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76 SMT007 MAGAZINE I MAY 2018 can carry out accurate comparison analysis on all solder joints of components on both sides of a PCB board within a couple of seconds or two minutes, leading to a conclusion whether solder joints are qualified or not. BGA Assembly Process and Variation Source To use X-ray inspection system more effec- tively, control parameters of BGA assembly process and parameter control limitations have to be clarified. BGA assembly process conforms to the sequence in Figure 1. When BGA components' eutectic solder balls are assembled in solder paste during assembly process, their positions are usually corrected through self-alignment of liquid soldering tin. Thus, mounting precision seems not so essen- tial as fine-pitch lead components and the lead- ing control phase in BGA component assembly technology is solder paste printing and reflow soldering. In addition, variation in terms of solder joint shape and size is also associated with many other elements. It's almost impossible to eliminate all the variations, so the key point in manufactur- ing process control is to reduce variation in each manufacturing phase. The influence of different variations on final assembly products should be carefully analyzed and quantita- tively processed. With the whole process from BGA components to PCB assembly process considered, leading elements affecting solder joint quality are: • Volume of solder balls • BGA component pad size • PCB pad size • Solder paste volume • BGA component deformation during reflow soldering process • PCB deformation at BGA mounting area during reflow soldering process • Mounting placement accuracy • Reflow soldering temperature curve No matter what type of inspection device is used, there must be a basis when judg- ing whether solder joints are qualified or not. IPC-A-610C regulates the definition of accep- tance criteria of BGA solder joints in 12.2.12 item. Excellent BGA solder joints are required to be smooth, round, clear in edge and with no cavities. Diameter, volume, grayscale and contrast should be the same for all solder joints with position aligned and no displace - ment or twist. BGA Assembly Process Capability A type of BGA component is used as an example in the following discussion. This type of BGA components is PBGA (plastic ball grid array) with 520 pins and a size of 2-by-2 in, featuring eutectic solder balls and lever- aging no-clean flux. Six Sigma process capa- bility analysis is implemented to testify BGA placement accuracy, solder joint open circuits and short circuits occurring probability. The assumptions prior to calculation are: • No variations take place on BGA component pad or PCB pad • BGA components suffer from no deformation (reflow soldering process) • The average deviation is figured out in accordance with the average volume of solder joints after reflow soldering; • BGA component weight is assumed to be balanced by flotage and surface tension • Pad and eutectic solder balls should feature good solderability • All distributions are normal 1. BGA placement Standard SMT equipment is used to get BGA components mounted. Ordinary mount- Figure 1.