SMT Soldering Handbook surface mount technology 2nd phần 4 potx

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Figure 4.6 Working principle of a sprinkling fluxer

The cylinder need not be removed during rest periods and overnight. The flux

on it will of course dry, but rotating the cylinder for about 15 minutes before

starting work again will clear it. For longer breaks in production, the cylinder is

removed from the fluxer and cleared of flux with an appropriate thinner, which as a

rule is supplied by the flux vendor.

Rotating-brush sprinklers

Figure 4.6 explains the working principle: a rotating cylindrical brush, carrying

fairly stiff nylon bristles, and of a length corresponding to the width of the

solderwave, is arranged at right angles to the travel of the circuit board conveyor.

The lower portion of the brush dips in a container of flux. The sense of rotation is

contrary to the direction of travel of the board conveyor. Somewhat before the

bristles reach the apex of their rotation, they pass the straight edge of a blade, which

can be pushed into the path of the bristles so as to bend them backwards. Having

passed the blade, the bristles spring forward and fling the flux they have picked up

from the reservoir upwards against the underside of the circuit board which passes

overhead.

A sensor-actuated mechanism pushes the blade against the brush when a board

arrives abovethe aperture ofthe sprinkler and retractsit as soon asthe board has passed.

The width of the spray is governed by the length of the blade, which is adjustable to

match the width of the boards to be fluxed. The amount of flux delivered is governed

bythe controllable speed of rotation ofthe brush, whilethe depth ofimmersion ofthe

bristles in the flux determines the size of the flux droplets to some extent. It is

customary to keep the brush rotating during short breaks in production. During

longer breaks, the brush is removed and stored in a container filled with thinners, and

provided with a well-fitting lid. Shouldthe bristles harden by being left to dry in air, a

brief period of rotation in the fluxer will soften them again.

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Figure 4.7 Working principle of ultrasonic atomization

Sprayfluxers, which propel the flux droplets in a straight path and at some speed

against a circuit board, have occasionally met with some objections. Because of their

straight line of flight, some droplets may reach the upper surface of the circuit board

through apertures such as unoccupied through-holes, vias, or milled slots in boards

which are to be broken into separate units after soldering.

Stray flux on the upper side of a board is undesirable. It can cause problems with

relays, trimmers, or any other component which is sensitive to physical contamina￾tion. Directing the flight of the drops against the board at an angle reduces the

problem, but does not entirely eliminate it.

Another difficulty arose with the introduction of wavesoldering in an oxygen￾free atmosphere (Section 4.4). Blowing atomizing air into the oxygen-free machine

interior runs contrary to the concept, and atomizing with compressed nitrogen is

costly.

Ultrasonic spray fluxers

The development of ultrasonically driven fluxing systems was motivated by these

problems. With ultrasonic atomization, a metered supply of flux is fed to the

vibrating surface of an ultrasonic generator. The vibrational energy is transmitted to

the film of flux which forms on that surface and breaks it up into an aerosol of very

fine droplets, which form a cloud of aerosol above the generator (Figure 4.7).

With some ultrasonic fluxers, a gentle stream of nitrogen (or air with a conven￾tional wavesoldering machine) wafts that aerosol against the underside of the board

as it traverses the sprayzone. With others, the atomizing surface of the ultrasound

generator is so shaped as to gather the aerosol cloud and to propel it towards the

circuit board.

The fluxing head of some ultrasound systems traverses the width of the board in a

zig-zag pattern, as has already been described; with others the shape of the aerosol

cloud is given a fanlike shape, so that one or two atomizing heads suffice to straddle

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Figure 4.8 Flux-densitySolids-content curve

the width of a board. Sensor-actuated control of the width and duration of flux

application are common to all ultrasound fluxers.

Ultrasonic sprayfluxers are suitable for use with fluxes based on alcohol, but not

for waterbased fluxes. Water, being heavier and less mobile than alcohol, requires

more kinetic energy for its dispersal into fine drops than the normal ultrasonic

sprayhead can supply, at least in its present state of development.

4.2.2 Monitoring and controlling flux quality

The solids content of a flux, given its type and formulation, is its most telling and

decisive parameter. With the exception of one reservation which will be discussed

presently, there is a direct relationship between the density of a flux and its solids

content. Every flux has a characteristic density/solids-content curve, which ought

to be given in the datasheet supplied by the vendor.

As a rule, these curves are correct for a temperature of 20 °C/68 °F and, strictly

speaking, the flux sample should be warmed or cooled to that temperature before its

density is measured. Vendors can save their customers a good deal of time and

trouble if they provide flux-density/solids-content curves for a range of test tem￾peratures (Figure 4.8).

Whether and how often the flux density needs checking depends on the type of

fluxer used. Wavefluxers and foamfluxers, where excess flux runs back from the

circuit board into the flux reservoir, demand a regular check of the quality and

purity of the flux. With these systems flux is constantly exposed to the ambient air, if

not actively aerated. Solvent may evaporate, flux constituents may oxidize, mois￾ture may be absorbed, impurities in the form of solids or contamination may be

washed off the board surface back into the fluxer.

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