ROGERS RF PCB DESIGN AND MANUFACTURING PROCESS
How to design a good RF PCB Layout? Simple tips to design RF PCB layout. In this section, we will discuss the simple tips when you want to design a PCB layout for RF applications with rogers pcb material. Tips provided here don’t include talking about Smith charts, Q factor, S parameters, etc which need a lot of academic knowledge. Instead, we will discuss “the simple way” to design the RF PCB layout. Below are some simple tips to start with:
1. No single-shot perfect RF layout. If your design (e.g. antenna) does not perform as expected in your simulations, it is perfectly normal. It could happen because the antenna impedance is influenced by components located around it, and the impedance may shift in a way that cannot be predicted in software simulations. The best that you can do is adding a matching network that lets you tune the antenna in your final product. Moreover, not only antennas that require impedance matching, but also between different RF components or subsections on board need it for proper interfacing. And of course, iteration is a key!
2. Use 4-Layer Design. Multi-layer PCB is best. It’s not obligatory to use 4 layers in RF design. You can do the 2-layer design but you need to read some advanced RF concepts. If it is quite hard for you to make an advanced RF study of your circuit or because it takes so much time, then you can use 4-layer design as a solution. Don’t forget to put continuous grounds under traces. And consider material selection carefully. Standard FR-4 may not meet your needs (we will discuss more another material in the next section). Lastly, follow the signal stack below.
3. Make everything 50 ohms. Just in case it is your first time to design RF PCB layout and there is no available proper tools to simulate your design in 3D, then the best alternative that you can try is choosing components that have a characteristic impedance of 50 ohms on RF ports. Why 50 ohm? Because 50 ohm is the best value to do impedance matching. This includes a microstrip impedance calculation to know it’s resistance. Moreover, you can adjust the trace width properly to make the trace impedance on your PCB that carries RF signals becomes 50 ohms. You can calculate the trace width using online trace impedance calculators or microstrip impedance calculators. You can also use a CPWG (coplanar-waveguide-over-ground) structure to build 50Ω RF traces on PCBs. Lastly, actually it is quite easy to find components (such as antennas, filters, amplifiers, etc), with 50 ohms characteristic input/output impedance, for your project.
4. Lay out RF first – ALWAYS. RF traces are the first priority that you have to care about since it is EXTREMELY high frequency signal carrying structures. That’s why, if you put them at last, or if you try to put them when the board has already gotten rather clumsy, you will make compromises with the trace layout. And it can ended up make your design fail. Lastly, please make sure to have sufficient space around the signal trace for smooth bends and isolation of the RF signal.
5. Isolation is important. Isolating an RF trace is important. Ensure to make RF traces appropriately isolated from other high-speed signals (such as HDMI, Ethernet, USB differential pairs, clock traces for crystals, etc). Commonly, it is employed by “via stitching”. For example, do stitching vias around the RF trace to prevent it from interfering with other components on board. Please remember that inappropriate isolation will not render your design defunct. However, it will deteriorate the receiver performance and the average data throughtput in most cases. Therefore, utilize isolated vias for separate parts of a filter or matching network.
6. Keep inductance low. The ground inductance can cause a huge impact in your RF design. Grounding RF chipset through a single via or narrow ground trace could cause a massive ground inductance. And as we know, high-frequency does not like inductance. Therefore, do not forget to ground the RF chipset adequately. If the RF chipset is a QFN with ground pad, use at least 9 power vias. Then ensure a large and continuous ground plane under the chip and RF trace as well. If you have a free space on the top layer, do not forget to add a ground fill that is connected to the inner ground layer through as many vias as practically possible. But of course, don’t add a thousand vias! It would make a lot of pains to your PCB manufacturer. Lastly, use least number of vias in RF routing, max number of vias in RF grounding.
7. Use Plating/Copper. Use gold plating for RF components created from etch, with no copper inlands near the RF circuitry and no copper thieving near the RF circuitry. Then, ground both ends of copper pours, and stitch many vias wherever possible. The last, separate RF planes from all other planes.
8. Routing. For routing in RF PCB design, there are some points that you need to consider: (1) orient sensitive traces orthogonally, (2) use short traces between the crystal and RF device, (3) keep interconnect traces separated as much as possible, (4) keep trace lengths to a minimum, and (5) adhere to proper corner routing. Below are some types of corner routing.
9. Don’t break the ground plane. Maybe sometimes you find a case when you have to design an RF system where you also have an audio or analog circuitry on the same circuit board, and the audio or analog circuitry is close to RF system. You may be trying to have a different ground plane to isolate the ground for the audio or analog part. But by doing that, you probably will make your RF section severely. And remember that if you break the ground plane under the RF trace, you may have a design that does not work. Therefore, here is an example of what you should NOT do:
Moreover, there are three best practices when you want to do grounding for RF PCB layout. First, connect the ground pins of the RF components to the RF ground plane as short as possible. Second, if the board includes multilayer ground planes, provide many ground vias wherever the signal trace makes a transition from one side of the plane to the other. And the last, consider the use of an RF fence/cover connected to ground. Below is the illustration of using RF cover.
10. Use Integrated Components. Always try to find an integrated component that meets your application. For example, use transceiver ICs like: nRF24L01+, ADF7242, AT88RF215, CC2650, CC1310, etc. The reason is that these components are at least already compact, reproducible, and also normally you can find the parameters of these components in the datasheet because every integrated components are already tested by the manufacture. And also it is better to use passive components like filters in a shape of integrated component which is much easier than design a discrete one. Moreover, simulation is strongly recommended if the RF component is being printed / down as part of the board.
11. Follow The Manufacturer Recommendations. Some times, the manufacturer will provide you with a reference circuit to match the impedance of output to 50 ohms, including the recommended solder pattern in its datasheet. Therefore, just follow this recommendation!
How to select the right PCB raw material? Does plain old FR-4 PCB material, which is also known as “Glass Epoxy”, match with requirements for RF designs? This question maybe comes up to your mind in many times and again. Although some engineers have been using FR-4 material for years to build not only protoboards, but wireless radios, RF test fixtures, and RF test equipment, we are not going to say that FR-4 does not have limitations. Instead, when we understand the limitations then we can make better cost/performance tradeoffs for all our designs.
So what are the limitations of FR-4? One of the limitations is that FR-4 has unstable Er (dielectric constant) over frequency. Besides, FR-4 also has issues with Loss, lead-free processing temperatures, and perhaps thermal conductivity as even low-power RF can consume a lot of power if the active circuits are biased to provide very high linearity. And because many FR-4 materials are not made specifically for RF performance, as a consequence the Er can be different between manufacturers and vary from lot to lot. Sometimes Er is not even specified by some material manufacturers! Does it means that FR-4 PCB material and “Glass Epoxy”-like materials can not be utilized for RF design? So how to choose a PCB material?
Basically, PCB material selection depends on many factors, some of which are:
Circuit design that needs impedance stability
1 Signal Loss tolerance
2 Operating temperature (Stability vs temperature, Temperature expansion, etc)
3 Heat sinking ability, because even a low power RF system can emit a lot of power)
4 Soldering / Assembly Temperature (Lead Free)
5 Product Cost
However, maybe not all of the above points are applicable to your project. Therefore, there are some available choices of material itself that you can choose, they are:
1 Plain old FR-4 material which has a higher loss and not tightly controlled Er
2 Better specified Er FR-4 derivatives which also could have a better loss
3 Specialized low-loss RF Materials which have well-specified Er values and also much lower loss
Although for some engineers it is a simple matter to read the datasheets and make a summary (e.g. spreadsheet, matrix) to make parameters comparison for these items, it can lead to a perplexing array of options. Especially for the person who is still new to RF world. For example, there are some people new to RF who used expensive exotic materials for even low-frequency non-critical applications simply because someone said that the application was “RF” and they immersed into “Over-Specify Mode” just to be safe. In fact, do they really need it? In high-volume cases, it is so hard to find anyone using really exotic materials in the under 6-GHz world since you will find materials that look just like regular old FR-4. In the low-volume but high-performance category, you will also find board material that looks like FR-4 and you will find higher-frequency materials, especially when the operating frequency exceeds 6 GHz. In the low-volume cases, performance may be so much important while the circuit designs might be more complex. Many of these products really use a tighter specified type of “Glass Epoxy” or exotic RF materials, which mainly for their repeatability and for the trace losses.
High-Performance RF PCB Materials. Suppose you want to build a 2.5-GHz Bluetooth module and the RF traces are about an inch long total, would you really consider the 0.3-dB signal loss, especially when you know that the antenna matching circuit possibly produce more loss than this? Maybe not. The next step up in improving on FR-4 is to use a high-performance material like Rogers PCBs (e.g. RO4350B) and others. As shown in Figure 6, the RO4350B PCB loss is less than half the loss of FR-4 at 6 GHz. While this may not be excessively significant and not worth the additional expense if your circuit works at under 6 GHz, at 10 GHz the losses are even less and FR-4 really begins to show its shortcoming.
These materials can work well up to the 20-GHz-plus range with really high-performance, plus having a very stable and repeatable Er. Besides, the Er of these materials is also usually much lower, being on the order of 3.6, and the Er is essentially flat with frequency since it is a highergrade “Glass Epoxy” material. If your circuit design uses distributed elements or matching networks in the multi-GHz range then there is really a no better choice than these types of materials for lot-to-lot consistency. Moreover, instead of based on glass epoxy, these materials usually have a ceramic filler which really improves the thermal conductivity. Huge numbers of these materials can likewise endure lead-free assembly temperatures well indeed. But of course, there is no free lunch! All of this performance comes with a cost; your board cost, to be specific. Another option to designing a multilayer PCB with all high-performance material is to develop a hybrid Glass Epoxy/high-performance material type board. This method is when you use a material like the high-performance Rogers RO4350B on the outside layers where the RF components and Microstrip traces use a lower-cost Glass Epoxy inside. In which, the power and control traces reside. This Hybrid-type construction works out quite well and can save a substantial amount on your board costs. Please make sure to check the details with your board manufacturer although you are already confident that the materials you want to use are compatible with each other.
So, which material should we choose? Well as discussed before that plain old FR-4 or improved Glass Epoxy can indeed be employed at all the common RF/wireless frequencies up to 7 GHz or more. If plain old FR-4 does not work for you for some reason, then you have the option of using a high-frequency, better specified “FR-4 like” Glass Epoxy material that won’t ruin the budget. If the total loss and circuit stability are of principal significance to you, or if your project requires to go above 10 GHz where FR-4 is truly getting entirely lossy per inch, then you can always use the exotic high-performance microwave materials like Rogers materials.
What is Rogers PCB material? Rogers PCB material is one type of high-frequency plate produced by Rogers Advanced Connectivity Solutions (ACS). Rogers company is the world’s leading manufacturer which focuses on high-performance dielectrics, laminates, and Pre-pregs.
Comparing to the conventional PCB plate epoxy resin, this kind of PCB is different. Most PCB plates are made of a material known as FR-4(Flame Retardant level 4), which is a glass fiber/epoxy composite with copper foil laminated on one or both sides. FR-4 material has a base standard of PCB substrates, which gives a widely effective balance between cost, manufacturability, electrical properties, durability, and performance. On the other hand, Rogers will provide us FR-4laminates (FR-4 core with copper laminate) since they are better known for cores with better high-frequency properties, such as PTFE (Teflon). Although Rogers materials are more expensive than fiberglass, they are less lossy at high frequencies. This makes this kind of PCB material is good for RF circuit boards. When the working frequency of the circuit is above 500MHz, the number of materials that can be chosen by the design engineer becomes significantly smaller. Usually, a radio engineer will use the term “Roger’s PCB” when they want to mention “Circuit board with Teflon cores”. But actually, Rogers also makes many types of PCB cores other than a circuit board with Teflon cores, while many companies also produce Teflon cores.
To sum up, there are several differences between the FR-4 material and Rogers material:
1. FR-4 material is less expensive than Rogers material.
2. Comparing to FR-4 material, Rogers material is better for high-frequency applications.
3. For dielectric constant (Dk), FR-4 has Dk value of about 4.5, which is lower than Rogers material with around 6.15 to 11.
4. In temperature management, Rogers material has a less variation comparing to FR-4 material
5. FR-4 material has a high Df (dissipation factor) than Rogers material, suffering more signal loss. 6. In impedance stability, Rogers material has a wider range of Dk values than FR-4 material.
Why rogers dielectric material? Rogers Corporation’s materials have some advanced electrical properties and performance which could be critical to your designs. Rogers material has some advantages: Low electrical signal loss and lower Dielectric loss, Effective-cost PCB fabrication, Improve impedance control, Wide range of Dk (dielectric constant) values (2.55-10.2), Low-cost circuit fabrication, Low outgassing for space applications, and Better thermal management.
Different kinds of Roger’s materials. By using Advanced Circuit Material, Rogers PCB wants to accommodate hardware engineers to build such circuits with the high-frequency, high-speed performance for wired & wireless communications. Rogers’ Products/Brands include RT/duriod® High-Frequency Laminates; RO4000® High-Frequency Circuit Materials; RO3000® HighFrequency Laminates; and TMM® Thermoset Microwave Materials. Each product has its characteristics and benefits. Below are the details of each Rogers’ product:
Rogers RT/duroid® high-frequency circuit materials are filled PTFE (irregular glass or ceramic) composite coversfor use in high reliability, aviation and defense applications. The RT/duroid types have a long industry nearness of providing high-reliability materials with predominant performance. This kind of material has several benefits:
1 Low electrical loss,
2. Low moisture absorption,
3. Stable dielectric constant (Dk) over a wide frequency range, and
4. Low outgassing for space applications.
RO3000 laminates are ceramic filled PTFE composites intended for use in the commercial microwave and RF applications. R03000 series laminates are circuit materials with very consistent mechanical properties regardless of the dielectric constant selected. Due to this characteristic, when designing multi-layer boards with varying dielectric constants, there will be very little issues if any at all The dielectric constant VS temperature of RO3000 series materials is very stable. RO3000 laminates also are available in a wide range of dielectric constants (3.0 to 10.2). The most common applications are:
1. Surface mount RF components, 2.
GPS antennas, and
3. Power amplifiers.
RO4000 laminates and pre-pregs possess favorable properties that are highly useful in microwave circuits and instances where controlled impedance is needed. This series of laminates are very price optimized and are also fabricated using standard FR4 processes which makes it suitable for multi-layer PCBs. Additionally, it can be processed lead-free. The series of RO4000 laminates offer a range of dielectric constants (2.55-6.15) and are available with UL 94 V-0 flame retardant versions. The most popular applications of this are:
1. RFID chips,
2. Power amplifiers,
3. Automotive radars, and
Rogers TMM® thermoset microwave laminates fuse dielectric constant uniformity, low thermal coefficient of dielectric constant (Dk), and a copper matched coefficient of thermal expansion. Because of their electrical and mechanical stability, TMM high-frequency laminates are perfect for high-reliability strip-line and micro-strip applications. This kind of material has several benefits:
1. Wide range of dielectric constants (Dks),
2.Excellent mechanical properties, cold flow, and resists creep,
3. Exceptionally low thermal coefficient of Dk,
4. Coefficient of thermal expansion fit to copper taking into account high reliability of plated through-holes,
5. Available copper clad in larger formats, allowing the use of standard PCB subtractive processes,
6. No wreck to materials during fabrication and assembly processes, Resistant to process chemicals,
7. Thermoset resin for reliable wire bonding,
8. No specialized production techniques required,
9. TMM 10 and 10i laminates can replace alumina substrates, and
10. RoHS compliant, environmentally friendly.Below is a table that shows the characteristics of various types of PCB materials.
Rogers PCB applications. Moreover, since nowadays 5G Technology developments are growing quickly, various devices request high-frequency PCBs and RF PCBs with high performance which need not only low electrical noise but also low signal losses. And Rogers PCB material is a perfect choice to match the technological characteristics, besides it is also cost-effective for this purpose. Some Rogers PCB applications are:
1 .Automotive Radar and Sensors
2. Microwave equipment of all kinds.
3. Cellular Base Station Antennas
4. RF Identification (RFID) Tags
5. 5G Station
6. Microwave point to point (P2P) links
7. LNB’s for Direct Broadcast Satellites
How is the PCB Manufacturing Process? Although PCBs are small, the manufacturing process of a PCB is quite extensive. Whether you want to make PCB by yourself or send it to a PCB manufacturer (e.g. PCBway, seeedstudio, etc), there are some crucial steps along with the development of the board. Because each step is so critical to the process, we will discuss in detail the manufacturing process of a PCB. Printed circuit boards are usually made with copper. Despite depending on the requirements, the copper is plated to a substrate and carved away to expose the design of the board. And because of multiple layers, they must be lined up and bonded together for a secure fit.
1. Design. As discussed in previous sections, before you begin manufacturing the PCB, you need to have a design of the board. The design process is commonly produced by using computer software. And you can use a trace width calculator to help with a majority of the details needed for inner and external layers.
2. Printing Design. In this step, you can use a special printer called a plotter printer to print the design of the PCB. This printer produces a film that shows the details and layers of the board. Once your design is printed, there will be two ink colors used on the inside layer of the board: Clear Ink to show the non-conductive areas; Dark Ink to show the conductive copper traces and circuits.
3. Creating the Substrate. In this term substrate is the insulating material (epoxy resin and glass fiber) that holds the components on the structure. And in this step substrate begins forming by passing the materials through an oven to be semicured. Copper is pre-bonded to the two sides of the layer and afterward carved away to show the design from the printed films.
4. Printing the Inner Layers. Then the design is printed to a laminate, the body of the structure. A photo-sensitive film is made from photo-reactive chemicals. It will harden when exposed to ultraviolet light (the resist) covers the structure. This will help adjust the blueprints and the genuine print of the board. Holes are drilled into the PCB to help with the adjustment process.
5. Ultraviolet Light. When adjusted, the resist and laminate go under ultraviolet lights to solidify the photoresist. The light uncovers the pathways of copper. The dark ink from before forestalls solidifying in regions that will be removed later on. The board is then washed in an alkaline solution to expel the abundance photoresist.
6. Removing Unwanted Copper. In this time to expel any undesirable copper that stayed on the board. A chemical solution, like the alkaline solution, destroys the undesirable copper. The solidified photoresist stays flawless.
7. Inspection. The recently cleaned layers should be investigated for alignment. The holes drilled earlier help adjust the inner and outer layers. An optical punch machine drills a pin through the holes to keep the layers arranged. After the optical punch, another machine will review the board to guarantee there are no deformities. From here on out, you will not be able to correct any missed errors.
8. Laminating the Layers. Now, you will see the board come to fruition as the layers are combined. Metal clamps hold the layers together as the overlaying procedure starts. A pre-preg (epoxy resin) layer goes on the arrangement basin. At that point, a layer of substrate goes over the pre-preg followed by a copper foil layer and more pre-preg resin. At last, there is more copper layer employed, which is the press plate.
9. Pressing the Layers. A mechanical press is then utilized to press the layers together. Pins are punch through the layers to keep them appropriately adjusted and made sure about, these pins can be removed relying upon the technology. If correct, the PCB will go to the covering press, which applies pressure and heat to the layers. The epoxy dissolves inside of the pre-preg that, along with the pressure, intertwines the layers.
10. Drilling. Holes are drilled into the layers by a computer-guided drill to uncover the substrate and inner panels. Any residual copper after this step is removed.
11. Plating. The board is currently fit to be plated. A chemical solution intertwines the entirety of the layers together. The board is then altogether cleaned by another series of chemicals. These chemicals additionally cover the board with a thin copper layer, which will saturate the drilled holes.
12. Outer Layer Imaging. In this step a layer of photoresist, like Step 3, is applied to the outside layer before being sent for imaging. Ultraviolet light solidifies the photoresist. Any undesired photoresist is removed.
13. Plating. Much the same as in Step 11, the panel is plated with a thin copper layer. Then, a thin tin guard is coated to the board. The tin is there to ensure the copper of the outside layer from being scratched off.
14. Etching. A similar chemical solution from before expels any undesirable copper under the resist layer. The tin guard layer secures the required copper. This step established the PCB’s connections.
15. Solder Mask Application. The entirety of the boards ought to be cleaned before the solder mask is applied. An epoxy is applied with the solder mask film. The solder mask applies the green color you ordinarily observe on a PCB. Any undesirable solder mask is expelled with ultraviolet light, while the wanted solder mask is prepared on to the board.
16. Silkscreening. Silkscreening is an imperative advance since this procedure is the thing that prints critical information onto the board. When applied, the PCB goes through one final coating and curing process.
17. Surface Finish. The PCB is plated with either a solderable finish, contingent upon the necessities, which will make better results to the bond/quality of the solder.
18. Testing. Before the PCB is viewed as complete, a technician will play out an electrical test on the board. This will affirm the PCB functions and follows the original blueprint designs.
How is the SMT Assembly Process? The process part after the PCB manufacturing process, of course, is components assembly. Since nowadays PCB manufacturer like PCBway is also able to do component assembly, in this section we will discuss in more details about the assembly process at the manufacturer.
Introduction to Surface Mount Technology. Surface Mount Technology is an area of electronic assembly used to mount electronic components to the surface of the printed circuit board (PCB) as contradiction embedding components through holes likewise with conventional assembly. SMT was created to diminish fabricating expenses and furthermore to utilize PCB space.
What are SMD’s? Surface mount device or SMD is the term utilized for the electronic components utilized within the surface mount assembly process. There is a wide scope of SMD component packages available on the market and come in numerous shapes and sizes – a selection can be seen below:
Surface Mount Assembly Process. The smt process begins during the design stage when a wide range of components are chosen and the PCB is designed utilizing a software package, for example, Orcad or Cadstar (others are available). Realize that the process begins at this phase as this is the best an ideal opportunity to fuse as many design features as possible that will make production straight forward and head-ache free. Regularly, circuits are taken from the schematic design phase to PCB layout with the main considerations being the usefulness, which obviously is significant, but design for manufacture and assembly (DFMA) ought to in a perfect world be fused. When the PCB design has been settled and components chosen the following stage is to send the PCB data away to a PCB manufacturing company and components purchased in the most suitable way to facilitate automation. There are several steps in the SMT process:
1. Machine programming – Gerber / CAD to Centroid / Placement / XY file. Having received the PCB panels and components the following stage is to arrange the various machines utilized with the manufacturing process. Machines such as the placement machine and AOI (Automated Optical Inspection) will need a program to be made which is best created from CAD data however frequently this is not accessible.
2. Solder Paste Printing. The first machine to arrange in the manufacturing process is the solder paste printer which is intended to apply solder paste utilizing a stencil and squeegees to the suitable pads on the PCB.
3. Solder Paste Inspection (SPI). Most solder paste printing machines have the alternative of including automatic inspection but, depending on the size of the PCB, this process can be tedious thus a different machine can regularly be preferred.
4. Component Placement. Once the printed PCB has been affirmed to have the right measure of solder paste applied it moves into the following piece of the manufacturing process which is component placement. Each component is picked from its packaging using either a vacuum or gripper nozzle, checked by the vision framework, and put in the programed location at high speed.
5. Pre-Reflow Automated Optical Inspection (AOI). Following the component placement process check that no slip-ups have been made and that all parts have been correctly placed before reflow soldering
6. First Article Inspection (FAI). One of the numerous difficulties for sub-contract manufacturers is the verification of the first assembly to the clients’ information or first article inspection (FAI) as can be very tedious.
7. Reflow Soldering. When all component placements have been checked the PCB assembly moves into the reflow soldering machine where all the electrical solder connections are formed between the components and PCB. It is applied by heating the assembly to an adequate temperature.
8. Post-Reflow Automated Optical Inspection (AOI). The last piece of the surface mount assembly process is to again check that no mistakes have been made by utilizing an AOI machine to check solder joint quality.
9. Through-hole Assembly – Selective Soldering. Albeit selective soldering is part of the through-hole assembly process, there are numerous aspects that influence the surface mount assembly and PCB design, for example, component position.
10. Process Verification using X-Ray Inspection. Because of many solder joints being shrouded, it is critical to have a non-damaging technique of inspection available that can be utilized to verify the soldering processes are conveying the ideal outcomes. X-ray inspection gives an approach to check underneath components such as BGA’s, amount of voiding present within solder joints and furthermore can be utilized to verify the solder hole fill after the soldering process.