Use the Best PCB Layout Tools to Specify Fabrication Materials
PCBs are layers of copper foil and laminate. Industry standards for material selection are found in IPC-2221 and available in Altium Designer for inclusion in design documents.
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Printed Circuit Boards (PCBs) are made by stacking layers of copper between layers of pre-impregnated (prepreg) epoxy/laminate. The most common prepreg in use today is FR4 for its flame-retardant and easy sourcing characteristics. It is rated to provide a high degree of electrical insulation in both dry and humid conditions while maintaining mechanical strength. FR4 is a grade of material and is typically made with a combination of materials composing a nonconductive glass-epoxy layer.
Materials other than glass-reinforced epoxy laminate are available although their cost is prohibitive for most manufacturing and design requirements. The Institute of Printed Circuit Boards, the industry standard providing certification for PCBs, offers guidelines for choosing materials in IPC-2221, the Generic Standard on Printed Circuit Board Design. The standard contains a wealth of information on base materials including prepregs and adhesives. Other material considerations for solder paste and plating are covered, in addition to markings and legends.
Employing qualities of PCB laminates for design features such as dielectric properties, creation of embedded passives, or thermal management is also possible. Electrical properties are typically controlled in the PCB by varying material width and thickness. Tables are available specifying parameters for all comparison of PCB materials, including FR4, within IPC-2221. Fabricators are also able to provide material properties of prepreg offerings for use as design features for your circuits.
Define layer materials and thicknesses
The industry primarily uses grade FR4 nonconductive material between layers of copper to build printed circuit boards. FR4 is a NEMA grade designation for glass-reinforced epoxy laminate material. The designation represents the ratio of fiber to resin and indicates characteristics such as flame retardant, dielectric constant, loss factor, tensile strength, shear strength, glass transition temperature, and z-axis expansion coefficient. FR4 is flame retardant making it suitable for safety requirements, and it is robust in varying temperature and humidity environments, increasing the quality of performance.
The laminates are placed between layers of copper and become substrates with dielectric properties relevant to the circuits assembled onto the boards. Designers specify substrate thickness to meet dielectric requirements for the circuits. IPC-2221 contains tables specifying dielectric constant for varying thickness of FR4 and other laminate materials possible for use in PCB substrate layers.
Another material consideration when designing PCBs is the thickness of copper plating. Typical designs use 1 oz copper for power and ground planes to accommodate multiple signal paths during circuit operation. With multiple currents passing through either the power and ground plane simultaneously during operation enough copper must be designed into the plane. Typically, power and ground planes are specified to 1 oz of copper. Signal planes are typically designed to ½ oz copper given one electrical signal traveling through them at any one time. For high-speed traces, more or less copper may be specified to meet impedance characteristics for the trace. Other material considerations include hole plating, solder paste, and solder mask.
Fabricators continue to discover materials better suited to high-speed signals. Materials can negatively affect signal integrity and nonwoven glass, and other, laminates are being found to reduce interference of the signal to maintain integrity. These materials are suitable for radar and microwave but the materials are more expensive to produce. Consulting with fabricators is a good plan for finding the sweet spot between signal integrity and design cost to manufacture.
View electrical and mechanical material layers
Select Your Layers and Use View Configuration to Display
There are Presets within the tool that let you choose the number of layers. Up to 32 layers may be specified for electrical and mechanical characteristics. Established signal and plane layers and their location within the design are managed here. Mechanical layers with fabrication notes are available to view.
You may also use the views to analyze your electrical and mechanical layers for material needs to meet design use. Design for Manufacture includes consideration of common fabrication materials versus the need for signal integrity. Additional material characteristics to analyze against your design would be dielectric properties such as breakdown voltage and the substrates ability to prevent leakage currents.
- Choosing FR4 and apply to pricing for your design.
- Use fabrication knowledge when choosing materials for design in manufacturing including use for RF and microwave.
- Employ material selection to drive design refinement for your circuits.
Layers of a PCB are built with laminates to build the substrate and with copper to support signal propagation. Fabricators start with a core which is made with a substrate and copper foil. The core is composed of substrate with copper foil adhered to both planar sides of the substrate. Signal traces are etched from the copper using vector information contained in gerber files. To build more layers, additional cores are etched specific to the artwork and then adhered together with prepreg. Prepreg is typically the same material used as the substrate in the core. The same material is used for impedance matching characteristics to maintain design integrity. For multiple layer boards, several cores are etched with the design and laminated together with prepreg to produce the final layer stack.
The layers are heated and pressed together, causing the prepreg to form around the etched traces. This leaves no gaps within the material. Industry machines are controlled to ensure that the layers remain uniform. Without careful fabrication techniques, layers become uneven and opportunity for shorting increases.
Create your stack and specify dielectric constants and overlays
Use the Layer Stack Manager to Specify Materials in Layout
Each layer of your design may be specified for material and thickness with Altium’s Layer Stack Manager. Material properties such as dielectric, copper, and solder mask are detailed for eventual communication to your fabricator in the assembly documents. Properties such as thickness, clearance from plane edge to board edge, coverlay expansion, and component orientation on the outer layers is specified. Materials for solder and paste mask are detailed, along with drill files and plating needs.
- Learn from a fabricator why spread glass and FR4 is popular for PCBs
- Build your layer stack for rigid and rigid-flex PCBs.
- Discover best practices for PCB material selection.
Getting your designed stack documented for communication with fabrication houses occurs in Altium’s Layer Stack Manager. Layer Names, type of layer and its material designation, thickness, dielectric type and constant are all specified in the Layer Stack Manager. The information is available for publication to assembly drawings for communication with fabrication house. Altium Designer’s Draftsman tool provides information on copper and layer stack material dielectrics required for realizing the design.
Materials for solder paste and hole plating are communicated as well. Use of RoHS materials is becoming the norm as removal of lead and other toxic materials become mandatory worldwide. With lead-free solder and solder paste, PCB materials withstand greater temperatures during manufacture so any special instructions can be added here.
Assembly instructions and fabrication details are enumerated on the general purpose mechanical layers built into the layer stack. Special layers within the Layer Stack Manager are dedicated to specify silkscreen, solder and paste mask, drill information, keep-outs and connection layers. Each item in the Layer Stack Manager is accessible with a double-click followed by edification.
Use Layer Stack Manager to Itemize PCB Materials
Communicating material specifications to fabricators happens with Altium’s Draftsman tool. Draftsman takes information the Layer Stack Manager and displays material properties explicitly on the assembly drawing. Fabricators use this information to make the printed circuit boards.
- Analyze copper effects for your plane pours and signal integrity concerns.
- Use Altium Designer copper distribution tools for all features on your PCB.
- Advance your ability to itemize and display specific layer information.
An important step in PCB design is specification of materials for communication to fabricators. Altium’s Layer Stack Manager is the tool that offers up to 80 layer templates to specify electrical, mechanical, special, or other layers necessary for fabricating a printed circuit board.