Copper, rightly called the “poor man’s gold,” is a malleable, ductile, thermal, and electric conductor resistant to corrosion, plus it’s relatively inexpensive. Even though copper is a good conductor of heat, it has a high melting point, which is why wires and certain electronic parts are mostly made of copper as they will not heat up quickly or melt. It is a great choice to use in Printed Circuit Boards because it can transmit signals easily without losing electricity on its electronic route. From a manufacturing point of view, this results in manufacturer savings, as excessive amounts of copper do not have to be used.
Making Copper Clad Boards
Printed Circuit Boards – PCBs with bare copper are called copper clad boards or raw boards. These are usually flat, non-conductive fusion materials lying under layers of copper circuitry that are either placed internally or as a cover on outside surfaces.
Copper clad boards are made using Copper Clad Laminate –CCL, also called cores. To do so, the phenolic plastic sheet is immersed in a resin and electronic glass fiber mixture (or any other reinforcing material) to clad (cover) it with copper on either one side or both sides (depending on the intended purpose and laminated in a hot press. The thickness of the phenolic sheets that industry suppliers generally supply is as three-ply laminates in 1/16″ and 1/8″ thicknesses.
FR-1, FR-2, FR-3, (flame retardant) and XPC, XXXPC (non-flame retardant) are commonly used paper-based copper clad boards. Mainly refers to the insulating base material composed of two different reinforcing materials for the surface layer and the core layer.
Types of copper clad laminate (CCL) boards
There are several categories of copper-clad laminate boards available based on different classification standards.
1. Reinforcing material/insulation material and Structure-based CCL Classification:
- Paper-based CCLs such as XPC,
- FR-4, FR-5 are glass fiber cloth-based CCLs
- CEM-1, CEM-3 are compound CCLs
- Special material base CCLs (metal-base CCL, ceramic-base, and so on)
2. Applied Insulation Resin Classification:
3. Performance/Intended Purpose Classification
- General performance CCL
- Low CTE (Coefficient of Thermal Expansion) CCL
- Low dielectric constant CCL
- High heat resistance CCL
4. Mechanical Rigidity Classification
5. Thickness Classification – not including copper foil thickness
- Standard thickness CCL (at least 0.5mm thick)
- Thin CCL (0.5mm or less thickness)
Printed Circuit Boards
Printed circuit boards are used in almost all electronic products. They are used to create conductive tracks, pads, and other features that mechanically support and electrically connect electronic components. Simply put, a PCB is a plastic board that has been reinforced with glass fibers. There are inter-connecting copper lines and pads attached to it, which are etched (cut) from an overlying copper layer laminated to the reinforced plastic board. These copper lines, called traces, allow the flow of electrical charge through the PCB and provide current to components systematically soldered into place on the board. These copper traces act like wires that conduct electrical power to the required components.
Application of Printed Circuit Boards
PCBs are an integral feature of almost all electronic gadgets, whether these are used domestically or industrially. They are also utilized to manufacture certain electrical products, such as passive switch boxes. Some common examples of PCBs application are:
- Computing devices such as laptops, desktop PCs, workstations, servers, and GPS devices
- Communication devices such as smartphones, smartwatches, tablets, radios, and walkie-talkies.
- Scanning equipment such as CT scanners, Ultrasound scanners, and X-Ray screens.
- Medical monitoring devices such as heart rate and blood pressure monitors and blood glucose monitors.
- Entertainment devices such as currently used TVs, iPods, and PlayStations; as well as stereos, DVD players, VCRs, and games consoles from the past.
- Modern home appliances (especially IoT-based ones) such as microwaves, refrigerators, alarm clocks, and coffee makers.
- General aviation and transport vehicles, such as military aircraft, helicopters, airplanes, spacecraft, and UAV (unmanned aerial vehicles, such as drones) as well as modern trains, buses, and cars.
The Use of Phenolic Plastics in PCBs
The material used for a Printed Circuit Board is generally composite resin insulated sheets made from a phenol and aldehyde combination (phenolic plastic). Although phenolics are used for many industries; however, their electrical insulating properties make them widely popular in the electronics industry. Phenolic plastics are also able to maintain their electrical and mechanical properties at high operating temperatures, which makes them so popular in the industry too. Certain phenolics can bear up to 550°F continuous operating environment temperatures.
PCBs can have a simple one to two layers of copper for low-density application purposes. For high-density applications, the layers of copper plating can go up to fifty or more. Multi-layer boards carry sandwiched copper coatings between layers of insulating material. The copper conductors between layers are connected by vias (copper-plated holes working as electrical connectors throughout the insulating substrates)
The initial PC boards
Circuit boards in the 1960s were tan-colored and made of pressed paper and phenolic resin. They were based on one-sided copper clad boards, and all excess unwanted copper was scraped away. Tracing on these was quite wide, at minimum 2-3 millimeters, and spacing was in the same order.
Mounting holes were punched in, finally adding and soldering components in their designated places. At the very start, all processes on creating these circuit boards were conducted manually, but soon upgraded to mechanical inserting and wave-soldering. Phenolic boards were not initially fire-resistant but eventually received a NEMA designation of FR-2 (Flame Resistant 2) for their synthetic resin bonded paper base characteristics.
How to determine CCL Quality
Though a small-sized component by itself, a Copper Clad Lamination board can strengthen or undermine the function of any product it is integrated into. It is imperative that mechanical testing be performed under various conditions to gauge the total strength of task-specific laminates. Therefore here are certain aspects to check in order to conclude if the CCL will function well.
Scratches, wrinkles, bubbles, dents, resin points, and pinholes may occur in the copper coating/ copper foil unintentionally during the manufacturing process. These cause low performance of the CCL and consequently the PCB. Technically, in order to perform optimally, the Copper Clad Lamination board should be flat, even, and smooth in appearance.
Strain test results are a reliable way to measure a PCB’s mechanical deformation. However, physical performance testing of a CCL must include dimensional stability, heat resistance (including thermal stress, Td, T260, T288, T300), peel strength (PS), bending strength, punching quality, etc.
One of the primary functions of a PCB is its conductivity, so electric performance has to be designed/tested/monitored carefully to check surface resistance, insulation resistance, arc resistance, volume resistance, dielectric constant (DK), dielectric loss tangent (Df), dielectric breakdown voltage, electric strength, and Comparative Tracking Index (CTI).
The chemical performance of a CCL should match standard requirements of:
- Flammability, to check ability to withstand fire
- Chemical reagent resistance, to measure the CCL’s ability to retain its original properties upon being exposed to a chemical reagent.
- Tg (Glass Transition Temperature at which carbon 30-50 chains turn from rigid, solid-state to more flexible, rubbery state),
- Z-axis coefficient of thermal expansion (Z-CTE) to check how much it would expand, dimensional stability to measure linear changes as a result of the change in temperature, etc.
It has to cater to the requirements in terms of water imbibition (water absorption by a material and its subsequent changes such as swelling), pressure vessels cooking test (to check for strength and leaks if placed in a pressure vessel), and other specified tests.
Copper Clad Lamination boards are the foundation of all PCB boards. Hence their size specifications are directly related to the PCB itself. Note that the size of CCLs comprise all dimensions in order to meet specific requirements, i.e., length, width, diagonal deviation, and warpage.
PCB Protective Coatings
If a PCB is intended for use in extreme environments, it requires a thin polymeric film (conformal coating) applied to it by dipping or spraying the PCB with the polymer after the components have been soldered. This polymeric conformal coat prevents rusting, corrosion, and current leakage or short-circuiting due to condensation.
One of the initial conformal coats used was wax, which clearly needed an upgrade. The latest modern conformal coats usually are dilute solutions of acrylic, silicone rubber, epoxy, or polyurethane into which the PCBs are dipped. Conformal coating is applied for plastic to be sputtered (deposited on the surface by ejecting plastic particles in quick, rapid bursts) onto the PCB in a vacuum chamber. It should be noted, however, that using conformal coatings makes PCB maintenance and servicing extremely difficult.
PCB Transportation and Handling
Assembled Printed Circuit Boards are most often static sensitive and should be only transported in antistatic bags, and any handling on the PCBs should be conducted if the user is properly grounded (earthed). Mishandling may lead to transmission of accumulated static charges within the board, subsequently damaging or destroying components.
Damaged boards may even work for a brief period. However, they will cause intermittent operating faults, completely fail to function, or limit the environmental and electrical conditions under which the board functions efficiently. Static discharges can blow the copper traces or alter their functionality, especially since modern PCBs have very finely etched paths. Multi-Chip Modules that comprise several integrated circuits together on one board and are used as a more extensive integrated circuit are especially susceptible to static charges, as are microwave PCBs.
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