2. 1. –Introduction to PCB Board Design
When starting the design of a pcb board, the functional requirements imposed by the system, by the client, etc. must be taken into account. , as well as the commercial economic aspect of the project.
But there are also a series of factors limited by the rules of making and by the facilities available to the factory (machinery, tools, etc.), to be taken into account when choosing the pcb board.
A circuit or a family of printed circuits that meet an optimal design will be obtained, when these requirements have been combined, in a detailed analysis. Many times from this analysis the conclusion is drawn that it is difficult or impossible to arrive at the ideal design. Below are the most important considerations that must be taken into account, in the choice of pcb board, to find consistency and interactions between design rules, manufacturing facilities and the final economic result:
- a) Specification of the product and estimate of its cost.
- b) Equipment Life.
- c) Electronic requirements (voltages, gains, impedances, etc.).
- d) Manufacturing methods:
- dl) Compatibility with the existing Factory Plant.
d2) Size of the order to be produced
d3) Degree and type of mechanization used.
- e) Subsequent operations:
- f) Maintenance
f1) Operational requirements.
f2) Repair requirements.
f3) Minimum degree of maintenance required.
- g) Materials and Components
- gl) Acquisition sources
g2) Delivery dates
In the design of a pcb board, it will be necessary to start by choosing the simplest category according to its density, attending to the requirements expressed in the corresponding specification. Electrical and environmental factors and the use or destination of the circuit board must be taken into account.
The lower the category of the board, the lower the cost and the less important in terms of quality and quantity the problems that arise in design and manufacturing.
The orientations on the use of the methods of design and distribution of the elements of the circuit in the base circuit board, must be presided over by the DISTRIBUTION RULES stated in this chapter.
2.2. – DISTRIBUTION METHOD. BASIC MODEL DRAWING.
Knowing the schematic, with the greatest precision and clarity possible, the distribution of the components of the circuit will be drawn in its simplest form, placing the elements with the idea of reducing the crossing points of the interconnections to zero.
The distribution will be made based on successive initial drafts or sketches, which gradually incorporate improvements, until the next step in drawing generation can be carried out. It is not uncommon to specify 3 or 4 sketches, and even more, in circuit boards of higher categories.
It is useful to use templates that serve as a basis for the assembly of components and that, together with the point-to-point connection of the schematic, help organize the interrelationships of the circuit, trying to comply with the design rules expressed in the chapter.
For an initial sketch it is common to start from a paper copy of a Basic Model Drawing that contains elements common to a family of pcb boards, thus saving time in this design step.
This mode of operation is used both for the manual production of model drawings, and when preparing sketches for digitization, prior to the use of plotting machines.
The Basic Model drawing generally records the following impressions:
a)circuit board outline.
- b) Reference Systems (with all their indications and marks).
- c) Printing and labeling of the connectors.
- d) Position of the support hole.
- e) Normalized configuration of holes.
- f) Ground and power planes.
- g) Connector outlet positions, as specified by the product.
- h) Test knots, as assigned by the product test specification.
These impressions can be common, repetitive or fixed and as such are expressed in the drawing.
In the method of distribution of a pcb board, the following steps can be followed as a guide, which can be completed in the order indicated or in another, increasing or reducing their number.
- a) Study of the schematic and the complete and detailed list of the components of the circuit.
a.1) Dimensional parameters (templates) of the circuit board and components
a.2) Electrical and mechanical parameters
- b) If a paper copy of the Basic Model Drawing is available, it will be taken as a starting point. If it is not available, assign their positions to all the fixed elements of the circuit (list of components) making the distribution bearing in mind the Distribution Rules for pcb boards.
- c) Find the repetitive parts of the Circuit and place them looking, in principle, for a uniform geometric distribution, coupling, on the other hand, the mechanical components (switches, pins, etc.).
- d) Decide on the type of base material for thecircuit board. This depends on its density, external environmental conditions, assembly conditions, recipient equipment, etc.
- e) Set the sizes of holes and knots and the widths of the conductors.
- f) Keep in mind the density classification, trying to avoid limit configurations.
- g) Locate components according to their input and output requirements
- h) Consider all the electrical, mechanical, structural and environmental requirements, especially taking into account:
h.1) Insulation resistance and/or dielectric properties of the base material for higher frequency or impedance circuits.
h.2) Voltage drops and temperature rise of the conductors.
h.3) Environmental operating and storage conditions (humidity, dust, temperature, vibrations, shocks, etc.)
h.4) Mutual coupling and inductive and capacitive effects.
h.5) Uniform distribution of the weight of the components on the surface of the base circuit board.
- i) Special reinforcements, special insulation, critical ground connections, temperature dissipation.
- j) Optimum use of the useful space of the circuit board for the routing of the conductors, remembering that they must be as short as possible.
- k) Take into account in the placement of the components, the necessary separation of the same to make feasible the use of the tools and devices that must be used in the assembly operations. Of course the components must not interfere physically, in accordance with electrical, thermal and repair requirements.
- l) The most delicate components must be placed in the correct place, to avoid harmful electrical couplings in the operation of the circuit.
All these requirements must be taken into account as a guide to follow in design, in accordance with the product specification. They serve both for manual processes and for digitization processes, as mentioned above.
The identification and accuracy of the finished pcb board can be, at most, equal to that of the Original Model Drawing.
The designer must be aware of the most important manufacturing parameters and reprographic processes of the pcb boards that can be used.
23. -DISTRIBUTION RULES.
Usually when a printed board is designed, there is a product specification that meets all the necessary requirements for the engineer to be able to generate all the information needed to manufacture it:
- a) Geometric characteristics of the circuit boards: External dimensions, details of their contour (notches). Hole blocks, their location relative to the contour or to a reference point. circuit board thickness and flatness limits.
- b) Conductive printing, its configuration on one or both sides, components that must be welded. Dimensions of nodes and conductors. Identification Records.
- c) Base material (your specification). Metallic holes. Protective finishes.
All this with the precise detail and for all kinds of circuit boards. both for the repetitive and for the special ones.
Below are a series of numerical data and design rules that the engineer needs to know to generate Model Drawings. This in turn, in the Factory, will be the starting material to begin the production process of the pcb board.
The smallest possible number of different sizes of holes is usually used for assembly and connection. The attached table shows the nominal diameters and their tolerances.
|Nominal diameters in mm.||non metallic||metallic|
To obtain a certain economy in drilling or punching operations, it is recommended not to use more than 3 diameters for ordinary holes.
The thickness of the metallic coating film will be for copper:
20 µm. min for holes diam. <= 1/2circuit board thickness.
25μm. min for holes diam. > 1/2circuit board thickness.
For tin/lead: 10µm. minimum.
Position of the center of the mounting holes, of the components with respect to the separation from the origin. tolerances.
Spacing <= 150mm. – Tolerance 0.1mm.
Separation > 150 mm. – Tolerance 0.2mm.
The distance between holes is expressed in the following table, with its tolerances.
|Distance between holes (d)||Distance tolerance (mm.)|
|Classes 11 and 21||Classes 12, 22 and 23|
|50 <= d < 100mm.||±0.2||±0.1|
The tolerance on grooves and notches is ±0.1 mm. in its two dimensions, whether or not they are metallic.
The following table specifies the nominal diameters of the conductive nodes corresponding to the nominal diameters of the holes. The nominal welding reserve node is also indicated. All this in the different types of circuit board.
|mm.||Lessons||diam. hole rating
of the node
of the node
The tolerances on the minimum widths of the conductors for the model drawings and different classes of circuit board are indicated in the attached table.
|Printed board class||eleven||twenty-one||12||22||13||23|
|min width in mm||0.8||0.8||0.6||0.5||0.4||0.4|
|Tolerance in mm.||±0.04||±0.04||±0.03||±0.03||±0.02||±0.02|
In the finished printed board and after the etching processes the conductors must have the following widths and tolerances.
|Nominal width in mm.||0.4||0.5||0.6||0.8||1.0||1.3||2.6|
|Maximum reduction in mm.||0.05||0.05||0.06||0.08||0.10||0.13||0.26|
Separation between conductors in the model drawing.
|PI class||11 and 21||12||13||22||23|
between conductors in mm.
Separation between conductors in finished pcb board and after the etching processes.
It must not be less, throughout the pcb board, than the following minimums in mm.
|PI class||11 and 21||12||13||22||23|
between conductors (mm.)
Distance of the conductors to the edge of thecircuit board: It will not be less than 2mm.
The conductive crowns on the faces must comply with their limitations, with respect to the holes that correspond to them, as follows:
|IP class||Minimum radius width (a) of the conductive ring,
formed by a node and its hole (ignoring edge defects).
|21,22 and 23||0.05mm|
Conductor thickness (for non-metallized circuit boards).
|Nominal thickness in mm.||Minimum thickness at reception||Minimum thickness before welding|
2.4. –LIMIT CONFIGURATIONS OF THE CLASSES ACCORDING TO THEIR DENSITIES
The dimensions that appear in the following examples have been obtained using the data from the previous sections.
2.4.1. –Limit configurations for circuit boards. Class 11.
Figure (a) shows the minimum possible gap between two holes of 1.2mm. in diameter, located in the grid of a module. Also listed is the only possible node size for that configuration.
Figure (b) shows the only configuration in Class 11 that allows the passage of a conductor between two 1.2 holes separated by 2 modules.
2.4.2. – Limit configurations for circuit boards. Class 12.
Figure (c) shows the minimum separation allowed between two holes located in a ½ or 1 module grid. For said separation, the maximum nodes and holes that can be used are listed.
Figure (d) shows the passage of a conductor between two 0.8 mm holes. located on a 1½ module grid.
2.4.3. – Limit configurations for circuit boards. Class 21.
Figure (e) shows the minimum distance allowed between two holes located in a 1-module grid. Likewise, the maximum holes and nodes compatible with said separation are indicated.
Figure (f) shows the passage of a conductor between two 1.2 mm holes. located in a grid of 2 modules. For this configuration, the only possible node is indicated, as well as the maximum width of the conductor.
2.4.4. – Limit configurations for circuit boards. Class 22.
Figure (g) indicates the minimum separation allowed between two holes located in a 1-module grid. The maximum possible nodes and holes are also listed.
Figure (h) shows the passage of a conductor between two 1.2 mm holes. located on a ½ module grid. For this configuration, the minimum node and the maximum conductor allowed are indicated. If the width of the conductor is reduced to another value within the margin indicated in the section corresponding to circuit boards without metallized holes, the diameter of the nodes or the separation between conductors can be increased.
Figure (i) shows the passage of a conductor between two 1.2 mm holes. located diagonally in a 1-module grid. The only supported node and conductor sizes for that configuration are also listed.
2.4.5. – Limit configurations for circuit boards. Class 23 and 33.
Figure (j) shows the only configuration that allows the passage of a conductor between two 0.8 mm holes, located in a 1-module grid.
2.4.6. – Typical applications of the different types of circuit boards.
The class of a printed board is fundamentally determined by the type of compactness of the components mounted on it.
The following tables indicate the possible limit configurations for each class, as well as the type of components that define them.
2.5. – RULES OF DISTRIBUTION OF COMPONENTS ON THE BASE CIRCUIT BOARD.
The rules below are to be followed on manually or computer-aided boards, and are specifically observed by technicians involved in printed board design.
A large number of the design rules depend on the direction in which the board is carried on the solder wave (molten tin bath over whose surface the board with its assembled components is passed to be soldered). That direction of travel is called the “transport direction in the circuit board weld.”
2.5.1. –Space between components.
The maximum size of the body of the components, admitted in the specification, must be taken into account. This will be the starting point to study the space between components.
In general, a minimum distance of 0.5 mm is allowed around the components for the purpose of inserting them. However, under certain circumstances (heat dissipation, high voltages, etc.) it may be necessary to space the components further, depending on the requirements and size of the components.
In that case, the special distances required must be defined by the designer on the printed board assembly drawing.
In the cases of high-density printed boards that do not have special requirements, isolated components with diameters less than 5 mm can be assembled without special precautions.
Example: Let there be two components A and B. (See fig. below).
It will be T(min)= ½ max. body diameter of A + ½ max. body diameter B + x(0.5 mm.). The value of T must always be rounded to the modulus dimension (M) or ½ (M).
Fig.2.5.1. Greater space, than the nominal, between components for isolation purposes.
2.5.2. –Overlapping of components.
The overlapping of components on the printed board, as indicated in the figure, is not allowed. This restriction is necessary in order to allow the replacement of a particular component without the need to move other adjacent components. In this sense, the wires used as bridges are also considered components.
Fig.2.5.2. Overlapping between components.
2.5.3. -Space between components and location holes of the circuit board.
The following figure shows the distances between centers that must exist between the locating holes and the closest mounting holes for speakers in the automatic mounting of components.
Fig.2.5.3. Placement in location hole circuit board.
2.5.4. -Layout of components.
Correct component orientation on the printed board also makes it easier to trace tracks later and reduces the overall cost of the printed board.
In order of preference, here are four different ways to arrange the axial components:
Configuration A. -All the axial components are mounted with their larger side parallel to the side of the circuit board that carries the connector and the same for the polarized components (diodes, etc.), with their polarity oriented, in all of them, in the same direction. In configuration 1A (less preferred) the polarized components could be oriented differently.
Configuration B. -All axial and polarized components have their larger side vertical to the side of the circuit board that carries the connector. The polarized components all have the same orientation. In Configuration 2B (less preferred) the polarized components may have different orientations.
Configuration C. -(Little preferred) .Most of the components are oriented parallel to the side of the board with connectors, but some are vertical to this side. The polarized components are not all oriented in the same direction.
Configuration D. -(Should be avoided). Most of the components are parallel or perpendicular to the side of the board that carries the connectors; but some of them are placed perpendicular to thecircuit board, that is, standing up. This Configuration makes the board considerably more expensive and should only be used when absolutely necessary.
Layout of Integrated Circuits (DIP). They should all have their polarity oriented the same way and the preferred orientation is with the polarity marking facing the connectors side of the board.
LAYOUT OF COMPONENTS
|A. PREFERRED ARRANGEMENT||B. PREFERRED ARRANGEMENT IF ‘A’ IS NOT POSSIBLE|
|C. LITTLE PREFERRED PROVISION||D. NON-PREFERRED ARRANGEMENT, MUST AVOID|
LAYOUT OF COMPONENTS
INTEGRATED CIRCUITS (DIP)
LOCATION OF THE HOLES IN THE GRID
TO REACH THE MAXIMUM DENSITY OF COMPONENTS ON THE CIRCUIT BOARD:
– All holes in the Y direction must be on the grid (1M=2.54mm.)
– All holes in the X direction in multiples of ½ M=1.27 mm.
– Typically: X1=1 ½ M.
2.5.5. -Mounting distance for axial terminations of components.
The following values will be used to determine the minimum mounting distance.
The axial terminations of the components will be submitted as indicated in figure 2.5.5. respecting parallelisms and forms.
- a) Non-glass body components with terminations of diameter <= 0.8 mm.
Min. distance “S” = max. body length s/spec. + 4 mm. Round to the next multiple of the module (M = 2, 54 mm).
- b) Glass body components and all components with conductor diameter > 0.8 mm.
Min. distance “S” = max. length of the body s/spec. + 5 mm. Round up to the next multiple of the module (M = 2.54 mm).
2.6. -TRACKING RULES.
The quality of a printed board, from the point of view of metallization, weldability of the circuit board holes and flatness, depends on the shape and distribution of the conductive print. The original drawing must satisfy the following rules:
2.6.1. -Conductive printing and shape.
- a) On each face of the circuit board the conductors must be distributed in the same direction and as long as possible. On the weld face, – whenever possible, the conductors should be oriented in the direction in which the circuit board moves in the welding machines, that is, parallel to the longest edges of the circuit board.
- b) The changes of direction of the conductors must be forming angles not less than 45º. Acute angles are not allowed.
- c) The width of the conductors must be as wide as possible, narrowing only in special and necessary cases.
2.6.2. -Distribution of conductive printing.
- a) The conductive print must be distributed over the entire surface of the circuit board in a uniform manner.
- b) Conductive printing areas on double-sided boards should be as equal as possible. To satisfy points a) and b), additional areas must be included in the model drawing. These areas are not part of the electrical circuit but ensure a good distribution. During metallizing processes these additional areas aid in current distribution so that the electrodeposition is more evenly distributed.
2.6.3. -Configuration of aids for current distribution.
- a) Current distribution aids consist of blind traces or crosshairs with lines approximately 6 mm apart. and 1mm wide. These blind zones should be positioned as close as possible to the conductive print, but never closer than 1mm. away.
A typical distribution of blind lines is indicated in the figure.
- b) On the welding face, the lines of the blind grid must form 45º angles with the direction of movement of the welding machine. A typical case is shown in the figure.
- c) A blind line of 1 mm. wide can completely surround the conductive print. This blind trace should be located as close as possible to the conductive print but outside the outline of the finished board. The figure shows a typical case of application of the blind outline outline of thecircuit board.
- d) The blind lines must not be constituted by a filled area. This is done because if the blind traces occupy a larger area than the areas of the conductive print, it can lead to non-uniformity in the metallics. The purpose of the blind traces is to protect the current distributions in the conductive print throughout its entire area. The mesh of the grid is therefore recommended to be 6 mm. Figures a , b and c show several typical cases with this technique.
Fig. a)circuit board with correct application of blind lines.
Fig. b)circuit board whose design does not require blind lines.
Fig. c)circuit board whose design is not correct. Filled areas should be gridded.
2.6.4. -Influence of the conductive print on the weldability of the holes.
A circuit board hole will have good weldability when its conductive area on the weld face is greater than on the component face. This is achieved in two ways:
- a) By an appropriate method of connecting the conductors to the node. Two examples are shown in the figure:
If a certain number of conductors start from a node, they must be arranged, on the component face, so that they start from a point separated from the node, as indicated in the figure.
- b) By selection of the size of the node. On the weld face an isolated hole must be surrounded by a larger node than on the component face; or, it must be connected to a blind conductor, as indicated in the figure.
Improved weldability by selecting the diameter of the node.
2.6.5. -Contacts for edge connectors.
Edge connectors allow PCBs to be connected quickly and easily. They can be of the direct or indirect connection type.
18.104.22.168. -Direct connection connectors.
The female connectors receive the edge connector printed on the board by simple plugging. For this purpose the connector contacts must be gold circuit board and. so specially made to fit the female connector. Tolerances in circuit board thickness must be taken into account.
22.214.171.124. -Indirect connection connectors.
In this case the multiple contacts of the connector are soldered to the board. The circuit board does not require a special shape and the tolerances are not so critical.
126.96.36.199. -Comparison between the types of connectors.
From the point of view of printing, the indirect connector is cheaper. The contacts do not need to be gilded and therefore the metallizing process is easier. The dimensions of the circuit boards are not critical and there is no special requirement regarding the thickness of the circuit board. The direct connector, on the other hand, requires tighter thickness tolerances. This is sometimes difficult and therefore the reason for a higher rejection rate.
The disadvantages of indirect compared to direct is that it requires more space in addition to its cost and the assembly operation.
To perform electrolytic gold plating of the edge connector, it is necessary to connect all its contacts to a conductive strip located outside the contour of the printed board. This interconnecting bar must be shown on the model drawing.
2.7. – METHOD OF CONNECTION BETWEEN LAYERS
The methods used for electrical interconnection between faces of a printed board are reflected in the attached table and must be selected for each particular case. The following table indicates the different methods to be used, depending on the type of circuit boards and the insulating support.
|connection method||insulating support||multilayer circuit boards||Lead allowed in the same hole|
|Phenolic paper||epoxy paper||epoxy glass|
|Thread in “C”||X||X||X||X|
Any of the three methods can be placed under components as long as they are properly insulated and separated from the printed board surface.
2.7.1..- Connection method in “C”.
It consists of a bare wire passing through a hole in the pcb board , folded over each face of the pcb board and soldered to the conductive print on both sides. The wire is not welded to the nodes immediately adjacent to the hole, but its ends are positioned so that the difference between the thermal expansions of the wire and the base material causes a slight additional bending of the wire, rather than lifting. (detachment) of the nodes or a breakage of the welded joint.
The figure shows a transverse connection section with “C” thread.
There are automatic and semi-automatic machines for inserting and forming “C” wires.
The design of the pcb boards to use the connection between faces with “C” wires must meet the following requirements:
- a) The nominal diameter of the holes must be 1.2 mm.
- b) No point of conductive printing on both sides of the pcb board, electrically connected to the wire or insulated from it, will be less than 1.5 mm nominal distance from the center of the hole.
- c) The shape and nominal dimensions of the nodes for the connection of the “C” wire, on both faces of the circuit board, are indicated in the following figure.
- d) The “C” wires of a printed board must be oriented according to the direction of the simultaneous welding, as indicated in the figure. However , it is recommended that all the threads in “C” be aligned in one direction only.
- e) In order to leave enough space for the “C” wire insertion tool, no holes should be placed within the oval area indicated in the figure. For example, once hole A is located, hole B will be allowed but hole C will not.
- f) The base materials in which the “C” thread connection can be used are the following:
– Phenolic paper.
– Epoxy paper.
– Epoxy glass.
2.7.2. -“V” connection method.
It is a preformed “V” thread that is inserted into a hole in the printed board. The wire is folded over the component side and soldered onto the conductive print on the component side. Bonding with the conductive print on the solder face is made by the solder position that fills the space between the “V” wire and the conductive foil. Such a section is shown in the figure.
The design of the pcb boards to use the connection between faces with thread in “V”. must meet the following requirements:
- a) The nominal diameter of the holes for “V” threads must be 1.2 mm.
- b) A hole used for the passage of a “V” wire cannot also be used for the passage of a component terminal.
- c) On the weld face of the circuit board there should be a circular node around the hole. The diameter of this node will be in accordance with the standards, that is, the minimum nominal diameter of the node for each class of circuit boards is the
|Class||Minimum Nominal Node Diameter|
- d) The spacing between adjacent “V” wires is limited by the normal rules governing the spacing of nodes, holes, and conductors.
- e) The “V” threads of a printed board must be oriented according to the direction of the simultaneous welding.
- f) The base materials in which the “V” thread connection can be used are the following:
– Phenolic paper
– Epoxy paper.
– Epoxy glass.
2.7.3. -Method of connection by metallized holes.
The most widely used interlayer connection method for double-sided and multilayer PCBs is circuit board holes. It consists of a hole on whose walls metal is deposited by chemical and electrolytic procedures, making the different layers through which the hole passes conductive.
On a printed board, if a connection between layers is made using this method, the rest of the connections between layers must be made using this same procedure.
The requirements that must be met in the design of circuit boards with circuit board holes are as follows:
- a) The circuit board holes can be used not only to connect the layers to each other but also to house the terminals of the components.
- b) The diameter of the holes must be selected according to the specified diameters. As a general rule, the diameter will never be less than one third of the nominal thickness of the circuit board. If the hole is also used for the passage of component terminals, its diameter must meet the appropriate requirements.
- c) The metallized holes must have a node in each of the outer layers. In the inner layers (multilayer circuit boards) a node will only be placed if an electrical connection with this layer is required. The minimum nominal sizes of the nodes will correspond to those specified for the type of circuit board.
- d) circuit board holes should only be used in epoxy fiberglass base material.
Main functions of circuit board holes.
The main functions of a circuit board hole are as follows:
1) Improve the fixation of the components.
2) Electrical connection between layers.
- a) When a hole in a printed board is not metallized, the only bonding force between the component terminal and the conductive print (node) is the adhesive that joins the conductive area with the insulating support. (See Figure).
If the density of the circuit board is high, when the diameter of the nodes is reduced, their adhesion force is reduced, thus not guaranteeing the fixation of the component terminals.
The metallization of the hole provides greater adhesion force in the soldering of the terminal of the components. See figure below.
- b) The electrical connection between layers is the fundamental basis of the circuit board hole as well as keeping the component terminal available for welding.
The circuit board holes must meet the following requirements:
In single-sided circuit boards, the function of the metallized hole is only to provide a better union of the component, although in this type ofcircuit boards it is uneconomical.
In double-sided circuit boards made with paper base material, the metallized hole can be made only in order to improve the union of the component, but not for continuity purposes, due to the low reliability that exists in this type of material, with respect to metallized. There is only a guarantee of continuity through the component terminal. Therefore, the union of the hole with the terminal must be visible and accessible or by making a hole parallel to the previous one that remains outside the component. The figure shows a detail of these procedures.
These methods are not recommended.
In double-sided circuit boards made of fiberglass base material, not only a connection between faces but also a good bond between component and hole is achieved.
The possibility of metallizing the holes in a uniform way, with an adequate thickness, depends on this compromise between the diameter and the thickness of the circuit board.
2.8. -ELECTRICAL STANDARDS FOR DESIGN.
2.8.1. -Width of conductor.
This dimension depends on the following parameters.
- a) Load current.
- b) Thickness of the conductor (copper foil).
- c) Separation between conductors.
- d) Type of base material.
- e) Maximum allowable temperature rise.
- f) Component assembly method.
The first three parameters define “the amps per unit area of the conductor”.
The next two, together, determine “the current density”.
Figure 2.8.1. It serves to know the relationship between the widths of conductors and the elevation of temperatures for circuit boards of different thicknesses of copper.
It refers to the cases in which the basic insulating material of the circuit board is phenolic paper, epoxy paper or epoxy fiberglass.
General conditions for the use of the abacus in figure 2.8.1:
- a) There is good ventilation or forced cooling is provided.
- b) The ratio, conductor width to conductor separation, will be greater than 1:2.
When this condition is not met, the current level must be reduced by 30%.
- c) The thickness of the copper will be uniform and counting on the successive operations, to which it has to be subjected in the manufacturing process, it will be necessary to estimate an excess increase, for this dimension, due to possible wear.
Fig. 2.8.1.: Maximum admissible intensity of the copper conductor.
2.8.2. -Temperature rise as a function of the current through the conductor .
In figure 2.8.2. the relationship between the current through a conductor, with a given section, and for a rise in temperature in the conductor is represented. The chart provides a means of relating the conductor width of the 1:1 scale model drawing to the thickness of the copper base laminate. The sum of the maximum ambient temperature in which the circuit board will be and the temperature rise due to the passage of current in the conductor must not exceed the maximum temperature stability of the insulating support. This limit will be indicated in the specifications and physical characteristics of each insulating support.
To select the width of the conductor, once calculated using the graph, you must choose the closest value and always in excess of the previously recommended widths.
2.8.3. -Separation between conductors, and width of conductors on the Model Drawing.
When thinking about the separation between conductors, the following factors should be taken into consideration:
- a) Potential difference between conductors.
- b) Peak voltage.
- c) Surface resistance of the base material.
- d) Environmental conditions at the destination of the equipment, temperature, humidity, dust, etc.
- e) Surface coatings that must be designed and equipped.
As a general rule, the minimum width of the conductor will be 1 mm. , in all cases where there are no density problems.
For special cases in low voltage (maximum 24 V direct current), the width of the conductor can be reduced to 0.3 mm. , and up to 0.2mm. , for processedcircuit boards. In these cases, you can increase the width when the driver leaves the compromise zone, on his route.
However, conductor widths should be kept as large as possible, allowing for imperfections that always occur at the edges.
The same can be said with the separation of conductors: It will be the greatest possible to reduce rejections in the Inspection, the more frequent the smaller the separation. The Factory and Inspection processes are complicated when the separation of conductors is minimal and therefore costs and delivery times increase.
Multilayer circuit boards.
In multilayer circuit boards, the limitations in the dimensioning are complicated, so that they admit the value of the peak of the potential difference between the conductors.
2.9. –CHARACTERISTICS OF THE CIRCUIT
It is not intended to make an exhaustive study of the characteristics of the circuit that need to be considered in the design of printed boards. The information given here is not complete; Likewise, the values and equations given should be used only as a guide, for an estimate of the approximate values of each of the characteristics. All the parameters of the circuit boards have a great interdependence and therefore the values measured empirically are not easy to coincide exactly with those calculated.
This is especially true in high-frequency regions where complete analytical prediction of plaque characteristics is impossible. Thus, the data that we provide, in this regard, is not complete and only provides orders of magnitude of parameters, to be taken into account in the design.
2.9.1.- Within the varied range of qualities of insulating supports, the fundamental characteristics of those materials most widely used today are indicated below. Average thickness of 1.58 mm.
|Properties||Phenolic paper||epoxy paper||epoxy glass|
|Surface resistance M Ohm. (*)||10 3||10 3||10 3|
|Volumetric resistance M Ohm. (*)||10 4||10 5||10 6|
|Water absorption, max. on %||0.75|