Huashengxing

The deformation causes during PCB board processing are very complex and can be divided into two types of stress: thermal stress and mechanical stress. The thermal stress is mainly generated during the pressing process, while the mechanical stress is mainly generated during the stacking, handling, and baking of the plates. Below is a brief discussion in the order of the process.

Copper clad laminate material: Copper clad laminates are double-sided panels with symmetrical structures and no patterns. The CTE of copper foil and glass cloth is almost the same, so there is almost no deformation caused by different CTEs during the pressing process. However, the size of the copper-clad laminate press is large, and there is a temperature difference in different areas of the hot plate, which can lead to slight differences in the curing speed and degree of the resin in different areas during the pressing process. At the same time, the dynamic viscosity under different heating rates also varies greatly, resulting in local stress caused by differences in the curing process. Generally, this type of stress will maintain equilibrium after compression, but will gradually release and cause deformation in future processing.

Compression: The PCB compression process is the main process that generates thermal stress, and the deformation caused by different materials or structures is analyzed in the previous section. Similar to the lamination of copper-clad laminates, local stresses caused by differences in the curing process can also occur. Due to thicker thickness, diverse pattern distribution, and more semi cured sheets, PCB boards also experience more and more difficult to eliminate thermal stresses than copper-clad laminates. The stress present in the PCB board is released during subsequent drilling, shaping, or grilling processes, resulting in deformation of the board.

Solder mask, character baking process: Due to the fact that solder mask ink cannot be stacked on top of each other during curing, PCB boards are vertically placed in a rack for baking and curing. The solder mask temperature is around 150 ℃, just above the Tg point of low to medium Tg materials. The resin above the Tg point is in a high elastic state, and the board is prone to deformation under its own weight or strong winds in the oven.

Hot air solder leveling: The temperature of the tin furnace during the hot air solder leveling of ordinary boards is 225 ℃~265 ℃, and the time is 3S-6S. The hot air temperature ranges from 280 ℃ to 300 ℃ When the solder is leveled, the board is fed into the soldering furnace at room temperature, and then subjected to post-treatment water washing at room temperature within two minutes after being discharged from the furnace. The entire hot air solder leveling process is a sudden heating and cooling process. Due to the different materials and uneven structure of circuit boards, thermal stress is inevitably generated during the heating and cooling process, leading to micro strain and overall deformation in the raised area.

Storage: PCB boards are usually stored in shelves during the semi-finished stage. Improper adjustment of shelf elasticity or stacking of boards during storage can cause mechanical deformation of the boards. Especially for thin plates below 2.0mm, the impact is more severe.

In addition to the above factors, there are many other factors that can affect PCB deformation.

Countermeasure for improvement

How can we prevent the board from bending and warping when passing through the reflow oven?

1. The effect of reducing temperature on board stress

Since "temperature" is the main source of stress on the board, reducing the temperature of the reflow oven or slowing down the heating and cooling speed of the board in the reflow oven can greatly reduce the occurrence of board bending and warping. However, there may be other side effects that matter.

2. Using high Tg sheet materials

Tg is the glass transition temperature, which is the temperature at which a material transitions from a glassy state to a rubbery state. The lower the Tg value of a material, the faster its board begins to soften after entering the reflow oven, and the longer the time it takes to become a soft rubbery state, resulting in more severe deformation of the board. The use of higher Tg sheets can increase their ability to withstand stress and deformation, but the cost of the material is relatively high.

3. Increase the thickness of the circuit board

Many electronic products have reduced the thickness of the board to 1.0mm, 0.8mm, and even 0.6mm in order to achieve a thinner and lighter design. This thickness is difficult to maintain when the board passes through the reflow oven without deformation. It is recommended that if there is no requirement for a thinner and lighter design, it is best to use a thickness of 1.6mm, which can greatly reduce the risk of board bending and deformation.

4. Reduce the size of circuit boards and decrease the number of spliced boards

Since most reflow furnaces use chains to drive the circuit board forward, larger circuit boards will deform and dent in the reflow furnace due to their own weight. Therefore, it is recommended to place the long side of the circuit board as the board edge on the chain of the reflow furnace to reduce the deformation caused by the weight of the circuit board itself. The reason for reducing the number of spliced boards is also based on this. When passing through the furnace, try to use narrow edges perpendicular to the furnace direction to achieve the lowest amount of deformation.

5. Used furnace tray fixture

If the above methods are difficult to achieve, the final solution is to use a reflow carrier/template to reduce deformation. The reason why reflow carriers can reduce board bending and warping is because whether it is thermal expansion or contraction, it is hoped that the tray can hold the circuit board in place until the temperature of the circuit board drops below the Tg value and begins to harden again, while maintaining its original size.

If a single-layer tray cannot reduce the deformation of the circuit board, an additional layer of cover must be added to clamp the circuit board between the upper and lower trays, which can greatly reduce the problem of circuit board deformation during reflow soldering. However, this furnace tray is quite expensive and requires manual labor to place and recycle the tray.