1. Reduce the effect of temperature on PCB board stress
Since the “temperature” is the main source of stress in the board, as long as the temperature of the reflow furnace is lowered or slow the speed of warming up and cooling down in the reflow furnace, the board bending and the board warping can be greatly reduced. However, there may be occurs other side effects, such as solder short circuit.
2. Use high Tg plate
Tg is the glass transition temperature, that is, the temperature at which the material changes from a glass state to a rubber state. The lower the Tg value, the faster the board starts to soften after entering the reflow furnace, and it will take longer to become soft rubbery. The deformation of the board will of course become more serious. The use of a higher Tg sheet can increase its ability to withstand stress deformation, but the price of the material is relatively high.
3. Increase the thickness of the board
In order to achieve a lighter and thinner purpose, many electronic products’ boards have a thickness of 1.0 mm, 0.8 mm, and even a thickness of 0.6 mm. Such a thickness is difficult to keep the board from deforming after passing through the reflow furnace. Thus it is recommended that if there is no thin and light requirement, the board can use a thickness of 1.6mm, which can greatly reduce the risk of bending and deformation of the board.
4. Reduce the size of the board and the number of panel
Since most reflow furnace use chains to drive the board forward, the larger the size of the board will be deformed in the reflow furnace due to its own weight, so try to place the long side of the board as a board edge on the chain of the reflow furnace, the deformation of the recess caused by the weight of the circuit board itself can be reduced. And the number of the panels is also reduced for this reason, that is to say, when get through the furnace, the narrow side is used to cross the furnace direction as much as possible to reach the lowest amount of depression.
5. Use the reflow carrier/template
If the above methods are difficult to achieve, use a reflow carrier to reduce the amount of deformation. The reason why the over-tray can reduce the bending of the plate is that whether it is thermal expansion or contraction, the furnace tray can fix the circuit board, and after the temperature of the circuit board is lower than the Tg value, it can maintain the original size after starting to harden again.
If the single-layer carrier cannot reduce the deformation of the circuit board, it is necessary to add a layer of carrier to clamp the circuit board with the up and down two-layer carriers. So that the problem of deformation of the circuit board over the reflow furnace can be greatly reduced. However, the tool and labor costs of using the reflow carrier are high, and manual labor is required to place and recycle the carrier.
6. Use Router instead of V-Cut
Since V-Cut will destroy the structural strength of the panels, try not to use the V-Cut board or reduce the depth of the V-Cut.