Corrosion products can come from anywhere and moisture is the element that makes it happen. My first thought without seeing the product would be to conformal coat the product, to reduce the amount of moisture reaching the components.

Secondly I would check the cleaning process used after the soldering operation of these boards to make sure they are clean and no flux residues are left behind from that operation. The ability of flux to absorb moisture is high and under the right condition can create all kinds of corrosion and dendritic growth.

These are two quick thoughts, hope it helps.

It sounds as if your product is subjected to a condensing environment in its end application. In such cases, the best approach would be apply some sort of a conformal coating or potting compound to the board after the assembly process but before it is sent out to the field. These types of products are commonly used to act as a moisture barrier in several varieties of electronics applications.

A conformal coating will solve this problem for you. There are many different conformal coatings on the market, the most suitable will depend on your application and processing capabilities. If you can supply me with more information on your application and what facilities you have in production then I can offer more assistance.

Alternatively, you can visit Electrolube website for more information. Depending on the quantity of boards you produce you may want to look at an acrylic, such as APL in aerosol form, as a starting point.