In electronics, printed circuit boards, or PCBs, are used to mechanically support electronic components which have their connection leads soldered onto copper pads in surface mount applications or through rilled holes in the board and copper pads for soldering the element leads in thru-hole applications. A board design may have all thru-hole components on the leading or element side, a mix of thru-hole and surface install on the top only, a mix of thru-hole and surface mount parts on the top and surface area mount components on the bottom or circuit side, or surface area install elements on the top and bottom sides of the board.
The boards are likewise used to electrically connect the needed leads for each element using conductive copper traces. The element pads and connection traces are engraved from copper sheets laminated onto a non-conductive substrate. Printed circuit boards are developed as single sided with copper pads and traces on one side of the board only, double agreed copper pads and traces on the top and bottom sides of the board, or multilayer styles 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 production process. A multilayer board consists of a number of layers of dielectric product that has been fertilized with adhesives, and these layers are utilized to separate the layers of copper plating. All of these layers are aligned 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 innovations.
In a typical 4 layer board style, the internal layers are typically utilized to supply power and ground connections, such as a +5 V aircraft layer and a Ground airplane layer as the 2 internal layers, with all other circuit and component connections made on the top and bottom layers of the board. Very complex board designs may have a a great deal of layers to make the numerous connections for different voltage levels, ground connections, or for linking the numerous leads on ball grid selection devices and other large integrated circuit bundle formats.
There are typically two types of material used to construct a multilayer board. Pre-preg product is thin layers of fiberglass pre-impregnated with an adhesive, and remains in sheet form, normally about.002 inches thick. Core product is similar to a very thin double sided board in that it has a dielectric material, such as epoxy fiberglass, with a copper layer transferred on each side, usually.030 thickness dielectric product with 1 ounce copper layer on each side. In a multilayer board style, there are 2 techniques utilized to build up the wanted number of layers. The core stack-up method, which is an older technology, uses a center layer of pre-preg material with a layer of core product above and another layer of core product below. This combination of one pre-preg layer and 2 core layers would make a 4 layer board.
The movie stack-up technique, a newer innovation, would have core product as the center layer followed by layers of pre-preg and copper material developed above and listed below to form the last variety of layers needed by the board style, sort of like Dagwood constructing a sandwich. This method permits the maker versatility in how the board layer densities are combined to satisfy the finished product density requirements by varying the number of sheets of pre-preg in each layer. Once the material layers are finished, the entire stack goes through heat and pressure that triggers the adhesive in the pre-preg to bond the core and pre-preg layers together into a single entity.
The procedure of manufacturing printed circuit boards follows the actions listed below for most applications.
The process of figuring out products, procedures, and requirements to fulfill the client's requirements for the board design based on the Gerber file information supplied with the order.
The procedure of transferring the Gerber file data for a layer onto an etch withstand film that is placed on the conductive copper layer.
The conventional procedure of exposing the copper and other areas unprotected by the etch withstand movie to a chemical that removes the unprotected copper, leaving the protected copper pads and traces in place; newer processes utilize plasma/laser etching rather of chemicals to get rid of the copper material, allowing finer line definitions.
The process of lining up the conductive copper and insulating dielectric layers and pushing them under heat to activate 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. Info on hole area and size is included in the drill drawing file.
The process of using 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. Prevent this procedure if possible because it adds cost to the ended up board.
The procedure of applying a protective masking material, a solder mask, over the bare copper traces or over the copper that has had a thin layer of solder applied; the solder mask protects against environmental damage, provides insulation, safeguards versus solder shorts, and safeguards traces that run between pads.
The procedure of covering the pad areas 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 parts have actually been positioned.
The process of using the markings for component classifications and part lays out to the board. May be applied to simply the top or to both sides if components are installed on both leading and bottom sides.
The process of separating several boards from a panel of similar boards; this procedure also allows cutting notches or slots into the board if required.
A visual evaluation of the boards; likewise can be the process of checking wall quality for plated through holes in multi-layer boards by cross-sectioning or other techniques.
The procedure of looking for continuity or shorted connections on the boards by methods using a voltage in between various points on the board and identifying if an existing flow occurs. Depending upon the board complexity, this procedure may require a specially designed test component and test program to integrate with the electrical test system utilized by the board maker.