PCB PRACTICALITIES
The second aspect of service after fault diagnosis is repair and component replacement. Ease of repair and access varies enormously between different make, type and vintage of equipment. Unsoldering of PCB-mounted components, especially multi-legged
types, is most easily done with a small hot soldering iron and commercially prepared desoldering wick, which consists of resin- impregnated copper braid to absorb old solder as it is melted. A useful alternative, especially for use with double-sided and plated- through-hole boards, is the solder pump, a spring-loaded vacuum device; the most effective is the type in which it is combined with a mains desoldering iron whose bit is made in the form of a hollow nozzle.
The very tightly packed double-sided PCBs used in cameras, cam- corders and portable videorecorders are the most difficult to deal with – see Fig. 1.3. Many of their components are SMD (Surface- Mounting-Device) types, which are soldered direct to the print panel on both sides of the board. These pinhead-size transistors, resistors, links and capacitors are glued to the board surface in manufacture, then soldered to the print in a bath-wave operation. To remove them, one or more small hot soldering irons are required to simultaneously release all connections. For multi-legged SMDs like miniature ICs it is usually necessary to use a specially designed desoldering station with hot-air blower and heat shields for both de-soldering and subsequent fitting of the replacement chip. It is important to be sure of the diagnosis before changing components on these high-density assemblies, and to closely consult the manufacturer’s service information in all cases.