SMT Soldering Handbook surface mount technology 2nd phần 3 doc

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efficiency of the flux is derived from the resulting wetting curve. The solderbath

contains a 63% Sn solder (for instance to JSTD-006, Sn63Pb37C), held at a

temperature of 250 °C/480 °F.

Corrosive action

The test for corrosive action is again confined to observing what a flux will do to

copper during soldering, or what the residue which is left on the copper will do in a

moist atmosphere.

In ISO 9455–13 flux residue, left on a copper coupon after having melted a small

amount of 60% tin solder together with the flux under test, is stored in a humid

atmosphere, at 40 °C/645 °F and 91% to 95% relative humidity, for three days.

Corrosion is deemed to have occurred if the flux residue has changed colour, or if

white spots have appeared in it.

In ISO 9455–5, a drop of the flux to be tested is placed on a flat glass slide, on to

which a thin film of copper, with a thickness of 0.05 m/0.002 mil (500 angstrom)

has been deposited by an evaporation technique, a so-called ‘copper mirror’.

Copper mirror slides are commercially available. The slide with the drop of flux on

it is kept in a humidity chamber at 23 °C/73 °F and 50% relative humidity for 24

hours, and then examined. If the copper mirror has disappeared underneath the

flux, it is deemed to have failed the test. A flux which passes the copper mirror test is

an ‘L-type’ (low activity) flux, which group comprises all R-type fluxes, most

RMA, and some R. If some of the copper mirror has gone, it is an ‘M-type’

(medium activity) flux, which may still be an RMA, but is mostly RA and

sometimes a watersoluble or a synthetic activated flux. If the copper mirror has

disappeared completely, the flux is an ‘H-type’ (high activity). Watersoluble and

synthetic activated fluxes fall in that group. An important aspect of flux classification

relates to the surface–insulation–resistance (SIR) properties of a flux (ISO 9455–17,

not yet issued).

Halide content

Determination by analysis

Ifa halide-free fluxis specified,somestandardsgive adetailedanalyticalprocedurefor

quantitatively determining the halide content of the flux. If this exceeds 0.05% by

weight of the rosin content of the flux, calculated as Cl, the flux does not conform to,

for example, a BS 5625 halide-free flux. If it exceeds 0.5% calculated Cl on the solids

content of the flux, it does not conform to an ANSI/J-STD-004 flux of type LI.

Silver-chromate test

This is a qualitative yes/no test, and does not indicate a specific halide percentage.

Silver chromate (AgCrO

) is a brick-red substance, which turns white or yellow in

the presence of a halide. Silver-chromate impregnated testpaper is commercially

available. If such a piece of paper turns white or yellow when a drop of the flux

under test is placed on it, halide is deemed to be present, and the flux cannot be

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classed as ‘L0’ or ‘L1’ to J-STD-004. There is a problem, though: certain acids and

amines (which may well be free of halide) are also capable of causing the colour of

silver-chromate paper to change. Because this test is relatively insensitive, a flux

with up to 0.05% halide will still pass it as ‘halide-free’.

Beilstein test

This test, which is mentioned in ANSI/J-STD-004, is more sensitive than the

silver-chromate test, but it is a qualitative test and gives no indication of the actual

quantity of halogen present. Its drawback is that it will also respond to any non-ionic

halogen in a halogenated solvent, should any such be contained in the flux.

The Beilstein test detects the presence of halogen in an organic compound. It

requires a small piece of fine copperwire gauze, which is heated in an oxidizing

flame (e.g. the blue part of a bunsen-burner flame) until it ceases to turn the flame

green. It is withdrawn, allowed to cool, and a small amount of the flux under test is

placed on it. It is then put back into the flame. If the flame turns blue-green, the flux

contains traces of halide. If not, it is deemed to be halide-free. The Beilstein effect

depends on the formation of a volatile copper halide. (F. K. Beilstein, Russo￾German chemist, 1838–1906.)

Solubility of flux residues

The average flux user needs guidance on how to assess the ease with which the

residue of the flux he is using, or wants to use, responds to the cleaning method he is

using or intends to use. The international standard ISO 9455–11: 1991 (E) is

relevant to this problem.

This standard describes a method of heating a sample of the flux on a dish-shaped

piece of brass sheet up to 300 °C/570 °F for a given time, placing the sample in a

humidity chamber for 24 hours and then immersing it in the solvent which is to be

used for cleaning. The presence of any residual flux left after cleaning is indicated by

the ability of the cleaned test specimen to form an electrolytic cell.

Surface insulation resistance (SIR) of the flux residue

By definition, the residue from a ‘no-clean’ flux remains on the board. Obviously,

not only must it cause no corrosion, but its presence must not interfere with the

functioning of the circuitry by lowering the surface insulation resistance (SIR) of

the board between adjacent conductors: a leakage current of 10A between

neighbouring IOs of a high-impedance microprocessor is enough to cause it to

malfunction (see Section 8.1.1). A number of tests to measure the SIR after various

soldering and cleaning procedures have been devised over the years. They are

described in Section 8.6.3.

J-STD-004 includes a method for testing the flux residue for its moisture- and

surface-insulation resistance. The relevant ISO working group is expected to

complete its deliberations on the same subject in about three years’ time (informa￾tion from BSI, London, April 1997).

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Tackiness of the flux residue

Finally, the residue from a no-clean flux must be dry and not sticky or ‘tacky’ under

normal temperature and moisture conditions. Tackiness is tested by applying

powdered chalk to a fluxed coupon which has undergone a specified temperature

regime. If the powder can be removed with a soft brush, the flux has passed the test.

3.5 Soldering heat

Conventional soldered joints are made with molten solder. Hence, the soldering

temperature must always be at least above the melting point of the solder, i.e. above

183 °C/361 °F. The immediate environment of the joint, and sometimes the whole

assembly, must be brought up to the soldering temperature too. The exact tempera￾ture needed depends entirely on the soldering method used. It is rarely less than

215 °C/420 °F and is often much higher.

3.5.1 Heat requirements and heat flow

Heat is a form of energy, which is usually measured in one of the following ways.

One calorie (1 cal) raises the temperature of one gram of water by 1 K (which is the

same temperature difference as 1 °C, Section 5.4.2). One calorie equals 4.187 joule,

or in units which are meaningful in the context of soldering, 4.18 watt.seconds

(W.sec).

Table 3.12 indicates the amounts of heat required in some common soldering

situations. In this context, it is useful to know the heat conductivity of the various

materials involved, so as to be able to gauge the speed with which the heat input

spreads within an assembly (Table 3.13).

The figures given in Tables 3.12 and 3.13 are worth studying. Table 3.12 shows

that organic substances like FR4 have a much higher specific heat than metals. This

has an important bearing on most soldering situations. The greater part of the

soldering heat expended in making a joint is not used to heat the metallic joint

partners, but to heat the FR4 epoxy board on which the copper laminate sits. Hence

the need to preheat the boards before they pass through the solderwave (Section

4.3), but also the benefit of preheating the circuit board, at least locally, when

soldering single multilead components (Section 5.7), or before carrying out repair

work, i.e. desoldering and resoldering single components (Section 10.3).

The list of heat conductivities is equally illuminating. The heat conductivity of

epoxy is two orders of magnitude lower than that of the ceramic substrate of a

hybrid assembly. Hence the need for taking the thermal management of SMDs,

which are mounted on an epoxy board, much more seriously than that of hybrid

constructions, which were initially the beginnings of SMD technology.

The figures also show how even the narrowest air gap prevents the flow of heat

between two hot bodies. Hence the need to have a drop of molten solder on the tip

of a soldering iron or thermode, or at least some flux on the joint to bridge that gap

(Section 5.7).

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