Reducing copper dissolution in lead-free assembly
lead-free soldering typical conveyor speeds
Selective soldering have decreased, increasing the solder
processes contact times with all copper surfaces.
There are two types Some suppliers have modified the design of
of selective soldering
the wave ducting to increase the potential
systems, a fixed noz-
contact time, thus avoiding a reduction
zle where the board
in conveyor speed. The dissolution rate in
assembly/connec-
wave soldering will therefore be associated
tions move through
with the wave temperature, conveyor speed
the flowing solder,
and contact time.
or systems with a
dedicated series of
PCB solder levelling
solder nozzles. The
With the exception of electroless nickel
first type is the most
immersion gold (ENIG), all board finishes
popular for both
like silver, tin, copper OSP and solder
small and medium
levelled board will experience some level
volume users, and
of copper dissolution during assembly.
the second type are
The nickel in the ENIG finish has a much
used in high volume
slower dissolution rate compared to copper
Figure 7. Main variables affecting the dissolution of copper in soldering processes. dedicated equip-
and normally the 2-4 µm of nickel under
ment. The board
the gold will not be removed. Typically any
solder levelling process will remove copper
connector terminations are held in contact
assembly is lowered
during application of a fresh coating of
with the molten solder. When all solder
into contact with the solder, simultane-
either tin/lead or lead-free solder. There
joints are in a liquid state, the component
ously soldering multiple connectors in one
may be more evidence of copper removal
can be removed from the printed board.
step. Prior to soldering, selected areas are
on a vertical levelling process as the bottom
This process is used as an alternative to a
fluxed and the board is pre-heated. On
of the process panel will be in contact with
vacuum de-soldering iron, which de-solders
some systems, there is no pre-heat or the
the solder for a longer period of time than
each pin individually.
profiling of the board is not done correctly.
the top. On a horizontal system contact
When a solder fountain is used,
This leads to the use of longer soldering
times will be the same across a panel,
the plated through holes will still be
times or engineers increasing solder tem-
blocked after removing the component
perature and contact time, both resulting
similar to the wave solder process. Solder
terminations. The solder must be blown out
in increased copper dissolution rates.
temperature, dwell time, solder alloy, cop-
of the holes when still in a liquid state or
per previously absorbed and flow rate will
the board must be subjected to a second de-
Wave soldering
affect the amount of copper removed in a
soldering operation. Most solder fountains
Most wave soldering systems use a lambda
single pass.
used for rework do not have pre-heat. This shape wave, where the solder flows in the
In the early days of lead-free, it was
will extend the solder contact times on opposite direction to the board travel.
common practice to double dip panels
thermally challenging boards. Some waves When the board assembly contacts the
due to inadequate heat input or the
are pumped with a near static flow, some wave, the solder also starts to flow in both
performance of the flux. The use of this
have faster flowing waves to increase heat directions. Typically during wave solder-
process was an attempt to improve the
transfer and maintain better control of the ing the board assembly is pre heated to
surface coverage and visual appearance,
solder bath temperature. Generally the
100-120°C measured on the topside of the
but can result in further copper removal.
faster the flow-rate the higher the copper
board, the base of the board may be 20-
Today a double dip may be conducted for
dissolution. The design of the nozzle and
thermally demanding boards, however the
30˚C higher.
distribution of pressure across the ducting
With the
will also impact copper removal rates.
introduction of
16 – Global SMT & Packaging – October 2009 www.globalsmt.net
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