Article 2. The Wetting Balance as a Process development tool.

Using the Wetting Balance to determine a suitable VOC Free flux

In this, the second of a series of three articles we detail the use of a wetting balance as a process development tool. Although traditionally regarded as a QC/QA tool, to those in the know, the wetting balance can be used in all areas that pertain to soldering and solderability.

A customer of ours on the West Coast asked for some help with a liquid flux change. The assembly process consisted of top side paste / reflow / bottom side glue dot / wave utilizing an OA type flux with subsequent post wash (aqueous). Being environmentally conscious they wanted to change to a no-clean process and to make matters even more interesting decided to go all the way to a VOC free no clean. This change would reduce the consumption of process water, reduce the waste stream of metal contaminated discharge, as well as negating the need for additional air discharge scrubber / oxidizer purchase. Most customers that I have known over the years have taken the more traditional approach of changing from aqueous post washing to a VOC containing no-clean. The first generation of VOC free fluxes left a lot to be desired but the current generation is proving to be a viable alternative producing excellent quality and first pass yields.

The problem: which flux to chose? The traditional method of flux evaluation would be to schedule line time, run product and evaluate the results. This costs a lot of money and is subject to inspector interpretational error. Furthermore it does not produce data necessary to refine the assembly process – how quickly did the solder wet, how long to reach maximum wetting force (a function of its ability to deal with oxides present on the surface of the components/board), are there signs of dewetting - did the flux protect the assembly through the wave soldering process.

 Solution: use the wetting balance to short list potential candidates. The starting point is to identify the activation level that the assembly can tolerate given the constraints of the design density and the operating environment. The customer found five fluxes from four manufacturers that met requirements with activation levels the same as per ANSI-J/STD-004. The wetting balance provided a means to run a screening experiment on the five fluxes that produced useful information and more important, was very cost effective.

Upon receipt of the five flux samples an initial series of tests were run to determine the surface tension of each of the fluxes. From our first article it will be remembered that this is a vital piece of information necessary to accurately determine the contact/wetting angle. To determine the flux surface tension requires the preparation of known good coupons – something that has excellent wettability. The coupon used is 10 mm x 10 mm copper foil of ½ oz weight (19 micron in thickness) that is die cut from a sheet of electro-plated copper. The foil as supplied is not coated with any anti-tarnish agents or zincating processes normally used in their process and thus is quite oxidized. However, the oxidation is simple copper oxide and thus readily reducible.

Ten coupons per flux are punched from the foil sheet. Each is subjected to 1 minute in Acetone (to remove non-polar contaminants), allowed to dry, dipped into a 20% v/v of nitric acid for one minute (is a mild etch that removes the oxide layer) and finally into DI water for one minute to remove the acid residues. Finally, the coupon is carefully dried by blotting with a paper towel. The coupon is then dipped into the flux; excess removed and due to the nature of VOC free is suspended over the solder pot for 1 minute – to ensure that the coupon is dry prior to immersing in to the solder bath. The immersion is 90 degrees incident to the solder surface and the immersion depth is 3 mm with a dwell time of 10 seconds. The surface dross is automatically removed prior to each immersion. This is repeated 10 times per flux and the highest value is chosen as the surface tension. An example of the surface tension graph for flux B is shown below.

This process is repeated for all five fluxes. Coupons were prepared using the above technique, which were used as test samples for the wetting test. As could be expected all fluxes performed well and there was no noticeable difference between them.

More coupons were prepared and oxidized by heating at 160º C for two hours.

The results of the tests are shown below

As can be seen, two fluxes, A and B did not perform (no wetting) two, D and E appeared similar and C has max wetting force. Fluxes C and E were chosen as potential candidates.

After further testing and then under actual operating conditions Flux E was the most suited to the process.

To be fair to the manufacturers of the various fluxes the author admits that the test was extreme. The samples were very heavily oxidized and would not represent normal operating conditions. Our customer wanted a quick test that gave them a starting point. We used this example as it clearly shows how the machine is used to compare the fluxes. Under normal circumstances the test would be run using a variety of surface finishes and the components that normally cause the most problem during assembly. Each sample is tested on the wetting balance in “as received” condition. Should all the fluxes appear to perform at the same level then it becomes necessary to stress the components. The type of component determines the type of stressing; Heat, RH or Steam and the operator would choose the degree of stressing. Samples would be incrementally stressed until a noticeable difference in the fluxes was detected. Actual operating conditions will determine the final choice.

Summary; faced with a choice of fluxes that all offered the same activation level the user would be faced with a series of expensive tests that use actual production time, equipment, components and chemistry. In the event that all conditions were favorable it may have been impossible to detect differences in the flux. By using the wetting balance all this work is done off line where the operator, using objective data, can replicate any number of desired conditions. Ultimately the final decision will be made under operating conditions however the user can be confident that the top choices will meet most requirements.

In our next and last article we will demonstrate the use of the wetting balance with an inert gas (Nitrogen) unit. With the mandate of Lead Free assembly fast approaching the use of Nitrogen Inerting in the assembly process must be considered.