Solder Alloy and Flux Pairing

The alloy and the flux are chosen as a pair, because the flux has to remove the oxide at the temperature where the alloy melts. A pair that is mismatched produces joints that look acceptable and wet poorly, or a residue that cannot be qualified.

The pairing is specified by the paste supplier for a reason, and departing from it should be a deliberate decision with a test rather than an experiment on the line.

What the Flux Has to Do

The flux removes the oxide from the surfaces, protects them from re-oxidation and lowers the surface tension so that the alloy wets. It has to be active during the window in which the alloy is liquid.

Below the activation temperature, the flux does nothing useful, while above its limit it burns and leaves a residue that is difficult to remove. The window between the two is the working range.

Activation Temperature and the Profile

The activation has to fall inside the thermal cycle, which means the flux chemistry and the profile are linked. A flux that activates at a high temperature needs a profile that reaches it before the alloy melts.

A profile that is too cool leaves the flux partly unreacted, which produces poor wetting and a sticky residue. This is the mechanism behind many defects that are attributed to the material.

Lead Free and Tin Lead Pairs

Lead free alloys melt at higher temperatures, so a lead free flux is formulated to activate higher and to survive longer at temperature. Using a tin lead flux with a lead free alloy is a common and avoidable error.

The reverse combination is less damaging and still not correct, since a lead free flux may be too aggressive for a tin lead process and leave more residue.

Residue and the Cleaning Decision

The residue that remains depends on the flux, the profile and the volume applied, and it determines whether cleaning is required. A no clean claim applies to a specific combination, not to the flux alone.

Where the residue cannot be qualified for the environment, cleaning is the alternative, and it requires a flux that can be removed. This is described for residue risk.

Surface Finish Interaction

The finish reacts with the flux, since an organic coating dissolves into it while a metallic one does not. A change of finish therefore changes the flux requirement, as described for surface finish selection.

Where a finish is substituted, the flux should be re-qualified rather than carried over on the assumption that both are solderable.

Testing the Pair

The test is a wetting balance or a spread test on a coupon with the production finish and the production profile. The result should be compared with a limit rather than judged by appearance.

The test is quick and it settles a question that would otherwise be argued from experience. It should be run whenever either the alloy, the flux or the finish changes.

Compatibility With the Process

A flux may also be incompatible with the equipment, since some chemistries attack the pumps, the nozzles or the filters. The equipment supplier’s recommendation should be checked.

The interaction is easy to overlook and expensive to discover, particularly where the flux is used in a spray or a foam system.

Records

The records should link the alloy, the flux and the profile to the boards, which is what allows a wetting problem to be traced. A record that names only one of the three is incomplete.

They belong with the process evidence described for manufacturing processes.

Process Control and Verification

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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.

Checks Before Release

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 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.

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.

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.

FAQ

Can a flux be used outside its specified alloy? It can physically, and the result is usually poor wetting or a residue that fails the cleanliness requirement.

What test proves the pair? A wetting balance or spread test with the production finish and profile, compared with a limit.

Does the finish matter? It does, because the flux reacts with it, so a finish change requires a re-qualification.

Why does a change of paste lot cause a defect? Usually because the flux chemistry has changed slightly, which moves the activation window.