onto the prepared pad array requires skillful use of a highly accurate vision-alignment system with high resolution and magnification. The smaller the pitch and size of the components, the more precise working must be. Finally the newly placed SMD is soldered onto the board. Reliable solder joints are facilitated by use of a solder profile which preheats the board, heats all the connections between the unit and the PCB to the melting temperature of the solder used, then properly cools them. High quality demands or specific designs of SMDs require the precise application of solder paste before positioning and soldering the unit. The surface tension of the molten solder, which is on the board's solder pads, tends to pull the device into precise alignment with the pads if not initially positioned totally correctly. Reflowing and reballing Ball grid arrays (BGA) and chip scale packages (CSA) present special difficulties for testing and rework, as they have many small, closely spaced pads on their underside which are connected to matching pads on the PCB. Connecting pins are not accessible from the top for testing, and cannot be desoldered without heating the whole device to the melting point of solder. After fabrication of the BGA package, tiny balls of solder are glued to the pads on its underside; during assembly the balled package is placed on the PCB and heated to melt the solder and, all being well, to connect each pad on the device to its mate on the PCB without any extraneous solder bridging between adjacent pads. Bad connections produced during assembly can be detected and the assembly reworked (or scrapped). Imperfect connections of devices which are not themselves faulty, which work for a time and then fail, often triggered by thermal expansion and contraction at operating temperature, are not infrequent. Assemblies which fail because of bad BGA connections can be repaired either by reflowing, or by removing the device and cleaning it of solder, reballing, and replacing. Devices can be recovered from scrapped assemblies for reuse in the same way. Reflowing as a rework technique, similar to the manufacturing process of reflow soldering, involves dismantling the equipment to remove the faulty circuit board, pre-heating the whole board in an oven, heating the non-functioning component further to melt the solder, then cooling, following a carefully determined thermal profile, and reassembling, a process which is hoped will repair the bad connection without