How to prevent PCB corrosion in long-term storage
· PCBRacks.com engineering team
Bare boards don't come with an expiration date, so they tend to sit on a shelf until someone needs them. When they finally go to assembly months later, the pads don't take solder the way they should, because the finish on the pads has aged.
How each finish ages
How fast a board ages depends on its finish. OSP, the organic coating, breaks down with time and heat. Immersion silver tarnishes when it meets sulfur, and cardboard and rubber bands both carry it. Immersion tin slowly turns into tin-copper intermetallic, faster when it's warm. ENIG and HASL hold up longer. Fabs often quote about six months for OSP, tin and silver, and about twelve for ENIG and HASL, but go by your own fab's date code and figures. Every finish ages faster when the board is warm, damp and handled.
You won't see green corrosion. You'll see a wetting problem at assembly, on a board that looked perfect going in.
What speeds it up
Moisture does, especially moisture that cycles. Steady humidity works slowly. A room that isn't heated at night, where moisture condenses on the boards and dries off again every day, works fast.
Fingerprints do too. Skin leaves salts and oils that pull water from the air, and they sit right on the pads. A handled board can corrode in the shape of the fingers that held it.
Boards touching each other make it worse too. Resting face to face, they trap moisture and dirt between them and hold it there. Air on both faces is about the cheapest corrosion control you'll find, and a slotted rack gives you that by holding each board on edge in its own slot.
Storing boards for months
If boards are going away for more than a few months, seal them in a moisture barrier bag with desiccant and a humidity indicator card, and heat-seal it. Bags made to MIL-PRF-81705 Type I are the usual choice. Anything you won't touch this quarter is better sealed and boxed than sitting open on a shelf.
Sealing protects more than the finish. Stored boards absorb moisture into the laminate, and that moisture can blister or delaminate them at reflow. Baking dries a board out, but it's hard on OSP and immersion finishes, so it's better not to need it. IPC-1601, in plain terms explains how bare boards take on moisture and how to package them.
Boards that go in and out of use every week can't live sealed. Keep those in conditioned space, on edge in slotted racks, and handle them by the edges. If the storage area isn't climate controlled, seal what you can until it is.
A quarterly check
Once a quarter, take a scrap board from the oldest stock you hold and test its solderability. The dip-and-look method in J-STD-003 is the standard way, and if you don't have that setup, solder a few pads by hand. Discoloration on the pads is a warning. Poor wetting means the stock has aged, and it's much better to find that out now, while the boards can still be refinished or reordered, than on the line with a job waiting.
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