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24 SMT007 MAGAZINE I JANUARY 2018 Feature Article by Russell Poppe JJS MANUFACTURING There is a continuing trend towards smaller, more densely-populated printed circuit board (PCB) assemblies in electronics manufac- turing. This is not necessarily because the PCB assembly needs to be smaller, but new designs are using significantly more ball grid array (BGA) and other types of devices with hidden solder connections, such as quad flat no-leads (QFN) and land grid arrays (LGAs). Such devices often have performance and cost advantages over larger packages with leads, so the trend is likely to continue. Automated optical inspection (AOI) is an established, key process control in the SMT industry that greatly increases confidence in the quality of the finished product. But what do you do about devices where you cannot opti- cally see the solder connections? X-ray inspec- tion provides the answer. Using X-ray as an in-process control can help remove the risk of producing assem- blies that are impossible or uneconomical to repair due to misplaced 'hidden connection' devices. Reworking a misplaced device can be time consuming and may cause other issues on the assembly, for example with surround- ing components on the PCB due to local heat- ing. Rework might also exceed the maximum number of solder reflow cycles allowed for double-sided assemblies. Finding a failure later in the process, for example at JTAG or functional test, incurs additional lost time and cost in diagnosis and re-test. So, when should you use X-ray? It should certainly be part of the 'first-off' inspection process, helping to ensure the oven profile is optimal for the leadless devices. It might then be sensible to check a sample of assemblies as they go through production; a few from the start, middle and end of the batch is typi - cal. Alternatively, an 'in line' process might be used, though it is worth noting that X-ray inspection—even if automated—is relatively slow. In practice, placing leadless devices, especially BGAs, is quite straightforward and normally causes few issues, so X-ray should be employed thoughtfully. X-ray inspection can also help reduce end- of-line manual inspection, for example on fine- pitch devices that cannot be fully covered by AOI (depending on what type of system you have), or where other BGA inspection methods such as an Ersascope might have been used. Another great benefit of X-ray inspection is in resolving quality issues. X-ray allows inspec-

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