PCB News - EMC Design of Hybrid Integrated Circuit

Hybrid integrated circuit is an integrated circuit formed by a combination of semiconductor integrated technology and thickness (thin). The process hybrid integrated circuit is a basic line made of thick film or thin film elements on the chip using a film forming method., And combine discrete semiconductor chips, monolithic integration or small-scale integration on the same substrate, and then external packaging. It has the characteristics of large, high, and good electrical performance.

As the size of the circuit becomes smaller, the function increases, and the operating frequency continues to increase, the electromagnetic interference phenomenon of the circuit board is becoming more and more prominent, and the problem of the electronic board has become the key to the normal operation of an electronic board. Electromagnetic design has become the key to system design.

1. Principles of electromagnetic principles

Electromagnetic interference refers to the normal operation of electronic equipment and power supply under certain electromagnetic interference, and it is also the ability of equipment and power supply to limit their own electromagnetic interference and avoid interference with other electronic equipment.

There are three basic conditions for the occurrence of any electromagnetic interference: first, there must be an interference source, that is, or equipment that produces harmful electromagnetic fields; second, there must be a way to spread the interference. It is generally considered that there are two ways: the way is the way and the radiation. Therefore, To solve the problem of electromagnetic interference, the three factors of electromagnetic interference should be solved one by one: the intensity of the interference of the potential interference factor; blocking the propagation path of the interference; reducing the sensitivity of the system to the interference.

The electromagnetic interference in the hybrid integrated design includes interference, crosstalk interference, and interference. When solving the EMI problem, first determine whether the collection path of the emission source is latent, radiating, or crosstalk. And if there is a complete circuit connection between the interference source and the sensitive device, interference will appear at the two poles. Radiation interference is emitted between a wire that transmits high-frequency signals.

2. Electromagnetic design

When determining the electromagnetic emission design, the functional test should be performed first, and the electromagnetic meeting requirements should be tested in the circuit in the scheme. If the parameters are not met, modify the parameters to achieve the index, such as power and operating frequency, followed by protective design. The second is to do protective design, including filtering, shielding, grounding and lap design. The third is to make the adjustment design of the layout, including the inspection of the overall layout, the layout inspection of components and wires, etc. Often, the electromagnetic foreign design of the circuit includes: the selection of technology and selection, the layout of the circuit device and the wire, etc.

3.1. Selection of crafts and objects

Hybrid integration has a variety of manufacturing processes to choose from, single-layer film, thin film thick film and fine co-fired thick film. The thin-film process can produce high-density hybrid thin-film materials with small size, power and high current density., With the characteristics of price, stability, reliability and flexibility, suitable for high-speed and high-power circuits. But it can only be made in a single layer and the cost is high. The high-thickness film process can be manufactured at a lower cost. Because the power electromagnetic layer and ground layer of the power supply can be set, the electromagnetic interference capability of the circuit board can be improved, and the electromagnetic interference capability of the circuit board can be improved. The distance between is only the distance between layers. In this way, the path of all signals on the surface can be reduced, so that a deep understanding of the differential mode can be achieved.

The one-point co-firing process has more advantages and is the mainstream technology of existing passive integration. Transmission characteristics. In addition, it has good characteristics with thin film technology, and can realize deep and high-performance synthetic circuits in combination.

The active device of the hybrid circuit generally chooses the chip, and if there is no bare chip, the corresponding packaged chip can be used. For the EMC characteristics obtained, the surface-mount chip is used. Choose the chip on the basis of meeting the product technical indicators. When HC can be used, AC, CMOS40 HC is used. The capacitor should have a low trigger on the loop, so as to avoid large attenuation of the signal.

The encapsulation of the hybrid circuit can be welded with Kovar metal and the cover, parallel seam, has a good shielding effect.

3.2. The layout of the circuit

When dividing the layout of hybrid microcircuits, three main factors must be considered first: the number of inputs/outputs, performance and cost., The power dissipation per square inch is not more than 2W.

In respect of piety, in principle, components related to each other should be damaged, digital circuits, analog circuits, and power circuits should be operated independently, and high-frequency circuits should be separate circuits from equipment. Noise-prone, low-current, high-current circuits. Etc. Need to stay away from the logic circuit. The main interference and light source sources such as circuit circuits and high-frequency circuits should be placed separately and far away from sensitive circuits. The output chip should be close to the I/O outlet of the hybrid circuit.

High-frequency vibration consumes small vibration connection lines, which are susceptible to interference due to propagation parameters and mutual electromagnetic interference. They cannot be close to each other, and input and output are reduced. Interface and low-sodium signal chip. produce.

In a mixed circuit, the power supply and the grounded lead on the substrate are called the power supply and the grounded lead on the substrate. It is best to distribute the power and grounded I/O evenly. The mounting area of the bare chip is connected to the most substrate plane.

In the mixed circuit of ingredients, specific circuit boards are arranged between the layers of the circuit boards, but they generally have the following characteristics:

(1) The power supply and the ground layer are distributed in the surrounding layers. The visible layer can well suppress the common mode radio frequency interference external to the circuit and the diffusion of the sound wave power supply.

(2) The inner power plane and the ground plane antenna can generally be connected to each other as a power board on the ground, using the interlayer capacitance as the power supply of the power supply, and at the same time, the grounding shields the current of the plane power supply.

(3) The layers should be prepared with the power supply or the ground to produce a cancellation effect.

3.3, the layout of the wire

In circuit design, it is often only to improve or pursue the impact of equipment on equipment, and the impact on equipment configuration, so that a large number of signals are radiated to interference, which may cause more space interference problems. Therefore, good prevention of interference. It is the key to the success of the design.

3.3.1 The layout of the ground wire

The ground wire is the basic reference point for circuit work, and can also be used as a low-impact path for signals. The ground wire mainly affects the ground of the digital circuit, and when the digital circuit outputs a low wire, it is sensitive to the noise of the ground wire. Misoperation caused inspiration and radiation emission. Therefore, the focus of these interferences lies in our understanding of the details of the ground wire (digital circuit, abstract ground wire induction is important).

The layout of the ground wire should pay attention to the following solar energy:

(1) According to different power supply voltages, the digital circuit and the analog circuit are respectively set with ground wires.

(2) Common ground wire is typical. When using the simultaneous thickness film process, you can set the ground plane by yourself, so that the thick winding range can also reduce the length of the antenna. Wire shield.

(3) Comb ground wires should be avoided. The structure makes the signal complex, which will increase radiation and sensitivity, and the commonality between the chip and the chip may also cause circuit errors.

(4) For the neural emergency chip, the time difference that will appear on the ground line should be designed as a closed loop to improve the noise tolerance of the circuit.

(5) Circuit boards with both analog and digital functions. The analog ground and digital ground are usually separated and connected only at the power source.

3.3.2, the layout of the power cord

Generally speaking, in addition to interference directly caused by electromagnetic radiation, electromagnetic interference caused by power lines is common. Therefore, the layout of the power cord is also very important. Generally, the following rules should be followed:

(1) The power cord is basically close to the ground wire and connected by winding, the mold radiation is small, and the signal line is disturbed alternately. Different power cord connections are connected to each other.

(2) When using precise technology, the analog power supply and the digital power supply exist separately to avoid mutual interference.

(3) The power plane and ground plane can adopt complete dielectric, speed and speed, and should have a low dielectric constant desktop. The power plane should be close to the ground and placed on the ground plane to act on the cosmic clock tubes distributed on the power plane.

(4) Decoupling should be carried out between the power input of the chip and the ground connection. The decoupling drive introduces a 0.01uF chip capacitor, which should be installed close to the chip to make the basic concept of the circuit component of the decoupling paste capacitor.

(5) When choosing a SMD chip, choose a chip with a power source close to the ground, which can further decouple the external interface of the coupling capacitor to achieve electromagnetic induction.

3.3.3, the layout of the signal line

When using a single-layer film process, a suitable method for the housing is to lay out the ground wire first, then feel the key signals, such as high-speed optical fiber signals or sensitive circuits close to the ground, and finally desire other input signal wires. It is better to arrange the signal according to the flow of the signal so that the signal on the stage occurs.

If you want to minimize EMI, let the signal line be close to the signal line formed with it to reduce the size of the loop and avoid interference. The low-signal channel is close to the high-density signal channel and the unfiltered power line, and the noise-sensitive connection line is parallel to the high-current, high-speed switching line. Possibly, turn all key traces into striplines. Incompatible signal lines (analog, digital and high-speed and low-speed, high-current) and low-current, high-voltage and low-voltage, etc.) should be kept away from each other and should not be routed in parallel. The brief lengthened and shortened the length.

The induction earphone with conduction band is proportional to the length and the logarithm of the length, and the width is inversely proportional. Therefore, the conduction band should be very short, and the length of each address line or data line of the internal components remains unchanged. The low-speed signal input as the input circuit can be reduced, and the high-speed signal input should be as small as possible.

In a certain thick film process, in addition to adhering to the rules of single-layer performance, you should also pay attention to:

Try to design a separate ground plane, signal layer layout and ground layer. When it cannot be used at the same time, it must be set on high-frequency or sensitive lines. Signal lines distributed on different layers so that they can be connected to each other. Reduce the signal lines between the lines and the surrounding interference lines in each layer; the signal on the same layer maintains a certain strength, and the corresponding ground loop isolation is used to reduce the signal lines between the lines. A high-speed signal line should be limited to the signal line on the same layer. Too close to the boundary of the substrate, otherwise it will produce characteristic changes, and it is easy to increase the sense of boundary and increase inspiration.

3.3.4, the layout of the circuit board

A selective selectivity of 160MHz with cyclic signal energy with a rising edge of 2ns. .

Regarding the layout of the circuit board, there are the following precautions:

(1) Do not use a daisy chain structure to transmit interface signals, but a type structure, that is, all loads are directly connected to the interface driver.

(2) The conduction band connected to the input/terminal of the crystal oscillator should be as short as possible to reduce noise interference and all output effects on the crystal oscillator.

(3) The vibrating ground wire should be connected to the top with a wide and short conduction band; the nearest digital ground of the crystal should be separated from the crystal and there should be fewer vias.