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OOOH Colors, It Must Be Lead Free
John Maxwell, Director of Product Development
Feb. 22, 2006 |
Lead free reflow solder presents users with a myriad of new experiences including
matte/dull/grainy solder joints instead of shiny smooth ones, higher reflow temperatures,
board de-lamination/blisters, increased plastic package cracking, potential tin whiskers
and chip terminations changing color after reflow soldering.
It was unusual to see chip terminations change colors when tin lead solders were used but
with the introduction of lead free reflow soldering and the corresponding increases in
reflow temperatures terminations are now changing colors. Two conditions are present
when reflow temperatures are increased for lead free solder alloys that leads to
discoloration. Reflow temperatures are above the melting point of tin (Sn MP is 232oC).
Air temperatures commonly used in forced convection reflow systems are high enough to
both melt the tin plating on the termination allowing it to be pulled into the solder joint
due to solder joint liquid solder surface tension leaving behind the exposed nickel barrier.
Now those metal oxide colors will be visible due to high air temperatures during reflow.
Metal colors are due to thin oxide films, the higher the temperature or longer the
exposure during soldering, colors will change. Colors may change at lower temperatures
than those listed if metals are exposed for long periods of time (minutes). Table 1 lists
oxide temperatures used for tempering tool steel from an old edition of Machinist’s
Handbook that are commonly encountered as set points in forced convection reflow
ovens. Those same colors are used for tempering stainless steel alloys that are made with
high nickel content and only cobalt separates iron (Fe) from nickel (Ni) in the periodic
table. These colors can be used as a guide for temperatures that chip terminations have
been exposed to. We can use this table as a guide in understanding why chip terminations
are changing colors after lead free reflow soldering and or rework. |
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| Most chip termination base metals can change color including both nickel and palladium
silver. A chip cross section drawing is in Figure 1 showing the termination structure. The
tin plated on the termination has no where to go and stays in place if just the chip is
heated. But if the parts are reflow soldered to an assembly then colors become visible
especially when assemblies undergo double sided reflow or if rework is performed on an
assembly. This is due to the surface tension of the liquid solder during reflow pulling the
tin plating from the top of the chip terminations as shown in Figure 2. |
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Discoloration is observed even when tin lead solders are used after multiple exposures to
reflow or rework. These colors are more pronounced when lead free solder alloys are
used due higher temperatures. The matte tin plated on terminations and leads melts and is
pulled into the solder joint leaving behind a thin tin/nickel intermallic layer. This layer
readily oxidizes resulting in discolored terminations. Even parts reflow soldered in
nitrogen blanket reflow systems can be discolored due to the presence of oxygen. These
systems are not oxygen free but have enough residual oxygen to discolor the exposed
nickel. The discoloration is not as pronounced as those systems that do not use a nitrogen
blanket but is still present.
Solder joint reliability is not affected as only the exposed portion of the termination
oxidizes. Metallurgical bonding occurs where the solder fillet is in contact with the
component termination and solder pad resulting in a reliable solder joint. What does
occur is that solder joints may be difficult to rework due to oxidation on the exposed
termination due to stable nickel passivation. |
| Notice: Specifications are subject to change without notice. Contact your nearest Johanson Dielectrics
Sales Office for the latest specifications. All statements, information and data given herein are believed
to be accurate and reliable, but are presented without guarantee, warranty, or responsibility of any kind,
expressed or implied. Statements or suggestions concerning possible use of our products are made
without representation or warranty that any such use is free of patent infringement and are not
recommendations to infringe any patents. The user should not assume that all safety measures are
indicated or that other measures may not be required. Specifications are typical and may not apply to all
applications. |
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