PCB distortion is a common problem in PCBA mass production, which will have a considerable impact on assembly and testing, resulting in electronic circuit function instability, circuit short circuit/open circuit failure.
The causes of PCB deformation are as follows:
1. Temperature of PCBA board passing furnace
Different circuit boards have maximum heat tolerance. When the reflow oven temperature is too high, higher than the maximum value of the circuit board, it will cause the board to soften and cause deformation.
2. Cause of PCB board
The popularity of lead-free technology, the temperature of the furnace is higher than that of lead, and the requirements of plate technology are higher and higher. The lower the TG value, the more easily the circuit board will deform during the furnace. The higher the TG value, the more expensive the board will be.
3. PCBA board size and number of boards
When the circuit board is over reflow welding machine, it is generally placed in the chain for transmission, and the chains on both sides serve as support points. The size of the circuit board is too large or the number of boards is too large, resulting in the depression of the circuit board towards the middle point, resulting in deformation.
4. Thickness of PCBA board
With the development of electronic products in the direction of small and thin, the thickness of the circuit board is becoming thinner. The thinner the circuit board is, it is easy to cause the deformation of the board under the influence of high temperature when reflow welding.
5. The depth of v-cut
V-cut will destroy the sub-structure of the board. V-cut will Cut grooves on the original large sheet. If the V-cut line is too deep, the deformation of PCBA board will be caused.
The connection points of layers on the PCBA board
Today’s circuit board is multi-layer board, there are a lot of drilling connection points, these connection points are divided into through hole, blind hole, buried hole point, these connection points will limit the effect of thermal expansion and contraction of the circuit board, resulting in the deformation of the board.
Solutions:
1. If the price and space allow, choose PCB with high Tg or increase PCB thickness to obtain the best aspect ratio.
2. Design PCB reasonably, the area of double-sided steel foil should be balanced, and copper layer should be covered where there is no circuit, and appear in the form of grid to increase the stiffness of PCB.
3. PCB is pre-baked before SMT at 125℃/4h.
4. Adjust the fixture or clamping distance to ensure the space for PCB heating expansion.
5. Welding process temperature as low as possible; Mild distortion has appeared, can be placed in the positioning fixture, temperature reset, to release the stress, generally satisfactory results will be achieved.
Post time: Oct-19-2021