Can lead-free and lead-containing PCBs be cleaned in a single process?
Can lead-free and lead-
containing PCBs be cleaned
in a single process?
Dipl. Ing. Stefan Strixner, ZESTRON Europe, Ingolstadt, Germany
introduction
To answer the first question, ZESTRON
This article describes a study
The introduction of lead-free materials
already conducted several cleaning studies
recently conducted by Zestron
enforced by the European RoHS and WEEE
on lead-free soldered assemblies in the past.
for better understanding the
regulations has brought upon countless
They showed that in most cases, cleanliness
risks of cleaning lead-free and
changes to the electronics manufacturing
requirements can be met without changing
industry. After the introduction on July 1,
lead-containing assemblies
the cleaning agent or equipment, if modern
2006, entire production processes had to be
together in a single process.
cleaning agents are used.
adjusted to the new regulations and partly
To find an answer to the second ques-
By conducting cleaning
even had to be reevaluated.
tion, the cleaning process has to be analyzed
trials and laboratory analysis
Due to the implementation of lead-free
in more detail. ZESTRON therefore recently
as well as analyzing customer solder pastes, electronic assemblies have to
initiated a study in which lead-free and
bath samples, the risk of a
be reflowed at higher temperatures, which
lead-containing assemblies were cleaned in
cross contamination of the
leads to intensively baked-in flux residues in
the same processes using the same family of
lead-free assemblies with lead
the ovens as well as on assemblies. To avoid
products (in this case MPC® Technology
was examined. The study
oxidization as a result of the higher reflow
based products), allowing the potential risk
showed that using modern
temperatures, lead-free pastes contain an
of a cross contamination of lead-free and
cleaning technologies, the
increased activator content, which in return
lead-containing assemblies to be examined.
impacts the potential risk of corrosion
limits for lead content on
or electrochemical migration. As there is
lead-free assemblies set forth
lead contamination on lead-free
also an increased rosin content included
assemblies?
by RoHS and WEEE are not
to reduce void rates, altogether a signifi-
The big concern about cleaning lead
exceeded.
cantly higher amount of residues has to be
containing and lead-free assemblies in one
removed in the cleaning process.
process is whether lead introduced into the
Thus lead-free solder paste formulations
cleaning process by cleaning lead-containing
go hand in hand with increased cleaning
assemblies might cross-contaminate lead-free
Keywords: Lead-Free Produc-
demands.
assemblies so that they exceed the allowed
tion, Handling Tin-Lead Parts,
Before the new regulations came into
lead concentration levels.
Defluxing Process, RoHS &
effect, water- and solvent-based cleaning
The current legislation restricts the level
WEEE Compliance, Cleaning
processes were used worldwide to clean lead
of lead in lead-free assemblies to 0.1% by
containing solder pastes. These solder pastes
Technologies
weight. The limitation also includes other
are still in use for high-level security boards
heavy metals such as cadmium (0.01%),
in industries such as automotive and aero-
mercury (0.1%), etc.
space. When starting production of lead-free
If metal ions are present in the cleaning
assemblies, users are confronted with two
agent, the possibility of chemical reactions
main questions:
of the metal ions with the lead-free assem-
1. Are there any process changes neces-
blies exists. Extent and impact of such inter-
sary to meet the required cleanliness
actions however depend on several factors:
levels?
• Solubility of tin and lead in the
2. Is there any risk of contaminating
respective cleaning agent
lead-free assemblies with lead by
• Temperature of the cleaning agent
cleaning lead-free and lead contain-
• pH-value of the cleaning agent
ing assemblies together? Is it neces-
• Exposure time
sary to source a separate cleaning
• Concentration of the involved spe-
machine for the lead-free assemblies
cies
to meet RoHS requirements?
Most current defluxing processes rely on
28 – Global SMT & Packaging – November 2008
www.globalsmt.net
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