SMT Soldering Handbook surface mount technology 2nd phần 10 pdf

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Figure 9.3 The yes/no nature of soldering success

Bridges and solderballs

A circuit board cannot function if it contains a short circuit, i.e. a solder bridge.

Wavesoldering without bridging demands special techniques, such as optimizing

the configuration of the wave (Section 4.4.4) and the board layout (Section 6.4.1).

Boards with a pitch below 1 mm40/mil are difficult to wavesolder without faults,

unless soldered in a nitrogen atmosphere.

With reflowsoldering, especially of fine-pitch boards, the type of paste and its

quality and the precision of the printing of it are key factors in achieving soldering

success (Section 5.2.3).

Solderballs need not necessarily be classed as soldering faults. If a solderball sits

between two neighbouring footprints on a fine-pitch board, it can constitute a

shortcircuit, and prevents the board from functioning. Elsewhere, solderballs repre￾sent potential shortcircuits, and as such reduce the reliability to an extent which is

difficult to quantify. How solderballs are to be regarded is very much a matter of

individual company policy.

Even a single soldering fault on a board prevents it from functioning, and there

are only two options: correct it or scrap the board. The choice between them

depends on several factors, which will be discussed in Section 10.1. What must be

stressed here is the following.

The nature of the soldering fault

The existence of a soldering fault is an objective fact. A joint is either soldered or it is

not soldered. A bridge is either there or else there is none. The soldering fault

presents a ‘yes/no situation’ (Figure 9.3). To pronounce upon it is in the nature of a

verdict upon an observed fact, and two or more inspectors must necessarily reach

the same verdict.

Because of its objective ‘yes/no’ nature, the success/fault verdict can be entrusted

to an automatic quality assessment system, which may be based on opto-electronic

inspection or functional electronic testing (Section 9.5.5).

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Figure 9.4 Some soldering imperfections

9.2.2 Soldering perfection and soldering imperfections

Assessing soldering perfection presents an inspector with a fundamentally different

situation: imperfect soldering does not prevent the affected circuit board from

functioning, but it can be seen as endangering or reducing its reliability. It may also

affect its saleability where the buyer has specified precise criteria.

Criteria for perfection may include the following features (Figure 9.4):

Wetting angle

Joint profile and amount of solder on a joint

Alignment or displacement of components

If an imperfection disqualifies the product in the eyes of the customer, it becomes

a soldering fault, because it makes the product unsaleable. A product which is

unsaleable does not work as far as the vendor is concerned.

Being saleable is the first function any manufactured product must fulfil. A

product which is not saleable in the market for which it has been made does not

function from the point of view of its maker (unless it is still saleable elsewhere for

less profit or at a loss). The offending feature must be corrected, or else the product

must be scrapped.

In contrast to the unequivocal yes/no verdict upon the verifiable fact of soldering

success or fault, a pronouncement upon the soldering perfection of a joint represents

a judgement, which is necessarily subjective. The judgements arrived at by different

inspectors represent points along a scale, which separate the ‘perfect’ or ‘acceptable’

from the ‘imperfect’ or ‘non-acceptable’ (Figure 9.5). On either side of the

accept/reject divide are areas of doubtful acceptability and false alarm.

It has been found that only 44% of the quality judgements on the same set of

soldered boards, made on two different days by the same inspector, agree with one

another. The quality judgements of the same boards made by two different inspec￾tors overlap by only 25%, while those made by three inspectors overlap by 14%

(A. T. & T. Bell, Burlington, N. Carolina).

To sum up: deciding between soldering success and a soldering fault amounts to a

verdict. Deciding whether soldering perfection has been approached sufficiently is a

matter of judgement, and the making of this decision can be automated only with

great difficulty.

The blowhole problem

Blowholes in wavesoldered throughplated joints, caused by ‘gassing’ of the walls of

the hole, are a special form of imperfection (Figure 9.6).

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Figure 9.5 The perfect/imperfect judgement scale

Figure 9.6 Blowholes in a throughplated wavesoldered joint

The causes of gassing of throughplated holes, and the measures which are needed

in order to avoid it, are by now well understood. Gassing can be prevented by

ensuring the smoothness of the drilled holes and the continuity and adequate

thickness of the copper plating on their walls. It can be cured by a suitable heat

treatment of boards which are liable to form blowholes before using them.


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Because blowholes do not interfere with the functioning of a circuit board, they

are soldering imperfections rather than soldering faults, though their presence or

absence is an unequivocal yes/no situation. Searching investigations have shown

that they do not affect the life expectancy of joints or their reliability, in any way.

Corrective soldering can only mask, but not fill, a porous hole, and it is bound to

shorten the life expectancy of the joint.


9.3 Practical examples of soldering faults

The nature of a soldering fault means that a circuit board is faulty and cannot

function until every single fault on it has been corrected. Therefore, the most

important task of any quality-control system is to find every one of them. In Tables

9.1–9.3, the various types of soldering faults are listed and illustrated. For complete￾ness’ sake, faulty throughplated joints are included.

9.4 The ideal and the imperfect joint

The criteria of perfection in a soldered joint go back to the days of handsoldering.

They have to do with two parameters: first, the wetting angle between the solder

and the substrate; and secondly the amount of solder in or on the joint. Together,

they determine the so-called joint profile. The ideal handsoldered joint has a ‘lean’

profile: the solder meniscus has a concave shape, so that the sharp wetting angle can

be seen clearly. Also, the contours of the ends of the joint members must be visible,

so that an inspector can be sure that, in the case of the leadwires of inserted

components, the wires do in fact project through the hole and that all leads have

been properly tinned (Figure 9.7).

The criteria of perfection of wavesoldered and reflowsoldered joints on circuit

boards go back to these early days. They deal with surface contours, surface areas,

the relationships between distances. It is possible to base judgements like good/bad,

acceptable/unacceptable or beautiful/ugly on these criteria, provided every inspec￾tor can refer to a set of pictures or samples of ‘perfect’ and ‘imperfect’ joints. It is

difficult and certainly expensive to derive a clear yes/no verdict unless precise,

time-consuming and therefore expensive measurements of individual joints are

made. It is equally difficult, if not impossible, to base an automatic, opto-electronic

inspection system on a ‘good/bad’ or ‘beautiful/ugly’ situation instead of a ‘yes/no’

one. Tables 9.4 and 9.5 illustrate practical examples of perfect and imperfect joints.

There is one instance where an imperfection can become a fault: ‘fat’ joints with

too much solder at the ends of a melf or chip-capacitor can cause the ceramic body

of the component to crack under the mechanical stresses caused by temperature

fluctuations during service. Fat joints hold the component as in a vice, lean joints

can yield.

9.5 Inspection

No circuit board should leave its soldering stage without having been inspected. To

inspect meansto view or examine closely and critically.This impliesthat inspection is

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