December, 2015
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Page 59
Solderability and Tinning: Does the Industry Know the Difference?
By Joseph Federico, Vice President, NJMET, Inc. A
particular dilemma facing the industry today stems from the difference in purpose of solder-
ability and tinning services. While NJMET has had the privilege of pro- viding these services to the aero-
initial resistance will be replaced by wetting forces “pulling” on it. This is normally displayed on a time versus force plot. A stronger and quicker pulling force is desirable and indi- cates good solderability.
Tinning Tinning is a method of replacing
the existing surface finish of a solder- able termination with a solder finish
of the desired alloy. The tin in both the tin-lead and lead-free alloys is a very aggressive metal, considering its ability to dissolve other metals. When a sample is dipped into the molten sol- der bath, the tin in the bath will first dissolve and remove the surface fin- ish. Some of the basis metal will be removed as well. The amount of basis metal removed is a function of the duration of the immersion, whether
the solder bath is static or dynamic, and to a lesser extent, the tempera- ture of the bath. When the sample is removed from the solder bath it will have a fresh coating of the solder alloy in place of the original surface finish. The removal of the basis metal (disso- lution) is a greater concern when using lead-free alloys as the higher tin percentages (usually around 97 per-
Continued on next page
SWB-2 dip wetting tester from Malcom.
space, military and medical fields, often, purchasing and engineering personnel confuse the two. Some companies plan to use solderability test samples, and install them into instrumentation without a precise requirement. There is also concern about how to properly perform these processes on components with a lead- free finish versus a tin-lead finish. Tinning is the process of dip-
ping the electronic component termi- nations into a bath of molten solder alloy. This creates a fresh inter- metallic layer between the solder and the base metal, providing a highly solderable surface finish. Solderability testing is designed
to determine how well molten solder will flow or “wet” on the solderable surfaces. This testing is necessary because the solderability of the ter- mination surface finishes tends to degrade over time. This solderability degradation is normally caused by contaminants, the most common of which is oxidation.
Solderability Testing The most common solderability
tests fall into two categories, qualita- tive and quantitative. The “dip and look” test is the most common quali- tative solderability test. In use for more than 50 years, this test involves taking a sample (in some cases sub- jected to steam aging or accelerated aging), and dipping and withdrawing it from a molten solder bath in a con- trolled way. Once removed from the molten solder, the sample is inspect- ed for the percentage of solder cover- age. Coverage of 95 percent is gener- ally considered acceptable. The most common quantitative
solderability test is the wetting bal- ance test. This test is a controlled immersion of a sample into a molten solder bath while measuring the forces encountered by the sample. The sample will initially encounter some resistance as it contacts the surface of the molten solder. As the solder begins to wet the sample, the
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