Issue link: https://iconnect007.uberflip.com/i/1124788
68 SMT007 MAGAZINE I JUNE 2019 The biggest impact on defects is the pitch of a component. defects. A much lower first-pass yield in a com- plex board may be a better indicator of qual- ity than a higher first-pass yield in a simpler board. We should use both methods to bench- mark our quality level. Component Types, Pitch, and Defect Levels It is important to know the relationship between the type of components and the defects levels in them. For example, one of the interesting findings in the study was close rela- tionships between types of components and defects. For example, even though we have almost 70 years of experience in wave solder- ing through-hole components, they had a very high defect level (4,000 PPM). Compare that to 1,400 PPM for gull-wing devices and only 600 PPM for BGAs. The average of all these compo- nents was 1,100 PPM. The biggest impact on defects is the pitch of a component. Pitch is defined as the distance between the centers to the center of adjacent pins, not the distance between adjacent pins. Pitch of a component determines the selection of solder ball powder size, the type of paste, and the kinds of printers and pick-and-place machines you need to process them. For exam- ple, the study found that PPM levels varied for different pitches of a gull-wing device. The findings are in Table 1. It goes without saying: Be careful when using pitches below 20 mils or 0.5 mm. Targeting Defect Levels in Your Products I have seen a very low level of defects, such as 6 PPM in one Intel product and 4.5 PPM at one EMS company in Malaysia. However, even at these companies, not every product had such a low defect rate. While less than 50 PPM is very rare, it is not uncommon or impossible to have less than 100 PPM. However, as high- lighted in my May 2019 column, you need to not only have in-house written DFM and pro- cess documents but also (and more impor- tantly) follow them. Also, as mentioned earlier in this column, some components, such as through-hole for wave soldering and gull-wing components with pitches below 0.5 mm, will make it harder to achieve lower PPM levels. Use such compo- nents only when you don't have other options. Achieving the low PPM defect level is impor- tant but not sufficient for high yield and reli- ability. You need to have the right kind of defect, which will be the subject of my next column in July. Stay tuned; it's only one month away. SMT007 References 1. Oresjo, S. "Year 1999 Defect Level and Fault Spectrum Study," SMTAI Proceedings, 2000. Ray Prasad is the president of Ray Prasad Consultancy Group and author of the textbook Surface Mount Technology: Principles and Practice. Prasad is also an inductee to the IPC Hall of Fame—the highest honor in the electronics industry—and has decades of experi- ence in all areas of SMT, including his leadership roles implementing SMT at Boeing and Intel; helping OEM and EMS clients across the globe set up strong, internal, self-sustaining SMT infrastructure; and teaching on-site, in-depth SMT classes. He can be reached at smtsolver@rayprasasd.com and has an upcoming SMT class July 22–24, 2019. More details at www.rayprasad. com. To read past columns or contact Prasad, click here. Table 1: QFP defect levels vs. pitch.