What is PCB?
A PCB is a printed circuit board, also known as a printed wiring board. It is used in electronics to build electronic devices. A PCB serves two purposes in the construction of an electronic device; it is a place to mount the components and it provides the means of electrical connection between the components.
A PCB starts out as a thin, non-conducting sheet of material. The most common material used is a glass fiber epoxy laminate material. A thin layer of copper is then chemically deposited on each side of this material.
The next step is to â€œprintâ€ the connection diagram onto the PCB. The connection diagram is the wiring required to connect the components. In the very early days of electronics, these connections were in fact done with wires. This is the reason PCBs are also sometimes referred to as printed wiring boards. The â€œprintingâ€ is usually done by photographically transferring the image to the board. This image is â€œprintedâ€ with an acid resistant material.
What is PCBA?
PCBA is know as a printed circuit board assembly. In assembly the bare board is populated with electronic components to form a functional printed circuit assembly(PCA), sometimes called a â€œprinted circuit board assemblyâ€ (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 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 BGApackages.
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 controlin this stage of PCB manufacturing.
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 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.
When boards fail the test, technicians may desolder and replace failed components, a task known as rework.