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SMT007-July2019

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30 SMT007 MAGAZINE I JULY 2019 The main standard for acceptance criteria of defects in electronics assemblies is J-STD-001. IPC-610 provides a visual representation of the acceptance requirements established in J-STD- 001. This standard (J-STD-001) has very few photos and is full of text and tables targeted at quality and process engineers. If you go through these standards, there are hundreds of different types of defects. There are week-long classes and certification programs for these standards. However, if you think about it, there are only two types of defects that matter for the functioning of electronic assemblies: shorts or bridges between adja- cent leads or opens between adjacent leads. All of the test methods, such as in-circuit test (ICT) and functional tests only look for shorts and opens because other types of defects can- not be detected by these tests and they don't affect the functionality of the boards. Thus, it goes without saying that we can simplify IPC- 610 and J-STD-001, but don't hold your breath . What Defect Should You Aim For? As I just mentioned, ICT and functional tests can only find opens and shorts; they are not looking for any other types of defects because they cannot find them. If the defects that you see don't fall into open or short category, you can safely classify them as "others." Exam- ples of "others" are insufficient, tombstoning, drawbridge, part movement, etc. Bridge is the most objective type of defect. No two people will disagree if you show them a bridge. Meanwhile, opens, especially if hidden, can easily escape even ICT and functional tests because they are most likely to make intermit- tent connections due to pressures exerted dur- ing bed of nail testing or ICT. There is another type of open called insuf- ficient solder joint. Many people will disagree with me calling insufficient joint an open joint, but we all agree that insufficient solder joint is not a total open today but will most likely become an open tomorrow or months later. Therefore, we might as well call it an open. Unlike bridges, insufficient solder joint is the most subjective defect you can find. It is very likely that two well-trained inspectors will dis- agree whether an insufficient solder joint is acceptable or not. To be on the safe side, you might as well consider it an open. Industry Findings: Which Defect Is More Prevalent? In my last column, I quoted a paper by Stig Oresjo [1] and Table 1, which shows the level of defects depending on the pitch of a gull-wing device. Based on Table 1, it is obvious that lower pitches will result in much higher defects because lower pitch devices increase the com- plexity of the manufacturing processes, such as handling, printing, placing, and soldering. The same paper by Oresjo also analyzed the types of defects for all the components: • Opens: 46% • Shorts: 22% • Insufficient: 17% As I noted earlier, insufficient solder joint is an open about to happen in the near future. So, if we combine open and insufficient, almost two-thirds of defects (63%) are opens, 22% are shorts, and the remainder are 11%. This is the average of all types of components in Orse- jo's paper. The results in the paper on gull-wing devices were even more significant: • Opens: 65% • Insufficient: 11% • Shorts: 16% • Others: 8% If you combine opens and insufficient, the number is alarming: 76%. Shorts are only 16% of the total. Table 1: QFP defect levels vs. pitch.

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