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MAY 2018 I SMT007 MAGAZINE 77 ing equipment is capable of recognizing BGA eutectic solder ball images with placement process capabilities covered as in Figure 2. Based on the above data, the maximum placement deviation is 6.53mil when process capability is Six Sigma. Since diameter of pad is 28 mil, the placement deviation can be neglected among components' self-alignment deriving from surface tension when solder paste is melted. When it comes to BGA compo- nents placement process, it conforms to Six Sigma level. 2. Solder joints with open circuits Assembly process tends to see open solder joints due to insufficient eutectic solder ball collapse. As far as PBGA with 520 pins are concerned, eutectic solder balls are balls with diameter of 30 mils whose standard deviation is 5003 mils (with volume participated in) and volume is regulated to be 14,1303 mils. Diam- eter of BGA and PCB pad is 28 mil with its solder paste thickness being 6 mil. Therefore, the average height of BGA solder ball edge is approximately 24 mils. As Six Sigma capabil- ity reflecting solder ball volume variation is concerned (Figure 3). After reflow soldering, the height of solder- ing bonding support determined by average volume of solder joints is 19 mils. As process capability is set to be Six Sigma, solder paste thickness is measured to be 4 to 8 mils. More- over, BGA solder balls will be collapsed into solder paste for 3 mils, which leads to the following calculated data: • Minimum thickness of solder paste below solder balls = 3 mils • Minimum collapse = 7 mils • Minimum incorporated collapse = 10 mils • Minimum security deviation generated to stop open circuits from taking place = 2.2 mils When above variations can be controlled into certain ranges, BGA reflow soldering process can achieve Six Sigma. Unfortunately, deformation at BGA compo- nents and PCB usually leads to height incon- sistency of soldering bonding during BGA reflow soldering assembly. BGA components and PCB pad features differences that lead to process variation. All in all, even though all variations are taken into consideration, open soldering joints will still take place. Thus, X-ray inspection system can be used to carry out defect inspection on open solder joints. 3. Solder joint bridging (short circuits) The same method can be used to estimate the influence of short circuits of soldering joints on assembly process capability. Solder joints differ from each other in terms of diam- eter and measured data indicates that bonding volume of each solder joint is in the range from 12800 to 192503 mils under Six Sigma process capability. As a result, the height of minimum soldering bonding support is 15 mils and then maximum soldering bonding diameter can be as much as 38.5 mils. When it comes to BGA components with pitch of 50 mils, solder joint bridging will hardly take place. Statistical Process Control Analysis Effective BGA assembly process control leads fewer variations occurring to solder connec- tions. In practical assembly process, however, Figure 2.

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