A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board (PWB) or etched wiring board. Printed circuit boards are used in virtually all but the simplest commercially produced electronic devices.
A PCB populated with electronic components is called a printed circuit assembly (PCA), printed circuit board assembly or PCB Assembly (PCBA). In informal use the term "PCB" is used both for bare and assembled boards, the context clarifying the meaning.
Alternatives to PCBs include wire wrap and point-to-point construction. PCBs must initially be designed and laid out, but become cheaper, faster to make, and potentially more reliable for high-volume production since production and soldering of PCBs can be automated. Much of the electronics industry's PCB design, assembly, and quality control needs are set by standards published by the IPC organization.
2.5.2 PCB Design Process & Workflow:
There are 11 steps to the PCB design process & work flow which we cover in the PCB design guide.
Step 1: Finalize your Circuit Design – Everything starts with the circuit design. Without a circuit there is no need for a PCB. In the old days most circuits were hand drawn and later captured electronically. In today’s world of modern computing, the circuit design is capture directly into a schematic. For the sake of clarity we’ve added this as step 1 in the PCB design guide.
Step 2: Choose PCB Design Software – Many times electrical engineers don’t have a choice when it comes to choosing the PCB design software. Their companies have invested thousands into software packages and all their legacy design are captures in those packages, and therefore are “stuck”. However the hobbies has several choices. Its important to choose a package that is first and foremost easy to use, but also capable of completing the PCB design as some packages won’t be able to handle the complexity.
Step 3: Capture Your Schematic – As mentioned earlier its likely that the circuit design is being captured electronically from the start. In general “capturing the schematic” is the process by which each component is drawn electronically and are interconnected with each other.
Step 4: Design Component Footprints – Once the schematic is complete its time to draw the physical outline of each of the components. These outlines are what are placed on the PCB in copper to allow the components to be soldered to the printed wiring board.
Step 5: Establish PCB Outline – Each project will have restrictions related to the board outline. This should be determined in this step since an idea of component count and area should be known.
Step 6: Setup Design Rules – With the PCB outline and PCB footprints complete is just about time to start the placement. Before placement thought you should setup the design rules to ensure that components or traces aren’t to close together. This is only one example as there are probably hundreds of different rules that can be applied to a PCB design.
Step 7: Place Components – Now its time to move each component onto the PCB and begin the tedious work of making all those components fit together. This is where you’ll find that PCB design is really a jigsaw puzzle.
Step 8: Manual Route Traces – It’s necessary to manually route critical traces. Clocks. Power. Sensitive analog traces. Once that’s complete you can turn it over to Step 9.
Step 9: Using the Auto Router – There are a handful of rules that will need to be applied for using an auto router, but doing so will save you hours if not days of routing traces.
Step 10: Run Design Rule Checker – Most PCB design software packages have a very good setup of design rule checkers. It’s easy to violate PCB spacing rules and this will pinpoint the error saving you from having to respin the PCB.
Step 11: Output Gerber Files – Once the board is error free it’s time to output the Gerber files. These files are universal and are needed by the PCB fabrication houses to manufacture your printed circuit board.
Chemical etching is done with ferric chloride, ammonium per sulfate, or sometimes hydrochloric acid. For PTH (plated-through holes), additional steps of electro less deposition are done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.
The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recalculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates.
The etchant removes copper on all surfaces exposed by the resist. "Undercut" occurs when etchant attacks the thin edge of copper under the resist; this can reduce conductor widths and cause open-circuits. Careful control of etch time is required to prevent undercut. Where metallic plating is used as a resist, it can "overhang" which can cause short-circuits between adjacent traces when closely spaced. Overhang can be removed by wire-brushing the board after etching
2.5.4 Component assembling
After the printed circuit board (PCB) is completed, electronic components must be attached to form a functional printed circuit assembly, or PCA (sometimes called a "printed circuit board assembly" (PCBA). In through-hole construction, component leads are inserted in holes. In surface construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.
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.
Soldering is the process of joining two or more similar metals by melting another metal having low melting point .In order to make solder accept the solder readily, the component terminals must be free from oxides and other obstructing films. Soldering flux cleans the oxides from the surface of the metal. The leads are cleaned chemically or by scrapping using a blade or a knife, small amount of lead is coated on the leaned portion of the lead and the bit of the soldering iron. The solder used is an alloy of tin and lead. Soldering iron is used to melt the solder and apply at the joints in the circuit. It operates at 230 v supply. Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the work piece. Soldering differs from welding in that the work pieces are not melted.
Figure 2.4 Soldering On PCB
There are two forms of soldering, each requiring higher temperatures and each producing an increasingly stronger joint strength:-
In the soldering process, heat is applied to the parts to be joined, causing the solder to melt and to bond to the work pieces in an alloying process called wetting. In stranded wire, the solder is drawn up into the wire by capillary action in a process called wicking. Capillary action also takes place when the work pieces are very close together or touching. The joint strength is dependent on the filler metal used, where soft solder is the weakest and the brassalloy used for brazing is the strongest. Soldering, which uses metal to join metal in a molecular bond has electrical conductivity and is water and gas-tight. There is evidence that soldering was employed up to 5000 years ago in Mesopotamia.
Hand-soldering tools include the electric soldering iron, which has a variety of tips available ranging from blunt to very fine to chisel heads for hot-cutting plastics, and the soldering gun, which typically provides more power, giving faster heat-up and allowing larger parts to be soldered. Hot-air guns and pencils allow rework of component packages which cannot easily be performed with electric irons and guns
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