Techniques for the construction of
Printed circuits
For the accomplishment of a practical assembly we must have knowledge about some steps to follow and other so many details that will help us to avoid the breakage of the components to mount, with its corresponding loss of money and time.
First that we must know it is the order in that we will develop our work, which, once chosen the project that we are going to realize, we will have to be safe of not conducting battles that are counter-productive with the normal development of our task.
The printed circuit:
We will begin with the construction of the printed circuit, this logically, after already having all the necessary elements for the construction of the same.
The first passage for the accomplishment of a circuit is the design or drawing on the paper of the interconnection, that is to say, the geometric disposition that are to have the conductors or tracks that will unite the different components electrically. This phase is of great importance and requires to dedicate to him all along necessary since any error that is committed will translate later in a problem that will be difficult to eliminate on the circuit already finished.
The design is realized assigning the spaces that will occupy the components, considering the dimensions of the same. Once realized this we will come to draw the interconnection tracks, with the logical precaution that they cannot be crossed. If the circuit is monoexpensive, the crossings of conductors will have to be realized by means of thread bridges located in the face of assembly of components, however if the circuit is double slide, the crossings will be realized by means of tracks in the same face that in the previous case.
With the realized design or we will come to obtain a photographic negative to natural size and in the circuits momocara we will cover by the face of copper with a photosensitive emulsion and envelope is placed previously she the negative with the image of the obtained design, soon exhibiting the circuit to the light, for which special lamps of high luminosity are used directly or the solar light. In this phase the zones set out to the light will be impressed solely, of the negative, that is to say, the conductive tracks.
Next it will be come to the developing, during which the zones will be eliminated where the photographic emulsion has not been impressed, being protected solely the conductive routes.
If we do not own of a photographic negative we will be able to by hand carry out the task drawing the tracks with indelible red directly on the copper face, this action is not best nor the most recommendable one since the finished level is not reached of that is obtained using the photographic negative, in addition we must have well-taken care of of which the red dries or before continuing with the following step.
After completing the photographic process (or manual) it is put under the circuit to a chemical attack with the purpose of to eliminate copper of the zones noncovered, being used for it ferric a water chloride dissolution.
This task will be realized placing acid in a plastic container or of glass and warming up it to Bath Maria until it reaches the temperature of 45ºC, being placed inside the plate by means of which we are going to obtain our printed circuit.
Once obtained the image wished on the laminate, having disappeared the nonuseful zones of copper, we come to eliminate the photographic emulsion of the tracks with a dissolvent (we realized if with indelible red we clean it the tracks with a cotton absorbed in alcohol), later it is let dry the circuit and one goes to the phase of drill of all the points where the terminals of the components will be inserted.
Once we have the circuit printed already constructed, we will have to come to the assembly of the components, for this is necessary to realize a previous preparation to them with the purpose of to thus facilitate its insertion in the circuit like also its later weld.
This previous preparation is realized in order to adapt, as well as possible, the form and dimensions of each component to the physical space that is going to arrange on the circuit. To this we will denominate it task pred-form of the components.
From the pred-form one we will be able to be avoided accidental risks of breakage of terminals, short circuits, and any other problem that can arise in a assembly when we do not consider these precautions.
The components of two terminals can be classified in: components with axial and component terminals with radial terminals. First they present/display the located terminals of exit on the ends of the body and aligned with this one, forming an imaginary line that would happen through its geometric center, the seconds they own his terminals in perpendicular form to the body in the ends of this one.
The elements of the second group will not need pred-form if the perforations in the printed circuit agree with the separation between the terminals, opposite case will be necessary to realize pring-form. Those of the first group require that always the pred-form one is realized.
In order to conduct this battle it is necessary to know what distances must bend the terminals for its insertion in the printed circuit, it previously requires to realize the measurement of the distance between the perforations for the assembly, being able to realize the same with a caliber or a graduated rule in millimeters. The components with axial terminals will mount parallels to the printed circuit and supported envelope generally he, when realizing the pred-form one will have to procure himself that the reference or value of the component that appears in the body is visible to facilitate therefore a fast identification after its insertion in the circuit.
The dubbing of the terminals can be realized by hand or with some special tools for this work.
When it is made by hand they must have present a set of precautions to avoid the breakage of the component:
- The dubbing will be realized with tiles of fine ends making pressure in the point of taken care of dubbing having special in not doing force to him to the element in himself, but only on the terminal.
- Force will not be exerted on the zone of union of the body with the terminal, since this one could be given off.
- It will not have to be the dubbing in an exaggerated right angle, but one will be to thus give a small curvature, being avoided to debilitate to the wire in that point.
- One will be to have certain esthetic in the assembly realizing a pred-form one in symmetrical form with respect to the body.
- One will be due to leave to a small portion of terminal between the body and the point of dubbing.
Shroud of components exists a special tool to carry out this denominated task, which, once fit the distance it enters dubbing points both, previously conducts all the operation in step being only avoided this way all the mentioned risks.
The components with radial terminals of exit can be mounted directly and present/display generally their terminals already cut to the suitable length, nevertheless sometimes is precise to realize conforming, when the distance between its exits does not agree with the separation between the holes of the printed circuit. In this case it will be come to measure the distance between holes and to realize in form manual the dubbing of the terminals to the measured distance.
It is very important to realize to the components a previous tin-plating to them with the purpose of to facilitate his later weld, and still more if the surfaces to weld are dirty or present/display something of oxide. In order to realize the previous tin-plating the soldering iron will be able to be used, applying directly with the end of the same small amount of tin that will be distributed uniformly on the terminal, will be due to avoid that the tin layer is very heavy since this will make difficult the insertion of the component in the printed circuit.
Components of greater power:
Before coming to the assembly we will have to identify those elements of the circuit that are able to produce during the operation a certain heat radiation to be put under conditions of use that force to them to dissipate a certain power.
These elements can be the resistance whose power is greater to 1/2 watt and also some transistors of average or discharge power.
With all these elements it will be tried to maintain a separation minimum of
5 mm since on the contrary it is possible to be gotten to damage to the same.
If resistance of high power with a relatively great body and its two exits located in the same end of the body appear, metallic supports will be used special that assure their mechanical fixation and prevent any movement that can get to break the terminals.
Assembly of the wires and cables:
The naked” or “bare” wire “that is going away to use to connect on the printed circuit the points require that it, will be cut with tiles custom-made necessary, realizing a dubbing to them in its ends of the same form which we saw for the terminals of the components, obtaining therefore the bridges apt for its assembly.
To cables of connection between printed circuits or these with the components located outside the same, we will have to cut them to the precise length, soon peeling his ends in a length of 4 or 5 mm with it tiles or pliers to peel.
In order to peel a cable the following recommendations are due to consider:
- It tiles will have to be perfectly fit to the diameter of the conductor so that the cover is cut solely and we avoid to damage to this one in inadvertent form.
- It will not have to be damaged the conductor in any point, to avoid later risks of breakage.
- It will be avoided to leave rest of the cover in the zone that there are bare, thus avoiding later problems with the weld.
For the treatment of the coaxial cables, once the bare one of the external cover has been realized, one will separate enmeshes pranced that trains the outer conductor, grouping all the threads of this one in a point and being in the open the conductive intern. Now a bare second will be realized on the cover of the conductive intern, trying to leave a certain length of cover that guarantees the isolation of the other conductor.
In case of being a parallel cable of two conductors, first we will separate an appropriate distance soon to conduct the operations already explained.
Assembly of the thermal dissipaters:
These are placed generally to some components and essentially to semiconductors.
This radiator will be placed on the component, before the assembly of this one on the printed circuit. When it is possible will be procured that the component-radiating set is mechanically subject to the circuit through same screw, conforming the terminals of a suitable form and adding a separator of the height sufficient to obtain to an isolation of the plate, realizing this in a material that does not pass on the heat.
There are some models of transistors and thyristors of power whose assembly is realized directly on a sufficiently great radiator so that he is able to evacuate the produced heat.
For his installation one is used an insulating lamina of mica that isolates the body of the transistor of the radiator, and washers and tubes with which the same effect is obtained, on the screws of fixation and the terminals of exit.
Normally the transistor and its dissipater are mounted separated of the printed circuit and they are placed in a zone of the equipment that has ventilation, thus to be able to evacuate the generated heat.
From these brief up to here treated concepts, we can to realize anyone of the explained practical assemblies in its respective section.
In them they are included, in case of being them necessary, other indications that there are to consider for the armed one of the circuit.
