· What is pcb assembly ?
In assembly the bare board is populated (or "stuffed") with electronic components to form a functional printed circuit assembly (PCA), sometimes called a "printed circuit board assembly" (PCBA). In through-hole technology, the component leads are inserted in holes surrounded by conductive pads; the holes keep the components in place. In surface-mount technology (SMT), the component is placed on the PCB so that the pins line up with the conductive pads or lands on the surfaces of the PCB; solder paste, which was previously applied to the pads, holds the components in place; if surface-mount components are applied to both sides of the board, the bottom-side components are glued to the board. In both through hole and surface mount, the components are then soldered.
There are a variety of soldering techniques used to attach components to a PCB. High volume production is usually done with a "Pick and place machine" or SMT placement machine and bulk wave soldering or reflow ovens, but skilled technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02 in. by 0.01 in.) by hand under a microscope, using tweezers and a fine tip soldering iron for small volume prototypes. Some parts cannot be soldered by hand, such as BGA packages.
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 the SMT 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. JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.
· While the power is on, in-circuit test, where physical measurements (for example, voltage) can be done.
· While the power is on, functional test, just checking if the PCB does what it had been designed to do.
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 exercise boundary scan test 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. The JTAG test architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes. JTAG tool 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.
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