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With the development of wireless communications and broadband networks, high-frequency PCBs are no longer simply metal conductors on top of insulating substrates to achieve interconnections. In many cases, the substrate and metal conductors have become part of the functional component. Especially in RF applications, where components interact with the substrate, the design and manufacture of HF PCBs is becoming more and more critical to the functionality of the product.
High frequency PCB manufacturers are also more involved in design related issues, especially in high frequency, high speed signal transmission. Similarly, high-frequency PCB designers must have an in-depth understanding of high-frequency PCB manufacturing techniques in order to produce qualified,high-performance high-frequency PCBs.
IPCB will introduce some of the parameters that we often contact,from shallow to deep to do some scientific and technological exploration,hoping to deepen the communication and exchange of design and manufacturing.
1Dielectric constant
Dielectric constant (Dk, ε, Er) determines the speed of propagation of electrical signals in the medium.The speed of signal propagation is inversely proportional to the square root of the dielectric constant. The lower the dielectric constant, the faster the signal travels. Let's make an analogy, as if you are running on the beach,the depth of the water submerged your ankles, the viscosity of the water is the dielectric constant, the more viscous the water is, the higher the dielectric constant, the slower you run.
The dielectric constant of HF RF PCB is not very easy to measure or define, it is not only related to the characteristics of the dielectric itself, but also related to the test method, test frequency, data status before and during the test. The dielectric constant also varies with temperature, and some specialised data have been developed with temperature in mind.Humidity is also an important factor affecting the dielectric constant, because the dielectric constant of water is 70, and very little moisture can cause significant changes.
For high-speed, high-frequency applications, the ideal data is an air dielectric wrapped in copper foil with a thickness tolerance of +/-0.00001’. As data development, everyone is working in this direction. For example, Arlon's patented Foamclad is very suitable for base station antenna applications.However,not all HF RF line design is the smaller the dielectric constant the better, it is often based on some of the actual design, some of the requirements of a very small size of the line, often need a high dielectric constant data, such as Arlon Arlon AR1000 used in miniaturised line design.Some designs,such as power amplifiers, often use a dielectric constant of 2.55 (e.g. Arlon Diclad527,AD255, etc.) or 3.5 (e.g. AD350, 25N/FR, etc.).There are also some with 4.5 dielectric constants (e.g. AD450) that have been converted from FR-4 designs to high frequency applications and want to stay with the previous design.
In addition to directly affecting the transmission rate of the signal, the dielectric constant also determines the characteristic impedance to a large extent,in different parts of the characteristic impedance matching is particularly important in microwave communications. If an impedance mismatch occurs,the impedance mismatch is also called VSWR (VSWR).
CTEr: Since the dielectric constant changes with temperature,and HF RF PCBs for microwave applications are often found in outdoor and even space environments,CTEr (Coefficenc of Thermal of Er) is also a key parameter. Some ceramic powder filled PTFE PTFE can have very good characteristics,such as CLTE.
2、Dissipation factor (Loss, loss tangent, Df, Dissipation factor)
Besides the dielectric constant, the loss factor is an important parameter that affects the electrical characteristics of the data. Dielectric loss, also known as loss tangent,loss factor,etc.,refers to the loss of signal, or energy, in the dielectric. This is because when high-frequency signals (which constantly change between positive and negative phases) pass through the dielectric layer, the molecules in the dielectric try to orientate themselves according to these electromagnetic signals, although in reality,since these molecules are cross-linked, they cannot really orientate themselves.However,the change in frequency causes the molecules to keep moving, generating a lot of heat and causing energy loss. Some materials,such as PTFE, have non-polar molecules, so they are not affected by changes in the electromagnetic field,which results in less loss.Similarly,the loss factor is also related to the frequency and test method,and the general rule is that the higher the frequency, the higher the loss.
The most intuitive example is the consumption of electrical energy in the transmission.If the circuit is designed for low losses,the battery life can be significantly increased. Battery life can be significantly increased.When receiving signals, using the loss of data, the antenna's sensitivity to the signal is increased,and the signal is clearer.
Commonly used FR4 epoxy resin (Dk4.5) polarity is relatively strong,at 1GHz,the loss of about 0.025,while PTFE substrate (Dk2.17) in this condition,the loss is 0.0009.Quartz-filled polyimide compared to glass-filled polyimide not only a lower dielectric constant, but also lower loss, because the content of silicon is more pure.
PTFE polytetrafluoroethylene molecular structure is very symmetrical,C-F bonding close,no polar groups.Therefore,with the electromagnetic field changes and the possibility of swinging is very small, expressed in the electrical characteristics of the loss is small.
3、Thermal conductivity
In many microwave fields, there are more high-power applications,the thermal characteristics of the data can greatly affect the reliability of the entire system.Therefore,the thermal conductivity coefficient should also be an aspect of our consideration. Some special high-reliability and high-power applications can also use metal lining (aluminium or copper base).
4、Fabricability DFM
We understand that PTFE Teflon data is relatively difficult to process,especially hole metallisation,which requires plasma or sodium naphthalene treatment to improve its activity,and PTFE Teflon is a thermoplastic data, which requires higher temperature for multilayer board processing.New low loss thermoset resins have been developed for HF circuits that can be processed in multiple layers without plasma activation,such as Arlon 25N/FR. They are now used in LNA, PA and antenna designs. Moisture absorption is also a consideration,as far as possible, the choice of moisture absorption of small data, electrical characteristics more stable.
5、Coefficient of thermal expansion (CTE)
Coefficient of Thermal Expansion (CTE) is usually abbreviated as CTE (Coeffecient Thermal Efficent),which is one of the important thermo-mechanical properties of the data.It is one of the important thermo-mechanical properties of data. It refers to the expansion of the data when it is subjected to heat.The actual data expansion refers to the volume change,but due to the characteristics of the substrate,we often consider the expansion in the plane (X-, Y-) and vertical direction (Z-) separately.
In-plane thermal expansion can often be controlled by reinforcement materials (e.g. glass cloth,quartz,Thermount),whereas vertical expansion is always difficult to control above the glass transition temperature.
Planar CTE is critical for mounting high-density packages.If wafers (typically with CTE of 6-10 ppm/C) are mounted on conventional high-frequency PCBs (CTE of 18 ppm/C), the solder joints may be overstressed and over-aged after many thermal loops.The CTE of Z-axis directly affects the reliability of hole plating, especially for multi-layer boards.
Usually the CTE of PTFE PTFE is larger,and it is not common to use pure PTFE PTFE for multilayer boards, but ceramic powder filled PTFE PTFE is often used. For example, Arlon's CLTE, LCCLTE, etc., the most representative application is the manufacture of up to 30 layers of multilayer PCBs used in global communication satellites.
6、Passive Intermodulation (PIM)
In RF front-end design, such as antenna and filtering, passive intermodulation is required, which is also related to the base material of HF PCB. Some companies use specific copper foils to keep the passive intermodulation within a certain range. The table below shows the difference between HF RF PCBs without passive intermodulation requirements and HF RF PCBs with specific PIM requirements.
Causes of Passive Intermodulation
Passive intermodulation is mainly caused by passive nonlinearities, which are usually of two types: those caused by metal contact and those inherent in the data itself. For example, coaxial cables and connectors are usually considered to be linear, but in high-power situations, their nonlinear effects show up. Slight non-linearities do exist in the contacts of cable braid, connector shackles and other metal joints. Each surface of these metal contacts has a thin insulating layer formed by metal oxidation, and it is this contact nonlinearity that produces the low-level passive intermodulation interference that can seriously degrade the performance of a receiver.
Metal contact nonlinearity is mainly due to loosening of the connection and corrosion, and its volt-ampere characteristic is a curve, the specific main mechanism is as follows:
(1) Non-linearity caused by poor installation techniques.
(2) Non-linearities related to high currents in metal contacts.
(3) Non-linearity associated with metal surface dirt, metal particles and carbonisation.
(4) Secondary electron multiplication effect through sand holes and microcracks in the metal structure.
(5) Electron tunneling effects and transistor behaviour through thin oxide layers (less than 50 Ao thick) in metal contacts.
(6) Thermal loops induced by strong currents in the relative motion of metal contacts. There is no strict boundary between linear and non-linear. Metal contacts are usually considered to be linear, but in high power cases non-linear effects are exhibited.
Non-linear effects cannot be completely eliminated, but can only be minimised as far as possible, the main reduction measures are:
(1) Maintain the smallest thermal loop, reduce the expansion and compression of metal materials generated by the non-linear contact.
(2) Minimise the number of metal contacts. For example, use choke connections or other dielectric connections to provide adequate current paths and keep all mechanical connections clean and tight.
3) Avoid the use of tuning screws or moving parts with metal-to-metal contacts in the current path as much as possible. If they must be used, they should be placed in low current density areas.
4) Improve the connection process of the data. Ensure that the connections are reliable and, as far as possible, free of gaps, contamination or corrosion.
5) The current density in the conductive path should be kept low. For example, the contact area should be large and the conductor block should be large.
Due to the complexity of the passive intermodulation problem, it is difficult to establish a high-power circuit model, and thus some analysis methods of nonlinear circuits can not be used, but for the metal contact nonlinear, a simple system can be used to represent, where X and Y denote the input and output signals (currents or voltages), respectively, through the single transfer function analogous to the entire process of generating the metal contact nonlinear, using the input-output method to analyse, the specific The input-output method is used to analyse the whole non-linear generation process of metal contact through a single transfer function, and the specific solution methods are power series method and Voltara series method. The power series method has the advantages of simplicity, fast calculation speed and easy implementation.
The selection of microwave data for HF RF PCBs is mainly through the dielectric constant, loss, thermal expansion coefficient, thermal conductivity, and several aspects of the selection of low-cost, low-loss thermosetting high dielectric constant ceramic filled with PTFE polytetrafluoroethylene.