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 mount applications or through rilled holes in the board and copper pads for soldering the part leads in thru-hole applications. A board design might have all thru-hole parts on the top or element side, a mix of thru-hole and surface install on the top just, a mix of thru-hole and surface area mount elements on the top and surface install components on the bottom or circuit side, or surface mount parts on the top and bottom sides of the board.
The boards are also utilized to electrically link the needed leads for each part utilizing conductive copper traces. The component pads and connection traces are engraved from copper sheets laminated onto a non-conductive substrate. Printed circuit boards are developed as single agreed copper pads and traces on one side of the board just, double sided with copper pads and traces on the leading and bottom sides of the board, or multilayer styles with copper pads and traces on the 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 product, such as FR-4 epoxy fiberglass, with copper plating on one or both sides. This copper plating is engraved away to form the real copper pads and connection traces on the board surfaces as part of the board production process. A multilayer board consists of a variety of layers of dielectric material that has actually been impregnated with adhesives, and these layers are utilized to separate the layers of copper plating. All of these layers are lined up and after that 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 four layer board style, the internal layers are often used to provide power and ground connections, such as a +5 V plane layer and a Ground airplane layer as the two internal layers, with all other circuit and part connections made on the top and bottom layers of the board. Extremely complicated board styles might have a a great deal of layers to make the various connections for different voltage levels, ground connections, or for linking the many leads on ball grid selection devices and other large integrated circuit package formats.
There are normally 2 kinds of material used to construct a multilayer board. Pre-preg product is thin layers of fiberglass pre-impregnated with an adhesive, and is in sheet kind, normally about.002 inches thick. Core material is similar to a really thin double sided board because it has a dielectric product, such as epoxy fiberglass, with a copper layer transferred on each side, typically.030 thickness dielectric material with 1 ounce copper layer on each side. In a multilayer board style, there are two approaches used to develop the preferred variety of layers. The core stack-up technique, 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 product below. This mix of one pre-preg layer and two core layers would make a 4 layer board.
The movie stack-up technique, a more recent innovation, would have core material as the center layer followed by layers of pre-preg and copper product built up above and listed below to form the last variety of layers required by the board style, sort of like Dagwood constructing a sandwich. This technique allows the manufacturer flexibility in how the board layer densities are integrated to fulfill the completed product density requirements by varying the number of sheets of pre-preg in each layer. When the material layers are completed, the whole stack is subjected to 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 making printed circuit boards follows the steps below for most applications.
The procedure of determining products, processes, and requirements to fulfill the client's requirements for the board style based upon the Gerber file details provided with the purchase order.
The procedure of moving the Gerber file information for a layer onto an etch resist movie that is put 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 eliminates the unguarded copper, leaving the safeguarded copper pads and traces in place; newer procedures use plasma/laser etching instead of chemicals to get rid of the copper material, permitting finer line meanings.
The process of lining up the conductive copper and insulating dielectric layers and pressing them under heat to trigger the adhesive in the dielectric layers to form a strong board material.
The procedure of drilling all of the holes for plated through applications; a second drilling procedure 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 applying copper plating to the pads, traces, and drilled through holes that are to be plated through; boards are put in an electrically charged bath of copper.
This is required when holes are to be drilled through a copper area but the hole is not to be plated through. Prevent 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 applied; the solder mask secures versus environmental damage, offers insulation, safeguards against solder shorts, and protects traces that run in between pads.
The procedure of coating the pad locations with a thin layer of solder to prepare the board for the eventual wave soldering or reflow soldering process that will occur at a later date after the parts have actually been put.
The procedure of applying the markings for part classifications and part details to the board. Might be applied to just the top or to both sides if elements are mounted on both leading and bottom sides.
The procedure of separating multiple boards from a panel of similar boards; this process also enables cutting notches or slots into the board if needed.
A visual examination of the boards; also can be the procedure of examining wall quality for plated through holes in multi-layer boards by cross-sectioning or other methods.
The procedure of looking for continuity or shorted connections on the boards by means applying a voltage in between various points on the board and determining if an existing circulation takes place. Depending upon the board complexity, this process might need a specifically designed test component and test program to integrate with the electrical test system utilized by the board maker.