Page 124 - Building A Succesful Board-Test Strategy

10 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

Figure 3-12 Many electrically identical components appear different to an AOI
system, (Courtesy GenRad.)

AOI suffers disadvantages when compared to conventional test as well.
Inspection times are generally longer because of the time required for x-y posi-
tioning and image evaluation. AOI cannot find numerous fault types that test can
find easily, and many users report a significant increase in false failures. Unlike con-
ventional test, an AOI system requires that the areas under scrutiny be visible by
line of sight, and a board containing components on both sides requires two inspec-
tion steps. (Instead of inspecting the same board twice, some manufacturers pass
the board between two AOI systems, thereby examining both board sides at the
same time.) Constructing a reasonably accurate inspection "program" requires a
good board or good-board simulation, not always easy during the often fast-chang-
ing period of final preparation for production.
Some companies are turning to AOI systems to examine BGAs before placing
them on boards. This inspection step confirms the existence and position of the
solder balls, as well as their diameter. Insufficient solder in the balls will likely pass
an electrical test after reflow, yet may pose a reliability problem for customers.
Even tested BGAs can lose solder balls or experience other problems during
handling. Other analysis techniques used after board assembly (such as x-ray)
can be expensive or impractical for detecting this situation. Adopting this step
can reduce the number of scrapped boards and devices, thereby lowering manu-
facturing costs.