Optimisation from a heat dissipation perspective begins with the PCB. PCBs using standard FR-4 material are limited to less than 135°C temperatures, although high-temperature versions (for use up to 180°C) are available. Those manufactured using bismaleimide triazine (BT), cyanate ester (CE) or polyimide materials can be used up to 200°C or sometimes even higher temperature, with quartz–polyimide boards useful up to 260°C. Boards made with PTFE resin have Tg greater than 300°C, but are not recommended for use above 120°C due to weak adhesion of the copper layer. Use of copper improves the PCB’s thermal characteristics since its thermal conductivity is more than 1000 times higher than the base FR-4 material.
Thoughtful design of the PCB along with logical placement of power-dissipating packages can result in big improvements at virtually no cost. Use of the PC board’s copper to spread the heat away from the package, along with innovative use of copper mounting pads, plated through-holes and power planes can significantly reduce thermal resistance.
While ICs are getting faster and more powerful, PC boards are shrinking in size. Today’s smaller PC boards (such as those found in cell phones and PDAs) and product enclosures with their higher speed demand more cooling than earlier devices. Their increased performance-to-package size ratios generate more heat, operate at higher temperatures and thus have greater thermal management requirements.
Most engineering materials are used to support mechanical loads only in applications where the use temperature in Kelvin is less than half the melting point. However, since the advent of surface mount technology, solder has been expected to provide not only electrical contact but also mechanical support at temperatures well in excess of this guideline.
In fact, at only 100°C, eutectic solder reaches a temperature over 80 per cent of its melting point and exhibits Navier–Stokes flow. Above this temperature, shear strength decreases to an unacceptable level and excessive relaxation is observed. In addition, copper-tin intermetallics can form between tin-lead solder and copper leads at elevated temperatures, which can weaken the fatigue strength of joints over time.
There are a number of solders that can be used at temperatures up to 200°C. Thus the temperature that a PWA can withstand is the lowest maximum temperature of any of the components used in the assembly of the PWA (connectors, plastic ICs, discrete components, modules, etc), the PCB and its materials, and the solder system used.
The shift to no-lead or lead-free solder as a result of the environmental and health impact of lead presents the electronics industry with reliability, manufacturability, availability and price challenges. Generally speaking, most of the proposed materials and alloys have mechanical, thermal, electrical and manufacturing properties that are inferior to lead-tin (Pb-Sn) solder and also cost more.
To date, the electronics industry has not settled on a Pb-Sn replacement. Pure tin is a serious contender to replace Pb-Sn. From a thermal viewpoint, lead-free solders require a higher reflow temperature (increasing from about 245°C for Pb-Sn to above 260°C for lead-free solder compounds) and thus greatly increase the possibility of component and PWA damage, impacting reliability.
Although the air immediately surrounding a chip will initially cool the chip’s surface, that air eventually warms up and rises to the top of the chassis, where it encounters other warm air. If not ventilated, this volume of air becomes warmer and warmer, offering no avenue of escape for the heat generated by chips.
By performing thermal analysis early in the design process, it becomes possible to ensure optimal component placement to protect against thermal problems. This, in turn, minimises or eliminates costly rework later. Modern electronic systems incorporate multitudes of components and subassemblies, including circuit boards, fans, vents, baffles, porous plates (such as electromagnetic interference (EMI) shields), filters, cabling, power supplies, disk drives and more. To help designers cope with this complexity, the most advanced thermal modeling solutions provide a comprehensive range of automated software tools and user-friendly menus that provide easier data handling, faster calculations and more accurate results.
Humidity is the volume or weight of moisture per unit of space—that is, water vapours present in the air. Absolute humidity is a measure of the actual water vapours in the air. It is measured in grams per cubic metre (gm/m3). Relative humidity is the ratio of absolute humidity to the theoretical maximum for a given temperature and pressure. It is expressed as a percentage. So, if the air holds half of what it could hold, the relative humidity is 50 per cent.
The devastating effects of humidity on electronic equipment are more often underestimated and misunderstood. The consequences of moisture ingress vary with the materials used as follows.
Primary humidity effects
These are the direct result of humidity on equipment or materials and include the following:
Humidity may degrade the performance of equipment operating in the infrared band and of some materials such as fabrics, some plastics and cellulose.