Immersion silver is a solderable surface finish applied to the bare copper of a printed circuit board. The role of the immersion silver is to prevent the copper from oxidizing. When soldering to a board with immersion silver a copper-tin intermetalic alloy is formed. The silver is absorbed into the solder joint. Immersion silver is applied as a last operation. It is common for it to be applied to individual boards in piece or pallet form after the parts have been removed from the process panel. This minimized handling contamination and visual defects to the plated surface.
Immersion Silver has the following advantages…
- Low cost.
- Flat co-planar surface to solder to.
- Highly solderable surface.
- Already in use with lead free solder pastes.
- Does not affect solder joint reliability.
- Compatible with no-clean assembly process.
- Wire aluminum bondable.
- Can be removed and replated.
The only drawback that really counts is that immersion silver is readily oxidised and is sensitive to sulfur. Most contract manufacturers stay clear of Immmersion Silver because they believe that their assembly yields shall go down. Handling is one concern they may have and they may want to charge more to process immersion coated boards. If they have their act together and have good handling procedures in place they shouldn’t need to do this. The other belief is that when the immersion silver becomes tarnished that it can not be soldered to. This is not entirely accurate. Printed circuit boards that are tarnished can still be soldered to. It all depends on how far the tarnish penetrates into the immersion silver surface. Lenora Toscano and Donald Cullen of MacDermid Inc. have done a nice study on the effects tarnish has on solderability. It can be downloaded from the following link…
The report shows that boards can still be solderd to. Unfortunately there is no magic wand available to most assemblers that can tell them how thick the tarnish is. Properly stored boards and handling procedures shall prevent immersion silver coated boards from becoming tarnished. However, when they do become tarnished you can still solder to them.
Now for the disclaimer. The success of soldering to the tarnished surface is dependant upon the contract manufacturer’s equipment and process controls in place. Now for the common sense answer. It shall still solder in most cases. Lets apply some common sense prior to throwing out the product. The way to verify if the tarnish is too thick is to run a solderability test. Take the board with the highest level of tarnish and use it as the solderability sample. If the immersion silver wets and meets the IPC solderability specification then the remaining boards shall be OK. If the sample fails then you need to decide what to do. Either scrap the remaining boards or find someone to strip off the tarnished metal and re-coat it with a fresh coat of immersion silver.