
(2) The depth of V-Cut and the connecting strip will affect the deformation of the panel
Basically, V-Cut is the culprit that destroys the structure of the board, because V-Cut cuts grooves on the original large board, so the V-Cut is prone to deformation. 2.1 Response Analysis of Lamination Materials, Structures and Graphics to Plate Deformation
The PCB board is composed of a core board, a prepreg, and an outer layer of copper foil. The thermal deformation of the core board and the copper foil depends on the thermal expansion coefficient (CTE) of the two materials; the thermal expansion coefficient (CTE) of the copper foil ) is about 17×10-6;
The CTE of ordinary FR-4 substrate in Z direction under Tg point is (50~70)×10-6;
Above the TG point is (250~350)X10-6, and the CTE in the X direction is generally similar to copper foil due to the presence of glass cloth.
Notes on TG points:
When the temperature of high Tg printed boards rises to a certain area, the substrate will change from "glass state" to "rubber state", and the temperature at this time 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, ordinary PCB substrate materials not only soften, deform, melt and other phenomena under high temperature, but also show a sharp decline in mechanical and electrical properties.
Generally, the Tg of the plate is above 130 degrees, the high Tg is generally greater than 170 degrees, and the medium Tg is greater than 150 degrees. Generally, PCB printed circuit boards with Tg ≥ 170°C are called high Tg printed circuit boards.
The Tg of the substrate is increased, and 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, there are more high Tg applications.
High Tg circuit board refers to high heat resistance. With the rapid development of the electronics industry, especially electronic products represented by computers, the development of high-functionality and high-multilayer requires higher heat resistance of PCB substrate materials as an important guarantee. The emergence and development of high-density mounting technologies represented by SMT and CMT have made PCB more and more inseparable from the support of high heat resistance of the substrate in terms of small aperture, fine line, and thinning.
Therefore, the difference between general FR-4 and high Tg FR-4 is that in a hot state, especially under heat after moisture 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.
Among them, the expansion of the core board with the inner layer graphics is different due to the difference between the graphics distribution and the core board thickness or material properties. When the graphics distribution is different from the core board thickness or material properties. deformed. When there is asymmetry in the laminated structure of the PCB board or the distribution of the graphics is uneven, it will cause a large difference in the CTE of different core boards, resulting in deformation during the lamination process. Its deformation mechanism can be explained by the following principle.
Assume that there are two core boards with a large difference in CTE, which are pressed together by prepreg. The CTE of A core board is 1.5×10-5/C, and the length of the core board is 1000mm. In the pressing process, the prepreg, which is used as a bonding sheet, goes through three stages of softening, flowing and filling graphics, and curing to bond the two core boards together.
Figure 1 shows the dynamic bonding curves of ordinary FR-4 resin at different heating rates. Under normal circumstances, the material starts to flow from about 90°C, and starts to cross-link and solidify after reaching the TG point. Before curing, the prepreg is in a free state. When the core board and copper foil are in the state of free expansion after being heated, the deformation can be obtained by their respective CTE and temperature change values.
Simulate pressing conditions, the temperature rises from 30°C to 180°C,
At this time, the deformations of the two core plates are
△LA=(180°C~30°C)x1.5×10-5m/CX1000mm=2.25mm-LB=(180°C~30°C)X2.5X10-5M/CX1000mm=3.75mm
At this time, because the semi-cured is still in a free state, the two core boards are long and short, do not interfere with each other, and have not yet deformed.
When 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 solidified state and cannot flow freely. The two core plates are combined together. When the temperature drops, if there is no interlayer resin Bound, the core board will return to the original length without deformation, but in fact, the two core boards have been bonded by the cured resin at high temperature, and cannot shrink at will during the cooling process, and the A core board should shrink 3.75mm , in fact, when the shrinkage is greater than 2.25mm, it will be hindered by the A core board. In order to achieve the force balance between the two core boards, the B core board cannot shrink to 3.75mm, and the A core board will shrink more than 2.25mm, so that the entire board Bending in the direction of the core plate B, as shown in Figure 2.
Schematic representation of deformation during lamination of different CTE core plates
According to the above analysis, it can be seen that whether the laminated structure of the PCB board, the material type, and the pattern distribution are even, directly affect the CTE difference between different core boards and copper foils, and the difference in expansion and contraction during the pressing process will pass through the solid sheet of the prepreg. The process is retained and finally forms the deformation of the PCB board.
二、2 Deformation caused during PCB board processing
The reasons for the deformation of PCB board processing are very complicated and can be divided into two kinds of stresses: thermal stress and mechanical stress. Among them, the thermal stress mainly occurs during the pressing process, and the mechanical stress mainly occurs during the stacking, handling, and baking processes of the plates. The following is a brief discussion in the order of the process.
Incoming copper clad laminates: The copper clad laminates are all symmetrical double-sided panels, without graphics, and the CTE of copper foil and glass cloth is almost the same, so there is almost no deformation caused by different CTEs during the lamination process. However, there are temperature differences in different areas of the large-sized hot plate of the copper-clad laminate press, which will cause slight differences in the curing speed and degree of resin in different areas during the pressing process; Local stresses due to process differences. Generally, this kind of stress will maintain balance after pressing, but it will be gradually released and deformed in the future processing.
Lamination: The PCB lamination process is the main process for generating thermal stress, and the deformation due to different materials or structures can be seen in the analysis in the previous section. Similar to the lamination of copper clad laminates, local stress caused by differences in the curing process will also occur. Due to thicker thickness, various pattern distribution, and more prepregs, the thermal stress of PCB boards will also be more difficult to eliminate than copper clad laminates. However, the stress existing in the PCB board is released in subsequent processes such as drilling, shaping or grilling, resulting in deformation of the board.
Baking process of solder resist, characters, etc.: Since the solder resist inks cannot be stacked on each other when they are cured, the PCB boards will be placed vertically on the shelf. The resin above the point is in a highly elastic state, and the board is easily deformed under its own weight or strong wind in the oven.
Hot air solder leveling: The temperature of the tin furnace is 225°C~265°C and the time is 3S-6S when the ordinary board is leveled by hot air solder. The temperature of the hot air is 280°C~300°C. When the solder is leveled, the board is put into the tin furnace at room temperature and then washed with post-processing water at room temperature within two minutes. The whole hot air solder leveling process is a sudden heating and quenching process. Due to the different materials and uneven structure of the circuit board, thermal stress will inevitably occur during the heating and cooling process, resulting in microscopic strain and overall deformation warping.
Storage: The storage of PCB boards in the semi-finished stage is generally firmly inserted into the shelf, and the adjustment of the shelf is not suitable, or the boards are stacked during the storage process, which 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 other factors that affect the deformation of the PCB board.
三、Prevention of PCB warping and deformation
The warpage of the circuit board has a great influence on the production of the 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 cannot 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 entire subsequent process. At this stage, the printed circuit board has entered the era of surface mount and chip mount, and the process has higher and higher requirements for the warpage of the circuit board. So we have to find the reason for the warping of the halfway gang.
1. Engineering design: matters needing attention in printed circuit board design:
A. The arrangement of prepregs between layers should be symmetrical, for example, the thickness between layers 1~2 and 5~6 of a six-layer board should be the same as the number of prepregs, otherwise it is easy to warp after lamination.
B. Multi-layer core board and prepreg should use products from the same supplier.
C. The area of the circuit graphics on the outer A side and B side should be as close as possible. If the A side is a large copper surface, and the B side only has a few lines, this printed board is easy to warp after etching. If the line area of the two sides is too different, some independent grids can be added on the thin side for balance.
2. Baking board before blanking: The purpose of baking the copper clad laminate before blanking (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board, and at the same time completely cure the resin in the board to further eliminate the remaining stress in the board. It is helpful to prevent board warping. At present, many double-sided and multi-layer boards still insist on the step of drying the board before or after blanking. However, there are exceptions for some sheet metal factories. At present, the baking time of each PCB factory is inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of the printed board produced and the customer's requirements for warpage. Cut into jigsaw boards and then dry or the whole piece of large material is baked and then unloaded. Both methods are feasible. It is recommended to dry the board after cutting. The inner board should also be baked.
3. The warp and weft directions of the prepreg: the warp and weft shrinkage rates of the prepreg are different after lamination, and the warp and weft directions must be distinguished when blanking and laminating. Otherwise, it is easy to cause warping of the finished board after lamination, and it is difficult to correct even if the pressure is applied to the board. The reason for the warping of multi-layer boards; many of them are caused by the random stacking of prepregs without distinguishing the warp and weft directions during lamination. How to distinguish between warp and weft? The direction of rolled prepreg is the warp direction, while the width direction is the weft direction; for copper foil boards, the long side is the weft direction, and the short side is the warp direction. If you are not sure, you can ask the manufacturer or Supplier inquiries.
4. Stress removal after lamination; take out the multi-layer board after hot pressing and cold pressing, cut or mill off the burrs, and then place it flat in the oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and make the resin completely curing, this step cannot be omitted.
5. Thin plate needs to be straightened during electroplating: 0.4~0.6mm ultra-thin multi-layer plate should be made with special clamping spokes when plate surface plating and pattern electroplating. After clamping the thin plate on the flying bar on the automatic plating line, use a The rods string the clamp wheels on the entire flybar to straighten all the boards on the spokes, so that the boards after electroplating will not be deformed. Without this measure, the thin plate will bend after electroplating a copper layer of 20 to 30 microns, and it is difficult to remedy.
6. Cooling of the board after hot air leveling: When the printed board is hot air leveled, it is impacted by the high temperature of the solder bath (about 250 degrees Celsius). After it is taken out, it should be placed on a flat marble or steel plate to cool naturally, and then sent to the post-processing machine for cleaning. . This is very good for preventing warping of the board. In some factories, in order to enhance the brightness of the lead-tin surface, the board is put into cold water for a few seconds after hot air leveling, and then taken out for post-processing. This kind of heat-cold impact is likely to warp certain types of boards. , layered or foamed. In addition, an air flotation bed can be installed on the equipment for cooling.
7. Treatment of warped boards: in well-managed factories, printed boards will be 100% flatness checked during final inspection. All unqualified boards will be picked out, placed in an oven at 150 degrees Celsius and baked under heavy pressure for 3 to 6 hours, and naturally cooled under heavy pressure. Then release the pressure and take out the board, and check the flatness, which can save some boards, and some boards need to be baked two to three times to be leveled. The pneumatic board warpage straightening machine distributed by Shanghai Huabao has a very good effect in remediating the warping of circuit boards through the use of Shanghai Bell. If the above-mentioned anti-warping process measures are not implemented, some boards are useless for baking and pressing, and can only be scrapped.
四、PCB board warping becomes standard
Please refer to IPC-A-600G No. 2.11 flatness standard for PCB board warping standards. For printed boards with surface mount components (such as SMT mounting), the distortion and bow standards are not more than 0.75%, and other types of boards are not more than 1.5%. %. The test method refers to lPC-TM-6502.4.22.
