What are Dielectric Call-outs?
Rush PCB makes printed circuit boards (PCBs), some of them very complex and diverse. These include multi-layered boards with up to 40 layers, but mainly boards with 4 to 12 layers being more popular. The stack-up holds the layers together. This includes copper cores with prepregs or dielectrics separating them. Although dielectrics have insulating properties, unlike insulators, dielectrics allow electric fields to pass through. Moreover, dielectrics can store electric charges, but insulators cannot.
Why is Prepreg Important?
The functioning of a capacitor best explains the importance of prepregs. As it is popularly known, capacitors store electrical energy. Usually, a dielectric separates the plates of a capacitor. Although the capacitor can store energy even when there is no dielectric or air replaces the actual dielectric, the presence of a dielectric change the functioning. With a dielectric present, the electronic field reduces and brings down the voltage withstanding capacity of the capacitor. This makes the capacitor more efficient when storing energy.
Application of a voltage across a dielectric polarizes it as a charged electron enters. The polarization hinders the motion of the electron and curtails its advance into the dielectric. The amount of polarization also influences the energy stored in the dielectric. Therefore, the properties of the dielectric or dielectric call-outs of the prepreg influence the nature of the PCB and the amount of power that it can handle.
Important Dielectric Call-Outs for Prepregs
Several characteristic values of the prepreg are important for differentiating them when designers use them in PCBs.
Peel Strength: The prepreg bonds the copper layer. The force necessary to peel the copper layer off the prepreg is its peel strength.
Volume Resistivity: The prepreg’s resistance to the leakage of electrical current. Important between adjacent layers in a multi-layered PCB.
Surface Resistivity: The prepreg’s resistance to the leakage of current along its surface. Important between adjacent copper traces on the same layer.
Moisture Absorption: The amount of moisture the prepreg absorbs when exposed for a certain period. Important as it reduces both volume and surface resistivity.
Dielectric Breakdown: When the applied voltage exceeds the breakdown voltage of the prepreg and makes it conductive, causing a large current flow.
Permittivity: The amount of energy the prepreg can store in an electric field.
Loss Tangent: Amount of resistive energy lost when propagating through the prepreg. Important for estimating the heat generated in the prepreg.
Flexural Strength: The maximum angle through which the prepreg can bend before breaking.
Arc Resistance: The prepreg’s resistance to the formation of a conductive path on its surface.
Thermal Stress: Stress on the prepreg due to changes in temperature.
Dielectric Strength: Voltage level that the prepreg can withstand before it breaks down and loses its insulating properties.
Flammability: The ability of the prepreg to extinguish any flames if the prepreg has caught fire.
Halogen Content: The number of Halogens present in the prepreg because of its manufacturing process. Lower amounts of Halogen are better.
Glass Transition Temperature: The temperature at which the prepreg starts changing from being solid to a softer state, but not yet melted.
Decomposition Temperature: The temperature at which the prepreg breaks down into its separate elements.
Coefficient of Thermal Expansion: The amount of expansion the prepreg will undergo in its Z-axis during change of temperature.
Time to Delamination: The time it takes for the prepreg to disintegrate into its separate elements when under a specific high temperature.
Comparative Tracking Index: A measure of the electrical breakdown characteristic of the prepreg. A high CTI indicates better insulation properties.
Rush PCB recommends considering all the above dielectric call-outs of a prepreg before specifying it for use in a multi-layered PCB. While preparing layers for multi-layered PCBs, the fabricator bonds a copper sheet to the prepreg. Primarily, the prepreg is not in its cured form. When bonding, the resin inside the prepreg melts and bonds to the copper layer. The same thing happens during curing, and the resin melts to act like an adhesive for holding the different layers together. Rush PCB selects the cores and prepregs for the stack-up, to reach the desired overall board thickness and the spacing between the layers.
Specifying PCB Stack-Ups for Rigid Boards
Manufacturers specify the resin content of the prepreg and the final thickness it will reach after bonding. This is important for the designer to know, as it decides the final board thickness. After this, the manufacturer lists specifications for the dielectric constant and dissipation factors.
For a simple PCB, it is enough for the designer to consider a high dielectric strength for the prepreg at the frequency of operation and an average dissipation factor. This simplifies the choice of the prepreg without entering deeply into physics and electrical engineering.
As a manufacturer, we at Rush PCB need to know a few things only: the overall board thickness, the finished copper weights, and the core material necessary. We adjust the prepreg to create a balanced construction or offset it for accommodating the requirements of controlled impedance.
For a four-layer stack-up, the PCB manufacturer can select the type of prepreg and adjust their thickness for a balanced build. Although stack-up specifications can be very detailed, this limits the freedom of the manufacturer and adds to the cost.
More complex boards, such as a 14-layer PCB stack-up may need changes to the prepreg thickness as the manufacturer calculates the impedance. For PCB manufacturers, generic stack-ups or simple stack-ups are better. The information we need for selecting the prepreg is usually:
- Core material necessary
- Finished copper weight
- Overall board thickness
It is very helpful to us if the designer allows equivalent materials when selecting the prepreg.
Specifying PCB Stack-Ups for Flex and Rigid-Flex Boards
Flex and rigid-flex boards require a different type of prepreg, usually known as no-flow prepregs. Although very similar to the regular prepregs, more cured resins make up the no-flow prepreg. This prevents the resin from overflowing at the flex to rigid transition areas. The other difference is the no-flow prepreg has Polyimide adhesive-less core added to it.
In a rigid-flex board, the stack-up usually has a mix of FR-4 core for the rigid part, prepregs, and Polyimide cores for the flex part. The information necessary for the board remains the same, the core material, finished copper weight, and overall board thickness. However, in addition, we also need the overall thickness of the flexible area and its copper weights.
When a PCB designer creates the layout of a circuit, they aim to achieve a reliable board that will function according to the requirement. To realize this aim, it is very important that the PCB manufacturer receives the pertinent information. This includes the PCB stack-up information together with the impedance requirements.
The information is also important for the manufacturer to generate the desired spacing between adjacent layers and the overall board thickness. Rush PCB recommends reliable communication between PCB designers and manufacturers to build successfully printed circuit boards of high quality.