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PCB Assembly Equipment Selection
Feb 28, 2018

  In the electronics manufacturing industry, printed circuit board (PCB) components are getting smaller and smaller, the assembly density is higher and higher has become a continuous trend of development. This trend is not necessarily due to the need to make PCB components smaller, but because the new design drastically uses more ball grid array packages (BGA) and other devices that have hidden solder connections, such as quad flat no-leads Package (QFN) and Bar Grid Display Package (LGA). Such devices typically offer some advantages in terms of performance and cost compared to larger pin packages, so the trend toward smaller, closer designs may continue.

  Automatic Optical Inspection (AOI) is a proven, key process control technology in the SMT industry that greatly enhances confidence in finished product quality. But what about the device's soldered connections that can not be seen with the naked eye? X-ray detection is the answer you are looking for.

  X-rays are used as a process control method to eliminate this risk, as the "hidden connection" devices are misplaced and the components produced are either not repairable or expensive to repair. Rework of misplaced components not only takes time, but can also cause other problems on the components, such as problems with surrounding components or PCBs due to localized heating. Rework is also likely to exceed the maximum number of reflow cycles that a duplex component can withstand. As a result of a late process failure, such as JTAG or functional testing, diagnostic and retesting can result in additional time and expense.

  So when should you use X-ray? Of course it should be used during the "first" test, which ensures that the heating curve of the furnace used is the best solution for leadless devices. After that, it is wise to pick samples for testing throughout the production of the assembly; usually several samples are taken at the beginning, middle, and end of a batch to start production. The alternative is to use an "online" process, but note that X-ray detection (even if automated) is relatively slow. During operation, the placement of non-pin devices (especially BGA) is simple and usually does not cause any problems, so X-ray detection should be used with caution.

  Figure 1: X-ray inspection without the aid of potentially destructive rework or microsection

X-ray inspection also reduces the need for manual inspection of the end of the line, such as fine-pitch devices that can not be fully tested with AOI, depending on the type of system used, or other BGA inspection methods.

  Another advantage of X-ray inspection is that it can solve quality problems. X-ray inspection eliminates the need for potentially damaging rework or microsection, both of which add cost and void components. Microscoping also requires the operator to guess where the problem lies based on what he or she has learned.

  You often hear someone say "it did not pass the test, I do not know where the problem lies, so it is definitely a problem with BGA"? The enhanced X-ray can provide X-ray layered imaging, or full 3D functionality, that examines the entire assembly to find any problems such as PCB layout or hole wall breakage, as well as lead-free components.

  In addition to the PCBA, X-rays can also perform non-destructive testing on other manufactured components, such as cable assemblies or other machined parts that require internal details. Even a certain degree of measurement can be carried out.