SMT Soldering Handbook surface mount technology 2nd phần 2 docx

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3 Soldering

3.1 The nature of soldering and of the soldered joint

Soldering, together with welding, is one of the oldest techniques of joining two

pieces of metal together. Today, we distinguish between three ‘metallurgical’

joining methods: welding, hard soldering (or brazing) and soft soldering. The term

‘metallurgical’ implies that at and near to the joint interface, the microstructure has

been altered by the joining process: what has happened has made one single piece of

metal out of the two joint members, so that electric current can flow and mechan￾ical forces can be transmitted from one to the other.

With both hard and soft soldering, the joint gap is filled with a molten alloy (an

alloy is a mixture of two or more pure metals) which has a lower melting point than

the joint members themselves, but which is capable of wetting them and, on

solidifying, of forming a firm and permanent bond between them. The basis of most

hard solders is copper, with additions of zinc, tin and silver. Most hard solders do not

begin to melt below 600 °C/1100 °F, which rules them out for making conductive

joints in electronic assemblies.

Soft solders for making joints on electronic assemblies were by tradition, until

recently, alloys of lead and tin, which begin to melt at 183 °C/361 °F. This

comparatively modest temperature makes them suitable for use in the assembly of

electronic circuits, provided heat-sensitive components are adequately protected

against overheating. With some of the lead-free solders which have now entered the

field (see Section 3.2.3) soldering temperatures might have to be either higher or

lower.

3.1.1 The roles of solder, flux and heat

Soft soldering (from here on to be simply called ‘soldering’) is based on a surface

reaction between the metal which is to be soldered (the substrate) and the molten

solder. This reaction is of fundamental importance; unless it can take place, solder

and substrate cannot unite, and no joint can be formed.

The reaction itself is ‘exothermic’, which means that it requires no energy input

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to proceed, once it has started. Soldering heat is needed to melt the solder, because

solid solder can neither react with the substrate (or only very slowly), nor flow into a

joint.

The reaction between solder and substrate is of crucial importance for both the

process of soldering, and for the resultant soldered joint. With a normal tin–lead

solder, only the tin takes part in the reaction. With lead-free solders, other alloying

components such as silver or indium may be involved as well. The reaction products

are so-called intermetallic compounds, hard and brittle crystals, which form on the

interface between the solid substrate and the molten solder. The bulk of them stay

where they have formed. They constitute the so-called ‘intermetallic layer’ or

‘diffusion zone’, which has a profound effect on the mechanical properties of the

soldered joint and on its behaviour during its service life.

Any non-metallic surface layer on the substrate, such as an oxide or sulfide,

however thin, or any contamination whatever, prevents this reaction, and by

implication prevents soldering. Unless the contamination is removed, the reaction

cannot occur. Unfortunately, under normal circumstances all metal surfaces, with

the exception of gold and platinum, carry a layer of oxide or sulfide, however clean

they look.

The soldering flux has to remove this layer, and must prevent it from forming

again during soldering. Naturally, the surface of the molten solder is also one of the

surfaces which must be considered here, because an oxide skin would prevent its

mobility. Clean solder can flow freely across the clean substrate, and ‘tin’ it. (The

expression ‘tinning’ derives from the fact that solder is often called ‘tin’ by the

craftsmen who use it, and not from the fact that tin is one of its constituents.)

It is important at this point to make it quite clear that the flux only has to enable

the reaction between substrate and molten solder to take place. It does in no way

take part in the reaction once it has arranged the encounter between the two

reaction partners. Hence it follows that the nature and strength of the bond between

solder and substrate do not depend on the nature or quality of the flux. What does

depend on the quality of the flux is the quality of the joint which it has helped (or

failed to help) to make. For example, if the flux did not remove all of the surface

contamination from the joint faces, the solder will not have been able to penetrate

fully into the joint gap, and a weak or open joint will result.

Thus there are three basic things which are required to make a soldered joint:

1. Flux, to clean the joint surfaces so that the solder can tin them.

2. Solder, to fill the joint.

3. Heat, to melt the solder, so that it can tin the joint surfaces and fill the joint.

3.1.2 Soldering methods

Handsoldering

The various soldering methods which are used with electronic assemblies differ in

the sequence in which solder, flux, and heat are brought to the joint, and in the way

in which the soldering heat is brought to the joint or joints.

Soldering 21

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Figure 3.1 The principle of handsoldering

With handsoldering, the heat source is the tip of a soldering iron, which is heated

to 300–350 °C/570–660 °F. A small amount of flux may have been applied to the

joint members before they are placed together. The assembled joint is heated by

placing the tip of the soldering iron on it or close to it. Solder and flux are then

applied together, in the form of a hollow solderwire, which carries a core of flux,

commonly based on rosin.

The end of the cored wire is placed against the entry into the joint gap. As soon as

its temperature has reached about 100 °C/200 °F, the rosin melts and flows out of

the solderwire into the joint. Soon afterwards, the joint temperature will have risen

above 183 °C/361 °F; the solder begins to melt too, and follows the flux into the

joint gap (Figure 3.1). As soon as the joint is satisfactorily filled, the soldering iron is

lifted clear, and the joint is allowed to solidify.

Thus, with handsoldering, the sequence of requirements is as follows:

1. Sometimes, a small amount of flux.

2. Heat, transmitted by conduction.

3. Solder, together with the bulk of the flux.

Clearly, this operation requires skill, a sure hand, and an experienced eye. On the

other hand, it carries an in-built quality assurance: until the operator has seen the

solder flow into a joint and neatly fill it, he – or more frequently she – will not lift

the soldering iron and proceed to the next joint. Before the advent of the circuit

board in the late forties and of mechanized wavesoldering in the mid fifties, this was

the only method for putting electronic assemblies together. Uncounted millions of

good and reliable joints were made in this way. Handsoldering is of course still

practised daily in the reworking of faulty joints (Section 10.3).

Mechanized versions of handsoldering in the form of soldering robots have

become established to cope with situations, where single joints have to be made in

locations other than on a flat circuit board, and which therefore do not fit into a

wavesoldering or paste-printing routine (see Section 6.2). These robots apply a

22 Soldering