PCB Tech - High-speed PCB Design GuideNine: How to master the characteristics of IC packaging

Placing the decoupling capacitor directly in the IC package can effectively control EMI and improve the integrity of the signal. This article starts with the internal packaging of the IC, analyzes the source of EMI and the role of IC packaging in EMI control, and then proposes 11 effective EMI control Design rules, including package selection, pin structure considerations, output drivers and decoupling capacitor design methods, etc., help design engineers choose the most appropriate integrated circuit chip in a new design to achieve the best EMI suppression performance. Existing system-level EMI control technologies include:

(1) The circuit is enclosed in a Faraday box (note that the mechanical package containing the circuit should be sealed) to achieve EMI shielding;

(2) Filtering and attenuation technology is adopted on the circuit board or the I/O port of the system to achieve EMI control;

(3) Strictly shield the electric field and magnetic field of the current circuit, or adopt appropriate design techniques on the circuit board to strictly control the capacitance and inductance of PCB traces and circuit board layers (self-shielding), thereby improving EMI performance.

EMI control usually requires a combination of the above-mentioned technologies. Generally speaking, the closer to the EMI source, the smaller the cost required to achieve EMI control. The integrated circuit chip on the PCB is the most important energy source of EMI. Therefore, if you can understand the internal characteristics of the integrated circuit chip in depth, you can simplify the EMI control in the PCB and system-level design.

PCB board-level and system-level design engineers usually believe that the EMI source they can reach is the PCB. Obviously, at the PCB design level, a lot of work can indeed be done to improve EMI. However, when considering EMI control, design engineers should first consider the choice of IC chip. Certain characteristics of integrated circuits, such as package type, bias voltage, and chip process technology (such as CMOS, ECL, TTL), etc., have a great influence on electromagnetic interference. This article will focus on these issues and discuss the impact of IC on EMI control.

1. The source of EMI

During the transition of digital integrated circuits from logic high to logic low or from logic low to logic high, the frequency of the square wave signal generated at the output is not the only frequency component that causes EMI. The square wave contains sinusoidal harmonic components with a wide frequency range, and these sinusoidal harmonic components constitute the EMI frequency components that engineers care about. The highest EMI frequency is also called the EMI emission bandwidth, and it is a function of the signal rise time rather than the signal frequency. The formula for calculating the EMI emission bandwidth is: F=0.35/Tr

Among them: F is the frequency in GHz; Tr is the signal rise time or fall time in ns (nanoseconds).

It is not difficult to see from the above formula that if the switching frequency of the circuit is 50MHz, and the rise time of the integrated circuit chip used is 1ns, then the highest EMI emission frequency of the circuit will reach 350MHz, which is much greater than the switching frequency of the circuit. And if the rise time of the IC is 500ps, then the highest EMI emission frequency of the circuit will be as high as 700MHz. As we all know, every voltage value in the circuit corresponds to a certain current, and also every current has a corresponding voltage. When the output of the IC changes from logic high to logic low or logic low to logic high, these signal voltages and signal currents will generate electric and magnetic fields, and the highest frequency of these electric and magnetic fields is the emission bandwidth. The strength of the electric and magnetic fields and the percentage of external radiation are not only a function of the signal rise time, but also depend on the control of the capacitance and inductance on the signal path between the signal source and the load point. Here, the signal source is located on the PCB The IC is inside the board, and the load is inside other ICs. These ICs may or may not be on the PCB. In order to effectively control EMI, it is not only necessary to pay attention to the capacitance and inductance of the IC chip itself, but also to the capacitance and inductance existing on the PCB.

When the coupling between the signal voltage and the signal loop is not tight, the capacitance of the circuit will decrease, so the suppression effect on the electric field will be weakened, thereby increasing the EMI; the current in the circuit also has the same situation, if the current Poor coupling between the same return path will inevitably increase the inductance on the loop, thereby enhancing the magnetic field, and ultimately leading to an increase in EMI. In other words, poor control of the electric field usually results in poor suppression of the magnetic field. The measures used to control the electromagnetic field in the circuit board are generally similar to the measures used to suppress the electromagnetic field in the IC package. As in the case of PCB design, IC package design will greatly affect EMI.

A considerable part of the electromagnetic radiation in the circuit is caused by voltage transients in the power bus. When the output stage of the IC jumps and drives the connected PCB line to logic "high", the IC chip will absorb current from the power supply and provide the energy required by the output stage. For the ultra-high frequency current generated by the continuous conversion of the IC, the power bus starts from the decoupling network on the PCB and ends at the output stage of the IC. If the signal rise time of the output stage is 1.0ns, then the IC must draw enough current from the power supply to drive the transmission line on the PCB in a short time of 1.0ns. The voltage transient on the power bus depends on the inductance in the power bus path, the current absorbed, and the transit time of the current. The voltage transient is defined by the following formula: V=Ldi/dt,

Among them: L is the value of the inductance on the current transmission path; di represents the current change in the signal rise time interval; dt represents the current transmission time (the rise time of the signal).

Since the IC pins and internal circuits are part of the power bus, and the rise time of the absorbed current and the output signal depends to a certain extent on the process technology of the IC, the above formula can be controlled to a large extent by choosing the appropriate IC. All three elements mentioned in.

2. The role of IC packaging in electromagnetic interference control

IC packages usually include: silicon-based chips, a small internal PCB, and pads. The silicon-based chip is mounted on a small PCB, and the connection between the silicon-based chip and the pad is realized through a bonding wire, and a direct connection can also be realized in some packages. The small PCB realizes the connection between the signal and power supply on the silicon-based chip and the corresponding pins on the IC package, thus realizing the external extension of the signal and power supply nodes on the silicon-based chip. The power and signal transmission paths that penetrate the IC include: silicon-based chips, connections with small PCBs, PCB traces, and input and output pins of the IC package. The control of capacitance and inductance (corresponding to electric field and magnetic field) depends largely on the design of the entire transmission path. Certain design features will directly affect the capacitance and inductance of the entire IC chip package.

First look at the connection between the silicon-based chip and the internal small circuit board. Many IC chips use bonding wires to realize the connection between the silicon-based chip and the internal small circuit board, which is a very thin flying wire between the silicon-based chip and the internal small circuit board. This technology is widely used because the coefficient of thermal expansion (CTE) of the silicon-based chip and the internal small circuit board are similar. The chip itself is a silicon-based device, and its thermal expansion coefficient is very different from that of typical PCB materials (such as epoxy resin). If the electrical connection points of the silicon-based chip are directly mounted on the internal small PCB, then after a relatively short period of time, the internal temperature of the IC package will cause thermal expansion and contraction, and the connection in this way will fail due to fracture . The binding wire is a lead method that adapts to this special environment. It can withstand a large amount of bending and deformation and is not easy to break.

The problem with using bonding wires is that the increase in the current loop area of each signal or power line will cause the inductance value to increase. A good design to obtain a lower inductance value is to achieve a direct connection between the silicon-based chip and the internal PCB, that is, the connection point of the silicon-based chip is directly bonded to the PCB pad. This requires the use of a special PCB board base material, which should have a very low CTE. The choice of this material will lead to an increase in the overall cost of the IC chip. Therefore, the chip using this process technology is not common, but as long as the IC that directly connects the silicon-based chip with the carrier PCB exists and is feasible in the design scheme, then Using such IC devices is a better choice.

Generally speaking, in the IC package design, reducing the inductance and increasing the capacitance between the signal and the corresponding circuit or between the power supply and the ground are the first choices for the process of selecting an integrated circuit chip. For example, small-pitch surface mount technology should be compared with large-pitch surface mount technology. IC chips packaged with small-pitch surface mount technology should be selected first, and these two types of surface mount technology package IC chips are better than via lead type packages. BGA packaged IC chips have the lowest lead inductance compared to any commonly used package types. From the point of view of capacitance and inductance control, smaller packages and finer pitches usually always represent improved performance.

An important feature of the lead structure design is the allocation of pins. Since the inductance and capacitance values depend on the proximity of the signal or power supply to the return path, enough return paths must be considered.

Power and ground pins should be allocated in pairs, and each power pin should have a corresponding ground pin adjacent to each other, and multiple power and ground pin pairs should be allocated in this lead structure. Both of these features will greatly reduce the loop inductance between the power supply and the ground, and help reduce voltage transients on the power bus, thereby reducing EMI. Due to habitual reasons, many IC chips on the market now do not completely follow the above design rules. However, IC designers and manufacturers have a deep understanding of the advantages of this design method, so IC manufacturers are more inclined to design and release new IC chips. Pay attention to the power connection.

Ideally, assign an adjacent signal return pin (such as a ground pin) to each signal pin. The actual situation is not the case, even the most avant-garde IC manufacturers did not allocate IC chip pins in this way, but adopted other compromise methods. In the BGA package, an effective design method is to set a signal return pin at the center of each group of eight signal pins. In this pin arrangement, each signal is between each signal and the signal return path. The difference is only one pin. For quad flat package (QFP) or other gull wing type ICs, it is unrealistic to place a signal return path in the center of the signal group. Even so, it is necessary to ensure that every 4 to 6 Place a signal return pin for each pin. It should be noted that different IC process technologies may use different signal return voltages. Some ICs use ground pins (such as TTL devices) as the signal return path, while some ICs use power pins (such as most ECL devices)