In electronic devices, printed circuit boards, or PCBs, are utilized to mechanically support electronic elements which have their connection leads soldered onto copper pads in surface install applications or through rilled holes in the board and copper pads for soldering the component leads in thru-hole applications. A board style may have all thru-hole elements on the top or component side, a mix of thru-hole and surface mount on the top side only, a mix of thru-hole and surface area mount components on the top and surface area install parts on the bottom or circuit side, or surface mount components on the leading and bottom sides of the board.
The boards are likewise utilized to electrically link the required leads for each element using conductive copper traces. The component pads and connection traces are etched from copper sheets laminated onto a non-conductive substrate. Printed circuit boards are designed as single sided with copper pads and traces on one side of the board only, double agreed copper pads and traces on the leading and bottom sides of the board, or multilayer designs with copper pads and traces on top and bottom of board with a variable number of internal copper layers with traces and connections.
Single or double sided boards consist of a core dielectric material, such as FR-4 epoxy fiberglass, with copper plating on one or both sides. This copper plating is engraved away to form the actual copper pads and connection traces on the board surfaces as part of the board manufacturing procedure. A multilayer board includes a variety of layers of dielectric material that has been impregnated with adhesives, and these layers are utilized to separate the layers of copper plating. All of these layers are lined up and then bonded into a single board structure under heat and pressure. Multilayer boards with 48 or more layers can be produced with today's technologies.
In a typical 4 layer board design, the internal layers are typically used to provide power and ground connections, such as a +5 V airplane layer and a Ground airplane layer as the two internal layers, with all other circuit and component connections made on the top and bottom layers of the board. Very intricate board styles might have a a great deal of layers to make the various connections for different voltage levels, ground connections, or for connecting the many leads on ball grid selection gadgets and other large incorporated circuit package formats.
There are usually two types of material utilized to construct a multilayer board. Pre-preg material is thin layers of fiberglass pre-impregnated with an adhesive, and is in sheet form, generally about.002 inches thick. Core material resembles a very thin double sided board in that it has a dielectric product, such as epoxy fiberglass, with a copper layer deposited on each side, generally.030 thickness dielectric material with 1 ounce copper layer on each side. In a multilayer board style, there are 2 techniques utilized to build up the wanted variety of layers. The core stack-up approach, which is an older innovation, utilizes a center layer of pre-preg product with a layer of core material above and another layer of core material below. This mix of one pre-preg layer and 2 core layers would make a 4 layer board.
The film stack-up approach, a newer technology, would have core material as the center layer followed by layers of pre-preg and copper product built up above and below to form the final number of layers needed by the board design, sort of like Dagwood developing a sandwich. This technique allows the manufacturer versatility in how the board layer densities are integrated to fulfill the completed product density requirements by varying the variety of sheets of pre-preg in each layer. As soon as the material layers are completed, the entire stack undergoes heat and pressure that causes the adhesive in the pre-preg to bond the core and pre-preg layers together into a single entity.
The procedure of producing printed circuit boards follows the steps below for most applications.
The procedure of determining products, processes, and requirements to satisfy the client's specifications for the board style based on the Gerber file details offered with the purchase order.
The process of transferring the Gerber file data for a layer onto an etch withstand movie that is placed on the conductive copper layer.
The conventional process of exposing the copper and other areas unprotected by the etch resist movie to a chemical that removes the unguarded copper, leaving the secured copper pads and traces in place; more recent procedures utilize plasma/laser etching instead of chemicals to get rid of the copper product, allowing finer line meanings.
The process of lining up the conductive copper and insulating dielectric ISO 9001 Accreditation layers and pressing them under heat to trigger the adhesive in the dielectric layers to form a solid board product.
The process of drilling all of the holes for plated through applications; a second drilling process is used for holes that are not to be plated through. Details on hole place and size is contained in the drill drawing file.
The process of applying copper plating to the pads, traces, and drilled through holes that are to be plated through; boards are positioned in an electrically charged bath of copper.
This is required when holes are to be drilled through a copper location however the hole is not to be plated through. Avoid this process if possible due to the fact that it adds cost to the completed board.
The procedure of using a protective masking product, a solder mask, over the bare copper traces or over the copper that has had a thin layer of solder used; the solder mask secures against ecological damage, offers insulation, protects against solder shorts, and safeguards traces that run in between pads.
The process of finishing the pad locations with a thin layer of solder to prepare the board for the eventual wave soldering or reflow soldering process that will happen at a later date after the components have been positioned.
The process of applying the markings for element designations and part details to the board. May be used to just the top side or to both sides if parts are mounted on both leading and bottom sides.
The process of separating multiple boards from a panel of identical boards; this process also permits cutting notches or slots into the board if required.
A visual evaluation of the boards; also can be the procedure of checking wall quality for plated through holes in multi-layer boards by cross-sectioning or other approaches.
The process of checking for connection or shorted connections on the boards by means using a voltage between different points on the board and determining if a present flow takes place. Depending upon the board intricacy, this procedure might need a specifically developed test fixture and test program to incorporate with the electrical test system used by the board producer.