Whenever it comes to choosing material for PCB designing, the very first question that comes to mind is “Should I use RF-4 material or Roger material for this application” or “Which Roger material should I use”, the answer to these questions is quite simple that is it totally depends on the application, Whether the user wants low cost or high reliability (low losses). There are multiple properties that need to be considered before choosing any material, some of them are
Decomposition Temperature (T D)
When a PCB is exposed to temperature that is above certain temperature threshold, it decomposes. During this process, the PCB losses round about 5% of its overall mass. This temperature range of decomposition is called decomposition temperature. It is best to use PCB material that can handle yours working temperature i.e generally between 200 ᵒC and 350 ᵒC. Ideally the decomposition temperature should be above this working temperature at which the soldering will be done.
Co-efficient of thermal expansion (CTE):
The rate at which the PCb material expands when exposed to temperature above glass transition temperature (Tg) is known as co-efficient of thermal expansion. It is measured in parts per million (ppm). The CTE for copper is 18 which means that it expands by 18 ppm per degree centigrade rise. The problem occurs when CTE of copper and substrate mismatches. Because if the CTE of substrate will be higher than copper then per degree centigrade, the expansion of substrate will be more than copper leading to de-lamination or solder joint failures. The materials with CTE of 70 ppm or less are usually recommended.
Dielectric constant 🙁εr)
Dielectric constant also known as relative permittivity of a PCB material defines the amount of charge a material can store. Greater the value of dielectric constant, greater will be the capacitance and hence greater charge will store resulting in greater voltage appearance across PCB. The of a material depends on frequency, usually decreases with increase in frequency. The materials with higher dielectric constant yields smaller circuits for a given operating frequency or vice versa. For a material to be appropriate for applications with high operating frequencies, it must maintain a stable dielectric constant over the operating range of frequencies used in that application.
Ideally, the PCB material should be the one with almost no moisture absorption when submerged in a liquid. The majority of materials offer absorption value between 0.01 percent and 0.20 percent. Higher the absorption percentage, higher will be the influence on the thermal and electrical properties of material.
Rayming PCB and Assembly is one of the proud assemblers of Roger PCB’s. The material’s that normally remain in stock are Rogers 4350B, Rogers 4003C, Rogers 3003, RT5880 and RT 5870. These materials are favorable for applications involving high frequency, high speed performance in both wired and wireless communication and noise sensitive circuits. The working frequency range and dielectric constant of all the materials is shown below,
|Material||Dielectric Constant||Frequency Range||Applications|
|R3003||3.00||8 GHz- 40 GHz||· Automotive Radar
· GPS Antennas
|R4350B||3.66||8 GHz- 40 GHz||· Cellular Base Station antennas
· Microwave point to point links
|R4003C||3.55||8 GHz – 40 GHz||· RF identification tags
· Automotive radars and sensors
|RT5880||2.20||8 GHz – 40 GHz||· Commercial airline broadband antennas
· Military radar systems
|RT 5870||2.33||8 GHz – 40 GHz||· Missile Guidance Systems
· Microstrip and Stripline circuits
These materials have been tested over the range of 8 GHz – 40 GHz by Rogers corporation using differential phase length method. The dielectric constant of all these materials vary slightly with every 10 GHz rise in frequency but choosing the material for PCB designing entirely depends on the application.
Let’s say a person wants to design an automotive radar he can pick R3003 or anyone from this list but if he will choose R4350B, it will decrease the size of circuit because higher the dielectric constant smaller will be the size of PCB for a specific operating frequency range. While on the other hand if RT5880 will be chosen, the overall circuit size will increase but there will be lesser power losses and noise will be negligible as compared to that produced in circuit with R3003 material.
Selecting a material is not simply a matter of choosing material based on just one parameter of dielectric constant. While choosing material for PCB designing engineers have to look on multiple properties of the material like Dissipation Factor i.e the power loss associated with the material and it increases with increase in the operating frequency. Second one is the Thermal Conductivity that tells about the heat conductive properties of the material. In case of amplifiers or missile guidance systems, improperly managed heat can cause mechanical and electrical variations in the properties of PCB material that can lead to serious problems.
The engineers at Rayming PCB and Assembly with their extensive expertise are always here to help our customers in choosing proper material for their RF application circuits in terms of cost versus performance.