In electronic devices, printed circuit boards, or PCBs, are used to mechanically support electronic parts which have their connection leads soldered onto copper pads in surface area install applications or through rilled holes in the board and copper pads for soldering the part leads in thru-hole applications. A board design may have all thru-hole parts on the leading or element side, a mix of thru-hole and surface mount on the top only, a mix of thru-hole and surface install parts on the top side and surface area mount parts on the bottom or circuit side, or surface area install elements on the top and bottom sides of the board.
The boards are also used to electrically link the needed leads for each component 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 just, double agreed copper pads and traces on the top 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 include 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 includes a variety of layers of dielectric product that has been impregnated with adhesives, and these layers are used to separate the layers of copper plating. All 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 technologies.
In a common 4 layer board design, the internal layers are often utilized to provide power and ground connections, such as a +5 V airplane layer and a Ground plane layer as the 2 internal layers, with all other circuit and component connections made on the leading and bottom layers of the board. Very complicated board styles may have a large number of layers to make the various connections for different voltage levels, ground connections, or for connecting the numerous leads on ball grid variety devices and other large integrated circuit bundle formats.
There are generally 2 kinds of product used to construct a multilayer board. Pre-preg material is thin layers of fiberglass pre-impregnated with an adhesive, and remains in sheet form, usually 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 deposited on each side, typically.030 density dielectric product with 1 ounce copper layer on each side. In a multilayer board design, there are two methods utilized to build up the preferred number of layers. The core stack-up technique, which is an older innovation, utilizes 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 approach, a more recent technology, would have core material as the center layer followed by layers of pre-preg and copper product developed above and listed below to form the final variety of layers needed by the board design, sort of like Dagwood developing a sandwich. This method enables the manufacturer flexibility in how the board layer ISO 9001 Accreditation densities are combined to satisfy the completed product thickness requirements by differing the variety of sheets of pre-preg in each layer. Once the product layers are finished, the entire stack undergoes 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 process of making printed circuit boards follows the steps below for many applications.
The procedure of figuring out products, processes, and requirements to fulfill the consumer's specifications for the board style based on the Gerber file info supplied with the purchase order.
The procedure of transferring the Gerber file information for a layer onto an etch resist movie that is put on the conductive copper layer.
The traditional procedure of exposing the copper and other locations unprotected by the etch withstand movie to a chemical that removes the unprotected copper, leaving the secured copper pads and traces in place; more recent processes use plasma/laser etching instead of chemicals to remove the copper material, enabling finer line meanings.
The procedure of aligning 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 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. Information on hole location and size is contained in the drill drawing file.
The procedure of applying copper plating to the pads, traces, and drilled through holes that are to be plated through; boards are placed 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. Avoid this procedure if possible because it includes expense to the completed board.
The process 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 ecological damage, offers insulation, protects against solder shorts, and secures traces that run in between pads.
The process of finishing 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 components have been put.
The procedure of using the markings for part designations and part outlines to the board. May be used to just the top or to both sides if components are installed on both top and bottom sides.
The process of separating numerous boards from a panel of identical boards; this process likewise enables cutting notches or slots into the board if required.
A visual evaluation of the boards; also can be the process of examining wall quality for plated through holes in multi-layer boards by cross-sectioning or other methods.
The procedure of checking for connection or shorted connections on the boards by ways using a voltage between various points on the board and identifying if an existing flow occurs. Depending upon the board complexity, this process may need a specifically designed test component and test program to integrate with the electrical test system used by the board maker.