Why do PCB boards warp during processing?

1. Reasons for PCB warping

The main reasons for PCB warping are as follows:

First, the weight and size of the circuit board itself are too large, and the support points are located on both sides, which cannot effectively support the entire board, resulting in a concave deformation in the middle.

Secondly, the V-cut is too deep, which causes warping at the V-cut on both sides. V-cut is a groove cut on the original large sheet, so it is easy to cause the board to warp.

In addition, the material, structure, and pattern of the PCB will affect the board warping. The PCB is pressed by the core board, prepreg, and outer copper foil. The core board and copper foil will deform due to heat when pressed together. The amount of warping depends on the coefficient of thermal expansion (CTE) of the two materials.

2. Warping caused during PCB processing

The causes of PCB processing warping are very complicated and can be divided into thermal stress and mechanical stress. Among them, thermal stress is mainly generated during the pressing process, and mechanical stress is mainly generated during the stacking, handling and baking of the board.

1. In the process of incoming copper clad laminates, since the copper clad laminates are all double-sided, symmetrical in structure, without graphics, and the CTE of copper foil and glass cloth are almost the same, there is almost no warping caused by different CTE during the pressing process. However, during the pressing process, due to the large size of the press, the temperature difference in different areas of the hot plate will cause slight differences in the curing speed and degree of resin in different areas during the pressing process. At the same time, the dynamic viscosity at different heating rates is also quite different, so local stress will also be generated due to different curing processes. Generally, this stress will remain balanced after pressing, but will gradually release and deform during subsequent processing.

2. During the PCB pressing process, due to the thicker thickness, diverse pattern distribution, and more prepreg, thermal stress will be more difficult to eliminate than copper clad laminates. The stress in the PCB board is released during the subsequent drilling, forming or baking process, causing the board to deform.

3. During the solder mask and silk screen baking process, since the solder mask ink cannot be stacked on each other during the curing process, the PCB board will be placed in the rack to bake the board for curing. The solder mask temperature is around 150℃, which exceeds the Tg value of the copper clad board, and the PCB is easy to soften and cannot withstand high temperatures. Therefore, manufacturers must evenly heat both sides of the substrate while keeping the processing time as short as possible to reduce the warping of the substrate.

4. During the cooling and heating process of the PCB, due to the unevenness of material properties and structure, thermal stress will be generated, resulting in microscopic strain and overall deformation warping. The tin furnace temperature range is 225℃ to 265℃, the hot air solder leveling time of ordinary boards is between 3 seconds and 6 seconds, and the hot air temperature is 280℃ to 300℃. After the solder is leveled, the board is placed in the tin furnace from the normal temperature state, and the normal temperature post-treatment water washing is carried out within two minutes after it comes out of the furnace. The entire hot air solder leveling process is a rapid heating and cooling process. Due to the different materials and non-uniformity of the circuit board structure, thermal stress will inevitably occur during the cooling and heating process, resulting in microscopic strain and overall deformation warping.

5. Improper storage conditions can also cause PCB warping. During the storage process of the semi-finished product stage, if the PCB board is firmly inserted into the shelf and the tightness of the shelf is not adjusted well, or the board is not stacked in a standardized manner during storage, it may cause mechanical deformation of the board.

3. Engineering design reasons:

1. If the copper surface area on the circuit board is uneven, with one side larger and the other side smaller, the surface tension in the sparse areas will be weaker than in the dense areas, which may cause the board to warp when the temperature is too high.

2. Special dielectric or impedance relationships may cause the laminate structure to be asymmetric, resulting in board warping.

3. If the hollow positions of the board itself are large and there are many of them, it is easy to warp when the temperature is too high.

4. If there are too many panels on the board, the spacing between the panels is hollow, especially rectangular boards, which are also prone to warping.