3. How are the discrete components attached? They have to be identified along with sockets, pins, connectors, and mounting devices. 4. What are the testing and troubleshooting con- siderations? They must be taken into account to make the board accessible for test measurements of all signals. 5. Will the board be mounted in an enclosure? How much space it will use and how it will be attached must be taken into consideration.
One approach to a printed circuit board design lay- out is to use paper dolls of each component that is to be mounted on the printed circuit board . Each paper doll is the exact size of the base of the com- ponent being used and permits correct component placement. Using the paper dolls, lay out the printed circuit board in such a way that the paper dolls duplicate the schematic diagram. During the layout process, the printed cir- cuit board will be viewed from the component side or top. Three issues address the best component placement: 1. Wastedboardspace—Keepcomponentsinahori- zontal or vertical plane, parallel to the board edge, to achieve a tighter layout. Avoid diagonal placements. 2. Insertion of polarized components—Try to group polarized components facing the same direction to aid assembly. 3. Ease of troubleshooting—Components should be easy to find; try to keep input on the left side of the board, output on the right, and power from top to bottom of the board.
ETCHING PRINTED CIRCUIT BOARDS There are two major techniques to remove copper from a copper-clad board to form a printed circuit board. The first technique involves using a mild acid such as ferric chloride or ammonium persulfate . The other technique uses a CAD-type program and a computer numerical control (CNC) machine to remove the copper and is referred to as a nonchemical process. To use the tray method, the copper-clad board with applied resist is etched in a glass tray by placing the board onto the surface of the acid. The copper side of the board should face toward the acid. Float it on the acid for approximately 15 to 20 minutes. The acid laden with copper sinks to the bottom, and fresh acid replaces it. In this fashion, no agitation is required. A different technique uses a 1-inch square of sponge to dab the acid on the printed circuit board. The acid is absorbed in the sponge by pouring a small amount on it. The sponge is dabbed continuously on the copper on the board until the copper is removed. Boards with thinner copper etch faster.
SOLDER AND SOLDERING IRONS Soldering is the joining of two pieces of metal with a solder alloy having a melting point below 800° Fahr- enheit (427oC). Solder used to include a combination of tin and lead in ranges from pure tin to pure lead and including all proportions in between. In elec- trical soldering, the alloy mix is usually 60% tin and 40% lead (60/40) or 63% tin and 37% lead (63/37) (Figure 49-7). For soldering
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- Winter '18
- Printed circuit board, Printed circuit boards