ESTRATEGIAS QUE SE PODRÍAN IMPLEMENTAR DESDE LA SALUD

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3.6 PRINTING OF CIRCUITS

The printing of circuits is carried out using either an etching process or an additive process. Both of these processes, are briefly described in the following paragraphs.

3.6.1 ETCHING PROCESS

In this process the copper foil is first cleaned, either chemically or mechanically, and is then coated with a photo-sensitive solution known as a 'resist', which has the property of becoming soluble when exposed to strong light.

A photographic positive of the circuit artwork is then placed over the sensitised board and time-exposed in a special printing machine. After exposure, the resist is washed away to leave unprotected areas of copper around the circuit pattern. The board is dried by a clean, oil and water free air blast. The complete board is then inspected to ensure that no resist has been removed from any part of the conductor pattern itself, and that no resist particles are present in areas which are to be etched away. The board is then placed in a bath which contains an etching solution, such as ferric chloride or ammonium persulphate, which etches away all the unprotected copper.

When the etching process has been satisfactorily completed, the board is thoroughly washed in water in order to remove all traces of etching solution, and is then dried and given a final inspection.

As printed circuit boards with the same circuit pattern are often required in large numbers, the simple 'print and etch' process is generally superseded by a screen printing process.

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC

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FUNDAMENTALS

3.6.2 ADDITIVE PROCESS

In this process, copper is deposited only in the areas where conductors are

required. To achieve this the base material is pre-coated with a suitable adhesive, the circuit holes are pre-fabricated, and the board is sensitised with a photo-resist solution. A negative of the circuit pattern is then screen printed onto the board so that the exposed areas define the conductor network. These exposed areas are chemically activated, and the board is then immersed in an electrolyses copper plating solution. After a period of time consistent with the deposition of the required thickness of copper, the board is removed from the bath. The major advantages of the additive process are: no chemical etching takes place, thereby eliminating

wastage of copper, the thickness of the deposited copper can be reduced and made more uniform, the conductor widths and spacing are less restricted, and the hole diameter can be reduced, thereby increasing the board area available for routing of conductors.

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

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FUNDAMENTALS

3.6.3 INSPECTION

After printing, circuit patterns are inspected with particular attention being paid to the following:

• Dimensional Accuracy and Condition of the Edges of Conductors

• Condition of the Pattern Surfaces

• Particles of Copper in Unwanted Areas

• Insulation Areas

• Lack of Resin Bond in etched Areas 3.7 SOLDERING METHODS

There are two main methods of soldering employed in connection with printed circuits boards, (a) hand soldering and (b) mass soldering.

3.7.1 HAND SOLDERING

This method is used for soldering joints separately, e.g. in limited batch production, and when a component or a wire is replaced after a test or a repair has been carried out. This method involves the use either of electrically heated hand irons, or of resistance type hand tools when the use of these is permitted.

3.7.2 MASS SOLDERING

In this method, all joints of a finally assembled board are soldered simultaneously, by bringing the board into contact with an oxide-free surface of molten solder, which is contained in a special type of bath. Mass soldering may be carried out in any one of five different ways:

Flat or Static Dipping - one edge of the board is first lowered on to the solder and the other edge is then lowered slowly to allow flux and solvent vapour to escape.

Wave Soldering – solder is pumped from the bottom of the solder bath through a narrow slot, so that a symmetrical 'standing wave' of solder is produced across the width of the bath. The circuit board after being fluxed, is then either manually or automatically passed against the crest of the solder wave by a conveyor.

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JAR 66 CATEGORY B1 MODULE 4

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FUNDAMENTALS

Weir and Cascade Soldering systems are of the moving solder type, the solder flowing down a trough by gravity, and then being returned to the main bath by a pump. In weir-soldering (diagram (a)) a circuit board is lowered on to the solder;

while in cascade soldering (diagram (b)) a board is conveyed across the crests of solder waves in a direction opposite to the solder flow.

Reflow soldering is an automated process also known as 'heat cushion' soldering.

It is applied particularly to circuit boards on which microcircuits and associated devices are to be assembled. These efficient but costly components require a special soldering technique, so that their full potential as surface-mounted devices can be realised. The reflow technique is generally recognised as the best method, since the soldered joints are easier to inspect and to remake when a faulty

component has to be replaced. In addition, soldering times and the risk of

overheating sensitive components are reduced, and distortion of leads is prevented.

The sequence of reflow soldering is shown in the diagram on the following page.

The leads of the circuit or component and the relevant lands on the circuit board, which have been pre-tinned by such methods as wave soldering or dip soldering, are first brought into contact with each other and accurately aligned. The sequence is then initiated by lowering the electrode on the lead to be soldered.

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JAR 66 CATEGORY B1 MODULE 4

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FUNDAMENTALS

Shortly before the electrode makes contact with the lead, the pre-set heating power is automatically switched on. The electrode is then pressed on to the lead under a load which gradually increases until the pre-selected value is reached. The solder melts, and in reflowing, it forms a 'cushion' through which the lead is pressed against its corresponding land of the circuit board. As soon as the cushion is

formed, the timing device cuts off the heating supply. After a 0.75 second delay, an air blast is delivered to cool the soldered joint, this accelerates the completion of the soldering process, and also improves the quality of the joint. At the end of the cooling period, the load is relieved, and the electrode is automatically raised ready for the next operation.