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58 SMT Magazine • June 2014 arTICLe The increasing density of modern assem- blies imposes new requirements on the packag- ing technology, especially BGA housings. How- ever, the terminals of those components escape physical probing and visual inspection. This gives rise to the question: How can technolo- gists ensure the quality of solder joints with ex- tremely reduced access? All That glitters is not gold The ever-progressing employment of surface mount technology (SMT) was further accelerat- ed by the introduction of BGA housings in the mid-1980s. This package's terminals are solder balls on its bottom side. Compared to wired ICs, BGA technology provides many advantages, such as: • smaller packages • increased packaging density • increased pin density • improved signal transmission characteristics • improved thermal linkage with the board. The latest packages of this type, e.g., VFBGA (very fine BGA), permit several thousand pins and a pitch of less than 0.5 mm. BGAs are as- sembled in a soldering process with many influ- encing parameters. This process normally leads to a partly matte-finished solder joint, which has to meet various mechanical and electrical criteria: • strong bonding between ball and board • high mechanical long-term stability • high structural integrity of the ball • high conductivity • high electrical signal integrity, and • high insulation strength between neighbouring pins. Even at this early point of discussion, the in- teraction between physical conditions and the resulting electrical properties is quite obvious. by Thomas Wenzel and Andreas Türk GoepeL eLeCTronIC The Trouble with BGA Solder Joints

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