Surface Finish Shelf Life and Solderability Monitoring
A surface finish has a shelf life, and the shelf life is a statement about solderability rather than about appearance. A board that looks unchanged after six months may have an oxide or an intermetallic layer that makes the joint unreliable, and the only way to know is to measure.
What Changes During Storage
The finish reacts with the atmosphere, with the sulfur in the air and with the packaging. An immersion silver tarnishes, an immersion tin grows an intermetallic with the copper underneath, and an organic coating slowly loses its protective function.
The rate depends on the temperature and the humidity rather than on the calendar, which is why a board stored in a controlled area has a longer working life than the same board in an open shop. Our board quality notes describe how the surface is judged.
Measuring Solderability
Solderability is measured by immersing a sample in molten alloy and assessing the wetting, or by a wetting balance that records the force against time. The balance gives a number and the dip gives a judgement.
Both are applied to a sample from the same batch, because the test destroys the area it touches. The sample is taken from the panel rather than from the board where the board cannot be spared. Our joint criteria notes describe the acceptance that the test supports.
The storage area is controlled for temperature and humidity, and the condition is recorded rather than assumed. A cabinet that is opened several times a shift has a different internal condition from one that is not.
The packaging matters as well, because a barrier bag with a desiccant slows the reaction. Where the bag is used, the indicator is read before opening, which is the only point at which the reading means anything.
A finish that is past its shelf life is not automatically scrap. It is a candidate for a bake, for a re-clean, or for a solderability test that decides the disposition.
The disposition rule belongs in the procedure rather than in a conversation. A rule that says test it, and what to do with the result, prevents both the scrapping of usable boards and the use of boards that should have been scrapped.
An immersion tin has the shortest practical life because the intermetallic grows regardless of the atmosphere. An electroless nickel gold is the most stable because the gold is inert, and its limit is usually the nickel underneath rather than the gold.
The rules therefore differ by finish, and a shop that runs several finishes needs a table rather than a single figure. Our fabrication notes notes describe how the finish is stated on the drawing.
Monitoring is a periodic solderability test on stock that has been in store for a stated period, rather than a test at the end of the shelf life. It gives a trend and it catches a storage problem before the stock is unusable.
The trend is the useful part, because a shop whose solderability falls faster than the finish data predicts has a storage problem rather than a finish problem.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Does a bake restore a finish? It removes moisture and it does not remove an oxide or an intermetallic, so the effect depends on what has happened to the surface.
Is a visual check enough? It catches tarnish and it does not catch an intermetallic that has grown under a good looking surface.
What does gopcb provide for finish shelf life? We provide a shelf life statement per finish, storage conditions recorded rather than assumed, periodic solderability monitoring on stored stock, a written disposition rule for expired material, and records that tie the finish to the batch.
