Microwave PCBs, as the name suggests, are designed and fabricated in such a way that they can handle much higher frequency signals with lesser or minimal loss, keeping signal integrity intact by keeping minimum cross-talks of signals.
In contrast to standard PCBs, microwave PCBs can cater to unusual challenges caused by high-frequency circuits. The microwave PCB fabrication and design require a lot of dedication, precision, and technological advancement of the designer and fabricator as well. As the fabrication of standard PCBs itself is a complex task, now for microwave PCBs, it has additional constraints, and limitations and requires exceptional understanding and modern equipment.
Generally, PCBs that can handle more than 1GHz frequency up to 30GHz are categorized as microwave PCBs and are commonly used in the aerospace industry, telecommunication, and medical devices. The microwave PCB fabrication procedure is almost similar to standard PCBs, but the only difference lies in its design like component placement, and trace routing, selection of substrate material. Some of the prerequisites for designing microwave PCBs are jotted down,
- It is important to keep the length of traces as short and straight as possible. Instead of 90°turn in traces, it is wise to use a 45° bend.
- While placing components, it is good practice to keep high-frequency I/O ports easily accessible, isolate digital components from analog, and place those components closer which requires high-speed traces.
- Poor grounding while designing the microwave PCB can result in some serious consequences and can cause crosstalk between two isolated circuits. So, it is important to ensure proper grounding where required, with thick traces.
- Assigning an appropriate layer is one of the major jobs of a microwave PCB designer, like assigning ground layers around sensitive tracks, to isolate different circuits with the help of power planes.
- Optimized routing helps to reduce noise in PCB. One of the examples of this is, eliminating excessive vias in PCB where they are not required to. Vias can possibly act as an antenna when placed nearer to high supply traces or other high-impedance components and may radiate EM waves to its surroundings, resulting in malfunctioning of circuits or even failure.
- Loop areas in microwave PCB have more impedance than linear tracks. For this reason, such loop areas can cause harmful effects on signal integrity. This problem can be mitigated by using more than one vias if required and placing them as close as possible to the pads of components.
- Transient current spikes are caused by switching transistors and microcontrollers, at the time of switching off. These spikes result in ESD and travel through ground planes; such noise is generally known as conducted noise.
- Clearance is the distance between two conductive traces. In microwave PCBs, proper clearance is required to mitigate crosstalk or unwanted coupling between different planes and traces. With the help of proper clearance, the designer can protect the fabricated microwave PCB from arcing which could result in failure of the PCB.
Fabrication Considerations for Microwave PCB:
Although the fabrication process of microwave PCB is generally almost similar to standard PCB fabrication, there are some considerations for microwave PCB fabrication, and it is important to have an eye on them during fabrication. Some of them are listed below.
1. Material Selection:
Substrate material directly impacts signal integrity and overall PCB performance. It is important to select the right substrate material. A substrate material with lower dielectric loss has a minimal signal attenuation factor and ensures higher signal integrity. Copper-clad thickness, thermal expansion coefficient, cost, operational frequency, and thermal stability are other factors that must be kept in mind while selecting the right substrate material. Material selection is critical in microwave PCB fabrication.
2. Impedance Matching:
It is the most important factor for maximum transfer of signal. This impedance matching involves both software designing and fabricating hardware. If there is no impedance matching, then skin effect loss usually arises and converts signal energy into heat which can be minimized by plating PCB gold. It is general practice to have ±5% or higher impedance tolerance for better transmission while keeping signal integrity intact.
3. Sequential Lamination:
Stacking layers by layers of PCB is sequential lamination. By doing this, the manufacturer can have control over the dielectric thickness and eventually over impedance. It is recommended to use bondply material for inner layers, which ensures consistency in impedance throughout the microwave PCB fabrication, resulting in minimal variation in impedance with improved signal integrity.
4. Plating of PCB:
It is considered a key factor for having a reliable microwave PCB. So, it is common practice to verify the plating of PCB and vias. Plating the PCB with gold helps to reduce the skin effect loss in microwave PCB.
5. Quality Assurance:
This involves visual inspection, automated optical inspection, and rigorous testing of PCB in different stages of PCB fabrication or even after fabrication, to track if the fabrication is according to required standards or not. For physical inspection, manual inspection or AOI can be performed to detect any short circuit, dry solder joint, unsoldered component, polarity of electronic components, or abnormalities in the PCB surface. To check the impedance characteristics of microwave PCB, time domain reflectometry is performed, it detects signal reflection and mismatching of impedance. To check the stiffness, robustness, and reliability of fabricated microwave PCB vibration and shock tests are usually performed. Other general electrical tests which are usually performed on standard PCB, are also performed on microwave PCB fabrication.
Challenges in Microwave PCB Fabrication:
PCB manufacturers encounter a different set of difficulties and challenges during microwave PCB fabrication. Microwave PCB requires more accuracy and precision than standard PCB, which means more attention to detail and advanced technological tools and methods are required by manufacturers to fabricate such PCBs. Unlike standard, conventional PCBs, manufacturers must keep in mind high frequency-related parameters like impedance and signal loss to have optimized microwave PCBs. Moreover, microwave PCBs can also be affected by different fabrication processes. Any miscalculation or deviation during microwave PCB fabrication can lead to undesired outcomes like faulty PCB, signal loss problems, and lower signal integrity. That’s why, microwave PCB manufacturer must consider these challenges, and try to mitigate their effect as first priority with utmost care.