III

Selecting Your Materials

Every design process begins with material selection, and this chapter focuses on selecting the right materials for your PCB design given the particular design requirements you outline in your specifications. We will be focusing largely on FR-4 as it is the most commonly used material for PCB design. If your specific material requirements are not listed in the sections below, please contact your manufacturer for further guidance

Basic Material Selection Process

When designing a PCB, there are several material choices to consider based on your unique design needs. Before selecting a material, it is recommended to first define the functionality and reliability requirements that your board must meet. See Figure 2 for a visual on how to begin your material selection process

Figure 2: Designer/End-User Materials Selection Map

Material Properties in Detail


Electrical Properties

The most critical properties to consider for electrical requirements are the electrical strength, dielectric constant, and moisture resistance. Refer to Figure 3 for a list of some of the more common materials and their associated property values. Remember to consult with your manufacturer for more specific data on electrical properties.

Figure 3: Typical Properties of Common Dielectric Materials
Copper Foil Types

Manufacturers will typically offer various types of foil for you to choose from, the most common being electro-deposited copper (ED Copper) and rolled copper. Rigid boards will typically use electro-deposited copper foil whereas rigid-flex boards will use rolled copper foil.



Copper Resistance Values

As boards get denser and more complex, it becomes increasingly important to calculate your copper’s distributed resistance. You can use the formula[2-5] below to easily compute the resistivity in your copper traces :


 
Current Carrying Capacity of Copper

In Figure 6 can be used as a reference to understand the current carrying capacity of internal layers for common copper thicknesses and temperature levels above ambient.

Figure 4: Encapsulated Conductor Widths


Finished Board Thickness

As part of your final material selection process, you will want to calculate your finished board thickness. This measurement is made from copper-to-copper and will represent your maximum finished board thickness.