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Manufacturing Issues
There is no drop-in substitute for eutectic tin-lead solder. All possible substitutes for
SnPb solder have different characteristics such as melting temperature, wetting angles,
durability, cost, availability, toxicity and manufacturability. Tin-lead solder has been
used widely for decades in solder, finishes, batteries, paints, cables, sealing glass, cathode
ray tube glass, piezoelectric devices and discrete components.
Given the global nature of the electronics industry and the impracticality for most companies
to simultaneously offer tin-lead and lead-free assembly parts and products, lead-free products
are being designed that are not even destined for EU markets or that are otherwise exempt. Yet,
despite movement by many companies in the electronics industry toward lead-free electronics,
most companies are not adequately prepared at this time for the transition, and the ban on most
uses of lead has created large infrastructure gaps in developing lead-free processes, including:
- Standardization of product specifications and processes - The industry needs to qualify and
optimize for large-volume manufacturing lead-free processes for all the possible production
parameters: surface finishes, component types and finishes, solder paste and flux composition,
reflow profile and reflow atmosphere, wave solder, rework and repair.
- Process equipment modifications and modernizations - In limited cases, lead-free assembly
has been shown to be possible with existing equipment. However, most companies will need to
upgrade at least some of their equipment and modify most of their processes. For example,
lead-free solder alloys tend to attack various parts of the wave soldering machines, such
as the solder pot, and pump impellers; equipment must either be replaced with more expensive
titanium-coated parts or epoxy-coated to prevent corrosion. The higher melting temperatures
of lead-free solder alloys mean a narrower processing window (i.e., the temperature range
within which the solder melts and flows properly but the components and boards are not damaged).
As a result, manufacturers must have greater control over their reflow ovens. Newer ovens are
equipped with extra zones, including post-reflow cooling zones.
- Processability of lead-free solders - Lead-free solders have higher melting temperatures
and different wetting characteristics than tin-lead solders and some paste formulations wet
better in nitrogen atmosphere than in air; some assemblers may therefore consider modifying
existing ovens that are compatible with nitrogen or purchasing new ovens. However, there is some
evidence that nitrogen can actually increase the rate of tombstoning (a phenomenon in which a
difference in surface tension at one end of a surface mount component during
melting/solidification will cause the part to stand up on one end, like a tombstone).
These fundamental differences in lead-free solder compared to SnPb solder, affect practically
every aspect of printed wiring board assembly:
- Stencil aperture - Because lead-free solders don't spread out as much, there is less risk
of solder paste bleeding off the lands and therefore some manufacturers are recommending
larger apertures can be used in order to help prevent stencil clogging. However, every solder
paste manufacturer is different and there are instances where this may not be recommended. It
should be approached on a case-by-case basis.
- Component placement - Because of different wetting angles, components are not as likely to
self-correct during reflow in the event of misalignment during placement.
- Reflow profile control - The higher melting temperatures mean that there is a narrower
processing window (the temperature range above which the solder paste melts and below which
the component is damaged). The type of flux chemistry is extremely important, so it is
important to pick solder paste suppliers carefully. In most cases, it will be possible to
use the reflow profile recommended by the solder past supplier. But in some instances, it
may be necessary to customize slightly depending on the type of components being used.
- Component moisture sensitivity levels - Finding compatible components for the lead-free
assembly process is by far the most significant and troubling aspect for many manufacturers.
The higher melting temperatures can damage vulnerable components, lowering the MSL ratings by
up to two or three levels. Component manufacturers are working hard to address these issues,
but there are many challenges in designing packages that can withstand the higher reflow
temperatures (up to 260 degrees C) without forsaking functionality. J-STD-020B addressed
this issue.
- High-melting temperature solder - Due to technology limitations, some component types
have been exempted from lead bans. For example, flip chips use high-content lead (to achieve
a specific high temperature necessary for the functionality). Therefore the packaged product
is permitted to contain lead, but it is important to understand that the second-level
interconnect (the point where the component is soldered to the board) is NOT exempt and
therefore must still be lead free.
- Component specifications - Because of higher processing temperatures of lead-free solder
alloys, some component packages are vulnerable to damage, especially "popcorning," a phenomenon
in which moisture that invariably penetrates the plastic molding vaporizes during reflow and
suddenly bursts through the material. Moisture sensitivity level ratings (MSL) are being
modified and new packaging materials are being developed, but these developments are in their
infancy and the industry needs to stay informed of these changes and will require assistance
on how to implement them. In addition, the current industry standards for ball grid array
connections are not applicable for most lead-free BGA applications; the current criteria are
based on eutectic tin-lead solder balls that collapse after reflow. Lead-free solder balls
behave differently and voiding is a major concern. The industry needs guidance on processing
criteria for BGAs.
- Reliability standards - Lead-free solder joints look different from standard tin-lead
solder joints: a reliable lead-free joint may appear duller, rougher and they wet differently
from a reliable lead solder joint. Thus, a solder joint that would otherwise be rejected by an
inspector might in fact be passable. The latest IPC reliability and testing standard (IPC-060c)
does not include sufficient information about lead-free assembly inspection. Several research
consortia (CALCE, UMass Lowell) and industry trade organizations (IPC, JEDEC) are investigating
lead-free reliability, but there are currently many questions about long-term reliability of
lead-free products and how to test and inspect them. Reliability engineers and technicians will
need to be re-trained about how to identify an acceptable lead-free assembly.
- Tin whiskers - The most common lead-free coating on the component leads is pure tin.
However, pure tin has a tendency to form whiskers - spontaneous columnar protrusions that
grow directly out of the tin presumably to relieve internal stress in the lattice. There is
concern that these whiskers can grow long enough to bridge leads and cause shorts. There are
many industry groups working on this issue to understand the whiskering mechanisms (no one yet
knows for sure why this happens) and to understand what mitigation techniques could possibly be
used.
- Board finishes - There are several lead-free board finishes that are viable options for
many companies, including OSP, hot-air solder leveling (HASL), electroless nickel immersion
gold (ENIG) and immersion silver. The most common is OSP (organic solder preservative).
Interestingly, immersion silver is increasingly becoming more popular, driven, not by lead-free,
but rather because immersion silver has excellent coplanarity which is extremely important as
pitch size continues to decrease.
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