3 Layout Planning and Design
3.9 Cooling Requirements and Packaging Density
3.9.1 Heat Sinks
Thermal management is an important aspect of the design of printed circuit boards. The design should accommodate the problems of heat distribution and heat removal in systems utilizing integrated circuits. For example, component density is often higher with SMT, resulting in greater power dissipation per square inch of PCB. In addition, closely spaced components make forced air cooling less efficient. Therefore, the air flow arrangement must be so designed that it can deliver the volume of air required to restrict the temperature rise of the board within the permissible limit. Reliability can thus get degraded unless special attention is paid to thermal management. For example, in multi-layer boards, all interconnections can be placed on internal layers and a heat sink of thick, solid copper or another material can be placed on the outer surfaces. Components can then be mounted directly on the metallic surface.
Sufficient free space should be provided around the heat sinks to improve efficiency. No bulky component should be mounted near the heat sink which may obstruct the free air flow. Generally, heat-generating components are raised to a higher level above the board. This prevents damage to the component and the board itself. In a vertically mounted PCB, two heat sinks should not be designed and mounted one above the other. In order to ensure maximum exchange of heat in heat sinks with unidirectional slots, the air flow must always pass the heat sink in the same direction as the slots are made.
3.9.2 Packaging Density
There is no simple formula to suggest the optimum packaging density on a PCB. For example, if the density is very low, a larger PCB area or a higher number of PCBs will be required to realize the same circuit. This will result in more volume of the equipment, more connectors and wiring with more parasitic influences on the working of the circuit, thereby degrading reliability while pushing up the cost. On the other hand, a very high packaging density will give higher circuit temperatures, more cross-talk, difficult servicing and maintenance, and probably a higher reject rate in PCB production. This again brings down the reliability and makes the cost higher.
The packaging density is usually dictated by:
a Purpose, use and application of equipment — whether fixed installation, portable or airborne;
a Heat generated and cooling arrangement — natural air flow, forced cooling or hermetically sealed unit;
a Type of components on board;
a Component technology-whether discrete, SSI, LSI, VLSI, or SMT; and
a Type of PCB used (interconnection density) — whether single-sided, double-sided, or multi- layered.
Multi-layer boards are preferred when the component densities possible with double-sided boards are not adequate. For example, in a double-sided board, a usual maximum is 2.0 TO cans per square inch which can be increased to more than 3. In some designs, it is possible to double the component density in the multi-layer boards as compared to double-sided boards, without appreciably increasing the volume of interconnections.
As general design guidelines, the packaging density of a PCB can be estimated from the number of component mounting holes per square inch of usable surface. This figure is 3–10 holes§in2 for single-sided boards, 10-20 holes§in2 for double-sided boards and more than 20 holes§in2 for multi- layer boards.
The packaging density is basically governed by:
a Board outline, size and form;
a Type of housing or enclosure in which the PC board will be finally mounted;
a Methods for mechanical attachment, i.e. card guides, stand-offs, etc.;
a Input/output termination, i.e. connector type, cable wire, etc.;
a Degree of support, i.e. retention and fastening;
a Card removal requirements, i.e. card extractors or special extract tools to aid in the removal of the board from enclosure;
a Desired accessibility for adjustable components;
a Heat dissipation requirements;
a Shielding requirements, i.e. circuit compatibility with other circuits and the environment;
a Type of circuit and its relationship to other circuits, i.e. the placement and area required;
a Environmental considerations such as shock and vibration humidity, salt spray, dust and radiation etc.; and
a Manufacturability, i.e. cost and case of manufacturing.
3.9.3 Package Style and Physical Attributes
Every electronic system consists of various parts including electronic components, interfaces, electronic storage media and the printed board assembly. The complexity of these systems is reflected in both the type of components used and their interconnecting structure.
Components are generally grouped into the following categories:
a Axial lead components;
a Radial lead components;
a Surface mounted devices; and a Electromechanical components.
Axial Lead Components
They are the most common type. They include resistors, some types of capacitors and diodes. For fixing the components, the leads are bent approximately 90° and inserted into the holes on the PCB
Layout Planning and Design 141
and soldered. The span between the two leads of these depends on the length of the body, lead diameter and the length of the lead upto the bend. If the axial component is required to be mounted vertically, one lead is bent vertically like a hairpin and is generally insulated with a sleeve.
Radial Components
They have leads perpendicular to their body and include certain capacitor types, variable resistors, active devices like ICs, transistors and some electromechanical components like switches, relays, connectors, etc. Transistors in TO packages are usually soldered to a cluster of pads (Figure 3.16).
ICs are often mounted on a base, which have leads spread so that they fall on a grid. Dual-in-line package (DIP) is the most commonly used IC package, standard DIP packages have 8, 14, 16, 20, 24, 28, 40, and 64 pins. They have two rows of leads with a lead separation of 2.5 mm within the row and a spacing of 7.5§10§15 mm between the rows.
1 3
2
1 3
2
\
TO-5 TO-18 TO-100
Fig. 3.16 Footprints TO-100 /TO-5/TO-18
Axial and radial components belong to the through-hole components type as they rely on a lead being inserted into drilled hole for the mechanical holding, and the soldering of the lead to the solder pad for the electrical connectivity.
The more complex components, as judged by the amount of input/output terminals they possess, the more complex is the interconnecting substrate.
Many peripheral leaded, lower I/O count devices such as memory and logic devices are being converted into area array packaging formats as either BGAs or fine pitch BGAs.
Surface Mount Technology (SMT)
Surface mount devices have leads with flat surfaces. These are soldered to solder pads which are called lands. The component is placed on the surface of the board instead of being inserted into them. Surface Mount Technology (SMT) has advanced to a stage wherein the majority of electronic components manufactured today are only available in SMT form.
BGA Packaging
Array packages such as BGA and fine pitch BGAs are now the latest technology component packages for I§O devices like memories, processors, and FPGAs. Ball and column grid arrays were standardized in 1992 with 1.5, 1.27 and 1.0 mm pitch. Fine pitch BGA array packages standards have established pitches of 0.8, 0.75, 0.65, and 0.5 mm. Area array packaging has the intrinsic value of making a coherent design. The signal I§O count for high performance BGAs is about 2.5 times of what is commonly required for BGAs used in hand-held products. BGA packaging is more useful for high frequency PCB design.
Electro Mechanical Components
This category of components includes relays, transformers, connectors, etc. In general, they do not follow any specific pattern of pin configuration, but have standard grided footprint, except transformers.