The Elements of A Leading-Edge Quality System



In electronic devices, printed circuit boards, or PCBs, are used to mechanically support electronic components which have their connection leads soldered onto copper pads in surface install applications or through rilled holes in the board and copper pads for soldering the element leads in thru-hole applications. A board style may have all thru-hole elements on the leading or component side, a mix of thru-hole and surface mount on the top side only, a mix of thru-hole and surface area install parts on the top and surface area install parts on the bottom or circuit side, or surface mount parts on the top and bottom sides of the board.

The boards are likewise used to electrically link the required leads for each element using conductive copper traces. The part 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 only, double agreed 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 number of internal copper layers with traces and connections.

Single or double sided boards include 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 actual copper pads and connection traces on the board surfaces as part of the board production process. A multilayer board includes a number of layers of dielectric material that has actually been fertilized with adhesives, and these layers are utilized to separate the layers of copper plating. All 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 common four layer board style, the internal layers are typically used to provide power and ground connections, such as a +5 V aircraft layer and a Ground aircraft layer as the 2 internal layers, with all other circuit and element connections made on the leading and bottom layers of the board. Extremely intricate board styles might have a a great deal of layers to make the various connections for various voltage levels, ground connections, or for connecting the many leads on ball grid selection devices and other big integrated circuit package formats.

There are usually 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 kind, generally about.002 inches thick. Core material resembles 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, normally.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 number of layers. The core stack-up technique, which is an older innovation, uses a center layer of pre-preg material with a layer of core material above and another layer of core material below. This mix of one pre-preg layer and 2 core layers would make a 4 layer board.

The movie stack-up approach, a newer technology, 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 number of layers needed by the board design, sort of like Dagwood building a sandwich. This approach allows the producer flexibility in how the board layer thicknesses are combined to satisfy the finished product density requirements by differing the number of sheets of pre-preg in each layer. As soon as the material layers are finished, the entire stack undergoes 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 process of producing printed circuit boards follows the actions listed below for the majority of applications.

The procedure of figuring out materials, procedures, and requirements to meet the client's requirements for the board style based on the Gerber file information supplied with the purchase order.

The procedure of moving the Gerber file information for a layer onto an etch withstand movie that is put on the conductive copper layer.

The traditional procedure of exposing the copper and other areas unprotected by the etch resist film to a chemical that removes the unguarded copper, leaving the protected copper pads and traces in location; more recent procedures use plasma/laser etching rather of chemicals to get rid of the copper material, allowing finer line meanings.

The process of aligning the conductive copper and insulating dielectric layers and pressing them under heat to activate the adhesive in the dielectric layers to form a strong board material.

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. Information on hole location and size is included 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 positioned 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. Prevent this procedure if possible because it includes expense to the ended up board.

The procedure of applying a protective masking product, a solder mask, over the bare copper traces or over the copper that has had a thin layer of solder used; the solder mask protects against ecological damage, supplies insulation, secures against solder shorts, and protects traces that run between pads.

The process of finish 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 actually been positioned.

The process of using the markings for component classifications and part outlines to the board. Might be used to simply the top side or to both sides if components are mounted on both leading and bottom sides.

The procedure of ISO 9001 separating multiple boards from a panel of similar boards; this process also permits cutting notches or slots into the board if required.

A visual inspection 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 approaches.

The procedure of looking for continuity or shorted connections on the boards by methods applying a voltage in between various points on the board and figuring out if an existing flow happens. Depending upon the board intricacy, this process might need a specially created test component and test program to incorporate with the electrical test system used by the board producer.