In electronics, printed circuit boards, or PCBs, are utilized to mechanically support electronic parts which have their connection leads soldered onto copper pads in surface area mount 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 components on the top or component side, a mix of thru-hole and surface area mount on the top only, a mix of thru-hole and surface install parts on the top side and surface mount components on the bottom or circuit side, or surface area mount components on the leading and bottom sides of the board.
The boards are likewise utilized to electrically connect the required leads for each component using conductive copper traces. The component pads and connection traces are engraved from copper sheets laminated onto a non-conductive substrate. Printed circuit boards are designed as single agreed 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 the 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 manufacturing procedure. A multilayer board consists of a number of layers of dielectric product that has been impregnated with adhesives, and these layers are used 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 normal four layer board design, 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 2 internal layers, with all other circuit and element connections made on the leading and bottom layers of the board. Really intricate board styles may have a large number of layers to make the various connections for different voltage levels, ground connections, or for linking the lots of leads on ball grid array devices and other large incorporated circuit bundle formats.
There are typically 2 types of product used to build a multilayer board. Pre-preg material is thin layers of fiberglass pre-impregnated with an adhesive, and is in sheet form, usually about.002 inches thick. Core product is similar to a really thin double sided board in that it has a dielectric material, such as epoxy fiberglass, with a copper layer transferred on each side, typically.030 density dielectric material with 1 ounce copper layer on each side. In a multilayer board design, there are two techniques used to build up the preferred variety of layers. The core stack-up technique, which is an older technology, ISO 9001 Accreditation uses a center layer of pre-preg product 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 technology, would have core product as the center layer followed by layers of pre-preg and copper product built up above and listed below to form the last number of layers required by the board style, sort of like Dagwood developing a sandwich. This technique permits the producer flexibility in how the board layer densities are combined to meet the finished product density requirements by differing the variety of sheets of pre-preg in each layer. Once the material 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 manufacturing printed circuit boards follows the steps below for most applications.
The procedure of determining products, procedures, and requirements to fulfill the client's requirements for the board style based upon the Gerber file details supplied with the purchase order.
The procedure of transferring the Gerber file data for a layer onto an etch resist movie that is placed on the conductive copper layer.
The traditional procedure of exposing the copper and other locations unprotected by the etch resist movie to a chemical that eliminates the unguarded copper, leaving the safeguarded copper pads and traces in place; more recent processes utilize plasma/laser etching instead of chemicals to get rid of the copper material, allowing 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 strong board product.
The process of drilling all of the holes for plated through applications; a second drilling procedure is utilized for holes that are not to be plated through. Details on hole place and size is contained in the drill drawing file.
The procedure of using 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 however the hole is not to be plated through. Prevent this process if possible since it adds expense to the finished board.
The process of applying a protective masking material, a solder mask, over the bare copper traces or over the copper that has actually had a thin layer of solder used; the solder mask protects against environmental damage, supplies insulation, protects against solder shorts, and protects traces that run in between pads.
The process of covering the pad locations with a thin layer of solder to prepare the board for the ultimate wave soldering or reflow soldering procedure that will occur at a later date after the parts have been put.
The process of applying the markings for part designations and element details to the board. May be applied to simply the top side or to both sides if components are mounted on both leading and bottom sides.
The procedure of separating multiple boards from a panel of identical boards; this procedure likewise permits cutting notches or slots into the board if needed.
A visual assessment of the boards; likewise 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 connection or shorted connections on the boards by ways using a voltage between various points on the board and identifying if an existing circulation occurs. Depending upon the board intricacy, this process might require a specially developed test fixture and test program to incorporate with the electrical test system used by the board manufacturer.