UNIT 7
Printed Circuit Board
Step 1: Design and Output
Circuit boards should be well-matched with, a PCB layout created by the designer using PCB design software. Universally used PCB design software includes Altium Designer, OrCAD, Pads, KiCad, Eagle, etc. formerly the PCB plan is agreed for manufacture, designers export the design into set-up their manufacturers support. The most recurrently used program is called extended Gerber.
PCB design software encodes information with copper tracking layers, drill drawing, apertures, component notations, and other options. All aspects of the PCB design experience checks at this point. The software performs oversight algorithms on the design to make certain that no errors go unobserved. Designers also inspect the arrangement with a view to elements concerning to track width, board edge spacing, trace and hole spacing, and hole size.
After a detailed inspection, designers promote the PCB file to PC Board Houses for production.
Step 2: From File to Film
PCB printing begins following designers output the PCB schematic files and manufacturers conduct a DFM check. Manufacturers use a particular printer called a plotter, which makes photo films of the PCBs, to print circuit boards. Plotters use exact printing technology to offer a thorough film of the PCB design.
The concluding product results in a plastic sheet with a photo negative of the PCB in black ink. Black ink represents the conductive copper parts of the PCB for the inner layers of PCB. The area of non-conductive material is denoted by the image of the remaining clear portion. The external layers pursue the reverse pattern: clear for copper, black refers to the region that'll be imprinted away. The plotter automatically develops the film, and the film is securely stored to prevent any unwanted contact.
Both layer of PCB and solder mask receives its individual clear and black film sheet. To attain the ideal grouping of all films, registration holes should be punched from side to side. The exactness of the hole occurs by adjusting the table on which the film sits.
Step 3: Printing the Inner layers:
The fundamental appearance of PCB comprises a protect board whose core material is substrate material. A powerful and dust-resistant starting point for the PCB is provided by substrate material. Copper is pre-bonded on both sides. The process involves whittling away the copper to divulge the design from the films.
The copper-sided shield is cleaned and passed into decontaminated surroundings. For the duration of this phase, no dust particles must settle on the laminate. Else Dirt may cause a circuit to be short or remain open.
The clean board receives a layer of a photo-sensitive film called photoresist. The photoresist comprises a layer of photoreactive chemicals that solidify after exposure to ultraviolet light ensuring a precise match from the photo films to the photoresist.
The photoresist on the copper is hardened when UV light passes through the clear parts of the film. The black ink from the plotter prevents the light from the attainment of the areas not meant to solidify, and they are slated for elimination.
An alkaline solution is used to wash the board that removes any photoresist left unhardened. The board is then dried.
The product emerges with resist precise covering the copper areas meant to remain in the final form. The board is examined by the technician to ensure that no errors occur during this stage. All the resist present at this point denotes the copper that will materialize in the finished PCB.
Step 4: Removing the Unwanted Copper
The copper solvent solution bath removes all of the uncovered copper. In the meantime, the preferred copper remains fully protected beneath the hardened layer of photoresist.
Now the solvent removes the unwanted copper, the hardened resist protecting the chosen copper wishes washing off through another solvent. The board now glistens with only the copper substrate necessary for the PCB.
Step 5: Layer Alignment and Optical Inspection
To ensure the layers are aligned it requires alignment punches. The layers are placed into a machine called the optical punch, which permits an accurate association so the registration holes are correctly punched.
Once the layers are placed together, it's impossible to correct any errors occurring on the inner layers. An automatic optical examination of the panel is done by machine to authenticate that it has no defects. The original Gerber file is compared with a scanned digital image using a laser scanner machine.
If the machine finds an inconsistency, the comparison is displayed on a monitor for the technician to assess.
Step 6: Layer-up and Bond
The layers are fused jointly. Outer layers must join with the substrate. The procedure is done in two steps: layer-up and bonding. A thin copper foil covers the top and bottom of the original substrate, which contains the copper trace etchings. Now squeeze in them together.
A technician places a prepreg layer over the alignment basin. The substrate layer fits over the position before the copper sheet is positioned. The complete process undergoes an automatic routine run by the bonding press computer.
Step 7: Drill
At last, holes are bored into the stack board. To locate the place of the drill targets, an x-ray locator identifies the appropriate drill target spots.
Before drilling, the technician places a board of buffer material beneath the drill target to make certain a clean bore is enacted. Every micro-movement of the drill is controlled by a computer. To identify the proper spots to bore are identified through the drilling file from the original design used by the computer.
The drills use air-driven spindles that turn at 150,000 rpm. The final affixation of these parts occurs later, after plating. Later than the drilling completes, the supplementary copper that lines the edges of the production panel undergoes deletion by a profiling tool.
Step 8: Plating and Copper Deposition
After drilling, the panel moves onto plating. The process fuses the dissimilar layers jointly using a chemical deposition. Later than a thorough cleaning, the panel undergoes a chain of chemical baths. For the duration of the baths, a chemical deposition process deposits a thin layer - concerning one micron thick - of copper over the surface of the panel. The copper goes into the recently drilled holes.
Earlier to this step, the inner surface of the holes simply exposes the fiberglass material that comprises the interior of the panel. By the way, the entire panel receives a new layer of copper. Notably, the new holes are covered. Computers manage the whole process of dipping, removal, and procession.
Step 9: Outer Layer Imaging
In this step, the photoresist is reapplied but this time the outer layers of the panel are imaged with PCB design. The layering is done in a germ-free room to avoid any contaminants from sticking to the layer surface, and then apply a layer of photoresist to the panel. The prepped panel passes into the yellow room. UV lights affect photoresist. Yellow light wavelengths don't take UV levels enough to influence the photoresist.
The course of action stands as an inversion to that of the inner layers. To end with, the outer plates suffer check up to make certain all of the undesired photoresists was detached during the preceding stage.
Step 10: Plating
As in Step 8, electroplate the panel with a thin layer of copper. The uncovered sections of the panel from the external layer photoresist stage obtain the copper electroplating. Subsequently the early copper plating baths, the panel usually receives tin plating, which permits the elimination of all the copper left on the board slated for elimination. The tin guards the section of the panel destined to stay behind covered with copper all through the next etching stage. Etching removes the unnecessary copper foil from the panel.
Step 11: Final Etching
The tin protects the preferred copper for the duration of this stage. The redundant uncovered copper and copper underneath the residual resist layer experience deletion. Yet again, chemical solutions are applied to take away the surplus copper. For the moment, the tin protects the prized copper during this stage. The conducting areas and relations are now correctly recognized.
Step 12: Solder Mask Application
Before the solder mask is functional to both sides of the board; the panels are cleaned and roofed with an epoxy solder mask ink. The boards obtain a flare of UV light, which passes through a solder mask photo film. The roofed portions stay unhardened and will go through removal. To end with, the board passes into an oven to cure the solder mask.
Step 13: Surface Finish
To introduce supplementary solder-ability to the PCB, chemically plate them with gold or silver. Some PCBs also acquire hot air-leveled pads throughout this step. The hot air leveling results in reliable pads. That process leads to the manufacture of surface finish. PCB Cart can process numerous types of surface finish according to the client’s specific demands.
Step 14: Silkscreen
The almost finished board receives ink-jet lettering on its exterior, used to point out all essential information pertaining to the PCB. The PCB lastly passes onto the final covering and curing stage.
Step 15: Electrical Test
As a finishing safety measure, a technician performs electrical tests on the PCB. The programmed method confirms the functionality of the PCB and its conventionality to the original design. A superior edition of electrical testing called Flying Probe Testing is used which depends on moving probes to test the electrical performance of each net on an exposed circuit board.
Step 16: Profiling and V-Scoring
The last step is wounding. Dissimilar boards are engraved from the original panel. The technique used is either centered on using a router or a v-groove. The v-groove cuts diagonal channels along both sides of the board and a router leaves small tabs along the board edges. Both ways allow the boards to effortlessly pop out from the panel.
Single-sided board
Printed circuit boards or PCBs are used to unite electronic components and these boards are frequently used in an extensive range of electronic devices. Single-sided printed circuit boards are a type of circuit board that has the conductive copper and components mounted on one side of the board and the conductive cabling connected on the other side. These PCBs are one of the mainly created boards as they are very easy and basic to make.
Single-sided printed circuit boards are commonly used in an arrangement of electronics and applications, including camera systems, printers, radio equipment, calculators, and much more.
Double-sided board
Double-sided PCBs are very alike to single-sided PCBs, apart from that they have two-sided traces with a top and bottom layer. These boards can build-up conductive copper and components on both sides of the circuit board, which allows the traces to cross over each other. This fallout in a higher density of circuits without the need for point-to-point soldering. As these types of circuit boards are more complex than single-sided PCBs, it can be more complex to produce.
The most well-liked types of PCBs are Double-sided circuit boards. There is a large number of applications in which double-sided PCBs can be used including lighting systems, vending machines, amplifiers, car dashboards, and many more.
A surface-mount device (SMD) is an electronic appliance whose components are positioned onto the exterior of the printed circuit board (PCB). This technique of manufacturing electronic circuit boards is based on surface-mount technology (SMT), which has chiefly replaced the through-hole technology (THT) especially in devices that have to be small or flat. SMT allows both sides of a PCB to be used when requisite.
A smartphone is an example of an SMD. It has components that must be packed very firmly in an extremely slim case, so it is not practicable to use THT components. SMT components can be smaller than THT components since they can have either smaller leads or no leads at all, which makes it easier to contract the components down.
Reference
i) Electrical Engineering Materials, Dr. G.P. CHHALOTRA & Dr. B.K.BHAT, Khanna Technical Publications.
