1.1. -THE BASIC FUNCTIONS OF A PRINTED PLATE ARE:
- Support your own components.
- Support your electrical interconnections.
All this following established rules in view of tolerances imposed by the nature of electronic equipment or systems.
In the design of printed circuits, we will find a series of variable factors that must be selected and combined in an optimal way in each case.
The placement of the components on the circuit base plate itself, the dielectric material of the base, the type of conductors, the number of layers of conductors, the rigidity, the density or compactness of the equipment on the plate, etc., properly combined will influence the performance, quality and cost of the product.
In the design, it will also be necessary to think about the Manufacturing conditions, creating adequate information for Manufacturing, knowing the means and costs that will intervene in the different operations to be followed, in order to find viable and profitable procedures.
1.2.- ADVANTAGES OF PRINTED CIRCUITS IN THE DESIGN OF ELECTRONIC EQUIPMENT, WITH RESPECT TO CONVENTIONAL CIRCUITS
- a) Space saving: Using printed connections takes up less space in the equipment than with the use of conventional connections.
- b) The conductors are permanently bonded to the base dielectric of the circuit, which also provides greater ease for mounting the components.
- c) It is normally impossible to break wires and produce a short circuit between wires.
- d) Given the high repeatability in the circuits, there is a uniformity of the electrical characteristics from assembly to assembly, increasing reliability?
- e) The volume and weight of the interconnections are significantly reduced.
Clear routes (tracks) of the conductors are produced that allow easy visual monitoring of them and greater organization and control of the space. This is all due to the flat shape of the conductive print.
- f) The identification of the parts of the circuit is simple and the coloring of the wires has been eliminated.
- g) Production processes in large series and highly automated techniques can be used.
- h) Operators may be employed with a minimum of training and skill.
- i) The clarity of the circuits allows, with visual aid, to simplify the verification processes as far as accuracy in the assembly of the components is concerned, thus minimizing errors.
- j) The maintenance of Electronic Equipment is more simplified, it is cheaper.
- k) In flexible plates, its flat and thin shape produces maximum savings in weight, space and cost. Up to 75% savings in volume and weight can be achieved, depending on your specific application.
1.3. -LIMITATIONS OF PRINTED CIRCUITS.
- a) The flat shape of the circuit requires a special skill in the design to locate the components and the interconnections.
- b) The long time spent in the design stage influences appreciably from the initiation of the design to the delivery of the final product.
- c) It costs too much work and money to introduce changes in the design when the established tools and manufacturing means are already available.
- d) Difficulties found in the repair of printed circuits.
1.4. – BASIC ELEMENTS OF PRINTED CIRCUITS.
- a) Insulating support.
- b) Holes for mounting components and/or interconnection.
- c) Interconnection connectors.
- d) Input and output terminals.
1.5. -CLASSIFICATION OF PRINTED PLATES.
1.5.1. – Categories of printed boards according to their density in components and interconnections.
Three basic categories are considered according to their densities in order from lowest to highest:
- a) Simple Face, with conductors on a single flat surface of the insulating base.
- b) Double Sided, with conductors on both sides of the insulating base, with metallized holes for the interconnection between sides, or other means.
- c) Multilayer with three or more layers of conductors separated by insulating material and usually interconnected through metallized holes.
1.5.2.- Densities of printed plates.
In any printed board, it is necessary to combine the limitation of its surface with the elements (components and interconnections) that must be equipped as indicated by the circuit. There are or may be a series of incompatibilities, given the diversity of sizes and shapes of its components, their number, the complexity of their interconnections, etc.
It is desirable, according to this, to know a measure that gives an idea of the order of the density of a printed plate and that allows typifying them.
The unit of density is taken as the number of holes, to mount components, per square decimeter of useful surface. This unit is not perfect, but it can serve as a reference to know, in a first approximation, the portion of the circuit that can be assembled effectively in each case.
Usually, the values indicated in the table correspond to the different classes of printed plates.
1.5. 3. -Classification system
There is a system for classifying printed boards by their densities, which provides the degree of concentration of conductors, knots and holes. This data, together with other factors such as the size of the plate, determine the tolerances allowed in the different phases of the design and manufacturing processes.
The classification system consists of two digits. The first digit represents the type of plate (number of layers and type of connections through them), and the second digit is related to the maximum local concentration of conductors.
To consider the amount of density of the printed plates, the following three variables are introduced:
- a) Nominal width of the conductors.
- b) Nominal separation between conductors.
- c) Difference between the nominal diameter of the nodes and the nominal diameter of the corresponding holes.
According to this, the second digit of the classification of a printed plate in design will be the smallest number for which the minimum values corresponding to the variables indicated above are satisfied over the entire plate.
1.5.4. –Minimum dimensional limits for each class of printed plate
Below, in separate tables, the minimum limits that define each class of printed plate are established, in terms of density, by the two-digit system.
- a) Plates without metallic holes.
The first digit of the classification of this type of plates will be 1, and the second will take the values 1, 2 or 3 according to the three parameters a), b) and c) indicated.
b) Plate with metallic holes.
The first digit will be 2 and the second will take the parameter values 1, 2, 3 or 4 according to the three parameters a), b) and c) indicated.
1.6. -MATERIALS USED IN THE BASE PLATE OR INSULATING SUPPORT.
They can be chosen among the following materials: according to the application of the printed plate.
- a) Rigid phenolic resins, with paper impregnated in them.
- b) Rigid polyester, with fiberglass impregnated in it. (
- c) Epoxy resin, with paper impregnated in it.
- d) Epoxy resin with fiberglass impregnated in it.
- e) Sheet Film of “mylar”, “teflon” or polyamides.
The choice, in each case, of the type of base material to be used will be made in accordance with the application and functions of the circuit to be supported.
The most used materials are a) and d). The so-called e) will be used in the case where mechanical rigidity is not an important factor, replacing the type marked d).
Materials should always be flame resistant.
The costs of these materials vary from the cheapest (phenolic resins with paper) to the most expensive (epoxy resin with fiberglass).
The cost differences of the materials are due to the physical, thermal and electrical properties of each type of material.
Materials type (a), (b) and (c).
Types (a), (b) and (c) are susceptible to puncture. The punching operation is economical when the manufacturing series are high. The use of these materials is limited to printed circuits whose holes are not going to be metallized.
These materials are not recommended for multilayer printed circuits, due to their poor dimensional stability; in boards with high conductor densities, breaks can occur inside the holes, due to the thermal shock, the terminals of the components are welded.
Materials type (d)
These materials are the most used in circuits that have metallized holes. Its dimensional stability is acceptable for boards with high conductor densities, with minimal breakage inside the metallized holes due to thermal shock. Holes in this type of material must always be drilled. There are difficulties in drilling, if they are made with a matrix, with the thicknesses normally used for printed circuits. The cutting, to size, of the plates must be done with a saw; shear or milling cutter since using a die is not recommended.
Type (e) materials
A great deal of work is currently being done to develop new types of base materials for flexible circuits. These materials in the form of a dielectric “film” have good electrical and mechanical properties. Normally these dielectric “films” have a layer of laminated copper and their use is widespread for multilayer circuits and hybrid printed circuits, whether or not they have metallized holes.
1.7. -THE SIZE AND SHAPE OF THE PRINTED CIRCUITS .
Normally, these two physical characteristics of printed plates are limited by the dimensions of the equipment for which they are intended and also by the tools and existing manufacturing facilities (machinery, facilities, etc.)
In order to reduce manufacturing costs, it is necessary to ensure that the choice of plates is made on standardized sizes for which the corresponding tools already exist (cutting elements, templates, accessories, etc.).
1.8. – COSTS.
The differences in cost between several small plates and a large equivalent are minimal. Large plates are more expensive to replace. Small boards need more connectors and have more waste.
1.9.- THICKNESS OF THE BASE MATERIAL.
The thickness is variable; it varies between 0.8 mm and 3.2 mm. For rigid plates (epoxy glass), the thickness of 1.6 mm. is the most used, the tolerance allowed in this case is +- 0.2 mm.
When the basic material is phenol or epoxy with paper, the admissible tolerance will be +- 0.14 mm.
The measurements are normalized in the following thicknesses: 0.8 mm. 1.0 mm. , 1.6mm. , 2.4mm. , 3.2mm. These values refer to nominal thicknesses of the finished printed plates.
1.10. -DEFORMATIONS OR WRAPS.
The base plate, with its plastic material, is subjected to temperatures that warp its original flat shape. The degree of deformation is higher for phenolic materials with paper and lower for epoxy resins with fiberglass.
The degree of warping also depends on the type (one-sided or two-sided) and size of the printed board, as well as the predominance of the metallic structure (conductors) and its balance on both sides (eg there may be ground planes and one side).
It is necessary to incorporate buttresses or ribs to minimize warpage. These are conveniently placed in the center or on the sides of the plate, prior to the simultaneous welding operation. The printed circuit connectors also serve as reinforcement, if their placement is studied.
1.11 .- HOLES .
Properly metallized, they are used to mount components and establish interconnections. They can be practiced by punching and drilling.
1.11.1. – Punching.
It is the most economical method when the same configuration of holes is repeated 50,000 or more times. It is used in cases where the basic material is paper or fiberglass.
Limitations for punched hole diameter and hole center spacing depend on the type and thickness of base material used.
1.11.2. – Drilled.
It is used almost exclusively for plates with epoxy fiberglass base material. It is a more expensive process than punching but there is economy if multiple numerically controlled drilling machines are available. There is no limitation on the diameter of the holes, but it is considered in practice, as a minimum limit of 0.6 mm.
Drilling tools, with perforated tapes for machine control, require less manufacturing time than punching tools.
For metallized holes, it is recommended that the diameter is not less than one third of the thickness of the circuit base plate. Under special conditions, the diameter can be reduced to one fifth of the thickness of the material.
1.12. -CONDUCTIVE PRINT.
The simplest process to obtain the conductors of a circuit is to etch them on the base laminate sheet. This requires a minimum of process steps and has been widely used in large production runs. The attack to obtain the circuit must be applied to one or both sides of the laminate.
To achieve the interconnection between the conductors on both sides, electrochemical procedures can be used. for metallizing holes.
To increase the density and complexity of the wiring, printed circuits are used with the conductors on both sides interconnected by metallized holes. These processes are used in industry for double-sided and multilayer circuits.
This requires special equipment for drilling and metallizing. So that the final product has the circuit conductors protected by metals resistant to corrosion, such as tin, lead, gold, etc. , which favor weldability during long storage times.
The manufacture of multilayer circuits: It is a combination of several processes. First, the conductive layers are individually printed and embossed, except for the outer ones, and then they are joined to form an integral panel. This panel is then processed as if it were a double-sided printed circuit with metallized holes.
The printing or the operation of depositing the model drawing on the copper of the insulating support can be done in two ways, fundamentally by serigraphy and by photoengraving. Either procedure has its own limitations though. in principle, these limitations are defined by the size of the manufacturing lot and by the conductive print density.
There are therefore two limits to determine which of the two processes should be followed. For high-density boards, with very close tolerances, in the case of class 23 and 33 boards, the process is limited to photoengraving. In plates whose manufacturing batches are small, below 10 panels, photo-engraving is also recommended, regardless of the classes of the plates.
1.13. -ENTRY AND OUTPUT TERMINATIONS.
There are basically two methods of termination between the conductive impressions of printed circuits and the interconnection of them with the system. The other method is to solder terminals for hard wiring of the PCB to the rest of the system. The other method consists of a quick connection through connectors.
The quick connection of printed circuits with connectors can be done in two ways, one by means of discrete connectors in the form of plugs and another by printed contacts on the edge of the board.
Direct plug-in contacts can be individually mounted or multi-mounted, using flanged clips and with a dielectric sandwiched between the connector and the plate.
The printed edge contacts of the conductive print are usually covered with supplemental plating that improves wear and extends their service life. These connectors are cheaper than the previous ones, but they limit the structure of the circuit by the width of the conductor and therefore the coupling for which the connectors are useful