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Rework (electronics)

Rework (or re-work) is the term for the refinishing operation or repair of an electronic printed circuit board assembly (PCBA). Mass processing techniques are not applicable to single device repair or replacement, so specialized techniques are required to replace defective components Ц most notably area array packages such as Ball grid array (BGA) devices. Reasons for rework X-ray picture of insufficient solder joints Insufficient or excessive solder joints that are created during assembly operations. Defective devices. Engineering parts changes, upgrades, etc. [edit]Hit the aim step by step Thermal profile of a lead free solder process Normally the entire rework includes a few adaptations, which must be done one by one. Obtaining a successfully reworked board requires of course no damage on surrounding units and the PCB itself. Consequently all parts of the treated conductor board are protected during the different processes. Further it is necessary to keep the thermal strain of the electronic assembly as low as possible and prevent unnecessary contractions of the board durability. [edit]The value chain of rework Dispensed solder paste on the pads of a QFPs Mask and spheres for reballing Separating a single SMD from the PCB after all the solder joints between these two parts have been molten is generally the first step and called desoldering. Afterwards the pad array on the conductor board is cleaned from old tin. It is quite easy to remove these rests by heating them up into the area of melting temperatures. In principle the precise placement of

the new unit onto the prepared pad array requires a highly accurate vision alignment system with high resolution and magnification which is controlled by a skilled operator. The smaller the pitch of the components the higher the demand on the precision equipment for the alignment. Finally the just now placed SMD is soldered on the board. Reliable and high quality solder joints are created by an adapted solder profile which heats all the connections between the unit and the PCBA up to the area of melting temperatures and cools them properly down. [edit]Extras X-ray picture of good solder joints. Good solder joints between BGA and PCBA The aforementioned value chain of rework can be supplemented by a few eminently processes. High quality demands or specific designs of SMDs require the application of solder paste before positioning and soldering the unit. Consequently much technical effort is needed to apply the new tin where required in the manufacturing of the board. Sometimes the desoldered BGA or CSP can be reused. The solder balls are removed and the sites "dressed" using a variety of different techniques. Most commonly used are solder braid and a soldering iron or heat based vacuum-based extraction with new spheres provided to the adjacent joints. This operation is called УreballingФ. Reballing of a BGA can be done via several methods including: 1. Using a set of mechanical fixtures for both the balls and the solder paste or flux 2. Using a BGA "preform" with embedded balls corresponding to the device pattern 3. Using semi-automated or fully automated machinery