Silver Tarnish: 5 Rules for Immersion Silver

Silver tarnish is the discolouration that appears on an immersion silver board when the metal reacts with sulfur compounds in the air. It is a surface film rather than a change in the deposit, and it is one of the few finish defects that can look alarming on the bench and still solder perfectly well.

That gap between appearance and function is what makes the topic difficult. A customer who sees a brown pad is entitled to ask questions, and the answer has to be based on the test that was run rather than on an assurance that the board is fine.

What Silver Tarnish Is and Is Not

The film is silver sulfide, formed when silver meets sulfur in the air. It grows slowly at room temperature and much faster in warm, humid conditions, and its colour moves from a pale tint through yellow and brown to a dark grey as it thickens.

It is not corrosion of the copper beneath, and it is not a plating defect. The distinction matters because the remedies are completely different: a tarnish problem is managed with packaging and storage, while a plating problem has to be taken back to the bath.

Immersion Silver and Its Thin Deposit

Immersion silver is a thin deposit, typically measured in fractions of a micron, that forms over the copper by displacement. Because it is thin, it is sensitive to both the copper surface underneath and to anything that touches it after plating.

The thin deposit is also why the finish works. It dissolves into the solder during reflow rather than staying as a separate layer, which keeps the joint simple and avoids the brittle intermetallics that a thick gold layer can create.

Sulfur, Humidity and the Storage Environment

Sulfur comes from many ordinary sources: cardboard, rubber bands, some papers, industrial air and even certain foods. A board packed against a cardboard sheet in a warm store will tarnish far faster than the same board in a sealed bag in the same room.

Humidity accelerates the reaction, so the storage area should be dry as well as clean. Where the climate is difficult, the answer is packaging rather than air conditioning, because the bag controls the local environment far more reliably than a room does.

Judging a Tarnished Pad

The judgement should be made against a boundary sample rather than by eye alone. A set of pads with a known, acceptable level of tarnish allows two operators to reach the same conclusion, and it settles most arguments before they reach the customer.

Where the level is beyond the boundary, the next question is whether the pad still wets. That is a solderability test rather than an inspection, and our dip and look notes describe the quick method that answers it.

Solderability After Tarnish

The flux in solder paste is designed to remove oxide and light contamination, and it will normally deal with a thin sulfide film. The film becomes a problem when it is thick, when the flux activity is low, or when the board has been stored long enough for the film to affect the whole surface.

The practical answer is to test rather than to guess. A board from the suspect lot, soldered with the production paste and profile, tells you more in half an hour than any amount of discussion about the colour of the pad.

Where a test is not possible, the safest course is to solder a sample panel and inspect the joints rather than to accept or reject the whole lot on appearance. A single panel from the top of the stack, soldered with the production recipe, gives a result that everyone can see and agree on.

Packaging That Buys Time

The packaging that protects a silver board is a barrier against sulfur and moisture, and it usually includes a desiccant and a sulfur absorbing material. The bag has to be sealed, because a bag that is left open or resealed with tape no longer controls anything.

Which packaging to use, and how long it buys, should be established by a trial rather than assumed. The same finish in two different bags can behave very differently, and the result depends on the environment the bag is stored in. Our packaging notes cover the general requirements.

Storage Rules and Shelf Life

A shelf life for a silver finish should state the packaging, the temperature and the humidity it assumes, because the number is meaningless without them. Once the bag is opened the clock changes, and many shops define a separate open bag life for the board after that point.

The rules should be written on the bag rather than in a manual, since the person opening it is the one who needs the information. Our shelf life notes describe how the same discipline is applied to components and paste.

Handling Before the Printer

Bare hands are the other source of contamination, and a fingerprint on a silver pad is both a tarnish promoter and a wetting barrier. Gloves, clean trays and a rule that boards are held by the edge remove most of the risk at almost no cost.

Boards should also be taken from the bag only when they are about to be used. Opening a bag early to stage a job, then leaving the boards on a bench overnight, is how a lot that passed at goods in becomes a lot that fails the print. Our board cleaning notes describe what can be recovered later in the process.

When Cleaning Helps and When It Does Not

A light tarnish can sometimes be removed with a mild chemistry or an approved wipe, and the pad then wets normally. The cleaning has to be matched to the finish, because an aggressive cleaner will remove the silver along with the film and leave bare copper behind.

Cleaning is not a substitute for packaging. A board that tarnishes in a week will tarnish again a week after cleaning, and the process should be corrected rather than repeated. The finish choice itself is worth reviewing, and our board finish notes compare the alternatives.

Records and Supplier Questions

A tarnish claim should be supported by photographs, the storage conditions, the packaging used and the result of a solderability test. With those four items the discussion can be about the finish rather than about a colour.

The batch number is the other essential item, because it lets the finisher compare with other customers and establish whether the problem is one shipment or one process week. The IPC finish standards define the tests and the acceptance criteria to quote.

FAQ

Does tarnish mean the board is scrap? Usually not. A thin film is normal on this finish and the flux in the paste will remove it. A thick, dark film on a board that has been stored badly is a different case and deserves a solderability test before use.

Can tarnish be prevented with a coating? Some finishes are protected with an organic layer that slows the reaction, and it is applied by the finisher rather than by the assembler. Where it is used, the coating has to be compatible with the paste chemistry and with any cleaning step.

Why does tarnish appear only on some panels? Packaging, storage position and the age of the lot all vary between panels, and the reaction is sensitive to all three. Panels at the top of a stack or nearest a cardboard divider usually show it first.