PCB Technical - How to improve PCB circuit board deformation

When circuit boards undergo reflow soldering, most of them are prone to board bending and warping. If it is serious, it may even cause components such as empty soldering and tombstones. How to overcome it?

The hazards of PCB circuit board deformation

In the automated surface mount line, if the circuit board is not flat, it will cause inaccurate positioning, 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 on which the components are installed is bent after soldering, and the component feet are difficult to cut neatly. The board cannot be installed in the case or the socket inside the machine, so it is also very annoying for the circuit board factory to encounter the 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 that are home to various components.

The PCB board is composed of copper foil, resin, glass cloth and other materials, and the physical and chemical properties of each material are different. After being pressed together, thermal stress will inevitably occur and cause deformation. At the same time, in the PCB processing process, it will go through various processes such as high temperature, mechanical cutting, wet treatment, etc., which will also have an important impact on the deformation of the board. In short, the reasons for the deformation of the PCB can be complex and diverse. How to reduce or eliminate the material characteristics Distortion or deformation caused by processing has become one of the most complicated problems faced by PCB manufacturers.

Analysis of the causes of deformation

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.

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. 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 prop up the entire board. If there are heavy parts on the board, or the size of the board is too large

If it is large, it will show the phenomenon of depression in the middle due to its own seed quantity, 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.

How to improve PCB circuit board deformation

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 heated and deformed when they are pressed together. The amount of deformation depends on:

The coefficient of thermal expansion (CTE) of copper foil and the coefficient of thermal expansion (CTE) of common FR-4 substrates.

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.

How to improve PCB circuit board deformation

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.

Whether the laminated structure, material type, and pattern distribution of the PCB board are uniform, directly affect the difference in CTE between different core boards and copper foils. The difference in expansion and contraction during the lamination process will be retained through the solidification process of the prepreg. Finally, the deformation of the PCB board is formed.

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.: As solder mask inks cannot be stacked on 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.