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High-Frequency PC Board

High-Frequency PC Board Applications, Specifications, and Challenges

Some electronic products need special signals for which you have to make a high-frequency PC Board. Such a circuit board can provide 500 MHz to 2 GHz frequency that is ideal for microwaves, a radio frequency, and certain mobile applications that involve high-speed designs.

Several electronic components and switches are complex and need to transfer signals at a fast speed which is provided by high-frequency PCBs. Such boards need special materials because ordinary materials can affect signal transmission due to a poor Er value. Designers have to consider certain factors while designing the high-frequency Printed Circuit Board that we will discuss below.

Understanding a High-Frequency PCB

PCB involves connections of different components through conductive paths to run a specific electronic item. Designers use copper to develop a conductive path in a PCB. Circuit boards also help in signal transmission in the case of Wi-Fi and other satellite systems. In other words, you need a high-frequency circuit board to connect multiple objects through signals.

High-Frequency PC Board
High-Frequency PC Board Applications, Specifications, and Challenges

Which Factors Affect The High-Frequency PCB?

The design of a high-frequency PCB is not that easy because certain factors influence it and you have to consider them. Such boards have complex fabrication due to high-frequency laminates. Besides, the circuit board has to manage different applications’ thermal heat transfer.
You can’t use any material for high-frequency PCBs because it influences signal transmission that can be fast or slow, depending on the material. Moreover, the change in a material’s Er value also affects a PCB’s impedance.
Similarly, the dielectric material also plays a role in the design of high-frequency boards. Manufacturers use different dielectric materials as mentioned below:
1. Roger’s
2. Teflon
3. FR4.
The Roger’s is not expensive, and its DF and DK values are also less than other materials. Besides, it is ideal for prototyping manufacturing and applications. Moreover, there is a minimum chance of signal loss due to this material.
Whereas Teflon is used due to its high frequency that is up to 5 GHz that enhances the speed of signals between different parts and objects.
On the other hand, the FR4 is ideal for RF applications that need a frequency from 1GHz to 10 GHz. But, the electric products having FR4 have certain drawbacks due to their limitations.
The best material for high-frequency PCBs is Teflon due to factors like water absorption, DK, and DF. Teflon is more expensive than other materials, but it is ideal for products that need more than 10 GHz frequency of signals.
What Are The Standard Specifications Of a High-Frequency PCB?
You have to consider certain materials to have a high-frequency board as mentioned above. Moreover, the change in a material’s Er value also affects a PCB’s impedance. PCBs are available in different frequencies and have certain specifications that we will discuss below.
PCB Size: It should be at least 6 mm x 6 mm, and can go up to 457 mm x 610 mm.
PCB Thickness: It ranges from 4 mm to 5 mm.
Type of Material. Generally, it should be RO4003C, Ro3003, RT5880, and Ro3010
Weight of Copper: It ranges from 0.5 oz. to 2 oz.
PP: It includes Domestic-25FR, Domestic-6700, and Roger’s 4450F.
Min Spacing: It should be at least 3 mils.
Solder Mask Colors: Some common colors, in this case, are yellow, red, white, green, and blue.
Sides of Solder Mask. They are according to the design files.
Silkscreen Colors and Size: The colors are mostly white, black, and yellow, whereas the sides are according to the files.
Impedance Clearance: It is either plus 10% or minus 10%, depending on the design.
High-Frequency PCB Finish: It can be immersion tin, gold, silver, or electroless nickel. All these finishes should be RoHS certified.
Annular Ring: It should be min 4 mil.
Diameter of Drilling Hole: It is a minimum of 6 mils.

All the above species are standard and may change according to the board design. Besides, most circuit boards are customized and designed according to your needs. It is hard to recognize the best high-frequency circuit board, however, the material and specifications can help you in this case. You can also get professional help from a qualified PCB designer and/or a circuit board manufacturer.

Top Tips To Develop The Best High-Frequency PCB

As you know high-frequency PCBs have a high density and integration than other PCBs, they need a thoughtful design and fabrication. Such boards are more scientific than traditional circuit boards, and we have some tips to help you create a reliable PCB.

1. The pins that exist between various layers of a high-frequency PCB should have minimal leads as an alternate. Besides, the lead between different pins should be small.
2. When it comes to high-frequency devices, there should not be more bends between their pins.
3. Make sure that loops don’t develop while wiring.
4. The impedance of signals should be compatible.
5. The power pins of an integrated PCB should have a high-speed decoupling.

Meeting the Challenges of A High-Frequency PCB

No matter how well-designed is your high-frequency circuit board, you have to face some challenges during its fabrication and assembly. Let’s discuss some common issues in this case.

Consider Scaling

A professional fabricator knows that the thickness of internal layers decreases during the lamination of a multi-layered PCB made if FR4. So, the manufacturer should evaluate the percentage of such a loss. This helps printed circuit boards manufacturers get the right dimensions after the lamination process is over.
Besides, the laminate material is not hard like FR4, so it reacts differently. You should know the behavior of each material. Besides, you should scale each thickness separately or it will affect the registration from drill to pad and layer to layer. The fabricator should know all the statistics in this regard.

Preparing Different Layers

A board with several layers is complex, as you have to prepare each layer to have a strong bond, especially in the case of Teflon. Soft material can get deformed during the aggressive preparation of a surface. Such a deformation results in wrong registration, turning a PCB into a scrap.
Replacing the Teflon becomes expensive and causes delays in fabrication. So, you must prepare the surfaces carefully to avoid such challenges.

Preparation of Holes

You need to prepare the hole before plating. Like, it should be free from debris or epoxy attached to its walls. A smooth surface helps have a well intact copper plating. However, ceramic or Teflon involves a different kind of hole preparation.
This process involves a lot of care like you should consider various parameters of the drill machine to avoid the smearing of the substrate. After drilling, the hole is treated through plasma that involves gases. Poor preparation of the hole before copper plating might result in poor signal transmission. Hence, a PCB should have clean holes to perform better.

Considering the CTE Rate

The designer also has to consider the CTE or coefficient of thermal expansion of different materials. Different materials have different expansion rates, besides this expansion can occur in any direction like x, y, or z, depending on the heat. You can have well-finished holes if the CTE is less.
The factor of CTE can cause issues during a hybrid PCB of several layers when you join the high-frequency materials with FR4. So, the CTE of the materials should be compatible, or different layers or materials will expand differently, creating an issue.
Other than layers, vias also have to face this issue. Hence, the plugging material of the vias should be compatible with other materials.

Compatibility

Some FR laminates are similar to the RF materials in terms of behavior, and you should understand it. For example, the ceramic impregnated boards are hard when you drill through the drill bits. The hit counts should be less, besides, the RPM and spindle settings should be customized.
Sometimes the holes have fingers, which are hard to remove, so the adjustment of drilling parameters is essential to reduce fiber.
So, you can meet all the above challenges if you design and fabricate a PCB carefully by approaching a prototype pcb manufacturer to verify your design.

Importance of a High-Frequency PCB

A high-frequency PCB is widely used in different industries, such as military, interchanges, gadgets, vehicles, PC, instrumentation, clinical, and other such fields. These circuit boards are more in demand than before, and 15% of circuit boards in the market come up with high frequency.

Final Words

Sometimes the electronic components and switches are complex and need to transfer signals at a fast speed which is provided by high-frequency PCBs. Such boards need special materials because ordinary materials can affect signal transmission due to a poor Er value.
You can’t use any material for high-frequency PCBs because it influences signal transmission that can be fast or slow, depending on the material. Moreover, the change in a material’s Er value also affects a PCB’s impedance. PCBs are available in different frequencies and have certain specifications.
Would like to know more about high-frequency PCB applications or pcb assembly services? Email us at: sales@pnconline.com

Minimizing Crosstalk in PC Board Layout

In this ongoing series on PCB layout from the design team at PNC, previous posts have looked at some of the initial steps to turn a circuit schematic into a manufacturable, reliable PCB. These posts have looked at  component placement, selecting appropriate trace widths, and BGA routing.   In this post we are going to take a deeper dive into methods for reducing crosstalk in the PCB design. After the power and ground have been routed, the next task is to route high speed signal traces, and the traces that could either generate or receive crosstalk.

 What is Crosstalk?

Crosstalk occurs when the signal on an aggressor trace on a PCB appears on a nearby victim trace, due to capacitive and inductive coupling between the two traces.  Typical aggressor signal traces are:

● High speed digital signals, especially clock signals
● Noise from switching power suppliers
● High frequency RF.

Victim signal traces, on the other hand, carry high impedance signals like op amp input lines or reset lines, or low impedance signals with long loops.   Low amplitude signals such as a sensitive analog measuring circuit traces are also susceptible.

Crosstalk occurs when aggressor trace and victim trace are close together and run in parallel for a distance.  The aggressor and victim(s) can be side to side on the same layer or on top of each other on adjacent signal layers. Coupling between traces on adjacent layers separated by just a thin section of laminate is called broadside coupling.

Minimizing Crosstalk in PC Board Layout
Minimizing Crosstalk in PC Board Layout

 

 

 

 

 

Printed Circuit Board Design guidelines to reduce crosstalk

There are several design rules to reduce crosstalk between signal traces.  Before applying these rules, the first step is to use the general guidelines described above to identify and flag any potential aggressor signal traces and their potential victims.

Since crosstalk occurs between two traces running in parallel, try to reduce the distance that the aggressor and victim traces run in parallel. Unfortunately, this may be difficult if the signals originate and terminate from the same locations.  To minimize broadside coupling try to orient the signal traces east-west on one layer and north-south on the second layer.

It is essential to have a broad contiguous ground plane directly under (or over) the signal layer.  A ground plane located between two signal layers can prevent broadside coupling. However, make sure that ground planes located on adjacent layers but not electrically connected do not overlap.  The overlapping ground planes separated by a dielectric form a capacitor, which can transmit noise from one ground plane to the other. This can defeat the purpose of separate ground planes if they were created to isolate the noisy elements of a circuit from the noise sensitive ones.

Increasing trce spacing

The most effective method of reducing crosstalk is to increase the spacing between the aggressor signal trace and the potential victim traces.  Like all electromagnetic radiation, electrical or magnetic coupling between the two traces drops with the square of the distance between them.  The amount of spacing required between the traces is dependent on the height of the traces above the ground plane.   The formula defining this relationship is from Douglas Brooks “Crosstalk Coupling: Single-Ended vs. Differential”   The coupling between two traces is proportional to:

Where S is the spacing between traces, and H is the distance from the trace to the ground plane.  Once H is defined by the lamination stack-up, the relative change in coupling can be easily plotted as a function of S.  Douglas Brooks looks in detail at the coupling between traces under several scenarios.  For those looking for some general guidance, a spacing of 5H is considered conservative.  The PC Board design team at PNC can assist designing a PCB stack up that will minimize the spacing needed between coupled traces, ensuring that crosstalk is minimized while maintaining routing density.

Finally, for very high speed digital signal traces, consider the use of differential pairs.  For many designers, the most common applications for a differential pair is for a high speed serial bus like USB, SATA, or HDMI.  The design rules for the layout of differential traces is beyond the scope of this post.

The most important part of reducing crosstalk in your PCB design is to first recognize in which signal traces crosstalk is likely to occur, then follow the guidelines above to minimize it.  PNC’s Printed Circuit Board designers have experience with high speed digital and RF circuits and can help you select the correct PCB layer stack-up and review your designs for areas where crosstalk is likely and suggest ways to minimize it. Request a design review from PNC today

3d-printed-pcb-prototype

Accelerate your New Product Development with rapid PCB assembly prototyping

The time from concept to prototype has accelerated remarkably in the past decade. 3D printed prototype components in a wide variety of materials are available in hours. Machined or sheet metal components are available from rapid prototype shops in only one or two days.

Prototype Printed Circuit Board Fabrication and assembly companies like PNC have followed this trend towards faster prototypes and can now provide complete assemblies in less time than ever before. PNC can fabricate and deliver a bare 10-12-layer PCB in just three days, and a simple double-sided board in just 24 hours.

However, even with the streamlining of PCB fabrication, the fully assembled PCBA is often the longest lead component in prototype designs primarily because of the sheer number and variety of passive and active components to be purchased and the demands of accurately placing and soldering those components. Sourcing the components on a typical PCBA BOM can take days in the best case and weeks in the worst case. Setting up and running the assembly job can add another few days, especially for double sided PCBs, and PCBs with a combination of surface mounted and through hole components.

Fortunately, there are some things that a product development team can do to reduce PCB assembly lead time.

First, do everything possible to reduce the impact of long component lead times. Plan to order the components as early as possible in the circuit design process. Deciding when to order components requires balancing the costs of scrapping some components as the design matures vs. the benefits of reducing the lead time for an assembled PCBA by days or weeks.

Second, reduce the time required to set up and build the prototypes by working with a full-service company like PNC. PNC has the capability to both fabricate the bare PCB and assemble the components. This means that the PCB fabrication team and assembly team can save time by working in parallel. While the PCBS are being fabricated, PNC’s engineers can create pick and place data, solder paste stencils and program the assembly equipment. When the PCBs are finished and the components arrive, everything is ready to begin assembly immediately.

The third way to save time with PCB prototypes is to minimize the number of PCB prototype iterations. Saving a full printed circuit board assembly prototype cycle is the most effective way to reduce the time from concept to mature design.

One way to reduce design iterations is by testing circuit designs as early in the design process as possible by building “Works Like” prototypes. “Works Like” prototypes are usually combinations of development kits, large one or two layer PCBs with larger SMT components that can be soldered by hand and various types of breadboards. In addition to testing the circuit, a “Works Like” prototype gives software developers an early platform to start developing code and debugging the circuit design. The result of testing early with rough prototypes is that you fix problems before you have invested the time in the full layout and prototyping process.

In parallel, the mechanical engineers can optimize cable routing and connector placement by printing 3D models of the PCBs, then epoxying actual connectors to the board model. This is an effective way to quickly try different options for cable routing using actual cables and connectors, since it is difficult to simulate the way actual cables behave with CAD software.

Experienced electrical engineers know that it is often poor connector access or cable interference that drive Printed Circuit Board layout redesigns as often as issues with actual circuit performance.

Once the circuit has been tested with the “Works-Like” prototype, and the board layout has been tested with 3D printed models, the last way to save time is to work closely with the PCB manufacturer to make sure that the PCB fabrication files are clean and complete, and that the BOM is accurate and matched with the circuit and layout to avoid placement mistakes.

This is another reason to select a full-service prototype pcb manufacturer like PNC. PNC can be a partner during the layout and design process, though fabrication and assembly ensuring the final design can be translated into a working prototype in the least possible time.