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Pcb assembly service that are provided by companies In assembly the bare board is populated with electronic components to form a functionalprinted circuit assembly(PCA), sometimes called a "printed circuit board assembly" (PCBA).
Inthrough-hole technologycomponent leads are inserted in holes.Insurface-mount technology(SMT) the
components are glued onpadsorlandson the surfaces of the PCB.
In both component leads are then mechanically fixed and electrically connected to the board bysoldering.
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There are a variety ofsolderingtechniques used to attach components to a PCB.High volume production
is usually done withSMT placement machineand bulk wave soldering orreflow ovens, but skilled
technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02 in.by0.01in.)
by hand under amicroscope, using tweezers and a fine tipsoldering ironfor small volume prototypes.
Some parts cannot be soldered by hand, such asBGApackages.
Often,through-hole and surface-mount construction must be combined in a single assembly because some
required components are available only in surface-mount packages, while others are available only in
through-hole packages. Another reason to use both methods is that through-hole mounting can provide
needed strength for components likely to endure physical stress, while components that are expected
to go untouched will take up less space using surface-mount techniques.For further comparison,
see theSMT page.
After the board has been populated it may be tested in a variety of ways:
While the power is off,visual inspection,automated optical inspection.JEDECguidelines for
PCB component placement,soldering,and inspection are commonly used to maintainquality control
in this stage of PCB manufacturing.
While the power is off,analog signature analysis,power-off testing.
While the power is on,in-circuit test, where physical measurements (for example, voltage) can
While the power is on,functional test, just checking if the PCB does what it had been designed
To facilitate these tests,PCBs may be designed with extra pads to make temporary connections.Sometimes
these pads must be isolated with resistors. The in-circuit test may also exerciseboundary scantest
features of some components. In-circuit test systems may also be used to program nonvolatile memory
components on the board.
In boundary scan testing, test circuits integrated into various ICs on the board form temporary
connections between the PCB traces to test that the ICs are mounted correctly.Boundary scan testing
requires that all the ICs to be tested use a standard test configuration procedure, the most common
one being the Joint Test Action Group (JTAG) standard.TheJTAGtest architecture provides a means
to test interconnects between integrated circuits on a board without using physical test probes.
JTAGtool vendors provide various types of stimulus and sophisticated algorithms, not only to
detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.
When boards fail the test, technicians maydesolderand replace failed components, a task
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