Looking for the High Frequency Design Techniques?

High frequency design is where you really need to consider the effects of parasitic inductance, capacitance and impedance of your PCB layout. If your signal is too fast, and your track is too long, then the track can take on the properties of a transmission line. pcb design If you don’t use proper transmission line techniques in these situations then you can start to get reflections and other signal integrity problems.

A “critical length” track is one in which the propagation time of the signal starts to get close to the length of the track. On standard FR4 copper boards, a signal will travel roughly 6 inches every nano second. A rule of thumb states that you need to get really concerned when your track length approaches half of this figure. But in reality it can actually be much less than this. Remember that digital square wave signals have a harmonic content, so a 100MHz square wave can actually have signal components extending into the GHz region.

In high speed design, the ground plane is fundamental to preserving the integrity of your signals, and also reducing EMI emissions. It allows you to create “controlled impedance” traces, which match your electrical source and load. pcb design services It also allows you to keep signals coupled “tight” to their return path (ground).

There are many ways to create controlled impedance “transmission” lines on a PCB. But the two most basic and popular ways are called Microstrip and Stripline.

A Microstrip is simply a trace on the top layer, with a ground plane below. The calculation involved to find the characteristic impedance of a Microstrip is relatively complex. trace, the height above the ground plane, and the relative permittivity of the PCB material. multi layer pcb design This is why it is important to keep the ground plane as close as possible to (usually) the top layer.

A Stripline is similar to the Microstrip, but it has an additional ground plane on top of the trace. So in this case, the trace would have to be on one of the inner layers. The advantage of stripline over microstrip is that most of the EMI radiation will be contained within the ground planes.

There are many free programs and spreadsheets available that will calculate all the variations of Microstrip and Stripline for you.

Some useful information and rules of thumb for high frequency design are:

• Keep your high frequency signal tracks as short as possible.
• Avoid running critical high frequency signal tracks over any cutout in your ground plane. This causes discontinuity in the signal return path, and can lead to EMI problems. Avoid cutouts in your ground plane wherever possible. A cutout is different to a split plane, which is fine, provided you keep your high frequency signal tracks over the relevant continuous plane.
• Have one decoupling capacitor per power pin.
• If possible, track the IC power pin to the bypass capacitor first, and then to the power plane. This will reduce switching noise on your power plane. For very high frequency designs, taking your power pindirectly to the power plane provides lower inductance, which may be more beneficial than lower noise on your plane.
• Be aware that vias will cause discontinuities in the characteristic impedance of a transmission line.
• To minimise crosstalk between two traces above a ground plane, minimise the distance between the plane and trace, and maximise the distance between traces. The coefficient of coupling between two traces is given by 1/(1+(Distance between traces / height from plane)^2))
• Smaller diameter vias have lower parasitic inductance, and are thus preferred the higher in frequency you go.
• Do not connect your main power input connector directly to your power planes, take it via your main filter capacitor(s).

Double Sided Loading

Loading components on both sides of a PCB can have many benefits. Indeed, it is becoming an increasingly popular and necessary option when laying out a board. There are two main driving factors behind a decision to go with double sided loading. The first is that of board size. If you require a particular board size, and all your components won’t fit on one side, then double sided load is an obvious way to go. The second reason is that it is required to meet certain electrical requirements. Often these days, with dense high speed surface mount devices packed onto a board,high speed pcb design  there is either no room for the many bypass capacitors required, or they cannot be placed close enough to the device to be effective. Ball Grid Array (BGA) devices are one such component that benefit from having the bypass capacitors on the bottom of the board.

 

 

searching for multi layer pcb design?

Multi layer  PCB Design

What is a Multilayer PCB?

A printed circuit board (PCB) is a thin board “printed” with electrical wires and made from fiber glass or similar material. PCBs are commonly used in computer devices such as motherboards, network interface cards, and RAM chips.multi layer pcb design They are relatively cheap and quite fast. When the PCB is fabricated with several layers placed over one another, it is known as a multilayer PCB. The multiple layers establish a reliable set of predetermined interconnections for the electronic circuits.

There are several techniques that can be used to accomplish this task. Some of these techniques are handicapped by their reliance on a large number of chemical processes to condition the substrate. A large number of chemical processes are needed to activate the through-holes and electrolytic copper plate between the adjacent layers. The general procedure is as follows:

1. Obtaining the material and equipment that you will need, such as drills and electrolytic copper-plating cell.
2. Formatting the copper substrates so that the orientation of each can be uniquely determined. This is sometimes known as patterning and can be done using a variety of methods.
3. Drilling the layers with specific drilling equipment to create holes or vias. These vias are plated with copper to form plated-through holes.
4. Properly cleaning the copper substrates on your board.
5. Electroplating the PCB substrate using acid copper electroplating.
6. Laminating the multilayer board.
Under high pressure and heat, the layers fuse together. The conductors will be separated, and signals and power between layers will be connected. pcb design  This technique ensures that all the layers are drilled and plated first, and then laminated. This can help reduce the number of chemical processes needed to accomplish this difficult task. Multilayer PCBs can have as many as 14 layers. However, this may be quite expensive, and it is more common for PCBs to have either 6 or 8 layers.

A multilayer PCB contains two reference planes and a signal via. The signal via allows a signal to flow through all the planes. A stitching via is connected to one of the planes next to the signal via and serves to reduce the area through which the signal passes through. This is very important as it may assist in reducing noise and cross talk.

Product functionality of the multilayer PCB depends on the interconnection between the layers. Thus it is very critical to be concerned about micro vias and overall HDI.

Multilayer boards can either be rigid or flexible. Rigid multilayer PCB technology can prove to be very expensive because of the expertise required and the expensive drilling equipment. Flexible multilayer PCBs make use of flexible circuits and reduce the size of the end product. pcb design services However, flexible multilayer PCBs must have fewer layers because an increase in the number of layers means a loss in flexibility of those layers.

Advantages of multilayer PCBs include high reliability and uniform wiring. However, the initial costs are higher than that of one-layered PCBs. Also, repairing a multilayer PCB is quite difficult.

Vias

With multi layer design comes the options of using different types of vias to improve your routing density. There are three types of vias – standard, blind, and buried.

Standard vias go through the whole board, and can connect any of the top, bottom or inner layers. These can be wasteful of space on layers which aren’t connected.

“Blind” vias go from the outside surface to one of the inner layers only. The hole does not protrude through the other side of the board. The via is in effect “blind” from the other side of the board.

“Buried” vias only connect two or more inner layers, with no hole being visible on the outside of the board. So the hole is completely buried inside your board.

Blind and buried vias cost more to manufacture than standard vias. But they are very useful, and almost mandatory for very high density designs like those involving Ball Grid Array (BGA) components.

Power Planes

It is good practice to use “power planes” to distribute power across your board. Using power planes can drastically reduce the power wiring inductance and impedance to your components. This can be vital for high speed digital design for instance. It is good design practice to use power planes whenever possible. They can even be used on double sided boards, if most of your signal tracks are on the top layer.

A power plane is basically one solid copper layer of board dedicated to either your Ground or Power rails, or both. Power planes go in the middle layers of your board, usually on the layers closest to the outer surfaces. pcb layout On a 4 layer board with complex power requirements it is common to dedicate one layer to your ground plane, and another layer to your various positive and negative power tracks. Your ground rail is usually your signal reference line, so a ground plane is first preference before a power plane is considered.

Many PCB packages have special Power Plane layers that are designed and laid out in reverse to your other normal tracking layers. high speed pcb design On a normal tracking layer, your board is assumed to be blank, and you then lay down tracks which will become your actual copper tracks. On a power plane however, your board is assumed to be covered with copper. Laying down tracks on a power plane actually removes the copper. This concept can take some getting used to.

 

Searching for the Multi layer pcb design?

Multi layer  PCB Design

What is a Multilayer PCB?

A printed circuit board (PCB) is a thin board “printed” with electrical wires and made from fiber glass or similar material. PCBs are commonly used in computer devices such as motherboards, network interface cards, and RAM chips. They are relatively cheap and quite fast. When the PCB is fabricated with several layers placed over one another, it is known as a multilayer PCB. The multiple layers establish a reliable set of predetermined interconnections for the electronic circuits.
There are several techniques that can be used to accomplish this task. Some of these techniques are handicapped by their reliance on a large number of chemical processes to condition the substrate. A large number of chemical processes are needed to activate the through-holes and electrolytic copper plate between the adjacent layers. The general procedure is as follows:

1. Obtaining the material and equipment that you will need, such as drills and electrolytic copper-plating cell.
2. Formatting the copper substrates so that the orientation of each can be uniquely determined. This is sometimes known as patterning and can be done using a variety of methods.
3. Drilling the layers with specific drilling equipment to create holes or vias. These vias are plated with copper to form plated-through holes.
4. Properly cleaning the copper substrates on your board.
5. Electroplating the PCB substrate using acid copper electroplating.
6. Laminating the multilayer board.
Under high pressure and heat, the layers fuse together. The conductors will be separated, and signals and power between layers will be connected. This technique ensures that all the layers are drilled and plated first, and then laminated. high speed pcb design This can help reduce the number of chemical processes needed to accomplish this difficult task. Multilayer PCBs can have as many as 14 layers. However, this may be quite expensive, and it is more common for PCBs to have either 6 or 8 layers.
A multilayer PCB contains two reference planes and a signal via. The signal via allows a signal to flow through all the planes. A stitching via is connected to one of the planes next to the signal via and serves to reduce the area through which the signal passes through. This is very important as it may assist in reducing noise and cross talk.

Product functionality of the multilayer PCB depends on the interconnection between the layers. Thus it is very critical to be concerned about micro vias and overall HDI.

Multilayer boards can either be rigid or flexible. Rigid multilayer PCB technology can prove to be very expensive because of the expertise required and the expensive drilling equipment. Flexible multilayer PCBs make use of flexible circuits and reduce the size of the end product. However, flexible multilayer PCBs must have fewer layers because an increase in the number of layers means a loss in flexibility of those layers.

Advantages of multilayer PCBs include high reliability and uniform wiring. However, the initial costs are higher than that of one-layered PCBs. Also, repairing a multilayer PCB is quite difficult.

Vias

With multi layer design comes the options of using different types of vias to improve your routing density. There are three types of vias – standard, blind, and buried.

Standard vias go through the whole board, and can connect any of the top, bottom or inner layers. These can be wasteful of space on layers which aren’t connected.

“Blind” vias go from the outside surface to one of the inner layers only. The hole does not protrude through the other side of the board.multi layer pcb design The via is in effect “blind” from the other side of the board.

“Buried” vias only connect two or more inner layers, with no hole being visible on the outside of the board. So the hole is completely buried inside your board.

Blind and buried vias cost more to manufacture than standard vias. But they are very useful, and almost mandatory for very high density designs like those involving Ball Grid Array (BGA) components.

Power Planes

It is good practice to use “power planes” to distribute power across your board. Using power planes can drastically reduce the power wiring inductance and impedance to your components. This can be vital for high speed digital design for instance. It is good design practice to use pcb design power planes whenever possible. They can even be used on double sided boards, if most of your signal tracks are on the top layer.

A power plane is basically one solid copper layer of board dedicated to either your Ground or Power rails, or both. Power planes go in the middle layers of your board, usually on the layers closest to the outer surfaces. pcb design services  On a 4 layer board with complex power requirements it is common to dedicate one layer to your ground plane, and another layer to your various positive and negative power tracks. Your ground rail is usually your signal reference line, so a ground plane is first preference before a power plane is considered.

Many PCB packages have special Power Plane layers that are designed and laid out in reverse to your other normal tracking layers. pcb layout On a normal tracking layer, your board is assumed to be blank, and you then lay down tracks which will become your actual copper tracks. On a power plane however, your board is assumed to be covered with copper. Laying down tracks on a power plane actually removes the copper. This concept can take some getting used to.

 

PCB Design

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board (PWB) or etched wiring board. Printed circuit boards are used in virtually all but the simplest commercially produced electronic devices.

A PCB populated with electronic components is called a printed circuit assembly (PCA), printed circuit board assembly or PCB Assembly (PCBA). In informal use the term “PCB” is used both for bare and assembled boards, the context clarifying the meaning.

Circuit properties of the PCB

Each trace consists of a flat, narrow part of the copper foil that remains after etching. The resistance, determined by width and thickness, of the traces must be sufficiently low for the current the conductor will carry. Power and ground traces may need to be wider than signal traces. PCB Design In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return.

For microwave circuits, transmission lines can be laid out in the form of stripline and microstrip with carefully controlled dimensions to assure a consistent impedance. In radio-frequency and fast switching circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; but they can be used as a deliberate part of the circuit design, obviating the need for additional discrete components.

Printed circuit assembly

After the printed circuit board (PCB) is completed, electronic components must be attached to form a functional printed circuit assembly,or PCA (sometimes called a “printed circuit board assembly” PCBA). In through-hole construction, component leads are inserted in holes. In surface-mount construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.

There are a variety of soldering techniques used to attach components to a PCB. High volume production is usually done with SMT placement machine and bulk wave soldering or reflow ovens, but skilled technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02 in. by 0.01 in.)by hand under a microscope, using tweezers and a fine tip soldering iron for small volume prototypes. Some parts may be extremely difficult to solder by hand, such as BGA packages.

Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques.

After the board has been populated it may be tested in a variety of ways:

While the power is off, visual inspection, automated optical inspection. JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.

While the power is off, analog signature analysis, power-off testing.

While the power is on, in-circuit test, where physical measurements (i.e. voltage, frequency) can be done.

While the power is on, functional test, just checking if the PCB does what it had been designed to do.

To facilitate these tests, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. PCB Layout The in-circuit test may also exercise boundary scan test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.

In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard. The JTAG test architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes. JTAG tool vendors provide various types of stimulus and sophisticated algorithms, not only to detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.

When boards fail the test, technicians may desolder and replace failed components, a task known as rework.

Design

Printed circuit board artwork generation was initially a fully manual process done on clear mylar sheets at a scale of usually 2 or 4 times the desired size. The schematic diagram was first converted into a layout of components pin pads, then traces were routed to provide the required interconnections. Pre-printed non-reproducing mylar grids assisted in layout, and rub-on dry transfers of common arrangements of circuit elements (pads, contact fingers, integrated circuit profiles, and so on) helped standardize the layout. Traces between devices were made with self-adhesive tape. The finished layout “artwork” was then photographically reproduced on the resist layers of the blank coated copper-clad boards.