In electronic devices, printed circuit boards, or PCBs, are utilized to mechanically support electronic parts 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 style might have all thru-hole components on the leading or part side, a mix of thru-hole and surface area mount on the top only, a mix of thru-hole and surface mount parts on the top side and surface area install elements on the bottom or circuit side, or surface area install parts on the top and bottom sides of the board.
The boards are also utilized to electrically connect the required leads for each element using conductive copper traces. The element pads and connection traces are etched from copper sheets laminated onto a non-conductive substrate. Printed circuit boards are created as single agreed copper pads and traces on one side of the board only, 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 top and bottom of board with a variable variety 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 surface areas as part of the board production procedure. A multilayer board consists of a variety of layers of dielectric material that has been fertilized 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 innovations.
In a normal 4 layer board design, the internal layers are frequently used to offer power and ground connections, such as a +5 V aircraft layer and a Ground aircraft layer as the two internal layers, with all other circuit and component connections made on the leading and bottom layers of the board. Very complicated board designs might have a large number of layers to make the various connections for different voltage levels, ground connections, or for linking the numerous leads on ball grid range devices and other large integrated circuit package formats.
There are typically 2 types of material used to build a multilayer board. Pre-preg product is thin layers of fiberglass pre-impregnated with an adhesive, and is in sheet kind, generally about.002 inches thick. Core material resembles an extremely thin double sided board in that it has a dielectric product, such as epoxy fiberglass, with a copper layer deposited on each side, usually.030 thickness dielectric product with 1 ounce copper layer on each side. In a multilayer board design, there are 2 approaches used to develop the desired number of layers. The core stack-up approach, which is an older technology, uses a center layer of pre-preg product with a layer of core material above and another layer of core material 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 innovation, would have core product as the center layer followed by layers of pre-preg and copper material developed above and below to form the final variety of layers required by the board design, sort of like Dagwood building a sandwich. This approach enables the producer flexibility in how the board layer thicknesses are combined to ISO 9001 Accreditation fulfill the finished item thickness requirements by varying the number of sheets of pre-preg in each layer. Once the product layers are finished, the whole stack is subjected to 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 listed below for most applications.
The procedure of identifying materials, procedures, and requirements to fulfill the customer's requirements for the board design based on the Gerber file details provided with the order.
The process of transferring the Gerber file data for a layer onto an etch withstand film that is put on the conductive copper layer.
The standard process of exposing the copper and other areas unprotected by the etch resist film to a chemical that gets rid of the vulnerable copper, leaving the safeguarded copper pads and traces in place; more recent procedures utilize plasma/laser etching instead of chemicals to get rid of the copper material, permitting finer line definitions.
The process of aligning the conductive copper and insulating dielectric layers and pushing them under heat to trigger the adhesive in the dielectric layers to form a solid board product.
The procedure of drilling all the holes for plated through applications; a second drilling procedure is used for holes that are not to be plated through. Details on hole location 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 placed in an electrically charged bath of copper.
This is needed when holes are to be drilled through a copper location however the hole is not to be plated through. Avoid this procedure if possible because it includes expense to the ended up board.
The procedure of using 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 versus ecological damage, supplies insulation, protects against solder shorts, and safeguards traces that run 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 procedure that will take place at a later date after the parts have been put.
The process of using the markings for component classifications and part details to the board. Might be used to just the top or to both sides if parts are installed on both top 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 needed.
A visual assessment of the boards; also can be the procedure of inspecting wall quality for plated through holes in multi-layer boards by cross-sectioning or other techniques.
The procedure of checking for continuity or shorted connections on the boards by methods using a voltage in between various points on the board and figuring out if a present circulation takes place. Depending upon the board complexity, this process may need a specially designed test fixture and test program to incorporate with the electrical test system utilized by the board maker.