How to Handle Dimensional Stability in Circuit Board Design
Stability in Circuit Board Design
A circuit board’s dimensional stability is important for several reasons. First, it helps keep the board flat and enhances surface mount component assembly. Second, it keeps the board from warping due to thermal stress. And third, it helps ensure that the board fits together properly with the enclosure. Dimensional changes in the board can cause a number of problems, including misalignment and alignment of holes and pins, and it can interfere with soldering.
Dimensional stability issues can affect the performance of a PCB, which is why a good quality circuit board should be designed to minimize the impact of these changes. To help do this, the design should incorporate geometric dimensioning and tolerancing (GD&T) from the outset. This will tell the designer where tolerances are acceptable and where tolerances are not, balancing the need for accurate positioning and fit with the need to avoid excessive shrinkage that can lead to a loss of accuracy or precision.
Using GD&T will also ensure that a PCB is manufactured to the appropriate size, allowing for adequate room for manufacturing process errors. The PCB should be sized to the maximum thickness that is permissible by the material manufacturer and any applicable standards, such as UL 283C for bare copper boards. This will reduce the need for reworking or rework stations that could increase production costs and decrease yields.
How to Handle Dimensional Stability in Circuit Board Design
Another important consideration in a PCB’s dimensional stability is the choice of the base material and copper layer. The underlying polyimide film can be stable or unstable, depending on its residual stresses and the coefficient of thermal expansion. The copper layer is also sensitive to temperature changes, which can cause the PCB to expand or contract and affect its dimensional stability.
For a more durable and cost-effective PCB, the core insulation layer should be made of high-density material. This will help to stabilize the circuit board against bending and twisting, and it will improve the signal transmission speed by increasing the dielectric constant of the board. The other insulation layers should be made of low-dielectric material to prevent the conductive copper from etching into the adjacent layers, which can cause a short circuit.
In addition to the use of high-density materials, it is also advisable to include a protective coating for the PCB to prevent moisture absorption, which can be a significant factor in the dimensional stability of a flexible circuit. The coating is typically made of a polyimide material, which can be more stable than polyester. In addition, the coated PCB can resist corrosion caused by oxidation of copper. The coating can also be used to protect the edges of a circuit board from damage during assembly and transport. This will help to reduce the incidence of rejected flexible circuits, which are typically caused by damage or by dimensional changes.
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