Solderability and tinning: does the industry really know the difference?

January 25, 2016 // By Joseph Federico
For many years NJMET has had the wonderful privilege of providing the electronic component industry with tinning and solderability services in the aerospace, military and medical fields. A particular dilemma which continues today is understanding the purpose of each service.

Often times, I find purchasing and various engineering personnel confusing the two services and most recently observed a few companies who were planning on using solderability test samples to be installed into instrumentation without a precise requirement. Moreover there is also a concern on how to properly perform these processes on components which have been manufactured with a lead free finish versus a tin lead finish.

Tinning in the electronic industry is the process of dipping the electronic component terminations into a bath of molten solder alloy, creating a fresh intermetallic layer between the solder and the base metal and 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 while in storage. This solderability degradation is normally caused by contaminants, the most common of which is oxidation.

So how does a customer know which process to request? Let's take a closer look at each one of these services and when they should be selected.

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. This test has been around in a similar form for over 50 years. The test involves taking a sample (in some cases subjected to steam aging/accelerated aging), and dipping it into, and withdrawing it from a molten solder bath in a controlled manner. Upon removal from the molten solder, the sample is inspected for the percentage of solder coverage. Solder coverage of 95% is considered acceptable.

The most common quantitative solderability test is the wetting balance test. This test involves 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 the sample contacts the surface of the molten solder. 

A wetting balance

As the molten solder begins to wet the sample, the initial resistance will be replaced by wetting forces “pulling” on the sample. This is normally displayed on a time versus force plot. A stronger and quicker pulling force is desirable. This would indicate that the sample has very good solderability.

These graphs from a wetting balance show a non wetting result (top), and a positive wetting result (bottom)