Inner layer automated optical inspection uses a computer to carefully examine the inner layer to look for incomplete patterns or resist that may still be on the surface. Oxide applied to the inner layer ensures better bonding of the copper foil and insulating epoxy resin layers between inner and outer layers.
The layup step in the multilayer PCB fabrication process happens when a machine helps to line up, heat and bond the layers together with a copper foil layer and insulating material between the inner and outer layers. Lamination uses heat and pressure to melt the bonding epoxy between the layers. Properly laminated PCBs will hold their layers tightly together with effective insulation between layers. When drilling multilayer boards after lamination, an X-ray ensures alignment of the drill bit.
These holes allow for connections to occur between layers of the multilayer PCB. Therefore, the accuracy of their placement and size in relation to the rest of the layer and the other layers is crucial. The holes from earlier are used to align the inner and outer layers. To align the layers, a technician places them on a type of punch machine known as an optical punch. The optical punch drives a pin down through the holes to line up the layers of the PCB. Following the optical punch, another machine performs an optical inspection to make sure there are no defects.
After the PCB has passed inspection — that is, neither the technician nor the AOI machine found any defects — it moves onto the last couple steps of PCB manufacture and production. The AOI step is crucial for the operation of the printed circuit board. Without it, boards that could have short circuits, not meet the design specifications or have extra copper that was not removed during etching could pass through the rest of the process. AOI prevents defective boards from going on by serving as a quality checkpoint midway through the production process.
Later, this process repeats for the outer layers after engineers finish imaging and etching them. At step six in the process, the PCB layers are all together, waiting to be laminated. The PCB laminating process is done in two steps: the lay-up step and the laminating step.
The original piece of the substrate is also covered in a layer of thin copper foil that now contains the etchings for the copper traces. The sandwiching of these layers is done using metal clamps on a special press table. Each layer fits onto the table using a specialized pin. A layer of the substrate is placed over the pre-impregnated resin, followed by a copper foil layer. The copper foil is in turn followed by more sheets of pre-impregnated resin, which are then finished off with a piece of and one last piece of copper known as a press plate.
Once the copper press plate is in place, the stack is ready to be pressed. The technician takes it over to a mechanical press and presses the layers down and together. If the layers are fixed properly, the PCB stack is taken to the next press, a laminating press. The laminating press uses a pair of heated plates to apply both heat and pressure to the stack of layers.
The technician needs to remove the top press plate and the pins from earlier, which then allows them to pull the actual PCB free.
Before drilling, an X-ray machine is used to locate the drill spots. When it comes time to drill these holes, a computer-guided drill is used to make the holes themselves, using the file from the Extended Gerber design as a guide. The plating process uses a chemical to fuse all of the different layers of the PCB together.
After being cleaned thoroughly, the PCB is bathed in a series of chemicals. Part of this bathing process coats the panel in a micron-thick layer of copper, which is deposited over the top-most layer and into the holes that have just been drilled. Bathing those holes in copper covers the walls of the previously drilled holes. However, this time the photoresist is only applied to the outside layer, since it still needs to be imaged.
However, while the process is the same, the outer layers get a plating of tin to help guard the copper of the outside layer. When it comes time to etch the outside layer for the last time, the tin guard is used to help protect the copper during the etching process. Any unwanted copper is removed using the same copper solvent from earlier, with the tin protecting the valued copper of the etching area.
One of the main differences between the inner and outer layer etching covers the areas that need removal. While inner layers use dark ink for conductive areas and clear ink for non-conductive surfaces, these inks are reversed for the outer layers. Therefore, the non-conductive layers have dark ink covering them, and the copper has light ink. This light ink allows for the tin plating to cover the copper and protect it. Engineers remove unneeded copper and any remaining resist coating during etching, preparing the outer layer for AOI and solder masking.
As with the inner layer, the outer layer must also undergo automated optical inspection. This optical inspection ensures the layer meets the exact requirements of the design.
A printed circuit board may have circuits that perform a single function, such as a signal amplifier, or multiple functions. There are three major types of printed circuit board construction: single-sided, double-sided, and multi-layered. Single-sided boards have the components on one side of the substrate. When the number of components becomes too much for a single-sided board, a double-sided board may be used.
Electrical connections between the circuits on each side are made by drilling holes through the substrate in appropriate locations and plating the inside of the holes with a conducting material. The third type, a multi-layered board, has a substrate made up of layers of printed circuits separated by layers of insulation.
The components on the surface connect through plated holes drilled down to the appropriate circuit layer. This greatly simplifies the circuit pattern. Components on a printed circuit board are electrically connected to the circuits by two different methods: the older "through hole technology" and the newer "surface mount technology. Gravity and friction between the leads and the sides of the holes keeps the components in place until they are soldered. With surface mount technology, stubby J-shaped or L-shaped legs on each component contact the printed circuits directly.
A solder paste consisting of glue, flux, and solder are applied at the point of contact to hold the components in place until the solder is melted, or "reflowed," in an oven to make the final connection. Although surface mount technology requires greater care in the placement of the components, it eliminates the time-consuming drilling process and the space-consuming connection pads inherent with through hole technology.
Both technologies are used today. Two other types of circuit assemblies are related to the printed circuit board. An integrated circuit, sometimes called an IC or microchip, performs similar functions to a printed circuit board except the IC contains many more circuits and components that are electrochemically "grown" in place on the surface of a very small chip of silicon.
A hybrid circuit, as the name implies, looks like a printed circuit board, but contains some components that are grown onto the surface of the substrate rather than being placed on the surface and soldered. Printed circuit boards evolved from electrical connection systems that were developed in the s. Metal strips or rods were originally used to connect large electric components mounted on wooden bases. In time the metal strips were replaced by wires connected to screw terminals, and wooden bases were replaced by metal chassis.
But smaller and more compact designs were needed due to the increased operating needs of the products that used circuit boards. In , Charles Ducas of the United States submitted a patent application for a method of creating an electrical path directly on an insulated surface by printing through a stencil with electrically conductive inks. This method gave birth to the name "printed wiring" or "printed circuit. In the , Paul Eisler of the United Kingdom patented a method of etching the conductive pattern, or circuits, on a layer of copper foil bonded to a glass-reinforced, non-conductive base.
Widespread use of Eisler's technique did not come until the s when the transistor was introduced for commercial use. Up to that point, the size of vacuum tubes and other components were so large that the traditional mounting and wiring methods were all that was needed.
These blueprints will be what you base the process off of. The design process is generally completed through computer software. Using a trace width calculator will help with a majority of the details needed for inner and external layers. A special printer called a plotted printer is used to print the design of the PCB. It produces a film that shows the details and layers of the board. When printed, there will be two ink colors used on the inside layer of the board:.
Now is when the PCB will start to form. The substrate, which is the insulating material epoxy resin and glass fiber that holds the components on the structure, begins forming by passing the materials through an oven to be semicured.
Copper is pre-bonded to both sides of the layer and then etched away to show the design from the printed films. The design is printed to a laminate, the body of the structure. A photo-sensitive film made from photo-reactive chemicals that will harden when exposed to ultraviolet light the resist covers the structure..
This will help align the blueprints and the actual print of the board. Holes are drilled into the PCB to help with the alignment process. Once aligned, the resist and laminate go under ultraviolet lights to harden the photoresist.
The light reveals the pathways of copper. The black ink from before prevents hardening in areas that will be removed later on. The board is then washed in an alkaline solution to remove the excess photoresist. Now, it is time to remove any unwanted copper that remained on the board.
Another method that is sometimes used for a PCB prototype is to print etch resistant inks onto the PCB using a silk screening process. With the complexity of electronic circuits increasing, it is not always possible to provide all the connectivity that is required using just the two sides of the PCB. This occurs quite commonly when dense microprocessor and other similar boards are being designed. When this is the case multilayer boards are required.
The manufacture of multi-layer printed circuit boards, although it uses the same processes as for single layer boards, requires a considerably greater degree of accuracy and manufacturing process control.
The boards are made by using much thinner individual boards, one for each layer, and these are then bonded together to produce the overall PCB.
As the number of layers increases, so the individual boards must become thinner to prevent the finished PCB from becoming too thick. Additionally the registration between the layers must be very accurate to ensure that any holes line up. To bond the different layers together the board is heated to cure the bonding material.
This can lead to some problems of warp. Large multi-layer boards can have a distinct warp on them if they are not designed correctly.
This can occur particularly if, for example one of the inner layers is a power plane or a ground plane. While this in itself is fine, if some reasonably significant areas have to be left free of copper. This can set up strains within the PCB that can lead to warping. Holes, often called via holes or vias are needed within a PCB to connect the different layers together at different points.
Holes may also be needed to enable leaded components to be mounted on the PCB. Additionally some fixing holes may be needed. Normally the inner surfaces of the holes have copper layer so that they electrically connect the layers of the board. These "plated through holes" are produced using a plating process. In this way the layers of the board can be connected. Drilling is then accomplished using numerically controlled drilling machines, the data being supplied from the PCB CAD design software.
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