Discussion on the solution to PCB warping

Sea: When the degree of distortion exceeds the standard, is there any other method besides using the method of adding gaskets to ensure installation?

Liu Haiguang: Use the system standard to evaluate how much the bridge curvature exceeds the standard, and then see how to correct the feasibility of heat treatment. Adding gaskets is generally a prohibited method

Zhu Mei: Can you share the heat treatment correction? Thank you

Liu Haiguang: Under the temperature condition of 100C (or 85C) in the oven, keep the temperature constant for 12~24H, and then naturally cool down to room temperature and take out..., PCBA depends on the warping area and the type and distribution of components, the bottom of the PCBA needs to be properly supported and the top is properly loaded (counterweight or pre-clamped), which can usually relieve or eliminate overload stress... Some high-power components will eliminate the assembly stress between the structural parts and the PCBA in this way even after assembly.

The thermal correction temperature curve is a trapezoid. If there is a large component with high heat capacity, a half-step can be made before the constant temperature to prevent the initial temperature from being too high and causing new stress concentration.

Liao Xiaobo:

Teacher Liu, the PCB warpage correction temperature condition you mentioned is too low, and it is difficult to have an effect, or the effect is difficult to last.

Whether the warpage of the printed circuit board can be leveled by heating and pressurizing is affected by many factors. The most likely to be effective is the warpage caused by the characteristics of the epoxy resin in the laminate. But only for this factor, if you really want to level it, the heating temperature must be raised to more than 20 degrees Celsius of the Tg value of the board, and the pressure/temperature/time curve of pressurizing and heating-constant temperature and pressure-constant pressure cooling can be effective. If the warpage is caused by asymmetric and uneven wiring on both sides of the PCB, unless the hardness of the board is improved after thermal correction, it will still warp after a period of time.

Liu Haiguang: Mr. Liao, the previous discussion was about PCBA with components. Thermal correction cannot reach the Tg temperature point, nor can it be completely flat. The IPC of the welded PCBA is based on reliability requirements, and its warpage is below 0.75% or 0.5%. The thermal correction of the electric assembly is equivalent to the aging of mechanical heat treatment. The main purpose is to effectively disperse the concentrated stress that has been generated...

This kind of welding exceeds the standard on one side, and the other side can no longer be mounted and welded

Liao Xiaobo: Yes, if the warpage of the printed circuit board assembly that has been assembled and welded with components exceeds the standard, the thermal correction of the electric assembly can indeed play a role in dispersing the concentrated stress. In the process of "customer service", I have cooperated with customers many times to use the "thermal assembly" method to implement "salvage" treatment on the warped printed circuit board assembly. Although it is suspected of "violating regulations", it is also helpless, but it is really effective. For high-reliability products, it is not worth promoting, but it is also a technical discussion.

Sea: Thank you teachers for your answers!