The high-frequency PCB material is based on using a synthetic thermoplastic fluoropolymer which has an excellent dielectric property at different smaller microwave frequencies.
For the design of PCB circuits with microwave frequencies, important characterizations that determine laminate circuit performance include dielectric constant (DK), dissipation factor (Df), thermal expansion coefficient (CTE), thermal dielectric constant coefficient (TCDk), and thermal conductivity. The most known high-frequency material for users of PCB laminates may be polytetrafluoroethylene (PTFE), a synthetic thermoplastic fluoropolymer with outstanding dielectric characteristics at microwave frequencies. PCB applications include RF antenna, WiFi (Carrier-Green and Licensed Access), IP infrastructure, power amplifiers, diplexers/multiplexers, testing, measurement, etc.
It is essential to have considerable expertise in manufacturing PCBs with these materials using PCBs produced from these goods.
Choosing a circuit material for a PCB is usually a compromise, frequently between price and performance. However, Printed Circuit Board materials are also chosen by two important factors: how well they suit the requirements of a final application and what work is needed to build the desired circuit with a certain material. These two variables may not mesh: one material may be suitable for a specific application but may provide difficulties in producing a circuit and vice versa.
However, by relying on concrete criteria intended to assess the appropriateness of the material for circuit manufacturing and to satisfy the requirements of an application, the process of choosing a PCB may be simplified for a specific application. The method will be illustrated using some of the most common high-frequency PCB materials, each of which reflects manufacturing characteristics and end-use compatibility.
For various applications, it is advisable to use FR4 material with defined layer buildup as required during the design. Additionally, the processing is faster with such material having improved dielectric properties. These also have lower dielectric constant, frequency, and temperature-independent along with lower loss-factor. There are high glass transition temperature, lower hydrophilic rate, and excellent thermal durability are considered as additional favorable properties.
The PTFE and Rogers materials are typically used for impedance-controlled higher frequency circuit boards. There is also the possibility of executing the design with the material combinations sandwich buildups. For achieving the higher frequency provided from the desired PCB type, there are special materials required and there are numerous substrate materials present which particularly support the design and could differ relying on signal speeds needed along with the circuit board application/environment.
The FR4 is the least expensive when comparing it with other high-speed dedicated materials in terms of pricing and Teflon is the most expensive. However, in recent times, it was noted that the FR-4 started to drop off in performance as soon as the signal speed edged higher than 1.6GHz. When it comes to Df, Dk, survivability in the environment, and water absorption, the newer generation substrates are the best choice.
There are newer generation substrates that can typically be used when the printed circuit board requires frequency above 10GHz. These substrates include Flex and Teflon as the best option since these have higher superior properties when comparing it with the traditional FR-4 material.
The high-speed substrates common supplies included Isola, Taconic, Dupont, Rogers, and Megatron materials. These materials typically have a lower loss and lower DK.
A variety of mechanical procedures are necessary for the production of high-frequency PCB materials. In general, plated-through-hole (PTH), multi-layer coating, and pcb assembly were the most important. The drilling procedure usually involves creating clean holes that are metalized subsequently to create troughs for the electrical connections from one conducting layer to another. Some issues related to the drilling process include smear, burring, and fracturing. Smearing may be fatal to PCB manufacturing using a PTFE-based material because the smear cannot be removed. Fracture of certain nonwoven glass hydrocarbon materials may be deadly.
However, this is not the case for most woven glass hydrocarbon materials. The PTH preparatory procedure is reasonably easy to specify for most non-PTFE materials, but PTHs for PTFE-based materials need specific processing. Ceramic-filled PTFE-based materials provide more forgiving PTH preparation choices. Non-ceramic PTFE materials need a specific procedure that may restrict final circuit outputs. Manufacturing multi-layer PCBs is challenging. One is that different materials frequently are linked together, and these different materials may have characteristics that hinder the operations of drilling and PTH preparation.
In addition, a discrepancy between certain material characteristics, such as the CTE thermal expansion coefficient, may cause dependability issues when the circuit is heat strained during assembly. The material selection procedure aims to identify a suitable mix of circuit materials for multi-layer PCBs that allow practical manufacturing and fulfill end-use criteria. Designers and manufacturers are provided with a wide range of materials to join the copper laminates that eventually form a multi-layer Printed Circuit Board.
The materials vary in dielectric constant, dissipation, and processing temperatures, as shown in Table 2. Lower lamination temperatures should generally be chosen. However, when a PCB is soldered or any other kind of heat exposure, it is required to employ a high-reflow (re-melt) bonding material that is thermal resistant and does not reflow at high processing temperatures.
Di-Electric Co-efficient (DK)
Make sure the substratum is composed of DK-like friendly materials.
Thermal expansion coefficient (CTE):
For materials, CTE is perhaps the most critical thermal feature. If the substrates contain distinct CTE elements, they may grow at various rates throughout the manufacturing process.
Many best practices are available in selecting the appropriate substrate and foil for your application in high frequency.
• Match Dielectric Constants – If you want a personality match, you want a Dk match on PCBs. If your PCB substratum is composed of resin and woven material, various Dks may exist. Non-uniform Dks will create issues in your substratum. You must verify with your manufacturer to ensure that you obtain as near as possible to all of your substrate Dks.
• Match Thermal Expansion Coefficient (CTE) – There are many temperature-related substrates properties. The CTE of your substratum component may influence your Dk. If your substratum components have varying CTEs, they may expand at various rates throughout the production or operation of the circuit. It may lead to problems in the manufacture of PCBs. During operation, the physical shape of the substratum may be changed and Dks not uniform. It leads to a broken connection in love.
• Tight Substrate Weave —The woven portion of your substratum must likewise be narrowly meshed. A loosely woven substratum causes different Dks to be killed.
• Don’t use FR4 —It would help if you also utilized a low-loss substratum. Some individuals still use FR4 for their high-frequency circuits, though. FR4 is not a suitable material for applications with high frequency; use anything else.
• Use the beauty of a smooth foil, and it is seemingly just skin-deep. A smooth copper foil ensures reduced resistive losses at the highest frequencies.
• Use a Conductive Foil – If your skin depth is low, make sure you’re not using weak conductors to complete copper. The current passes through these weak drivers and creates a poor circuit.
Various RF PCBs and microwave PCB applications are multi-layer businesses. By combining various materials, board characteristics may be fine-tuned to enhance electrical performance, thermal qualities, and cost balance. Composite boards, commonly referred to as multi-layer hybrid PCBs, may be challenging to compile since various PCBs’ production stages have to take care of many layer-to-layer interactions.
There are also other difficulties, like the PC Board mixed-signal design, which mixes analog and digital components, creating many variables. The future is certainly bright, with so many present uses and new markets for RF PCB’s and microwave PCBs. However, RF boards are complicated and must include numerous variables and show unique behaviors that are not communicated with their lower frequency relatives. It’s not “dark magic,” but it may be difficult.
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