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Gold coating of PCBs has been abandoned by most users
except when there is a good reason for retraining it.
On the other hand, gold plating of component leads is
still fairly common for at least two reasons:
- It is helpful during component packing.
- It provides a reliable electrical contact if the
component is assembled on a socket.
Pure gold (the so-called 24 carat gold) and low alloy
metal gold (99 to 99.9%+ gold), which may be electrodeposited
from acid or sulphite baths, usually have satisfactory
solderability.
However, gold is highly reactive both with tin and lead
at normal soldering temperatures.
Reaction with tin leads to the formation of several
intermetallic compounds, one of which, AuSn4, in turn
gives a eutectic with tin, which melts just above 200
°C (slightly below 400 °F). The gold-lead phase
diagram is also quite complex and includes a eutectic
(85% lead, 15% gold) which melts at about 250 °C
(482 °F).
In eutectic or 60/40 solder joints no gold-lead intermetallic
compounds are formed because gold reacts preferably
with tin.
If the gold content is under 20% by weight, above about
177 °C (350 °F) the actual solder is a ternary
Sn-Pb-AuSn2 system, which changes into a Sn-Pb-AuSn4
system upon cooling.
In normal wave soldering conditions, for instance at
250 °C (482 °F), gold is dissolved quickly by
molten solder. A coating of 2-3 microns may be dissolved
completely with a contact time of 1 second if it is
wetted by flowing solder. This is a typical case of
solubility because the soldering temperature is much
lower because the soldering temperature is much lower
than the melting point of gold (1063 °C).
The tin-gold intermetallic compounds dispersed into
the solder matrix have an acicular shape, which is detrimental
to solder joints because it forms a preferential cleavage
plane.
In general, the best approach is to keep the amount
of such compounds low by soldering:
- At the minimum possible temperature;
- with the minimum possible contact time.
A flash of gold (thickness approximately 0,5-1 micron,
i.e., 20-40 millionths of an inch) will be dissolved
completely, whatever the soldering parameters are. In
this case the opposite approach of high temperature
and long contact time is to preferred.
This will not only dissolve the gold layer completely,
but also disperse the intermetallic compounds in the
bulk of the solder fillet, so that the ductility of
the solder can reduce the danger of cleavage.
This means that the solder joint is established on the
base metal, which may be passivated through pores in
the gold layer without having been fluxed.
Good results are obtained only if the base metal (usually
nickel or tin-nickel) has been properly activated before
plating with gold.
For gold plated leads, the approach which produces
the most reliable joints is stripping of the gold layer
by fluxing and dipping the leads in a solder pot.
The geometry of the joint frequently plays an important
role in determining the reliability of a joint on gold
plated surfaces.
In some cases the soldering parameters must be established
by investigating the amount of gold dissolved and its
distribution in the solder fillet.
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