defect of the month
Resolving Intermittent Area Array Packages with Video Simulation
Excessive temperature or moisture may be to blame.
Open connections on area array packages can be difficult to spot, particularly if they are intermittent electrical failures like the examples in FIGURE 1. One or more open joints can occur between the package and solder sphere or at the PCB pad interface. Reflow soldering with either convection or vapor phase can cause packages to move and separate. This can be caused by warping of the package or in some cases minor popcorning due to moisture. Both faults can be simulated and recorded with video for reference.
Opens on a BGA package
BGA package closeup
Figure 1. Opens on a BGA package.
Secondary reflow of solder joints can cause intermittent connections of packages. This occurs when adjacent packages are reworked or when the board passes through wave or selective soldering under excessive soldering temperature. Using video simulation in the process can be helpful to solve process defects. In such instances, while soldering boards, the operator is also observing reflow optically or with x-ray. See for an example.

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Bob Willis
is a process engineering consultant; His column appears monthly.