Question: We have a customer requesting us to use Silver Immersion Finish boards that are at least 5 years old. We have no idea how the boards have been stored during the last 5 years, seeing they have been stored at our customer’s facility.
We understand the need for baking to remove moisture. Comments?
Answer: Baking the boards may or may not be necessary depending upon how they were stored and whether or not they were multilayer boards. The baking being suggested is to dry out the boards so they will not delaminate, however if they are double sided or two sided boards, baking should not be necessary. If they are multilayer then one has to address how they were stored, whether or not they were stored in a dry chamber or in the open environment of a storage department. Baking will impact the solderability of the board, especially if they are as stated immersion silver boards.
My recommendation is to assemble a board, bake it for at least 8 hours and process it through the wave solder process. If it solders then just bake the others and process them. If the board is a surface mount board, then the thermal ramp up is quite different and delamination most likely will not happen, so run a board and check it out. Depending upon the results of processing the board through the SMT process will determine whether or not the boards should be baked.
What about the metallurgy and quality of the immersion silver finish?
Answer: The quality of the boards is again dependent upon how the boards were stored over the past 5 years. More than likely the board surface has oxidized quite a bit and the surface appears blackish in color. This is due to the oxidation or reaction of the silver coating with any sulfur present in the area. If this is the case then a more aggressive flux will need to be used so that it can reduce the oxides on the surfaces of the board. It is not recommended to mechanically abrade the surfaces of the product to make the land areas more solderable as this will have other impacts on the reliability of the solder joint.
Solder testing will determine solderability. Comments?
Answer: Process the boards through the manufacturing process will definitely define whether or not the boards are solderable.
If the finish has tarnished is there any way to restore the immersion finish for better solder quality?
Answer: The use of a more aggressive flux, something in the OAH1 category should be able to clean the surface and allow it to solder, which will also allow the solder to flow on the land areas of the terminations.