Solderability Testing Methods For PCB Finishes
Solderability is the ability of a metallic surface to be wetted by molten solder under a defined set of conditions. On a bare board it depends on the final finish, on the amount of oxide that has grown since plating, and on contamination picked up during handling and storage. It is not a design property, and it cannot be judged from a specification sheet alone.
Because it degrades silently, solderability is verified by test. Three methods cover most practical needs: the dip and look test, the wetting balance, and steam aging to accelerate changes that would otherwise take months. The choice depends on whether a lot needs a pass or fail decision or a measurement of margin.
Why Bare Board Solderability Is Tested
Boards are stored for weeks or months between fabrication and assembly, and surfaces oxidise throughout that period. A finish that is perfectly solderable in the week it is plated may be marginal after a year in a warehouse, particularly if the packaging is not sealed. Assembly defects traced to wetting are expensive because they appear only at the end of the line.
Testing also protects the finish specification itself. If incoming lots are sampled, a supplier that quietly changes a plating bath or a rinse will be detected before production is affected. For long life products such as automotive or industrial controllers, the incoming test is the only practical control on a property that cannot be repaired later. How the finish is specified in the fabrication package is described under PCB design and fabrication.
The Dip And Look Method
In a dip and look test, a coupon or a production board is dipped in flux, immersed in molten solder for a fixed time at a fixed temperature, and then examined. The solder is usually held at 245 C for two to five seconds for a lead free alloy, and the resulting coverage is compared with a reference chart.
The result is graded by the percentage of the tested area that is wetted. Complete coverage with a smooth, bright film is acceptable; areas where the solder has dewetted and left the base metal exposed are not. The method is fast, cheap and easy to standardise, which is why it remains the most widely used check on a production floor.
Wetting Balance Measurement
The wetting balance gives a number instead of a picture. A specimen is suspended from a sensitive force transducer and lowered into the solder bath, and the vertical force on the specimen changes as wetting proceeds. The recorded curve rises through zero at the wetting time and approaches a steady value that represents the wetting force.
Two parameters are usually reported: the time to reach zero force, and the maximum force achieved. Short wetting times and large wetting forces indicate good solderability. The method is more sensitive than dip and look because it detects the beginning of degradation long before coverage becomes visibly incomplete.
Steam Aging And Accelerated Degradation
Steam aging simulates the oxide growth of long storage in a short time. Specimens are suspended above boiling deionised water for a defined period, commonly eight hours for a standard test, and are then tested by dip and look or by wetting balance. The treatment is intended to mimic the effect of roughly one year of natural aging.
The test is demanding and must be controlled carefully. Contamination in the water, variation in the steam temperature and condensation dripping onto the specimen all change the result. Steam aging is usually specified for products with a defined shelf life requirement, or when a supplier change or a process excursion needs to be evaluated.
Flux Activity And Its Influence
Wetting in a solderability test is the joint result of the surface condition and the flux activity. A strongly activated flux will wet a surface that a mild one cannot, so a test result is only meaningful when the flux is fixed and matches the flux that will be used in production. Changing flux between test and production invalidates the comparison.
This is why the flux is specified in the test procedure along with the solder alloy, the bath temperature and the immersion time. The link between flux choice and the final joint is described under lead-free versus leaded solder.
Interpreting Results And Setting Limits
Results are compared against acceptance criteria written into the inspection instruction. Typical limits are at least 95 percent coverage for a dip and look test, a wetting time below one second, and a wetting force above 200 micronewtons per millimetre of specimen width. The numbers should be derived from a baseline of known good material.
A single failing coupon is a warning, and a lot that fails repeatedly is a rejection. Because these tests destroy the sample, they are applied to coupons from the panel border rather than to saleable boards. Sampling frequency is chosen from supplier history and product criticality, as described under PCB design quality characteristics.
Process Control and Verification
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.
FAQ
Is solderability testing required for every lot? Not always. Incoming testing is common for critical products and for finishes that are sensitive to storage, while a stable supplier with a good history may be sampled less often.
Can a board that fails solderability be recovered? Often yes. A light flux cleaning or a reflow of the surface can restore wetting on mildly oxidised finishes, but heavy intermetallic growth or corrosion cannot be reversed and the lot should be rejected.
Which alloy and temperature should the test use? Use the same alloy as production, at a bath temperature within about ten degrees of the production peak, so the result reflects the real process rather than a laboratory condition.
