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Figure 1.33 Pin-through-hole connection from a chip to a printed circuit board.
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to the bottom half and wired to the lead frame. Ceramic packaging is normally only available as a pin-through-hole (PTH) device, whereas plastic packages have a wide range of card attachment technologies. The added cost of the ceramic packaging makes it typically only appropriate for prototyping circuits where the embedded microcontroller will be erased and reprogrammed. The technology used to attach chips to PCBs has changed dramatically over the past 20 years. In the early 1980s, most devices were only available in PTH technology packages (shown in Fig. 1.33) in which the lead frame pins are soldered into holes in the raw card. This type of attach technology has the advantage that it is very easy to work with little specialized knowledge or equipment is required to manufacture or rework boards built with PTH chips. The primary disadvantage of PTH is the amount of space required to put the hole in the PCB as well as the size of the chip s pins. Surface-mount technology (SMT) eliminates this disadvantage by soldering the chip s pins to the surface of the PCB using one of the two pin types shown in Fig. 1.34. The two SMT pin types offer advantages in certain situations. The gull wing pin allows for hand assembly of parts and easier inspection of the solder joints. The J lead reduces the size of the part s overall footprint. The shift from PTH to SMT is due to the reduction in PCB space required by SMT for a given number of I/O pins. The size difference of an SMT versus PTH component is shown in Fig. 1.35. Pin-through-hole components are normally built with pins 0.100in (100 thousandths of an inch) between pin centers. The measurement between lead centers is a critical one for electronics because it is directly related to how
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Figure 1.34 The different types of surfacemount technology pins used to solder a chip to a PCB.
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Figure 1.35 Comparison of PTH and SMT components sizes for the same number of pins.
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densely a board can be populated with electronic components. The SMT component shown in Fig. 1.35 has lead centers of 0.050in and the amount of board space is about 25 percent of the PTH component. This may seem like a considerable improvement, but consider that SMT components can be placed on both sides of a printed circuit board which means that eight SMT chips use up the same amount of PCB real estate as one PTH chip! The 0.050in lead centers quoted above are actually very large for SMT components. Modern SMT components can have lead centers as small as 0.16in (0.4mm). SMT parts with small lead centers are known as ne pitch parts. Further reducing board sizes for a given set of pins is the SMT technology known as ball grid array, in which the SMT pins are built from a two-dimensional array of small solder balls. Amazing as it sounds, there are chip packages with over 2,000 leads that take up no more than a square 1.25in (30mm) on a side. Assembly and rework of SMT parts are actually easier in a manufacturing setting than PTH. When the components are rst assembled onto PCBs, a solder/ ux mixture called solder paste is printed onto the SMT pads of the boards using a metal stencil with holes cut in the locations where the solder paste is to be put. A squeegee-like device spreads the paste over the stencil and deposits it on the card where there are holes. Once the paste has been deposited, the parts are placed onto the paste and then run through an oven to melt the solder paste, soldering the parts to the board. To rework a component, hot air (or nitrogen gas) is owed over the solder joints to melt the solder, allowing the part to be pulled off. SMT is easier to work with in a manufacturing setting; it is a lot more dif cult for the hobbyist or developer. Chip-on-board (COB) or glob top packaging is just what the name describes a chip is literally bonded to a printed circuit board with just a drop of encapsulant over it. COB parts are useful in applications that require a very small form factor for the nal product; because the chip is used directly, there is no overhead of a package for the application. Typical applications for COB include telephone smart cards and satellite or cable TV descramblers. There are two methods of COB attachment currently in use. The
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