Assessibility

Generally, PCBs can be grouped into four types: through-hole, surface-mounted, mixed, and flex-print. Prior to the appearance of ball-grid-array (BGA) components, majority of the PCBs have little or no accessibility issues since the component leads or pins are visibly soldered to the board by machine or by hand. There are exceptions, however, when a manufacturer decides to apply some kind of epoxy compound over certain parts of the PCB to provide support for weak spots or socketed components, or simply to mask off sensitive information to keep prying eyes away or discourage repair and rework.

SMT technology has, however, evolved to the point where even more precious board space savings can be achieved on the PCB, by transforming the through-hole pin-grid-array (PGA) a step further to the surface-mount BGA. Such progress is seen as inevitable and necessary in engineering expediency and business sense, but not necessarily appreciated and welcomed by the test and repair communities, since it severely hampered the ability to test and repair this type of PCBs using traditional methods and equipment.

The often hyped alternative solution to this predicament is boundary-scan or JTAG testing. Granted, many new ICs including the BGA types have in-built JTAG circuitries, but ensuring that a PCB meets the JTAG requirement will take additional efforts on the designer's part to implement to realize the design-for-testability (DFT) goal.

Accessibility of probe points is thus an important factor to consider before reverse engineering work can commence. You should therefore make an estimate of how much of the PCB's component leads or pins are accessible, compared to those that have been obstructed, hidden, or even buried intentionally. One other factor that adds to the problem of accessibility is conformal coating; we will look at it separately since it's quite a broad subject in its own right.