SMT Soldering Handbook surface mount technology 2nd phần 5 potx

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demand a solder with a different melting point or maybe a certain percentage of

silver, the user will be well advised never to change the specification of his

solder. The upheaval which would be caused by changing from the standard

tin–lead solder to a lead-free one (Section 3.2.3) explains the general reluctance

of the industry to adopt a lead-free technology, unless forced to do so.

4. The standard wavesoldering temperature of 250 °C/480 °F plus or minus a few

degrees is, like the conveyor angle, the result of over four decades of practical

wavesoldering experience. Without a compelling need, it is advisable not to

depart from it.

4.7.2 Choosing and monitoring operating parameters

Condition of the flux

Given that the choice of flux is settled, the contents of the fluxer should at all times

match the density and/or the acid value which is specified in the vendor’s data sheet.

Section 4.2.2 discusses in detail how this requirement can be met, by automatic

equipment if required. It is worth restating at this point that the success of

wavesoldering depends critically on the consistent quality of the flux, and that this

constancy is assured more easily with sprayfluxers than with foamfluxers.

Amount of flux per unit of board area

This parameter also affects the soldering success, though to a lesser degree than the

density and the activity of the flux. Too much flux means more solvent in the flux

cover and, unless the preheater is adjusted accordingly, a risk of boiling and

solder-prill formation as the board passes through the solderwave. If boards have to

be cleaned after soldering, too much flux reduces the cleaning efficiency. Too little

flux, uneven fluxcover or, worse, unfluxed patches inevitably cause soldering faults,

such as bridges, icicles, solder adhering to the board and open joints, especially with

low-solids fluxes. With these, the margin of error is much narrower than with

high-solids fluxes.

The thickness of the flux cover can be controlled to some extent with the various

types of sprayfluxer, but foamfluxers permit very little, if any, control over this

parameter. At the time of writing (1997), there is no equipment on the market for

automatically monitoring the thickness of the flux cover. A frequent visual check of

the overall appearance of the soldered boards is the best method of ensuring the

stability of this important factor. Automatic video surveillance of the output of a

soldering line should be capable of giving warning of a malfunction of the fluxing

unit.

Intensity of preheating

Insufficient preheat leaves too much solvent in the fluxcover, which is therefore

more liable to be washed off in the solderwave, leading to bridging or open joints.

This factor is particularly critical with double waves, where a substantial portion of

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the flux cover must survive the passage through the first, turbulent wave. Moreover,

if the board is too cool, the solder may not rise through all plated holes and form the

required solder meniscus on the upper board surface.

Too sharp a preheat can cause trouble with rosin-based fluxes: overbaking such a

flux will cause the rosin to polymerize. This reduces its mobility, so that it may

obstruct the solder in tinning all solderpads or in rising to the top surface of the

board. It will certainly make cleaning less efficient.

By contrast, fluxes with a low solids content and very little rosin, and the

so-called ‘no-clean’ fluxes (Sections 3.5 and 8.1) which are mostly rosin-free,

require more intense preheating to ensure that the flux coating is not washed off in

the double solderwave. With these fluxes, most vendors suggest that the underside

of the board should have a temperature of 120 °C/250 °F on emerging from the

preheating stage.

The methods of controlling the intensity of preheating are dealt with in Section

4.2.3.

Parameters of the solderbath and the wave

The level of molten solder in the machine should at all times be kept strictly at the

height recommended by the maker. Many machines are fitted with an automatic

solder feeder, which maintains the correct solder level. Failing an automatic level

control, the solder level must be regularly checked at intervals depending on the

usage of the machine, and if necessary topped up. Unless fitted by the maker, it is

advisable to install a simple solder-level sensor, which gives an audible or visible

warning as soon as the solder level drops below the maker’s danger mark.

If the solder level drops too low, dross and flux-residues which float on the

solderbath can be sucked into the inlet of the solderpump. Once in the solder

stream, they tend to deposit on the solder conduits and the pump impeller. These

deposits interfere with the steady running of the solderwave, as will be discussed

below. Particles of dross and flux which reach the wave nozzle emerge in the wave

as small, but conspicuous, black spots, which pop up in the wavecrest and finish up

on the surface of the solderjoints. Such dross or flux inclusions do not necessarily

threaten the function or reliability of the affected joints, but they are a legitimate

cause of rejection by quality control or by the customer.

The temperature of the solder is one of the most basic wavesoldering parameters.

The general suitability of 250 °C/480 °F for most wavesoldering tasks has been

mentioned already. Close adherence to this value is less critical than is often

assumed, an accuracy of ±2–3 °C/4–6 °F being quite sufficient. It is much more

important to guard against a slow, unnoticed upward or downward drift of the

solder temperature away from its set value. The temperature readout on the control

panel of the machine, together with its warning signals, may be misleading: software

or functional errors are not unknown. The safest way to guard against this danger is

to check the actual solder temperature halfway through every working shift by

checking it with a reliable, preferably occasionally re-calibrated, handheld tempera￾ture measuring instrument, with its sensor placed in the solderwave about 5–

10 mm/0.25–0.5 in below the crest.

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Figure 4.30 Checking the wave height. : Conveyor angle

The height of a wavecrest is directly linked to the speed of the solder pump,

which with most good machines has a slip-free, tachometrically controlled drive

which is protected against variations in the supply voltage. The waveheight and its

consistency across the whole width of the wave can be checked very simply by

sliding a piece of plain FR4, with gradations marked on it, across the length of the

wavenozzle while the pump is running (Figure 4.30). It is advisable to carry out this

simple check at the beginning of every shift. Some computer-controlled machines

are fitted with a sensor-operated surveillance of the height and integrity of their

solderwave(s).

The depth of immersion of a board into the crest of the solderwave is normally

equivalent to the thickness of the board. It is therefore important that the underside

of the board is strictly parallel to the line of the wavecrest to well within this

measure. This is easily checked by letting a piece of plain FR4 without copper

lamination, as wide as the largest board, run across the wave and stop briefly over the

wavecrest. The flattened wavecrest will be clearly visible through the translucent

FR4. If the board is parallel to the wavecrest, the width of the band formed by the

flattened wave will be the same across the whole breadth of the testboard (Figure

4.31).

If the board is not parallel to the wavecrest, the whole conveyor must be tilted

sideways until a parallel position is achieved. Provisions for carrying out this

adjustment are, or should be, a feature of every wavesoldering machine. As an

alternative to the FR4 board, many machine vendors can supply a plate of heat￾resistant borosilicate glass which carries a pattern of parallel lines to make it easy to

check the width of the wavecrest across the plate. To make sure that the glass plate

does not crack during this manoeuvre, it is advisable to pass it over the fluxer and the

preheater before arresting it over the wave. With FR4, this is not necessary.

Uneven or rough running of the solderwave, such as fluttering of the

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