e005064.pdf

GENERAL INTEREST

Lead-free soldering

Silver and copper instead of lead

In order to prevent heavy metals from being released into the environment, it’s

best to simply avoid the use of mercury, cadmium and lead. However, what

happens to the soldering of electronic components if lead is prohibited?

onwards, the guidelines prohibit not

only the use of lead, mercury and

cadmium, but also compounds of

valence-six chromium and halo-

genated flame retardants. This

makes it impossible to use solders

containing lead after this date.

The changeover has already

started in industry. The most com-

monly used soldering processes are

reﬂow soldering (used primarily with

SMD components) and wave solder-

ing. Lead-free alloys can be used in

both processes. However, this

causes changes to essential process

parameters, so a certain amount of

fine tuning is necessary in order to

obtain the best results when the

new alloys are used.

Lead-free alloys

The harmful environmental effects of heavy

metals have been known for a long time, but

only recently have any effective and publicly

visible measures been taken against them.

Mercury-free batteries, rechargeable batteries

without cadmium and lead-free petrol are the

most well known examples. There have also

been changes in the composition of solder,

which normally is an alloy of lead and tin. For

example, the use of solders containing lead

has been prohibited for food containers for

quite a while already, and lead-free solders

must now be used for domestic water lines

as well. In these two cases the critical factor

was the direct risk of lead poisoning, but in

the case of lead-based solders used

in the fabrication of electronic equip-

ment, the primary issue is the envi-

ronmental effects of a steadily

increasing volume of electronic

products.

Up to now, soft solder containing

around 60% tin (Sn) and 40% lead

(Pb) has almost always been used

for soldering electronic components.

Small amounts of copper and silver

are sometimes also present. With an

eutectic tin-lead alloy, which con-

tains 62% tin and 38% lead, the melt-

ing point lies at the relatively low

value of 183 °C. ‘Eutectic’ means that

the melting and solidiﬁcation points

are the same, which causes the sol-

der to change directly from the liquid

to the solid state (or the other way

around). With non-eutectic alloys,

there is a certain range between the

melting and solidification tempera-

tures, within which the solder has a

‘pasty’ or viscous consistency.

Electronic scrap

Revision 3 of the European guide-

lines for electronic scrap puts an end

to the use of heavy metals and other

environmentally questionable mate-

rials in the fabrication of electronic

equipment. From the January 1, 2004

Elektor Electronics

5/2000

GENERAL INTEREST

Possibly replacements for lead in

tin solder alloys are copper (Cu), sil-

ver (Ag), bismuth (Bi), antimony (Sb),

indium (In) and zinc (Zn). Although

a eutectic tin-bismuth alloy has the

lowest percentage of tin of all the

lead-free alloys, and is thus not sig-

nificantly more expensive than

tin/lead solders, it has the disadvan-

tage that its melting temperature is

too low (138 °C). All other substitute

alloys have a much higher percent-

age of tin (more than 90%), which

means that they not only have

higher melting temperatures but are

also more costly. Consequently, lead-

free solder is around twice as expen-

sive as conventional lead/tin solder.

Most lead-free solders are alloys

of tin with copper, silver or copper

plus silver. Of this group, a eutectic

tin/silver/copper alloy has the low-

est melting temperature. This alloy

is 95.5% tin, 3.8% silver and 0.7% cop-

per, and it melts at 217 °C. This is

still 34 °C higher than the melting

point of the usual tin/lead solder.

This relatively expensive alloy, which

goes by the designation

Sn95.5Ag3.8Cu0.7, appears to be on

its way to becoming the standard for

lead-free soldering.

There is also a whole series of

other tin/silver/copper alloys with

either a higher or lower percentage

of silver, ranging from 5% silver

down to 2% silver (in the latter case,

with 0.8% copper plus 0.5% bismuth).

The melting temperatures of these

alloys range from 221 to 240 °C. Pure

tin/silver alloys have a very high pro-

portion of tin (the eutectic alloy is

99.3 Sn/0.7 Cu), and the melting

point (227 °C) is nearly as high as

that of pure tin (232 °C).

how nicely you make your solder

joints, they always look like ‘cold’

joints.

more critical problem, since the cir-

cuit boards remain in the hot area for

a longer time. However, the use of

lead-free alloys does not increase the

marginal cost as strongly, since the

cost of the solder alloy is a relatively

small part of the overall cost of man-

ufacturing soldering powders and

pastes. The lower melting point and

better wetting characteristics of the

Sn/Ag/Cu alloy, as compared to

other alloys, are also advantages for

reﬂow soldering.

Sn [wgt.-%]

1000

960.5 °

900

800

11.5

(12.5)

724 ° 19.5 (21)

700

600

49.6 (52)

500

480 °

or (Ag)

22.85

(24.6)

(26.8)

400

300

221 °

232 °

200

96.2

(96.5)

100

18 °

0 0

000057 - 11

100

Sn [pt.-%]

Figure 1. Melting point diagram of a tin/silver alloy.

Suppliers:

Most mail-order distributors, such as Conrad, Far-

nell, RS Components, Maplin and so on, already

offer lead-free solders. In the UK, Multicore is the

best known brand.

Internet addresses:

www.alphametals.com/products/leadfree.html

www.lead-free.org

www.solderworld.com

www.multicore.com

Summary

Changing over to lead-free solder is

without doubt ecologically worth-

while, and it is in principle possible.

The fewest problems occur with

hand soldering, since the only thing

that changes is that lead-free wire

solder is more expensive.

For industrial fabrication of elec-

tronic equipment, changing over to

lead-free solder involves not only a

higher materials cost, but also mod-

ifications to the soldering process

when the new alloy is introduced.

These can be time-consuming and

expensive. The main areas of con-

cern are temperature and contami-

nation problems, as well as the wet-

ting characteristics of the alloy.

(000057-1)

Lead-free soldering

Changing over to lead-free soldering

presents the fewest problems with

hand soldering. Lead-free wire sol-

der made from Sn/Ag/Cu or Sn/Cu

alloys is also suitable for touch-up

soldering and repairing conventional

tin/lead solder joints. The somewhat

higher melting temperature (217 °C)

of the eutectic Sn/Ag/Cu alloy, which

is most commonly used, is hardly

noticeable in practice. What is

noticeable is that the solder joint is

not shiny after it cools off, but imme-

diately turns a dull grey. No matter

Figure 2. A few samples: the large rolls hold

wire solder made from the eutectic

tin/silver/copper alloy Sn95.5Ag3.8Cu0.7. They

differ only in the ﬂux content (3.5% for normal

use and 1% for SMD soldering). The smaller

rolls hold wire solder made from tin/silver

(Sn95Ag5) and tin/copper (Sn99Cu1) alloys.

The smallest roll is once again a

tin/silver/copper alloy.

Text (German original): Ernst Krempelsauer

5/2000

Elektor Electronics

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