Solderability and Storage Conditions

A board that soldered perfectly last month may not solder today, and nothing about the process has changed. What changed was the surface. Solderability decays with time and with the conditions the board has been stored in, and the decay is invisible until the first defect appears. Measuring it and controlling it is cheaper than chasing wetting defects through the line.

What Solderability Means

Solderability is the ability of a surface to be wetted by molten alloy under a defined set of conditions. It is not a property of the board alone; it depends on the finish, the flux, the temperature and the time. A finish with excellent solderability can still fail if the flux is too mild or the profile too cool.

The practical consequence is that solderability has to be specified with a test method and a reference condition. A statement that a surface is solderable is meaningless without knowing what flux and what temperature were assumed.

How Finishes Age

Every finish ages at a different rate and by a different mechanism. An organic coating protects the copper until it decomposes, an immersion silver tarnishes, an electroless nickel and gold layer changes as the gold diffuses, and a tin finish oxidises and grows intermetallic into the copper beneath it.

The rate is governed by temperature, humidity and the presence of contaminants such as sulphur compounds. A warehouse that is hot, humid or near a source of sulphur is a much harsher storage environment than the temperature figure alone suggests, and the comparison of finishes is set out in our article on surface finishes.

<img src="https://www.gopcba.com/wp-content/uploads/2026/09/271-1.jpg" alt="Wetting balance curve recorded during a solderability test” />

Oxide Growth and Intermetallic Formation

Oxide growth is a diffusion process, so it accelerates with temperature. A board stored at thirty five degrees ages far faster than one stored at twenty. The oxide that forms is what the flux has to remove, and once it exceeds what the flux can handle, wetting stops.

On a tin finish a second process runs in parallel: tin and copper interdiffuse and form intermetallic compounds at the interface. That layer grows with time and temperature, and it consumes the free tin that would otherwise be available to wet. A finish that looks bright can therefore have very little solderable tin left near the surface.

Storage Conditions and Packaging

Storage conditions should be specified, not assumed. Temperature, humidity and cleanliness of the storage area all matter, and so does the packaging. A moisture barrier bag with desiccant limits both water vapour and airborne contamination, while an open shelf does neither.

The clock that matters starts when the bag is opened rather than when the board was made. A declared shelf life is therefore a figure for sealed storage, and it should be paired with a floor life figure for the period after opening. Both belong in the specification.

Measuring with a Wetting Balance

A wetting balance measures the force on a specimen as it is dipped into molten alloy, and the shape of the curve describes how quickly and completely the surface wets. Time to zero force and maximum wetting force are the two numbers usually reported, and both degrade measurably as a surface ages.

The test is repeatable enough to trend, which makes it useful for incoming inspection and for storage studies. The methods, including dip and look tests for comparison, are described in our article on solderability testing.

Assembly Consequences of Degraded Wetting

Poor wetting does not always produce a visible defect. It produces fillets that are smaller than intended, joints that are mechanically weaker, and a process that is more sensitive to every other variable. The board may pass inspection while carrying joints with less intermetallic contact than the design assumed.

It also shifts the defect distribution. Joints on the edges of the process window fail first, which means the defects appear where the air flow is weakest or the thermal mass greatest rather than randomly. That pattern is a clue that the surface, not the profile, has changed.

Mitigation and Requalification

When solderability has degraded, the options are limited. Baking can remove moisture but does not restore an oxidised finish, and a more active flux can recover some of the loss at the cost of residue and cleanliness requirements. Neither restores a finish that has consumed its solderable layer.

The durable answer is to control storage so that the finish never degrades beyond the process window, and to verify by measurement rather than by date. A periodic wetting balance check on stored stock shows when a lot is approaching its limit, and the judgement framework is the same one used in PCB quality assessment.

Checks Before Release

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process.

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Verification and Records

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

FAQ

Does baking restore solderability? No. It removes absorbed moisture, which helps with delamination and with outgassing, but oxide and intermetallic growth are not reversed by heat.

How long can a board be stored? It depends on the finish and the conditions. A sealed bag in a controlled warehouse allows a much longer period than an open shelf in an uncontrolled one.

Is a visual check enough? No. A tarnished surface is obvious, but intermetallic growth beneath a bright surface is not, and that is the change that matters most.