In order to design highly reliable products, designers need to study the intended application environment and its impact on the electronic assembly. This article discusses thermal and humidity effects and protection measures in particular.
Apart from cabinet, an electronic assembly comprises electronic components assembled/soldered over a printed circuit board (PCB). When dealing with electronic assemblies, one must comply with certain standards with respect to its design, manufacturing and assembly. IPC, an association connecting electronics industries, is one of the organisations that develop such standards.
Based on performance, all the electronic equipment can be classified into three categories:
1. Class-1 general electronic products
These include consumer products, some computer and computer peripherals as well as general military hardware suitable for applications where cosmetic imperfections are not important and the major requirement is function of the completed printed board or printed board assembly.
2. Class-2 dedicated service electronic products
These include communication equipment, sophisticated business machines, instruments and military equipment in applications where high performance and extended life are required, and for which uninterrupted service is desired but is not critical. Certain cosmetic imperfections are allowed.
3. Class-3 high-reliability electronic products
These include commercial and military equipment where continued performance or performance on demand is critical. Equipment downtime cannot be tolerated, and equipment must function when required. Examples include life-support equipment and critical weapon systems.
These end-product classes have been established to reflect progressive increase in sophistication, functional performance requirements and testing/inspection frequency. There may be an overlap of equipment between classes. The printed board user needs to determine the class to which his product belongs. The contract shall specify the performance class required and indicate any exceptions to certain parameters, where appropriate.
Reliability can have different meanings to different people. The IPC reliability series describes it as “the state where an equipment will work as long as expected, under a variety of use conditions, with no greater than a predicted number of failures.” However, a general definition that has broad applicability is “time-dependent product performance that satisfies all customer expectations.”
In military, industrial, commercial and consumer electronics applications, certain equipment may contain devices that are highly sensitive to environmental conditions. Understanding the effects of these uncontrolled environmental conditions at the component and system levels and applying this knowledge during the design phase can improve the reliability of the equipment, thereby reducing failures and thus maintenance costs.
Some of the environmental conditions affecting electronic equipment and systems include moisture, dust, vibration, air cooling and heating. The focus of this article is to quantify the effects of temperature and humidity on electronic components and PCB performance.
As printed wire assemblies (PWAs) continue to increase in complexity, the risk of field failures due to unforeseen thermal problems also increases.
In the operation of an IC, electrons flow among tens of millions of transistors, consuming power. This produces heat, which radiates outward through the chip package from the surface of the die, increasing the IC’s junction temperature. Exceeding the specified maximum junction temperature causes the chip to make errors in its calculations or perhaps fail completely.
When IC designers shrink a chip and reduce its operating voltage, they also reduce its power dissipation and thus heat. However, shrinking a chip also means that heat-generating transistors are packed closer together. Thus, while the chip itself might not be as hot, its power density (the amount of heat concentrated on particular spots of the chip’s surface) may begin to increase. Efficient cooling methods are required to protect spots in the chip from heating. If heat is not properly removed or managed, it will shorten the IC’s overall life, even destroying the IC.
Heat buildup in an IC generally begins as junction temperature rises until heat finds a path to flow. Eventually, thermal equilibrium is reached during steady-state operating temperature, which affects mean time between failure (MTBF) of the device. A frequently used rule of thumb is that for each 10°C rise in junction temperature, there is a doubling of the failure rate for that component. Thus, lowering temperatures 10°C to 15°C can approximately double the lifespan of a device. Accordingly, designers must consider operating temperature as well as safety margin of devices.
Among discrete passive components, capacitors are the most sensitive to elevated temperatures. Lack of compact, thermally stable and high-energy-density capacitors has been one of the most significant barriers to the development of high-temperature systems. For traditional ceramic dielectric materials, there is a fundamental tradeoff between dielectric constant and temperature stability.
Capacitance of devices made with low-dielectric-constant titanates, such as C0G or NP0, remains practically constant with temperature and shows little change with aging. Capacitance of devices made with high-dielectric-constant titanates, such as X7R, is larger but exhibits wide variations with increase in temperature. In addition, leakage currents become unacceptably high at elevated temperatures, making it difficult for the capacitor to hold a charge.