PCB News - Analyze what are the reasons that affect the deformation of the PCB circuit board?​

The 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 various reasons and improvement methods that may occur.

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

Generally, a large area of copper foil is designed on the circuit board for grounding purposes. Sometimes there is also a large area of copper foil designed on the Vcc layer. When these large area copper foils cannot be evenly distributed on the same circuit board When it is installed, it will cause uneven heat absorption and heat dissipation. Of course, the circuit board will also expand and contract. If the expansion and contraction cannot be done at the same time, it will cause different stress and deformation. At this time, if the temperature of the board has reached At the upper limit of the Tg value, the board will begin 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 are rivet-like connection points (vias) between the layers. The connection points are divided into through holes, blind holes and buried holes. Where there are connection points, the board will be restricted. The effect of expansion and contraction will also indirectly cause plate bending and plate warping.

The weight of the circuit board itself will cause the board to dent and deform

Generally, the reflow furnace uses a chain to drive the circuit board forward in the reflow furnace, that is, the two sides of the board are used as fulcrums to support the entire board. If there are heavy parts on the board, or the size of the board is too large, It will show a depression in the middle due to the amount of seed, causing the plate to bend.

The depth of the V-Cut and the connecting strip will affect the deformation of the jigsaw

Basically, V-Cut is the culprit that destroys the structure of the board, because V-Cut cuts grooves in the original large sheet, so the V-Cut is prone to deformation.

2.1 Analysis of the influence of pressing materials, structures, and graphics on plate deformation

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

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

The Z-direction CTE of the ordinary FR-4 substrate at the Tg point is;

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

Notes on TG point:

When the temperature of a high Tg printed board rises to a certain area, the substrate will change from the "glass state" to the "rubber state". 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 base material maintains rigidity. That is to say, ordinary PCB substrate materials not only produce softening, deformation, melting and other phenomena at high temperatures, but also exhibit a sharp decline in mechanical and electrical properties.

Generally, the Tg of the board is above 130 degrees, the high Tg is generally greater than 170 degrees, and the medium Tg is about greater than 150 degrees.

Usually PCB printed boards with Tg≥170 degree Celsius are called high Tg printed boards.

As the Tg of the substrate is increased, 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, high Tg applications are more common.

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 multilayers 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 PCBs more and more inseparable from the support of 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 the mechanical strength, dimensional stability, adhesion, water absorption, and thermal decomposition of the material in the hot state, especially when heated after moisture absorption. 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 pattern is different due to the difference between the pattern distribution and 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, it will be different. Will be deformed. When the PCB laminate structure has asymmetry or uneven pattern distribution, the CTE of different core boards will vary greatly, resulting in deformation during the lamination process. The deformation mechanism can be explained by the following principles.

Suppose there are two core boards with a large difference in CTE that are pressed together by a prepreg, in which the CTE of the A core board is 1.5x10-5/ degree Celsius, and the length of the core board is both 1000 mm. In the pressing process, the prepreg, which is used as the bonding sheet, will bond the two core boards together through three stages of softening, flowing and filling with graphics, and curing.

The dynamic adhesion curve of ordinary FR-4 resin at different heating rates. Generally, the material starts to flow from about 90°C and cross-links and cures when it reaches above the TG point. The prepreg is in a free state before curing. At this time, the core The plate and copper foil are in a state of free expansion after being heated, and their deformation can be obtained by their respective CTE and temperature changes.

Simulate pressing conditions, the temperature rises from 30°C to 180°C,

At this time, the deformation of the two core plates are respectively

△LA=(180 degree Celsius~30 degree Celsius)x1.5x10-5m/ degree CelsiusX1000mm=2.25mm

△LB=(180 degree Celsius~30 degree Celsius)X2.5X10-5M/ degree CelsiusX1000mm=3.75mm

At this time, since the semi-cured is still in a free state, the two core plates are long and short, do not interfere with each other, and have not yet been deformed.

During 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 cured state and cannot flow freely. The two core plates are combined together. When the temperature drops, if there is no interlayer resin binding, the core The 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. The A core board should shrink by 3.75mm. 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 shrinks more than 2.25mm, so that the whole board is directed to the B core The direction of the board is warped.

According to the above analysis, it can be seen that whether the laminated structure and material type of the PCB board have been distributed uniformly, which directly affects the CTE difference between different core boards and copper foils. The difference in expansion and contraction during the lamination process will pass through the solid film of the prepreg. The process is retained and the deformation of the PCB board is finally formed.

2.2 Deformation caused during PCB processing

The reason for the deformation of PCB board processing is very complicated and can be divided into two kinds of stress: thermal stress and mechanical stress. Among them, the thermal stress is mainly generated during the pressing process, and the mechanical stress is mainly generated during the stacking, handling and baking of the plates. The following is a brief discussion in the order of the process.

Incoming copper clad laminate: The copper clad laminates are all double-sided, with symmetrical structure and no graphics. The CTE of copper foil and glass cloth is almost the same, so there is almost no deformation caused by the difference in CTE during the pressing process. However, the size of the copper clad laminate press is large, and the temperature difference in different areas of the hot plate will cause slight differences in the resin curing speed and degree in different areas during the pressing process. At the same time, the dynamic viscosity at different heating rates is also quite different, so it will also produce Local stress due to differences in curing process. Generally, this kind of stress will maintain balance after pressing, but will gradually release and deform during future processing.

Pressing: The PCB pressing process is the main process that generates thermal stress. The deformation due to different materials or structures is shown in the analysis in the previous section. Similar to the pressing of copper clad laminates, local stresses caused by differences in the curing process will also occur. PCB boards have more thermal stress than copper clad laminates due to thicker thickness, diverse pattern distribution, and more prepregs. The stress in the PCB board is released during subsequent drilling, shape, or grilling processes, causing the board to deform.

Baking process of solder mask, characters, etc.: Since solder mask inks cannot be stacked on top of each other when they are cured, PCB boards will be placed in a rack for curing. The solder mask temperature is about 150°C, which just exceeds the Tg point of medium and low Tg materials, Tg The resin above the point is highly elastic, and the plate is easily deformed under the action of its own weight or the strong wind of the oven.

Hot-air solder leveling: The temperature of the tin furnace is 225 degree Celsius~265 degree Celsius, and the time is 3S-6S when the ordinary board hot-air solder is leveled. The hot air temperature is 280 degree Celsius~300 degree Celsius. When the solder is leveled, the board is put into the tin furnace from room temperature, and the post-treatment water washing at room temperature will be carried out within two minutes after being out of the furnace. The entire hot-air solder leveling process is a sudden heating and cooling process. Due to the different materials of the circuit board and the uneven structure, thermal stress will inevitably appear during the cooling and heating process, leading to microscopic strain and overall deformation and warping area.

Storage: The storage of PCB boards at the stage of semi-finished products is generally firmly inserted in the shelf, and the tightness of the shelf is not adjusted properly, or the stacking of the boards during the storage process 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 factors that affect PCB deformation.