EMC design of hybrid integrated circuits

1. Hybrid integrated circuit

(Hybrid Integrated Circuit) is an integrated circuit made up of a semiconductor integrated process combined with a thick (thin) film process. The hybrid integrated circuit is formed by forming a thick film or a thin film element and an interconnection thereof on a substrate, and assembling and assembling a discrete semiconductor chip, a monolithic integrated circuit or a micro component on the same substrate, and then externally packaging. It has the characteristics of high assembly density, high reliability and good electrical performance.

As the size of the circuit board becomes smaller, the wiring density increases, and the operating frequency continues to increase, the electromagnetic interference phenomenon in the circuit becomes more and more prominent, and the electromagnetic compatibility problem becomes the key to whether an electronic system can work normally. The electromagnetic compatibility design of the board is the key to system design.

2. Principle of electromagnetic compatibility

Electromagnetic compatibility refers to the ability of electronic equipment and power supplies to work normally and reliably under certain electromagnetic interference environments. It is also the ability of electronic equipment and power supplies to limit their own electromagnetic interference and avoid interference with other surrounding electronic equipment.

Any occurrence of electromagnetic interference must have three basic conditions: first, it must have a source of interference, that is, a device or device that generates harmful electromagnetic fields; secondly, it must have a way to propagate interference, and it is generally considered that there are two ways: conduction coupling and radiation. Coupling method, the third is to have sensitive equipment that is susceptible to interference

Therefore, solving the electromagnetic compatibility problem should be solved one by one for the three elements of electromagnetic interference: reduce the interference intensity of the interference generating component; cut off the propagation path of the interference; and reduce the sensitivity of the system to the interference.

The electromagnetic interferences present in hybrid integrated circuit designs are: conducted interference, crosstalk interference, and radiated interference. When solving EMI problems, first determine whether the coupling path of the source is conductive, radiated, or crosstalk. If a high amplitude transient current or a fast rising voltage occurs near the signal-carrying conductor

Nearby, the problem of electromagnetic interference is mainly crosstalk. Conductive interference if there is a complete electrical connection between the source of interference and the sensitive device. Radiation interference occurs between two parallel wires that transmit high frequency signals.

3. Electromagnetic compatibility design

In the hybrid integrated circuit electromagnetic compatibility design, the first thing to do is to check the functional compatibility. In the circuit that has been determined by the scheme, check whether the electromagnetic compatibility index can meet the requirements. If it is not satisfied, the parameters should be modified to reach the index, such as the transmitting power and the working frequency. , re-select the device, etc. 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. Generally, the electromagnetic compatibility design of a circuit includes: selection of processes and components, circuit layout, and wiring of wires, etc.

3.1 Selection of processes and components

Hybrid integrated circuits are available in three fabrication processes, single-layer films, multilayer thick films, and multilayer co-fired thick films. The thin-film process is capable of producing small-sized, low-power, and high-current-density components for high-density hybrid circuits. It is high-quality, stable, reliable, and flexible, and is suitable for high-speed high-frequency and high-package density circuits. However, it can only be used for single-layer wiring and is costly. The multilayer thick film process can manufacture multilayer interconnect circuits at a lower cost. From the perspective of electromagnetic compatibility, the multilayer wiring can reduce the electromagnetic radiation of the circuit board and improve the anti-interference ability of the circuit board. Because a dedicated power plane and ground plane can be set, the distance between the signal and the ground is only the interlayer distance. In this way, the loop area of all signals on the board can be minimized, thus effectively reducing differential mode radiation.

Among them, the multi-layer co-fired thick film process has more advantages and is the mainstream technology of passive integration. It can realize more layers of wiring, easy to embed components, increase assembly density, and has good high frequency characteristics and high speed transmission characteristics. In addition, it has good compatibility with thin film technology, and the combination of the two can achieve a higher packing density and better performance of the hybrid multilayer circuit.

Active devices in hybrid circuits generally use bare chips. When there is no bare chip, the corresponding packaged chips can be used. For best EMC characteristics, surface-mount chips should be used. When selecting a chip, the low-speed clock should be selected as much as possible while satisfying the technical specifications of the product. Never use AC when HC is available, and CMOS4000 can use HC. The capacitor should have a low equivalent series resistance to avoid large attenuation of the signal.

The hybrid circuit package can be made of ruinable metal base and cover, parallel seam welding, and has a good shielding effect.

3.2 Circuit layout

When performing the layout division of hybrid microcircuits, there are three main factors to consider: the number of input/output pins, device density, and power consumption. A practical rule is that the chip-like component occupies 20% of the substrate, and the power dissipated per square inch is no more than 2W.

In terms of device layout, in principle, the related devices should be placed as close as possible, and the digital circuit, the analog circuit and the power supply circuit should be placed separately to separate the high frequency circuit from the low frequency circuit. Devices that are prone to noise, small current circuits, high current circuits, etc. should be as far away as possible from the logic circuit. The main interference and radiation sources such as clock circuits and high-frequency circuits should be arranged separately and away from sensitive circuits. The input and output chips should be located close to the I/O exit of the hybrid package.

High-frequency components should be shortened as much as possible to reduce distribution parameters and mutual electromagnetic interference. The susceptible components cannot be too close to each other, and the input and output should be kept as far away as possible. Place the oscillator as close as possible to the location where the clock chip is used and away from the signal interface and the low level signal chip. The components should be parallel or perpendicular to one side of the substrate, and the components should be arranged in parallel as much as possible. This not only reduces the distribution parameters between the components, but also conforms to the manufacturing process of the hybrid circuit and is easy to produce.

The power and ground extraction pads on the hybrid circuit substrate should be symmetrically placed, preferably distributing a number of power and ground I/O connections evenly. The mounting area of the bare chip is connected to the most negative potential plane.

When a multilayer hybrid circuit is selected, the interlayer arrangement of the board varies with the specific circuit, but generally has the following features.

(1) The power supply and the ground layer are distributed in the inner layer, which can be regarded as a shielding layer, which can well suppress the common mode RF interference inherent in the circuit board and reduce the distributed impedance of the high frequency power supply.

(2) The power plane and the ground plane in the board are as close to each other as possible. Generally, the plane is above the power plane. This can use the interlayer capacitance as the smoothing capacitor of the power supply, and the ground plane shields the radiation current distributed in the power plane.

(3) The wiring layer should be arranged as close as possible to the power source or ground plane to generate flux cancellation.

3.3 Layout of the circuits

In the circuit design, often only pay attention to improve the wiring density, or pursue uniform layout, ignoring the influence of the line layout on preventing interference, causing a large number of signals to radiate into the space to form interference, which may lead to more electromagnetic compatibility problems. Therefore, good wiring is the key to success in design.