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Bowing of printed circuit boards (PCBs) is often considered the top level of false positives because it is probably the least understood. Many new inspectors believe that having a perfectly flat rigid board as the standard is a fallacy. Understanding the causes and reasons for PCB bow and twist during the PCB design phase can help address the issue.
First, let's discuss the difference between the two terms bow and twist. A board with a bow problem will lift from the panel even though all four corners are in contact with the panel (think of the shape of a bowed weapon).
Examples of bent and warped printed circuit boards
Warp occurs when three PCB corners are in contact with the panel and the fourth corner is elevated. According to the IPC, both conditions require determining whether the degree of bend or warp is acceptable or unacceptable.
The answer is due to the materials and processes of manufacturing printed circuit boards. PCB laminates are made of fiberglass cloth and epoxy resin, and each layer has unique thermal expansion properties. Add a layer of copper to one or both sides of the PCB laminate, and you have to consider additional thermal expansion properties.
When the board manufacturer exposes the material to various etching and heat treatment processes, there is no guarantee that the laminate will exhibit a uniform reaction across all samples.
Because reactions vary from sample to sample, despite the strict manufacturing processes that PCB laminate manufacturers follow for all laminates, the IPC sets allowable tolerances. To account for normal variations, the finished product is expected to fall within defined parameters or tolerances, rather than exact numbers. These preset tolerances allow for smaller degrees of bend warp without affecting the performance of the board.
Other factors can affect bow and twist during board manufacturing, including:
• Additional part number features
• Higher board layer counts (additional materials = additional heat treatment)
• Material mix (i.e., using a high-frequency PTFE laminate with standard FR4 to control impedance values, resulting in an unbalanced stack-up)
A mix of copper weights hurts bow and twist because copper has a high coefficient of thermal expansion. Higher-density copper will expand toward areas of minimum resistance with greater force than lower-density copper areas.
A balanced stack-up has opposing thermal expansion values that oppose each other, helping to maintain even bow and twist forces. By making the stack unbalanced, one side with a larger thermal expansion value will affect the entire board. Solid layers of copper planes will expand differently than signal layers, especially if the signal density is less. Having all signals on similar layers on one side of the board, and all planes on the other is a recipe for disaster.
PCB manufacturers should also do their part to ensure that unprocessed laminates are properly stored to keep the material flat, and to be responsible for ongoing work to prevent awkward stacking of production panels. Manufacturers can also advise customers to steal copper when the design is prone to bending and flexing.
So how can we prevent the board from bending and warping when it passes through the reflow oven?
Since "temperature" is the main source of board stress, as long as the temperature of the reflow oven is reduced or the speed at which the board heats up and cools down in the reflow oven is slowed down, the occurrence of board warping and warping can be greatly reduced. However, there may be other side effects.
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 will soften after entering the reflow oven, the longer it will take to become a soft rubber state, and the more serious the deformation of the board will be. Using a higher Tg board can increase its ability to withstand stress and deformation, but the price of the material is relatively high.
To achieve the purpose of being thinner and lighter, the thickness of many electronic products has been reduced to 1.0mm, 0.8mm, or even 0.6mm. It is difficult to keep the board from deforming when passing through the reflow furnace with such a thickness. It is recommended that if there is no requirement for thinness, the board should preferably be 1.6mm thick, which can greatly reduce the risk of board bending and deformation.
Since most reflow furnaces use chains to drive the circuit board forward, the larger the size of the circuit board, the more likely it is to be dented and deformed in the reflow furnace due to its weight. Therefore, try to put the long side of the circuit board as the board side on the chain of the reflow furnace, which can reduce the dent deformation caused by the weight of the circuit board itself. The reason for reducing the number of panels is also based on this. That is to say, when passing through the furnace, try to use the narrow side perpendicular to the furnace direction to achieve the lowest dent deformation.
If the above methods are difficult to achieve, the last resort is to use a reflow pallet (reflow carrier/template) to reduce the amount of deformation. The reason why the reflow pallet can reduce the board bending and warping is that no matter if it is thermal expansion or contraction, it is hoped that the pallet can fix the circuit board until the temperature of the circuit board is lower than the Tg value and begins to harden again, and it can also maintain the original size.
Shenzhen Highqualitypcb SMT Processing: Understanding PCB bending and twisting provides us with parameters for acceptable standards and key prevention methods. Manufacturers need to understand the main causes of printed circuit board bending and bending, but they cannot eliminate the potential root causes of circuit board manufacturers. With a better understanding, there will be a better and more efficient circuit board design and manufacturing process.