VERY
HIGH TEMPERATURES AFFECT EVEN "STANDARD" POLYIMIDE LABEL MATERIALS.
POLYONICS'XF 581 AND 582 SOLVE THAT PROBLEM
Question
#1: What is the push for “lead-free” soldering all about?
The
European perspective on waste management and recycling as
described in the European Union’s Waste of Electrical and
Electronic Equipment (WEEE) and Restriction of Hazardous Substances
(ROS) strongly suggests that lead-free electronic assemblies
will be mandatory in st1:place>Europe
by 2008. Japan
is moving toward voluntary compliance by 2003 with a lead-free
initiative that focuses on environmental marketing of new
products such as mobile phones, consumer electronics and automotive
electronics (with the exception of recycled lead-acid storage
batteries). The viewpoint concerning lead-free electronic
assemblies in the U.S.
is somewhat different since lead usage in electronic solder
comprises less than 1.0% of total lead consumption. Despite
this fact, technological obsolescence of end-of-life (EOL)
electronic products resulted in 21 million used computers
being dumped into the solid-waste recycling process in 1998.
o:p>
An
additional concern is that although the electronic solder
contained in electronic products represents less than 1.0%
of total lead consumption, 28.5% of all lead bearing materials
going into municipal solid waste (MSW) in the U.S. is from
associated cathode ray tubes (CRT’s) contained in television
sets and computer monitors. If lead-acid
storage batteries could be 100% recycled and thereby removed
from MSW, electronic solder and CRT’s would represent more
than 83.7% of the remaining lead bearing source material.
Question
#2: What does this mean for PCB manufacturers?
The
pending legislation and market trends leading toward the implementation
of lead-free electronic assemblies have raised several issues
including the need to increase the thermal tolerances of electronic
components. Lead-free solder alloys such as tin-silver-copper
(SnAgCu) with a melting point of 217°
C, require higher processing
temperatures than traditional tin-lead (SnPb)
alloys thereby reducing the process window and focusing on
the need for rigorous control of the thermal process during
soldering. This paradox between higher melting points of lead-free
alloys and the thermal thresholds of electronic components
forms the basis of the lead-free
challenge. Raising component thermal tolerances will place
a significant economic burden on electronics manufacturers
since they will be faced with higher overall thermal processing
costs. This increase in costs associated with the thermal
process will drive electronics assemblers to maximize the
efficiency of their thermal processes. This is especially
true when soldering through-hole (TH) components in complex
mixed technology printed circuit boards (PCB’s), where greater
thermal demands are placed on the process capabilities of
existing soldering methods such as wave soldering.
Question
#3: What are the implications for the manufacturing processes?
Lead-free
soldering of surface mount technology (SMT) has been studied
extensively over the past several years while lead-free flow
soldering of TH components has received little attention.
However, in order for an electronic assembly to be considered
as truly lead-free, every step of the assembly process must
utilize lead-free materials including SMT reflow, TH soldering,
rework, field repair and EOL recycling infrastructure.
Since
the wetting characteristics of lead-free alloys are less than
that typically exhibited by SnPb alloys at lower process temperatures,
the selection of a good flux is mandatory
to assure good solderability. The flux selected must be able
to withstand exposure to the higher process temperatures required
for lead-free alloys. In general, fluxes used for
lead-free TH flow soldering must be able to withstand topside
PCB preheat temperatures as high as 130°
C and solder temperatures as high
as 280°
C for a minimum of 3 seconds of contact time
or longer. The major difference between SbPb and
lead-free flow soldering is the higher melting point required
by the lead-free alloy. Because lead-free alloys require higher
preheat temperatures, thermal shocking
of components as they enter the molten liquid wave
should not exceed 100°
C. For most applications, VOC-free, water-based fluxes,
applied with a spray fluxer or inkjet drop fluxer, are recommended
since they can withstand these higher processing temperatures.
Question
#4: What are the implications for the thermal profile of these
processes?
In
order to form quality solder joints, the flow soldering process
irregardless whether wave soldering or site-specific selective
soldering is used, must:
1)
raise the temperature of the base metals high enough to allow
sufficient wetting,
2)
provide adequate contact time for capillary action to take
place, and
3)
provide adequate thermal energy to create an intermetallic
layer, and a good electrical connection.
While
lead-free solder alloys require higher processing temperatures
due to the nature of their wetting properties, TH components
can be damaged if their internal threshold temperature is
exceeded by either rapid heating or excessive heating of the
PCB assembly during the soldering process.
This
is especially true in lead-free flow soldering since the process
temperatures are typically 30-40°
C higher than traditional SnPb alloy process temperatures
for which most TH components were designed. A minimum dwell
time is required to promote wetting of the through-hole component
lead while a maximum dwell time should not be exceeded that
could result in damage to the component. Likewise, a minimum
temperature is required for the solder to melt and flow, while
exceeding a maximum temperature could cause internal damage
to the component.
Question
# 5: What are some specific temperatures to be aware of?
There
are two areas of thermal increases:
PREHEAT
CYCLE: For most lead-free flow soldering applications,
the topside PCB temperature will increase
by as much as 125% to approximately 110- 130°
C to limit the thermal shock as the PCB contacts
the higher temperature of the lead-free molten solder. The
ideal method is to heat up the PCB as quickly as possible
to 100°
C and continue to heat the PCB with forced hot-air
convection preheating for optimal evaporation of the water
medium from the water-based flux.
SOLDER
POT:Due to the higher melting point of lead-free
alloys, the temperature of the solder pot will also increase
to improve solderability and shorten contact times. For AgSnCu
with a melting point of 217°
C, the solder pot temperature
will be between 250-270°
C or as high as 260-280°
C for SnCu.
Question
# 6: What is the impact of these changes on the circuit board
labels used on the PCB’s?
Each
label material used must be re-evaluated against these new
thermal profiles. If polyester was used in a location which
now experiences the higher temperatures (and renders it useless),
then the material must be changed to a different material,
such as PEI
or Polyimide
Label materials. The higher temperatures may
also require that standard polyimide labels be changed to
newer XF
582 Polyimide Labels,or one of the other NEW
Lead Free Labels offered by Polyonics.
The
following photo shows the effect of very high temperature
(600 °
F or nearly 300° C)
for 1 hour. The
temperature discolored the FR-4 epoxy board material, but
did not affect the kapton labels.
Polyonics
XF 552 Yellow Polyimide Label presents
yet another alternative to XF 581 and XF 582. The photo below
shows again the result of the 50 minute heat test at 600
°
F (nearly 300° C)
for 50 minutes.
