1-36
LEVEL III.—Drawers or pull-out chassis are level III units, as shown in figure 1-36. These are
designed for accessibility and ease of maintenance. Normally, circuit cards associated with a particular
subsystem will be grouped together in a drawer. This not only makes for an orderly arrangement of
subsystems but also eliminates many long wiring harnesses. Defective cards are removed from such
drawers and defective components are repaired as described in level II.
LEVEL IV.—Level IV is the highest level of packaging. It includes the cabinets, racks, and wiring
harnesses necessary to interconnect all of the other levels. Other pieces of equipment of the same system
classified as level IV, such as radar antennas, are broken down into levels 0 to III in the same manner.
During component troubleshooting procedures, you progress from level IV to III to II and on to level
0 where you identify the faulty component. As you become more familiar with a system, you should be
able to go right to the drawer or module causing the problem.
Q40. Resistors, capacitors, transistors, and the like, are what level of packaging?
Q41. Modules or submodules attached to a mother board are what packaging level?
Q42. What is the packaging level of a pcb?
INTERCONNECTIONS IN PRINTED CIRCUIT BOARDS
As electronic systems become more complex, interconnections between components also becomes
more complex. As more components are added to a given space, the requirements for interconnections
become extremely complicated. The selection of conductor materials, insulator materials, and component
physical size can greatly affect the performance of the circuit. Poor choices of these materials can
contribute to poor signals, circuit noise, and unwanted electrical interaction between components. The
three most common methods of interconnection are the conventional pcb, the multilayer pcb, and the
modular assembly. Each of these will be discussed in the following sections.
Conventional Printed Circuit Board
Printed circuit boards were discussed earlier in topic 1. You should recall that a conventional pcb
consists of glass-epoxy insulating base on which the interconnecting pattern has been etched. The board
may be single- or double-sided, depending on the number of components mounted on it. Figures 1-37 and
1-38 are examples of conventional printed circuit boards.
Multilayer Printed Circuit Board.
The multilayer printed circuit board is emerging as the solution is interconnection problems
associated with high-density packaging. Multilayer boards are used to:
reduce weight
conserve space in interconnecting circuit modules
eliminate costly and complicated wiring harnesses
provide shielding for a large number of conductors
provide uniformity in conductor impedance for high-speed switching systems
