|
Will cleaning be impacted by implementing a lead-free
soldering process?
Yes. A lead-free reflow process subjects the assemblies
to higher temperatures (because of higher melting temperatures)
compared to Sn/Pb. As in all post-reflow cleaning applications,
reflow temperature directly effects the cleaning process.
As a rule, higher reflow temperatures create a more challenging
cleaning process. In short, the hotter the flux gets,
the more difficult the cleaning process becomes.
In any soldering process, metal salts are formed during
the reflow process as the flux reacts with the oxides
on the board’s surface. These metal salts generally
are encapsulated into the flux and are easily removed
during the cleaning process. In lead-free reflow applications,
however, higher temperatures increase oxidation and,
thus, metal salts. Higher temperatures also reduce the
flux’s ability to encapsulate the metal salts,
allowing them instead to adhere to the board’s
surface, creating a white residue and additional cleaning
challenges.
In Sn/Pb reflow applications, water soluble fluxes are
normally removed without a chemical additive. In lead-free
applications, however, a chemical additive may be required
for all flux types because of the higher reflow temperatures
and the resulting flux volatilization. Flux removal is
a process consisting of a mechanical and chemical component.
Wash solution (water and chemical), delivered under pressure
is sprayed onto the board’s surface and under its
components. In addition to the normal challenges of ensuring
thorough under-component penetration (impingement), the
cleaning system must contend with a more volatilized
and more difficult to remove flux as well as additional
salts on the board’s surface.
One potential remedy is to increase the wash time and
the amount of time the solution is sprayed onto the board’s
surface. This potential solution may not be desirable
as most cleaning chemicals utilize a high pH formulation,
which over time may cause solder joint dulling. A more
effective solution is to increase the power in the wash
section of the cleaning system. If wash solution can
be delivered to the board’s surface under increased
pressure and flow rates, the result will be more cleaning
in less time. The increased pressures assist the chemical
in “solubilizing” the flux while the contact
time reduction prevents solder joint dulling.
|
