Disagrees on SACX performance.

The article "Pb-Free Solder Assembly for Mixed-Technology Boards" (CIRCUITS ASSEMBLY, March 2006) made several inaccurate and misleading statements in relation to the performance of the patented SACX alloy. First, the author wrote, "The SAC and SACX alloys dissolve most metals they come into contact with over time. For these alloys, new soldering equipment parts are necessary to keep equipment maintenance to a minimum." The consensus from wave solder machine manufacturers is that all high-Sn alloys will dissolve stainless steel, including SN100C; however, the rate of dissolution can vary between alloys. For SAC305 we recommend that the contact parts in the wave solder machine have a suitable protective coating; however, SACX behaves in a different way than SAC305 and therefore we advise that it can be used in untreated solder pots (see alphametals.com for specific advice).

Second, the author wrote, "SACX ... has a dull appearance and suffers from micro-cracks or the aforementioned hot tears/shrink holes. This can also lead to inspection problems." One of the specific advantages of SACX over SAC305 is the fact that SACX solder joints do not have micro-cracks or hot tear/shrink holes. A major consumer electronics OEM in Europe has chosen SACX specifically as it has wetting performance and reliability similar to SAC305 and does not have micro cracks. This OEM also commented that the matte appearance of the solder joints makes inspection much easier.

Finally, the author wrote, "When the recommended soldering temperature is used, wetting is very similar between SN100C and SACX." When using SACX we recommend soldering temperatures for single side boards of 255[degrees]C and for PTH 260[degrees]C. The chart in the article shows that at 255[degrees]C and 260[degrees]C the SACX alloy wets 30% faster than SN100C. This increase in wetting speed will improve hole fill.

Gerard Campbell

Global Product Manager--Metals

Cookson Electronics Assembly Materials

Karl Seelig responds: It is true that that high-Sn alloys lead to solder pot erosion. However, it is also known that certain elements added to these alloys reduce the aggressiveness of high-Sn alloys toward solder pots. One such element is nickel, an ingredient of SN100C. Antimony, found in the Castin alloy, is another element proven to slow erosion rates in wave solder equipment. We have data (Ed.: see graph online at circuitsassembly.com/cms/content/view/3355) that demonstrate the rate of corrosion of wave equipment when using various Pb-free alloys. Different Pb-free alloys do have different erosion rates. In spite of this, it is recommended that new equipment surfaces designed for Pb-free soldering be implemented to avoid erosion issues in the future.

Regarding the wetting times and temperatures, as the wetting balance study on page 44 of the March 2006 issue showed, SACX does not stand out as claimed. For point of reference in the article, SN100C is referred to as "Sn-0.7Cu+Ni"; SACX is "Sn-0.7Cu+(Ag+Bi). Furthermore, our study shows similar wetting forces (Ed: please see plot online). Finally, we have evidence that surface micro cracks do exist with the SACX alloy (Ed: see photo online). The dull surface, which is related to cooling rates, is due to an open grain. As a result of this dull surface, inspection criteria must be modified for the SACX alloy. As for SN100C, the same inspection criterion that applies for SnPb can still be maintained.

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