Proper selection of packaging approaches for an electronic system is determined not just by its function, but also by the chosen component types and the system's operating parameters. These include clock speeds, power consumption, heat management methods, and the environmental conditions where the system will be used. This section outlines the fundamental constraints that must be taken into account when designing the packaging for electronic systems.
The operating speed of electronic systems is a critical technical factor that affects interconnection design. With many digital systems operating close to, and even beyond, 100 MHz, there is a growing demand for packaging engineers to create innovative solutions to meet the increasing system speed.
The dielectric properties of substrate materials play a significant role in determining the speed of signal propagation, with the signal propagation being inversely proportional to the square root of the dielectric constant. It is essential for designers to consider the substrate materials' dielectric properties they intend to use to ensure optimal electrical performance. Additionally, the length of conductors affects the time of flight, and the signal propagation on the substrate between chips, and must be kept short to guarantee high-speed system operation.
For systems operating at speeds above 25 MHz, interconnections must have transmission line characteristics to minimize signal losses and distortion. A proper design of such transmission lines requires precise calculations of the conductor and dielectric separation dimensions, with exact PCB manufacture to ensure the expected accuracy of performance. For PWBs, there are two basic transmission line types:
With the increasing clock rates and the number of gates per chip, power consumption has become a growing concern. Some chips require up to 30 W of power, making it necessary to use more terminals to accommodate the power flow and ground planes. Typically, 20 to 30 percent of chip terminals are used for power and ground connections, but in high-speed systems, this may increase to 50 percent due to the need for electrical isolation of signals.
To address this issue, design engineers must provide adequate power and ground distribution planes within multilayer boards (MLBs) to ensure low-resistance flow of currents, especially in boards connecting high-speed chips consuming tens of watts and operating at 5V, 3.3V, or lower. Proper distribution of power and ground is crucial in reducing di/dt switching interference in high-speed systems and minimizing heat concentrations.
In some cases, separate bus-bar structures may be necessary to meet high power demands. By optimizing power and ground distribution in the system, design engineers can efficiently reduce power consumption and improve the overall performance of high-speed systems.
As electronic equipment operates at higher frequencies, ICs, modules, and assemblies can generate radio frequency (RF) signals. These signals, known as electromagnetic interference (EMI), can harm neighboring electronics or even other parts of the same equipment, causing errors, failures, and mistakes. Preventing EMI is crucial, and specific standards dictate acceptable levels of radiation, which are exceedingly low.
To ensure that their equipment does not exceed these limits, packaging engineers and particularly PWB designers must be well-versed in methods of reducing or eliminating EMI radiation. By doing so, they can safeguard their equipment against EMI and guarantee optimal functionality.
Understanding the end-use and market segment of an electronic product is essential in determining the appropriate packaging approach. The packaging designer must consider whether cost or performance is the driving force behind the product's use and where it will be utilized. For instance, if it is to be placed under the hood of a car, the designer must take into account the severe environmental conditions. On the other hand, if it is intended for office use, the operating conditions are typically benign. The severity of the equipment operating conditions has been classified by the IPC and is listed in Table 1.
TABLE 1. Realistic Representative-Use Environments, Service Lives, and Acceptable Cumulative Failure Probabilities for Surface-Mounted Electronics by Use Categories
|Worst-case use environment|
|Use category||Tmin, °C||Tmin, °C||∆T*, °C||tD, h||Cycles/year||Years of service||Acceptable failure risk, %|
|5-Industrial & automotive||-55||+95||20||12||185|
|ground & ship||&60||12||265||~5||~0.1|
The widespread digitization of electronic functions has brought together consumer, computer, and communication technologies, increasing the demand for electronic products and mass production. Consequently, product cost has become a crucial criterion in electronic system design, with design engineers required to analyze potential trade-offs and prioritize cost while complying with design and operation conditions.
Rigorous cost trade-off analysis is particularly important in the early stages of electronic product design, as around 60% of manufacturing costs are determined then, despite only 35% of the total design effort having been expended. Attention to manufacturing and assembly requirements, known as Design for Manufacturability and Assembly (DFM/A), can reduce assembly costs by up to 35% and PCB costs by up to 25%.
The most cost-effective electronic packaging designs require consideration of three key elements:
Design engineers can select the most cost-effective approach using various measurements that relate the effects of different PWB design elements to their costs. The article on PWB density and methods to increasea provide guidelines on how to optimize PWB designs in line with these considerations.