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Hot Air Levelling Technology Lead-Free
— offered by —
Procurement Trading Xchange for Interconection Carriers (pt_X_ic)
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The nearer the deadline draws
to the compulsory
implementation of lead-free regulations (ROHS), the
greater the attempts to achieve
lead-free solutions that
are comparable to the “all-purpose answer” SnPb in
terms of
technology and cost. Two systems are
particularly predominant: tin/silver/copper (SnAgCu) and,
more recently, stabilised tin/copper
(SnCuNi). Although
the level of efficiency of both systems is not yet completely
clear, the following
presents the latest information:
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SnAgCu |
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There is more experience in the field of silver systems. Here,
differentiation is made
between the Sn-3.5Ag-0.7Cu, Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu systems which
vary in
use from continent to continent.
However, the agreed expert opinion (IPC/Soldertech-Conference
Brussels 2003) is that
the technological differences between the three systems are negligible.
Sn-3.5Ag-0.7Cu is the most
common alloy in Europe.
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SnCuNi |
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Although there are even fewer field results for the stabilised
tin/copper system Sn-0.7Cu-
0.1Ni, it shows a lot of potential.
It is called “stabilised” because the addition of nickel is said to show an improved
hardening reaction in soldering.
The creation of needle-structured crystals is said to alter
to rounder structures, achieving
an improved soldering flow for wave soldering of PCBs (reduction
in torn soldering, less
electrical bridging).
This alloy was developed in Japan and is supplied in Germany by the license
holder, Balver Zinn.
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The Printed Circuit Board Manufacturer |
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The SnCuNi system is particularly interesting for the PCB manufacturer
as the
investment needed to convert from conventional lead-tin techniques to tin/copper is far
lower than for facilities providing the processing technology for chemically produced
surfaces.
Because, this is also a melting pot procedure. And only the solder’s tolerance with the
dipping-container needs to be further researched.
Our company is already working on solutions with
mechanical engineering
manufacturers. From May 2004, we plan to offer our customers SnCuNi is series
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Soldering Trials |
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As already mentioned, there has been insufficient field research.
Therefore direct
comparison of soldering characteristics has been undertaken in diverse trial campaigns
by expert and work groups (BDF Germany, Boeing USA).
Both systems enable smooth, shining surfaces and reliable soldering joins, using wave
soldering
as well as reflow.
The tin-copper system promises advantages in terms of the copper’s peeling
reactions
(leaching) on the circuit boards. The effect on the copper surface is halved.
Costs must be kept
in mind. The silver system is approximately 40% more expensive than SnCuNi due to the use of precious
metal; also, there is the ecological disavantage.
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Lead Sensitivity: |
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The problem of lead sensitivity during the lead-free conversion phase
in 2006 remains
unsolved.
Structural changes in lead-free soldering caused by remains of lead (even below 0.1%
weight)
in processing containers have been observed.
Structural coarseness occurs in SnAgCu if there
are stressful temperature changes even
well below 96 C° (degradation acceleration).
Lead secretion occurs in bordeline grains in SnCuNi
systems which, under mechanical tension, can create weak spots. Splitting is possible when soldering
heat cools.
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Conclusion: |
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There is no doubt that the “good old” lead/tin still
provides the best soldering results.
However, the two procedures presented above can be improved upon.
The temperature and processing
profiles in wave and reflow soldering are two points
where this could happen, so that almost the
same quality could be achieved.
In spite of the limited experience in the field, in our opinion,
the SnCuNi system offers
definite advantages over the silver systems both in cost and quality.
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The following table shows the characteristics
of both systems in direct comparison: |
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SnAgCu - Systems |
Sn-0.7Cu-0.1Ni (SnCuNi) |
Melting point
(eutectical point)
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217 C° |
227 C° |
| Procedure characteristic reflow/convect |
Tpeak = 230 C° |
Tpeak 240 C° |
| Procedure characteristics Wave |
Badtemp.: 255 – 265 C° |
Badtemp.: 255 - 260 C° |
| Ecolog. disadvantages |
Silver content |
no |
| Metal price |
12 – 13 € |
7 – 8 € |
| Leaching (Cu peeling) |
Aggressive high leaching rate |
Leaching Rate 0,5 von SnAgCu |
| Soldering error rate compared with SnPB |
Equal Solder bridging, more non-soldering |
Equal Solder bridging, more non-soldering |
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