Image:Smt closeup.jpg
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A closeup of the printed circuit board of an inexpensive Seitec USB BAR branded keydrive (circa 2004), showing various typical surface-mount technology components.
The large square component in the centre of the image is a 7x7mm 48-pin LQFP device. Each of the wires from this is around one quarter of a millimetre wide, with pins spaced every half millimetre. The large component on the left is a flash memory device, packaged in a 20x12mm 48-pin TSOP1 package (with pin dimensions the same as those of the LQFP device).
Two components show the limitations of the surface-mount technology. On the bottom of the photo is a 1-gang mechanical switch, which is typically operated by means of the tip of a pen. On the right side is a USB connector. Both of these must endure a moderate amount of external mechanical (mostly torsional) stress, inducing loads that exceed the very modest capacity of the solder bonds that hold on other SMT components. For this reason, both devices feature two solid black plastic plugs which fit into drilled holes in the circuit board. The USB connector further has two large metal vanes (connected to the grounded metal shell of the connector) which are also secured through PCB holes and further bonded with blobs of clear epoxy. Although this method of securing these devices is similar to the through-hole techniques that predate SMT, the electrical connections for both devices use the same solder pads as other SMT devices.
The circuit board is painted with a white decal layer that identifies the components (using codes that correspond with the devices on the circuit diagram from which the circuit board was derived). and shows their layout. In this board U3 denotes an integrated circuit package. The numerous tiny light-brown capacitors are denoted with C followed by a number (and outlined with round-ended boxes). The similarly tiny dark-brown resistors are indicated by an R followed by a number and outlined by a rectangular box. The layout software which produced the board tries to keep the numbers as close as possible to the corresponding device, but sometimes it must lay them out schematically some distance from the actual device (as is the case for the C11, C12, R5 decals, which are actually arranged to the right of the position of the corresponding components). The switch is mounted (electrically) on the reverse surface (where, unseen in this photo, is is denoted SW1, being the first and only switch on the board). Similarly the USB connector and the two through-hole test connectors (5 holes from one of these is shown in the bottom left corner) are denoted JP1, JP2, and JP3 (again unseen on the reverse).
To the right of resistors R1 and R2 are two vias, conductive copper channels holes drilled through the circuit board. These conduct the signals from the two resistors through the body of the circuit board either to an intermediate electrical layer or all the way through to components on the reverse side. Note that these appear to be coloured red; this is because, like the rest of the printed electrical circuitry on the board, they are covered with a thin coating of (in this case red) plastic insulator. It is possible to see the top conductive layer beneath the red insulator, particularly in the bottom area around C8 and C9.
For a fuller discussion of the components on this keydrive, see this article.
[edit] Licencing and authorship
This image is Copyright © 2004 by John Fader. It is dual-licenced under the following terms:
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. Subject to disclaimers. |
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This work is licensed under the Creative Commons Attribution-ShareAlike 2.0 License. |
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This file is an exact duplicate of another file from the Wikimedia Commons. Unless it is currently protected from editing, this media file may qualify for speedy deletion if it satisfies these conditions. |  |
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| current | 20:21, 7 December 2004 | 1,024×800 (184 KB) | John Fader (Talk | contribs) | (dual licence: {{GFDL}} and {{cc-by-sa-2.0}}) |
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