Understand the causes, hazards, and solutions of circuit board bending and warping

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.

Difference between bow and twist

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.

Why does the IPC allow bend or warp?

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 Effects of Bow and Twist

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

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.

Countermeasures for board warping and bending

So how can we prevent the board from bending and warping when it passes through the reflow oven?

1. The effect of reducing temperature on board stress

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.

2. Use high Tg boards

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.

3. Increase the thickness of the circuit board

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.

4. Reduce the size of the circuit board and the number of panels

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.

5. Use a reflow pallet jig

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.

Conclusion

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.