Printed installation

Basic concepts. In the SEM, the IET conclusions must be reliably connected in accordance with the electrical circuit diagram. This can be accomplished using both bulk conductors and printed wiring.

With printed wiring, the IET electrical connection was made using printed conductors — separate conductive strips in a conductive pattern (GOST 20406-75). The conductive pattern forms a conductor material obtained by selective etching of metal foil, metal deposition or application and subsequent processing of special pastes.

The configuration of the conductive pattern is determined by the electrical schematic diagram and design and technological limitations. With a non-planar connection graph, several conductive layers separated by dielectric layers are used to eliminate the intersections of the conductive paths. Electrical contact between the layers is carried out using interlayer connections — sections of conductor material passing through holes in the dielectric. Such a pattern is called multi-layered, and under the layer understand the elements of the printed pattern, manufactured in one technological operation.

The structural element on the surface or in the volume of which the conductive pattern is made is called the base. The board is a base material, cut to a specified size, containing the necessary holes and at least one conductive pattern.

Printed assembly — a board with IET attached to it and mechanical elements made by soldering and applied coatings.

The main reasons for the widespread use of printed wiring:

• possibility of electrical and mechanical connection of IET;

• the wide distribution of effective CAD systems;

• sophistication of technological processes and operations;

• high performance and low cost in serial and mass production;

• the ability to automate the installation and installation of IET.

The disadvantages of printed wiring include:

limited switching capacity of individual layers;

deterioration of board reliability indicators as the number of conductive layers and connections between them increase;

almost non-repairable multilayer structures;

the difference of parameters, especially parasitic, of a sample with a volumetric installation from the parameters of a printed-circuit product

the complexity of the development and correction without the use of CAD.

Types of boards. Depending on the purpose, the products with printed wiring are divided into circuit boards for cells and REMO, backplanes for intermodular installation, flexible printed cables, microassembly boards. Along with the usual (rigid) boards with a flexible base, which, due to the difficulty of installing IET and the possibility of bending, it is advisable to use as a backplane, have been developed. In turn, the commercially available flexible printed cable (GPC) is a strip of thin flexible dielectric with a printed pattern in the form of parallel running strips.

For the manufacture of film resistors and capacitors on the circuit board micro-assemblies (substrates) are applied along with conductive layers of resistive and dielectric materials.

Base materials, the number of conductive layers, and the technology of producing a conductive pattern are commonly used as other classification features.

Materials are divided into groups: layered dielectrics; ceramics; metal sheets coated with a thin dielectric layer. Boards with a base of laminated dielectrics are called the actual printed circuit boards (PP), from ceramics — ceramic plates (KP), from metal — integral boards (PI).

Currently, with low requirements for packing density and operating conditions most often use PP from layered dielectrics, which are characterized by low cost; a wide range of foil and non-foiled materials produced by the industry in the form of large-sized blanks; ease of machining by punching, drilling and cutting. The main disadvantage of layered dielectrics is low thermal conductivity, and in some cases, insufficient resistance to external influences.

Ceramics has a thermal conductivity of about twenty times greater than glass fiber, before annealing is easily processed. The possibility of warping ceramic bases during firing limits their maximum dimensions to relatively small values ​​(up to 120 mm). The use of metal bases covered with a thin layer of insulator is promising. When using bases made of aluminum alloys, this may be an oxide layer, and of low carbon steel, a glass-enamel layer or a thin polyimide film. Metal bases have sufficient rigidity, so that the geometric dimensions of the boards in this case limits not the resistance to mechanical stress, but the possibility of applying a large-sized printed pattern with the required accuracy. Aluminum oxide, enamels have good thermal conductivity, but greater dielectric constant (?? 10). The thermal conductivity of a polymide film is about one and a half times worse than that of fiberglass, however, with a film thickness of about 30 microns, the resulting thermal resistance of such a base is still significantly less than that of fiberglass laminate and even ceramics, and the low dielectric constant of polyimide (?? 3) reduces parasitic capacitances .

Depending on the number of layers, single, dual and multilayer boards are distinguished. In single-layer PCBs with a layered dielectric, the conductive pattern is located on one side of the base, and in double-layer PCBs, on two, therefore they are called unilateral (OPP) and double-sided (DPP) printed circuit boards. When using layered dielectrics and ceramics (before firing), it is easy to make holes in the bases, so multilayer printed circuit boards (MPP) and ceramic multilayer boards (ILC) are sets of mechanically combined NFPs or one-way KPs, whose conductive layers are connected using metallized vias. For the same reason, in multilayer boards with metal, and in many cases, ceramic bases, conductive and insulating layers build up on one side consistently on top of each other (layer-by-layer build-up).

There are boards with the so-called surface mounting technology (TMP), in which the IET pins are attached to the contact pads on their surface with an overlap. The ability to install the IET on both sides of the base, the processability of installation, the absence of holes determine the promise of motherboards with TMT. Mixed installation boards are used: IET installation on one side into the holes, and on the other, TMP.

When classified according to technological features, the boards are divided according to the methods of obtaining the selective pattern and the material of the conductive layer. The selective pattern can be applied using photolithography or screen printing using a stencil made on the basis of a fine grid. Yielding on the main indicators of meshography, photolithography is better in terms of the accuracy of reproduction of sizes and in the case of large figures.

The conductor layer can be obtained by subtractive, additive or semi-additive methods. Subtractive technology (selective removal of material) is less preferable than additive (selective deposition) because of the high consumption of copper, etchant and environmental pollution. Despite this, selective etching of copper foil has become widespread, especially in the manufacture of single and small-scale REFs, which is explained by the wide range of foil dielectrics produced by industry, the smoothness and relative simplicity of technological processes. In the additive process, selective copper deposition, thin film or thick film technology is used. A semiadditive process involves covering the entire surface of the dielectric with a thin layer of copper, applying a protective pattern and subsequent galvanic thickening of the copper conductors. The additive and semi-additive technology allows to metallize the holes.