Engineers use Printed Circuit Boards (PCBs) to mechanically support components in electronic equipment. PCBs also provide the necessary electrical interconnections between the components mounted on them with the help of copper pads and traces present on the PCB surface. Manufacturers make such PCBs from copper clad laminates, which consist of a base reinforcing material or substrate, with a sheet of copper pasted on one or both sides of the substrate. To realize a PCB, the manufacturer removes unwanted copper from the copper clad laminate, while retaining necessary copper pads and traces.
Fig 1: Copper Clad Laminate and Cross-Sectional View
Fig 2: PCB After Removing Unwanted Copper from Copper Clad LaminateTypes of Copper Clad Laminates
Two major types of copper clad laminates are available—one with the substrate made of organic materials and the other with the substrate made of inorganic material.
Substrates made of organic materials have the reinforcing material as glass felt, fiber paper, or glass fiber cloth, impregnated with resin adhesive. After the resin dries, fabricators cover the substrate with copper foil, bonding them at high temperature and pressure. Substrates made of organic material form the major chunk of copper clad laminates that the electronic industry uses to make PCBs.
The reinforcing material in substrates made of inorganic materials are either ceramic or metals such as copper, iron, or aluminum. Fabricators usually bond copper foils on such substrates with a thin insulating layer in between. The insulating layer has high thermal conductivity as the use of this type of PCB is mainly to address very high frequency operation or thermal management in equipment.
Classification of Copper Clad Laminates
Beyond the broad classification above, copper clad laminates with substrates made of organic material have further subdivisions depending on:
Material: paper base, glass fiber cloth base, compound type, etc.
Insulating Resin: phenolic, epoxy, polyester, etc.
Performance: high heat resistance, low dielectric constant, low coefficient of thermal expansion, chemical resistance, environmental performance, etc.
Mechanical Rigidity: Rigid and flexible
Copper Cladding: standard copper foil, HTE copper foil, LP copper foil, different thicknesses, etc.
Fabricating Copper Clad Laminates
For copper clad laminates, fabricators typically use prepreg, a reinforcing material already impregnated with a resin containing a proper curing agent. The resin comes in an unhardened, pre-dried form. The fabricator heats the prepreg and places it in a mold where they sandwich it between two layers of copper foils. If necessary, the fabricator adds additional resin to the sandwich. Heating the combination allows the resin to flow and makes the prepreg stick to the copper foil. Further combination of pressure and temperature is necessary to bond the prepreg to the copper foil.
Fig 3: Fabricating a Copper Clad Laminate
Lead-Free Copper Clad Laminates
In accordance with the RoHS regulations, manufacturers do not use substances such as PBDE and PBB when fabricating lead-free copper clad laminates. Moreover, prepreg manufacturers offer the PN system, which has Brominated Epoxy resin as the main resin, and Phenol-Formaldehyde resin as the curing agent to help fabricators make copper clad laminates conforming to the RoHS directives.
Selecting Copper Clad Laminates
PCB manufacturers select copper clad laminates based on several criteria of performance and requirements of the application:
Size: As copper clad laminates form the base of PCBs, their size must conform to the size requirements corresponding to the PCB. Manufacturers often set up smaller PCBs in arrays for ease of manufacturing. Parameters of concern include the length, width, diagonal deviation, and warpage of the copper clad laminate, and each parameter must meet specific requirements.
Appearance: Manufacturing and handling processes may subject the surfaces of a copper clad laminate to blemishes such as dents, wrinkles, scratches, resin points, bubbles, pinholes, etc. These blemishes not only reduce the visual aspect of the copper clad laminate, but may also lead to lower the performance of the derived PCB. Therefore, PCB manufacturers prefer copper clad laminates with a smooth and flat appearance.
Physical Performance: PCB manufacturers specifically look for dimensional stability, bending strength, peel strength, heat resistance, and punching quality in copper clad laminates. Electrical Performance: Properties of copper clad laminates that affect the electrical performance of a derived PCB include the dielectric loss tangent, dielectric constant, surface resistance, volume resistance, arc resistance, insulation resistance, electric strength, dielectric breakdown voltage, comparative tracking index, etc. A PCB designer must select the various parameters based on the application.
Chemical Performance: Depending on the application, selection of a copper clad laminate must meet requirements of resistance to chemical reagents, flammability, coefficient of thermal expansion in the Z-axis, glass transition temperature, dimensional stability, etc.
Environmental Performance: The copper clad laminate must meet the water absorption specifications.
Standards for Copper Clad Laminates
ASTM D1867 defines the standard specification for copper clad laminates that manufacturers use for making printed circuit boards. Covering twelve grades of copper clad laminates, ASTM D1867 requires the laminates to meet peel strength at elevated temperatures, crosswise and lengthwise flexural strength, flammability ratings, volume resistivity, water absorption, dielectric breakdown, dissipation factor, and permittivity. The laminates must further conform to tests for blistering, warp, and twist.
Manufacturers of copper clad laminates follow the IPC-4101C as their manufacturing standard, and the IPC-IM 650 for testing the copper clad laminates they fabricate.
Advantages of Copper Clad Laminates
Using copper clad laminates for PCBs allows designers take advantage of constant distance between the copper foils acting as power and ground planes. When distributing power to a system, this arrangement reduces the modal resonances by lowering the inductance between the two planes. This not only reduces the impedance of the system, but also decreases the amount of filter capacitors necessary.