Signal and Power Integrity Design for High-Density PCBAs

As demand for computing power seems unlimited, the hardware behind it is being pushed to the farthest limits of technological capability. This is driving the need for a deeper understanding of the effects of manufacturing on a printed circuit board assembly’s (PCBA) signal integrity (SI) and power integrity (PI). Both signal and power quality are essential for these electronic products, and measurements of SI and PI can help produce the highest performing systems. Properly regulated and applied, electrical power can support long product operating lifetimes. However, ensuring good SI and PI in both design and manufacturing is technically challenging. Power-supply inconsistencies and noise can degrade performance, resulting in such problems as latency in high-speed-digital (HSD) circuits, and HSD signals can be degraded by slight changes in temperature, material flexing, and hidden fabrication defects. Planning for SI and PI analysis during a product’s development stages and performing SI and PI testing as part of the manufacturing process, HSD products with high reliability and repeatability can be the result. As engineers at Benchmark have learned from experience, the test results obtained by integrating SI and PI analysis into both early design stages and automated manufacturing routines can guide product optimization. When pursuing customers’ most challenging electrical and mechanical printed-circuit-board assembly (PCBA) requirements, SI/PI testing can provide valuable insights to enhance product performance and reliability.

To achieve outstanding signal quality when designing and manufacturing HSD products, Benchmark has integrated SI/PI testing and analysis into the automated testing that accompanies product development, such as for the design and assembly of densely configured PCBAs. By tracking SI/PI as early as possible, data are available to mechanically, electrically, and thermally optimize PCBAs to meet or exceed challenging goals. For example, as PCBAs are required to handle higher power levels, SI/PI testing can identify potential problems in a circuit layout, such as locations for potential heat buildup. SI/PI analysis reveals problem areas in a layout to trigger the development of alternative layout configurations to conserve PCBA energy and boost power efficiency.

By performing SI/PI measurements from prototyping through production, potential layout problems can be corrected. Modifications in circuit assembly can be made to enhance product quality and reduce a customer’s manufacturing costs. SI/PI testing includes coolant-based chip testing on a PCBA, solder and flux analysis throughout the PCBA, and thermal analysis. Learning to interpret SI/PI measurement data at high power levels can help discover and avoid hotspots in PCBAs relying on high-density interconnects (HDIs), contributing to improvements in thermal management and product operating lifetime.

Providing power to increasingly complex, high-density circuits is neither routine nor simple. It requires careful planning, starting with the choice of PCBA materials. To achieve high circuit densities, multiple circuit layers require vias for signal and power interconnections. In addition to generating heat in high-density circuits, handling high power with poor PI can contribute to poor SI and degraded performance in HSD circuits. Power ripples can raise the levels of a circuit’s electromagnetic interference (EMI), especially for power rails in proximity to closely spaced parallel signal lines. But methodical SI and PI testing and analysis can uncover problems and monitor the consistency and effectiveness of an electronic manufacturing process for high-density circuits. SI/PI testing can also uncover signal and power irregularities due to ground bounce (changes in ground reference voltage) between layers of a multilayer PCBA and digital timing errors resulting from voltage jitter.

Because they are subject to crosstalk and interference, special attention should be given to SI/PI for closely spaced transmission lines. Signal lines interconnecting some PCBA’s electronic functions may cover a single PCBA layer, but in dense designs or powerful computing applications, signal lines quite often bridge multiple PCBA layers. A challenge in the design and manufacture of multilayer PCBAs is avoiding signal line coupling, especially for closely spaced parallel signal paths in dense PCBAs. For multilayer board layouts, vias interconnecting layers must avoid impedance mismatches that can disrupt the flow of current. Impedance mismatches cause signal reflections and degrade both SI and PI performance levels. 

As demand for artificial intelligence (AI) grows, PCBAs are capable of increased functionality in denser designs. SI/PI testing provides a means of evaluating the performance and reliability of a PCBA as narrower line widths are fabricated in denser circuit patterns. Whether as an integrated part of manufacturing or as part of a regular maintenance schedule, SI/PI testing helps identify and eliminate PCBA problem areas, such as junctions where even minimal loss can transform into damaging heat at high power levels. SI/PI analysis investigates signals and power supplies as interactive characteristics of a circuit, where often the most practical attempts at thermal management can impact the performance of a product (such as a need for additional vias and heat sinks). By following SI/PI testing warning signs for excess heat, practical thermal management can be implemented to modify a circuit layout, such as adding vias and heat sinks, to improve the heat flow away from a PCBA hotspot.

Because performance enhancements are often achieved through layout changes, EMI and electromagnetic compatibility (EMC) testing are also performed alongside SI/PI testing as part of manufacturing production at Benchmark. By reinforcing data from two-dimensional (2D) and three-dimensional (3D) EMI measurements with 2D and 3D EM circuit simulations, a customer’s product design can be modeled for expected SI and PI performance levels and EMI problems due to closely spaced components, power lines, and signal transmission lines. Computer EM simulations that can project SI and PI performance can help guard against layout problems that can be corrected prior to prototyping. 

Studying SI

SI refers to measured signal quality, such as frequency and amplitude accuracy for analog signals and timing accuracy and logic levels for HSD signals. A circuit’s SI can be analyzed using a source to generate known voltage signals and a test receiver to characterize changes that occur as the signals travel through a PCBA. Changes include time delays, loss of power, and increases in noise. SI can be degraded by long signal lines, including differential signal pairs used in HSD circuits, and by changes in impedance and impedance mismatches at junctions and interconnections along the signal path. Resonances caused by multiple signal reflections at impedance mismatches  may occur when a signal’s path length is equal to a multiple of a quarter of the signal wavelength and its reflected signals overlap with its transmitted signals. 

SI performance is subject to circuit layout and dimensions. As noted, long signal lines can yield timing errors, especially at Gb/s speeds, resulting in signal latency. The choice of PCBA circuit materials can also impact latency and timing, since a material’s frequency- and time-domain characteristics are often a function of temperature, whether it is the environmental temperature of an application or the rise in temperature resulting from the dissipation of heat from a circuit material’s dielectric losses at given signal and power levels. Proper thermal management is essential to maintain acceptable SI over a wide operating temperature range. 

Probing SI

Variations in transmission-line impedance cause signal reflections and variations in SI. Signal paths and power rails with tightly controlled impedance and the use of level, consistent ground and power planes are essential for good SI/PI. Signal linewidths and spacings must be tightly controlled to maintain consistent impedance, with layers of multiple-layer PCBAs interconnected by consistent vias. Circuit material characteristics, such as dielectric constant (Dk) and dissipation factor (Df),  must also be consistent as the foundation for signal paths with consistent impedance throughout a PCBA. Physical variations in circuit material, such as changes in conductor and dielectric thickness, also result in signal path impedance variations even for tightly controlled signal and power lines. 

Signal and power lines experience losses of energy as a function of length. Long lines are also candidates for changes from a nominal signal waveform or supply voltage due to variations in impedance. While they may be difficult to design and fabricate, creative circuit layouts with shorter signal lengths can help reduce energy losses and minimize opportunities for signal reflections and timing errors. Still, within densely packed PCBAs, signal paths must be spaced, with signal lines, power lines, and components placed to minimize EMI. EMI can result from power conversions within the PCBA and may be difficult to reduce without the addition of shielding or high-inductance coils. Some circuit layout tradeoffs may be necessary to achieve the best SI/PI performance. 

Making Measurements

For HSD circuits, timing is everything. In addition to signal path variations, HSD SI is very much subject to packaged device quality as well as the interconnection of a packaged device to a PCBA’s signal and power circuitry. Packaged components such as semiconductor integrated circuits (ICs) and surface-mount-technology (SMT) components feature interconnection dimensions that are affected by the slightest latencies, so the precision placement and consistent attachment of SMT and other packaged components can play a key role in achieving good PCBA SI. Especially for digital components, in which signal transfers are executed according to clock timing, uneven soldering can be a cause of excess latency. 

Timing is essential for many computer-controlled products, either via wireless communications such as WLAN or physical interconnections, such as Ethernet ports, Universal Serial Bus (USB) 3.0 or 3.1 connectors, or backplane connectors such as PCI Express (PCIe) connectors. For a product with poor thermal management, problems from heat build-up may not be apparent from standard manufacturing testing. But with SI testing, and a drop in the product’s SI during testing, timing issues caused by thermal buildup over time may be detected, prompting a modification to the circuit layout to avoid the thermal issues and the timing errors. 

Good PI requires consistent, ripple-free voltage for a PCBA and the components within its power distribution network (PDN). The PDN must deliver current from a power source and return current by means of a low-impedance conductive path. Power may come from an AC source using an on-board switched-mode power supply to convert the AC voltage to a DC voltage. But voltage converters (including DC-to-DC converters) can suffer transient voltage effects. These transient voltages react with any inductance within the supply rails, causing DC voltage ripple in the supply. Such DC voltage ripple effects, which are detected during PI measurements, can be disruptive to the timing of HSD circuits. Transient voltages can react with the inductance of the power rails to cause voltage ripple effects, leading to DC voltage ripple and to an electronic product’s poor PI performance. LDO regulators may also be part of a product’s voltage supply rail, although they are also potential sources of noise for the product’s PI evaluation. The power-line noise can affect all components connected to it, depending upon their location on a single-layer or multilayer PCBA. 

During prototyping and product development, SI and PI testing can help minimize energy loss, locate interference, and evaluate the severity of voltage deviations and ripple. It can be performed with fundamental test tools, such as digital voltmeters (DVMs), logic analyzers, and digital storage oscilloscopes (DSOs) to measure power rails and check for proper power-on timing in a product’s power supply. Additional measurements with instrumentation such as EMI receivers can check for crosstalk and EMI affecting both signal and power lines. Measurement of energy efficiency can be particularly useful for battery-powered systems requiring long operating times per charge. Maintaining good energy efficiency can also help minimize the amount of energy lost as heat and product design iterations required for safe dissipation of heat. 

Effective and efficient product development relies on a wide range of test and measurement capabilities. By integrating automated test equipment (ATE) into the manufacturing process, SI/PI can be optimized while a product is enhanced for electrical, mechanical, and thermal behavior. Through a start-to-finish approach, high-yield products can be manufactured that meet the most challenging regulatory requirements. In-house semiconductor fabrication capability and the capability to directly characterize SI and PI help enhance and speed the product manufacturing process. 

Extensive in-house measurement capabilities in the frequency, time, and power domains enhance the SI/PI product development process by providing additional insights into a particular layout or a design’s mechanical, electrical, even optical characteristics. By processing ATE data collected during manufacturing, such as frequency-domain measurements of signal loss (Fig. 1), computer-aided-design (CAD) software can model a product design in search of enhancements. By performing measurements in the time domain, tools such as eye diagrams (Fig. 2) can provide invaluable insights into the synchronization and timing of critical circuits and systems.

Figure 1. ATE used in manufacturing provides data for analysis tools, such as computer-aided-design (CAD) software that helped analyze the strength of transmit channels within a communications product, here with frequency in the horizontal scale (DC to 16 GHz) and signal magnitude (loss) in the vertical scale (0 to -10 dB).

Figure 2. Timing precision is evaluated with the aid of time-domain measurement tools such as those used to generate this high-resolution eye diagram. It features a vertical voltage scale of 200 mV/div with an adjustable offset (shown here at 0.0 mV) and a horizontal time scale of 100 ps/div, with a delay in this case of 0.250 ns.

By understanding SI/PI measurements and interpreting SI/PI test data, Benchmark’s engineers have been able to integrate SI/PI testing and EMI/EMC measurements into product development and automated manufacturing to help customers optimize products manufactured with Benchmark even prior to prototyping. Measured SI/PI data can reveal the benefits, for example, of repositioning a DC power supply within a product’s circuit layout for better delivery of DC power throughout one or multiple circuit boards. By teaming computer-aided-design (CAD) software with SI/PI measurement capabilities, SI/PI test data can guide component placement changes in a circuit layout to help reduce the lengths of power-supply rails and power-supply ripple. Performing SI/PI/EMC analysis as part of manufacturing and during the initial stages of design can eliminate the need for some prototyping stages in a product’s development and result in fewer design iterations and faster time to market.

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