Surface Finish Thermal Aging
A surface finish is not a passive coating. It is a thin layer of metal with its own diffusion behaviour, and it changes from the moment it is applied. Thermal aging is the process by which that change becomes a soldering problem, and it happens during storage as well as during the reflow cycle itself.
What the Finish Has to Do
The finish has to keep the copper solderable, provide a surface that wets within the profile, and stay thin enough not to affect the geometry of a fine pitch feature. It also has to survive the storage and the handling between fabrication and assembly.
Those requirements pull in different directions. Gold is an excellent barrier and a poor solder surface in thick layers, tin is an excellent solder surface and a poor barrier, and every finish is a compromise between the two.
Intermetallic Growth
Solder and copper react to form intermetallic compounds at the interface, and the layer grows with time and temperature. A thin layer is necessary for a good joint; a thick one is brittle and is the initiation site for a fatigue crack.
The growth rate roughly doubles for every ten degrees of temperature, so a board that has been through an extra reflow cycle has aged more than one that has not. Our article on solder joint fatigue estimation explains how the layer affects the life of the joint.
Storage and Its Effects
Storage ages the finish even without heat. Tin finishes oxidise, OSP layers degrade, and immersion silver tarnishes, and each of those changes slows the wetting action during reflow.
Packaging matters as much as time. A board stored in a paper sleeve in a humid room ages faster than one stored in a sealed bag with desiccant, and the difference is visible in the wetting balance. Our article on <a href="https://www.gopcba.com/solderability-storage-effects/” title=”solderability storage effects”>solderability storage effects covers the measurement.
Multiple Reflow Cycles
A double sided board goes through two reflow cycles, and a board with a mixed process may go through more. Each cycle grows the intermetallic layer and consumes part of the finish, and a thin finish can be entirely consumed.
When the finish is consumed, the solder no longer meets the surface it was designed to wet. The result is a joint that looks acceptable but dewets on a second pass. Our article on no clean versus water soluble paste describes the flux side of the same problem.
Finish Selection for Thermal Load
Where a board sees several reflow cycles, a finish with a thicker tin layer or a nickel barrier is more tolerant. The barrier layer stops the copper and the tin from meeting, which is why ENIG and similar finishes age more slowly than bare copper with OSP.
The trade is cost and process control. A nickel barrier has to be plated to a controlled thickness, and a thin barrier is worse than none at all. Our article on plating thickness explains how the layer is specified.
Verification and Records
The practical verification is a wetting balance test on a sample, or a solderability test after a simulated aging bake. Both give a number that can be compared with a limit rather than an opinion about appearance.
The records should tie the finish and the age to the lot. When a wetting problem appears, the date code and the storage history are the first things to check, and they are the only things that separate a material problem from a process problem.
Practical Recommendations
Keep the storage time short, keep the boards sealed, and specify the finish from the number of thermal cycles rather than from habit. A finish that is adequate for a single sided single pass board may be marginal for a double sided assembly.
Where a finish is changed, the change should be qualified on the assembly rather than assumed. Our article on solderability testing describes the qualification sequence.
Checks Before Release
Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure. The sequence of operations is part of the specification, because a different order produces a different result from the same steps.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.
The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.
Points to Confirm at First Article
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. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.
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. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
FAQ
Does gold thickness affect solderability? Yes. A very thin gold layer dissolves into the solder, but a thick one leaves gold in the joint and embrittles it, which is why the layer is specified to a tight range.
Can an aged board be recovered? Sometimes, by re activating the surface with a stronger flux, but the change should be qualified rather than applied by default, since it affects residue and reliability.
Is thermal aging only a storage issue? No. Heat drives the diffusion, so the reflow cycles and any bake contribute as much as the time the board spends on the shelf.
