Calls to the Helpline asking for help with soldering are on the increase. In the past few months, callers having problems soldering components with Palladium (Pd) over Nickel (Ni), Pure Tin (Sn) over Ni, Gold (Au) over Ni and Silver (Ag)/Platinum (Pf) over ceramic have hit the Helpline. The increasing numbers of components with leads and terminations employing Pb free finishes may be the reason.
Here is an example of one such call.

by Guy Ramsey
Caller:
"We are having trouble soldering to an Electroless Nickel, Immersion Gold (ENIG) board. Can you tell us what is wrong?"

Helpline: "There are a number tests that can point to something wrong with the process, the material selection, or the solderability. Assuming your material selection and process controls are adequate, we can assume there is a problem with the board, do you need to know more?"

Caller: “Yes, our supplier maintains the boards are good and we need an independent evaluation."

SOLUTION - Solderability testing provides interested parties with an indication of how good the surface finish is working. These tests can be as simple as a dip and look or may involve use of sophisticated equipment that attempts to quantify the performance of the solderable surface. These tests provide a necessary starting point for assessing the performance of Pb free component finishes.

Figure 1 shows the image of the board finish in question after solderability testing. The board finish had failed to protect the nickel barrier over the copper.

In the days of SnPb soldering, suppliers of materials for the electronics industry could design fluxing compounds optimized for one or two metal interfaces and generally satisfy the needs of the market. Soldering, most often, involves melting the surface finish and joining it to the lead or land pattern. This is no longer the case. The advent of Pb free materials and the diversity of PWB, leadless terminations and component lead finishes present real problems. Variation in the performance of the finishes may bring your manufacturing process to a stop.

While we are seeing an increase in the number of soldering related problems, the process end remains essentially unchanged by the introduction of Pb free surface finishes. We have soldered to copper (Cu), nickel (Ni) and the same Pb free frame alloys for many years. The intermetallic compounds will still be Sn based, generally SnCu, or SnNi depending on the basis or barrier metal. But, the material providing the protective finish, and the performance of those finishes, is different. The interactions of the Pb free alloy, flux, surface finish and solderable basis metal are quite different. Furthermore, the variety of interactions increases. Fluxing chemistries and process parameters must be optimized to respond to all the finishes employed in the design.

In the short term, manufacturers must learn to use Pb free components with SnPb solders. We have observed that good results are possible but process windows are a little smaller. It is also possible that your current flux will not produce the desirable results.

Most of the Pb free finishes are applied over a Ni barrier. Ni is plated onto the lead to prevent the underlying metal from diffusing through the surface finish, where it would be subject to oxidation (NiO, NiOOH). Ni, then, is the surface to which we solder. If the surface finish fails to provide protection, the Ni barrier itself becomes unsolderable.

While test methods have been established, absolute acceptance criteria have not. Thus far, we have been able to identify causes for soldering problems through careful analysis. We are working to correlate existing acceptance criteria with data tests using the new Pb free alloys. This should help users set up performance-based requirements.

Soldering to Ni requires slightly higher temperatures and longer dwell times. However, caution is advised. Some leadless terminations may leach off components when subjected to high temperatures and extended dwell.

Inspectors are going to see solder connections that look bad. Contact angles are going to be higher than desired. Lines of demarcation at the finish to solder interface may be clearly visible and yet, acceptable. You should still be seeing low contact angles and evidence of wetting. When in doubt, test again (components and board s should have been tested before you put them into stock). Remember, these finishes are not as rugged as Hot Air Solder Leveled (HASL).

For more information on Pb lead soldering contact the Helpline at 610-362-1320.