Immersion silver as lead-free circuit board surface
To meet the requirements of the lead-free regulation for circuit board assembly, it is necessary to have an overview of the issue.
In addition to the actual solder, both the circuit board surface and the component itself must be lead-free to the greatest extent possible (< 0.1% by weight).
There is a large selection of lead-free circuit board surfaces available, from lead-free HAL, immersion tin, immersion nickel-gold and OSP to immersion silver.
Further developments have been made recently, particularly for the surface on silver-organic basis, that make this surface a "first choice" for a large number of applications!
Würth Elektronik offers you these immersion silver circuit boards based on the latest technology.
The production process - and what makes it so interesting
The reaction is an immersion deposition (immersion silver) that guarantees constant layer thickness and excellent surfaces. That's an advantage that will pay off in the mounting process.
Further details are:
- typical layer thickness: approx. 0.25 – 0.35 µm
- very low process temperature (up to a max. of approx. 50ºC)
- low copper removal
- Extremely level and even surfaces
The image through the electron microscope shows: Very even surfaces are formed as a result of the production process described above. They are necessary for all fine pitch and HDI applications, but they also have a very positive effect on technically less sophisticated applications in the mounting process.
- Stable solder connections
The schematic structure of a solder joint with immersion silver shows that Ag diffuses into the tin-solder during the soldering process, and that the actual solder connection is thus formed between copper and tin. This diffusion process distinguishes immersion silver from other surfaces and makes the solder connection very reliable and stable.
- Low ionic contamination and electromigration
In the past it was said that silver surfaces have weaknesses in these areas. The latest research shows that the immersion silver surface used by Würth Elektronik compares very well. The values attained are very similar to other chemical surfaces.
- Good wetting characteristics with multiple reflow mounting
Due to increased layer thickness and optimized chemical composition, the immersion silver surface used by Würth Elektronik has reliable wetting characteristics even after multiple reflow passes.
- Excellent shelf life
Joint tests with the process manufacturer have shown that circuit boards coated in immersion silver using the Würth Elektronik method demonstrate excellent solderability when stored under typical climate conditions in mounting operations (up to 35°C and 85% rel. humidity) even over 12 months. During storage the circuit boards should be kept in the package provided by Würth Elektronik to exclude the possibility of contamination from the surrounding air.
Immersion silver surfaces in actual use
Meanwhile, Würth Elektronik actively serves over 300 customers (tendency rising) with immersion silver circuit boards who report excellent results in their mounting operations (e.g., reduction in costs due to malfunctions).
There are many good reasons for its use:
• very reliable lead-free surface for stable solder connections
• cost-saving potential compared with other lead-free surfaces
• excellent solderability
• reliable in combination with leaded and lead-free solders
• suitable for multiple soldering operations
• suitable for multiple reflow assembly
• 12-month shelf life for immersion silver