Download Lead-Free Soldering: A Challenge For Reflow Oven Manufacturers

ASSEMBLY
E L E C T R O N I C S
L E A D - F R E E
A N D
G R E E N
T O W A R D S
Lead-Free Soldering: A Challenge
For Reflow Oven Manufacturers
by Peter Franklin,
BTU Europe
With the Lead-free initiative
now in full swing within the
electronics industry, manufacturers are faced with finding
alternative materials and processes to Tin Lead soldering. TinSilver-Copper is widely regarded
as the most suitable alternative
for most applications although
its higher melting point with
regard to Tin Lead means that
several considerations, such as
the efficacy of reflow ovens, will
have to be taken into account as
the changeover takes place.
The transition from Lead-based
to Lead-free soldering continues
to be a major topic of interest
within the electronics assembly
industry. Several original equipment and sub-contract manufacturers have already developed
viable Lead-free processes for
production manufacturing. For
these companies, lines are run
with Lead-free or leaded solder
pastes according to product requirement and customer demand. However for many other
assemblers the spectre of Leadfree still poses a future challenge.
Ever since the Lead-free initiative was launched, electronics
industry supported groups have
been evaluating alternative materials and indeed alternative
processes to ‘Tin Lead’ soldering. Much attention has focused
on Tin-Silver-Copper materials, with Sn-Ag-Cu at approximately 4% Silver, 0.5% Copper,
currently regarded as the most
practical and affordable alternative to Tin-Lead for most applications. The ‘SAC’ material possesses many of the qualities of
eutectic Tin Lead - application
Figure 1 – Lead-free process impact
of the paste and placement of
components not usually requiring equipment upgrades. However the same may not be true of
the reflow solder oven.
Elevated melting point
SAC solder becomes liquidus at a
temperature of 217°C, some 34°C
higher than Tin Lead solder. This
elevated melting point translates
directly to the time temperature
profiles on the reflow solder oven
and becomes the limiting factor
in the reflow solder process window. Temperature tolerance of
the devices on the PCB remains
the same, typically 260°C, but the
temperature required to fix them
to the substrate has increased. A
reflow oven temperature profile
peaking at 210°C for Tin Lead
may peak at 240°C to 250°C for
‘SAC’ which means the process
window, formerly 40°C to 50°C,
is now around 10°C.
The challenges of Lead-free will
bring out the advantages of a
more technically enhanced reflow solder oven. The higher
peak temperature requires more
power in the heated zones and
reduced conveyor speeds (lower
throughput) to achieve the same
ramp rates. Typically an additional 11 to 12 seconds of heating and 11 to 12 seconds of cooling will be required to cope with
OnBoard Technology February 2006 - page 26
Figure 2 – A Pyramax Lead-free ready reflow oven from BTU
the additional 34°C of peak temperature. In a standard reflow
profile this equates to at least
a 7% reduction in throughput.
The oven process chamber configuration may cause other problems at reduced conveyor speeds.
Time above liquidus can become
too long, causing problems with
joint reliability and cosmetic appearance.
The use of Nitrogen atmosphere
will improve solder joint appearance in the Lead-free process,
but for low cost manufacturing
the added cost of using Nitrogen
may not make it a viable solution.
The difficulty in translating a
process to Lead-free will depend
on how demanding the product
is and the compromise that can
be made in terms of line speed
(throughput, cosmetic appearance and component choice).
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Figure 3 - Flux collected by the flux
manager is deposited in drip trays
that can be quickly and easily removed for emptying
Flux management has been the
maintenance focus for reflow
oven designers since the advent
of ‘no clean’ materials which volatise in the first heated zones of
the oven. These ‘airborne’ fluxes
will re-condense in the cooling
section causing contamination
problems for continued PCB
production. Lead-free presents
further challenges. The higher
temperature melting point demands more aggressive fluxes,
or fluxes which remain in contact with the board further into
the time temperature profile,
in order to protect against oxidation. Nitrogen atmosphere
Lead-free soldering solves some
of the problems associated with
higher temperature soldering
but as previously mentioned
OnBoard Technology February 2006 - page 27
ASSEMBLY
E L E C T R O N I C S
L E A D - F R E E
Flux management
The Pyramax Flux management
system extracts flux laden atmosphere from optimum positions
in the heated chamber, processes the atmosphere through heat
exchangers and filters before
re-cycling the gas back into the
oven chamber. Flux collected by
the flux manager is deposited in
drip trays that can be quickly
and easily removed for emptying. All flux manager maintenance can be performed without the need to stop the oven
running nor even interrupt PCB
production. The Pyramax was
specifically designed to be as
maintenance-free as could be
achieved. The only regular attendance required is to manage
the fluxes being processed and
this can be done without interrupting production. This low
maintenance feature and the robust design, combined with the
recycling of atmosphere when
running in Nitrogen, make the
Pyramax a very low cost of ownership production tool.
A N D
The BTU Pyramax range of reflow solder ovens are all Leadfree compatible as standard.
The ovens are designed to run
both Tin-Lead and Lead-free
time temperature profiles. The
‘designed in’ 350°C maximum
temperature allows the oven to
be worked at Lead-free temperatures 24/7 while still providing
excellent machine life. BTU’s
many years of experience in high
temperature furnace technology
has given them the necessary
expertise to select materials and
designs for the Pyramax range of
ovens, ensuring robust and reliable service with a minimum of
maintenance.
this will not be a viable solution
for all PCB assemblers.
G R E E N
Greater accuracy in temperature
control of the reflow oven is important to eliminate any temperature overshoot when the control
system calls for power – a function that modern digital temperature control loops perform
with great accuracy. Further
improvement can be achieved by
controlling the convection flow
rate in a convection heated oven,
as by doing so we can improve
the transfer of energy efficiency
from the oven heat emitters to
the PCB. This feature is key to
allowing complex assemblies and
densely populated boards to survive the transition to Lead-free.
Heat transfer can be improved by
increasing the convection flow
rate but only if the atmosphere
can be trapped and held in the
heater enclosure to ensure temperature equilibrium of all of the
convection flow in a heated zone.
Where the heater enclosure is
a plenum, a static pressure can
be generated slightly above atmospheric to ensure that all of
the atmosphere achieves the
required temperature before it
reaches the PCB. This static pressure may be measured and controlled using transducers in the
heater enclosure. Increasing the
efficiency of temperature transfer provides better temperature
uniformity between bare board
and components. As with Tin
Lead soldering, this temperature
delta between the most difficult
components to solder (usually,
but not exclusively the largest)
and the component with the lowest temperature tolerance is the
process window. This method of
controlling and manipulating
convection flow in the oven by
regulating the convection static
pressure independently in each
heated zone is an effective way to
achieve a wider process window
through increased heat transfer
efficiency and better temperature uniformity on the product.
Furthermore if we close loop the
convection control, a more stable, repeatable process can be
achieved.
The high melting point of SAC
solder paste effects more than
just the components on the PCB.
The board itself is more inclined
to warp at these elevated temperatures. Larger PCBs should,
where possible, be designed with
a highway for a ‘centre support’
to be used on the underside to
support the PCB throughout the
soldering process, especially if
they are very thin boards or have
cut outs of other features which
might undermine the rigidity of
the PCB.
T O W A R D S
Controlling convection flow rate