How to prevent PCBA board from warping?

1. The impact of lowering temperature on PCB board stress

Since "temperature" is the main source of board stress, as long as the temperature of the reflow furnace is lowered or the speed of the board heating and cooling in the reflow furnace is slowed down, the occurrence of board bending and board warping can be greatly reduced. However, other side effects may occur, such as solder short circuits.


2. Use high Tg plates

Tg is the glass transition temperature, which is the temperature at which the material changes from the glass state to the rubber state. The lower the Tg value of the material, the faster the board begins to soften after entering the reflow oven, and the time it takes to become soft and rubbery. It will also become longer, and the deformation of the board will of course become more serious. Using a plate with a higher Tg can increase its ability to withstand stress deformation, but the price of the material is relatively high.


3. Increase the thickness of the circuit board

In order to make many electronic products thinner and lighter, the thickness of the board has been reduced to 1.0mm, 0.8mm, or even 0.6mm. With such a thickness, it is really difficult to keep the board from deforming after passing through the reflow oven. It is recommended that if there is no requirement for thinness and lightness, the board should be 1.6mm thick, which can greatly reduce the risk of board bending and deformation.


4. Reduce the size of the circuit board and reduce the number of panels

Since most reflow ovens use chains to drive the circuit board forward, larger circuit boards will be dented and deformed in the reflow oven due to their own weight, so try to place the long side of the circuit board as the edge of the board. On the chain of the reflow furnace, the dent deformation caused by the weight of the circuit board itself can be reduced. This is also the reason for reducing the number of panels. That is to say, when passing through the furnace, try to use the narrow edge perpendicular to the furnace passing direction to achieve the minimum The amount of dent deformation.


5. Use oven tray fixture

If the above methods are difficult to achieve, the last step is to use a reflow carrier/template to reduce the amount of deformation. The reason why the reflow carrier can reduce the bending and warping of the plate 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 the original size.
If the single-layer pallet cannot reduce the deformation of the circuit board, you must add a layer of cover and clamp the circuit board with the upper and lower pallets. This can greatly reduce the problem of deformation of the circuit board after passing through the reflow oven. However, these oven trays are quite expensive, and labor is required to place and recycle the trays.


6. Use Router instead of V-Cut for split board use

Since V-Cut will destroy the structural strength of the panels between circuit boards, try not to use V-Cut split boards, or reduce the depth of V-Cut.