Keith,

Could you provide any references to published papers, etc.?

Thank you.

Regards,
Karl Sauter


[log in to unmask] wrote On 06/21/06 02:50,:

>Hi Hilbrand,
>
>You do not say which lead-free solder you were using in your attempt to dip
>solder fine transformer wire but I suspect it would have been a SnAgCu alloy.
>
>Rapid dissolution of copper in those alloys is a well established phenomenon,
>particularly at the high solder temperature you presumably have to use to
>burn off the polyurethane insulation from the transformer wire.
>
>As Pascal Obendorf has pointed out the rate of dissolution at a particular
>temperature is very dependent on the degree to which the alloy is already
>saturated with copper.  That is why, for example, the SAC305 alloy (Sn-3.0Ag-0.5Cu)
>is particularly aggressive towards copper in dip soldering, minipot rework and
>even wave soldering.   At 0.5% copper it is well under the saturation limit
>for copper even at 260C.   There are many reports of tracks and pads
>disappearing completely and of through holes being heavily eroded.
>
>Another factor affecting the rate of dissolution of the copper is the overall
>composition of the lead-free solder.   Copper does not dissolve directly in
>the solder but first reacts with the tin in the solder to form an intermetallic
>compound layer at its surface.   The rate at which dissolution of the
>underlying copper occurs is therefore dependent on the rate at which that
>intermetallic layer forms and dissolves.   It has been found that if that intermetallic
>(Cu6Sn5) has a trace of nickel in it (so that it is (Cu, Ni)6Sn5 its rate of
>growth and dissolution is substantially reduced.
>
>There are some commercial alloys available that combine both of these effects
>to produce a solder that can be safely used for dip soldering fine
>transformer wire.   They are basically a tin-copper alloy but with the copper level
>boosted to 3 or 4% and with the trace of nickel to further inhibit dissolution and
>improve the properties of the alloy as a solder.  They are now in very
>widespread use for dip soldering of transformer wire
>
>The following experiment gives an indication of the effect:
>
>80 micron transformer wire
>Solder temperature 400C
>After 5 seconds immersion:
>Sn-3.0Ag-0.5Cu alloy-  wire completely dissolved
>Sn-40Pb- wire eroded to 50 microns
>Ni-modified Sn-3Cu alloy: wire eroded to 68 microns
>
>Regards
>Keith Sweatman
>Nihon Superior Co. Ltd.
>
>
>
>
>
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--
Karl Sauter             Senior Staff Engineer, Engineering Technologies
                        Sun Microsystems Inc.
                        Office: (650) 786-7663 / x87663
                        E-mail: [log in to unmask]

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