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Archive >  December 2015 Issue >  Special Features: PCB and Test > 

Solderability and Tinning: Does the Industry Know the Difference?
SWB-2 dip wetting tester from Malcom.

A particular dilemma facing the industry today stems from the difference in purpose of solderability and tinning services. While NJMET has had the privilege of providing these services to the aerospace, 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 dipping the electronic component terminations 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 termination 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, qualitative and quantitative. The "dip and look" test is the most common qualitative solderability test. In use for more than 50 years, this test involves taking a sample (in some cases subjected to steam aging or accelerated aging), and dipping and withdrawing it from a molten solder bath in a controlled way. Once removed from the molten solder, the sample is inspected for the percentage of solder coverage. Coverage of 95 percent is generally considered acceptable.

The most common quantitative solderability test is the wetting balance 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 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 indicates good solderability.

Tinning
Tinning is a method of replacing the existing surface finish of a solderable 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 solder bath, the tin in the bath will first dissolve and remove the surface finish. 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 temperature 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 (dissolution) is a greater concern when using lead-free alloys as the higher tin percentages (usually around 97 percent) are more aggressive than the tin-lead alloys.

When to Use Solderability Testing
Solderability testing is a great tool to help determine if the surface finish of the selected components will provide the degree of wetting necessary for acceptable solder connections. If, for example, solderability testing results indicate less than optimal solderability, it could result in poor wetting and defective solder connections. This would then requires rework and additional processing. Solderability testing can also be used to determine the effects of storage conditions on the surface finish. By including accelerated aging, the testing can also estimate acceptable storage life. This kind of testing can also be used to verify conformance of the components as received from the vendor.

When to Use Tinning
To ensure the best wetting of solder connections, the terminations should be tinned just prior to the soldering process. Jim Raby, a pioneer of the industry and the founder of STI Electronics, once said, "Nothing solders like solder." Tinning just prior to the soldering process ensures that the solderable terminations have an excellent solder coating that has not yet had the chance to form an oxide layer on the surface. This used to be common practice for the manufacturing of high reliability electronic assemblies. When pre-tinning is accomplished just before the soldering process it eliminates the need for solderability testing.

Tinning is also the best method to rework terminations that exhibit degraded solderability. Even severely oxidized terminations can be reworked by a hot solder dip tinning process replacing the oxidized surface with a pristine solder finish.

Tinning in a tin-lead bath is one of the methods used for lead-free terminations that must be used in a tin-lead soldering process. The risk of whisker growth is greatly diminished by replacing the lead-free finish with a solder finish containing lead.

Lead-Free Soldering Compatibility
The 2003 rules — the "Waste Electronic Equipment Act" (WEEE) and The Restriction of Hazardous Substances (RoHS) act went into effect on July 1, 2006. These rules have had an enormous effect on consumer electronics. products for the defense and aerospace industries as well as medical electronics are "exempt" from these rules and continue to use lead bearing solders and avoid the use of tin plating. The main reason for this refusal of lead-free materials is the risk of tin whiskers and other reliability problems inherent in lead-free soldering. While there is yet no definitive answer to what causes tin whiskers, as little as 1 percent lead added to pure tin materials drastically reduces their occurrence.

Beginning July 1, 2006 RoHS legislation restricted the amount of Lead (Pb), Cadmium (Cd), Mercury (Hg), Hexavalent Chromium (Cr6+), Polybrominated Diphenylethers (PBDEs), and Polybrominated Biphenyls (PBBs) in electronic and electrical equipment. These chemicals present a risk to human health and the environment. Each EU member state adopts its own enforcement and implementation polices using the directive as a guide.

Even though RoHS is a European directive, the RoHS directive has a global effect. The directive indicates that anything covered by RoHS entering the EU must be compliant, whether they include cables made in China, parts molded in the USA, or PCBs from Japan. If a product is eventually destined for the EU, it is impacted by RoHS.


Contact: NJMET, Inc., 1240 Main Avenue, Clifton, NJ 07011 973-546-5393 fax: 973-546-1836 Web:
http://www.njmetmtl.com

Vice President and Director of Operations at NJMET, Joseph Federico has helped focus the company on electronic component procurement, testing, up-screening, and counterfeit detection. He has met with distributors around the world, as well as military and aerospace companies to raise the awareness of component counterfeiting. He would like to acknowledge Jim Raby, Dave Raby, Ray Cirimele, and Marietta Lemieus from STI Electronics, Inc., 256-461-9191, for their special help with this article.

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