Most of the circuit boards are prone to board bending and warping when reflow soldering. If it is serious, it may even lead to component empty soldering, tombstone, etc. How can we prevent PCB board from bending and warping in the reflow soldering? Let Kingford find it out for you.
Harm of PCB circuit board deformation
On the automated surface mount line, if the PCB board is uneven, it will cause inaccurate positioning, components cannot be inserted or placed on the board holes and surface mount pads, and even the automatic insertion machine will be damaged.The circuit board on which the components are mounted is bent after soldering, and the component legs are difficult to cut flat and neat. The board cannot be mounted on the chassis or the socket in the machine, so it is also very troublesome for the assembly plant to encounter the board. 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.
PCB deformation cause analysis
1. The uneven area of the copper plane on the circuit board will worsen the board bending and warping.
Generally, a large area of copper foil is designed for grounding on the circuit board. Sometimes the Vcc layer is also designed with a large area of copper foil. When these large areas of copper foil cannot be evenly distributed on the same circuit board When you go up, it will cause the problem of uneven heat absorption and heat dissipation speed. Of course, the PCB board will also expand and contract. If the expansion and contraction cannot be caused at the same time, it will cause different stresses and deformation. The upper limit of the Tg value, the board will begin to soften, causing permanent deformation.
2.The weight of the PCB board itself can also cause the board to sag and deform.
The reflow furnace uses a chain to drive the circuit board in the reflow furnace. That is, the two sides of the board are used as fulcrum to support the entire board. If the board has overweight parts, or the board size is too large, Because of its own amount, it will show the phenomenon of middle depression, resulting in plate bending.
1.Reduce the effect of temperature on PCB board stress.
Since temperature is the main source of board stress, as long as the temperature of the reflow soldering is reduced or the temperature of the board is raised and cooled in the reflow furnace, the occurrence of plate bending and plate warpage can be greatly reduced. However, other side effects may occur, such as shorted solder.
2.Use high Tg board.
Tg is the glass transition temperature, that is, the temperature at which the material changes from glass to rubber. The lower the Tg value, the faster the board starts to soften after entering the reflow furnace, and the time it takes to become soft rubber It will also become longer, and of course the deformation of the board will become more serious. The use of higher Tg plate can increase its ability to withstand stress and deformation, but the price of the material is relatively high.
3.Increase the thickness of the PCB board
In order to achieve thinner and lighter thickness of many electronic products, the thickness of the board has been left at 1.0mm, 0.8mm, or even 0.6mm. It is recommended that if there is no lightness, the board can use a thickness of 1.6mm, which can greatly reduce the board Risk of bending and deformation.
4.Reduce the size of the circuit board and the number of puzzles
Since most reflow furnaces use a chain to drive the PCB board forward, a circuit board with a larger size will sag in the reflow furnace due to its own weight, so try to put the long side of the circuit board as a board edge On the chain of the reflow furnace, the depression deformation caused by the weight of the circuit board itself can be reduced, and the number of puzzles is reduced for this reason. Low depression deformation.
5.Use oven tray fixture
If the above methods are difficult to achieve, it is to use a reflow carrier (template) to reduce the amount of deformation. The reason that the reflow tray can reduce plate bending and warping is that whether it is thermal expansion or cold shrink, the tray is desired You can fix the circuit board and wait until the temperature of the circuit board is lower than the Tg value to start to harden again, and then you can maintain the original size.
If the single-layer tray cannot reduce the deformation of the PCB board, you must add another layer of cover and clamp the circuit board with the upper and lower trays. This can greatly reduce the deformation of the circuit board through the reflow furnace.
6.Use Router instead of V-Cut subboard
Since V-Cut will destroy the structural strength of the board-to-board puzzle, try not to use V-Cut sub-boards or reduce the depth of V-Cut.