Solder Pot Alloy Addition: Level, Grade and Skim

Alloy addition is the routine of putting solder back into a wave solder pot, and it is one of the few operations where a small habit has a large effect on joint quality. The pot loses metal to the boards, to dross and to drag-out on the components. The pot is also the largest single piece of alloy on the line, so its condition is worth more attention than its size suggests.

Adding the right alloy at the right time keeps the pot inside its analysis window. Adding the wrong grade, or adding bars into a pot that is too low, can put the whole bath outside specification in a single shift. A top-up made with the wrong grade is impossible to undo, because the whole pot would have to be drained and recharged.

What Alloy Addition Does

The pot is a reservoir that is drained continuously by the process, so the level falls through a shift. The metal removed from the pot leaves as solder on the joints, as dross on the surface and as drag-out on the boards. Dross is part of the consumption, so a pot that produces more dross than usual will also need more alloy than the analysis predicts.

Replacing it keeps the wave stable, because wave height depends on pump depth and on the volume above the pump inlet. A pot that has dropped too far will produce a wave that is shorter and less stable than the profile assumes. A level probe or a float should be verified against a dipstick, because a probe that reads high will let the pot run low.

Pot Level and Why It Matters

Every machine has a working level band, and that band is set by the pump, the nozzle and the wave geometry. Below the band the wave height falls, and above it solder overflows at the nozzle or splashes onto the boards. The band should be marked inside the pot as well as on the gauge, so that the level can be judged with the machine open.

The level should be checked at the start of a shift and after any long run, and restored before it reaches the bottom of the band rather than after. The level and the alloy analysis should be read together, because both of them affect the wave. Bars that have been stored damp should be dried before they are added, because moisture under the surface causes spitting.

Bar Grade and Alloy Match

The bar grade must match the alloy in the pot, and the match includes the minor additions as well as the main elements. A lead-free pot must not be topped up with a tin-lead bar, and a pot charged with one supplier product should not be fed from another without checking the specification. The alloy specification should be kept at the machine, so that the grade can be confirmed without leaving the line.

Bars should be identified by their stamp and stored so that grades cannot be mixed at the machine. Incoming alloy verification is described in incoming alloy verification notes. A pot that has been idle should be stirred gently before a run, because the composition can stratify while it stands.

Adding Bars Without Thermal Shock

A cold bar dropped into a hot pot chills the local metal and can freeze the wave for a short period, which is why additions are usually made during a break or at the end of a run. Preheating a bar on the pot edge reduces the disturbance. Wetting the bar against the pot wall before it is released is a simple technique that removes most of the splashing.

Bars should be added slowly and away from the pump intake, so that the melting metal does not carry oxide into the pump. Adding several bars at once is faster but produces a larger disturbance. Skim should be removed with a tool that does not drag good metal out of the pot along with the oxide.

Skimming and Oxide

The oxide that forms on the surface is skimmed or pushed to the side, and the timing of that work matters. Skimming immediately after an addition removes fresh oxide before it can be drawn into the wave. The skim rate is a useful indicator, since a sudden increase usually means the pot is running hotter than it should be.

Skim should be kept separate from good metal, because it carries a higher oxide content and is normally sent for recovery rather than returned. Dross control is described in solder pot dross control work. Impurity analysis should be done on a schedule rather than on suspicion, because the trend matters more than the reading.

Contamination and Impurity Limits

Copper, iron, zinc and gold all accumulate in a solder pot, and each has a limit above which the joints become dull, brittle or grainy. Copper is the most common, because it dissolves from the board and from the plating on the pads. A pot that has reached its impurity limit can sometimes be diluted with fresh alloy, but the arithmetic has to be done properly.

The addition of fresh alloy dilutes the impurities as well as restoring the level, which is one reason a pot that is topped up regularly stays inside its window longer. Impurity management is described in solder alloy impurity control notes. Adding alloy to a running pot should be a written procedure, because an improvised addition is difficult to reproduce.

Adding Alloy to a Running Pot

On a high volume line the pot may have to be topped up while it is running. In that case the bar should be held at the pot edge until it is hot, added in small pieces and placed as far from the nozzle as the pot allows. The wave should be checked against the profile after any addition, since the level and the composition both affect the height.

The operator should watch the wave for a minute after the addition, because a cold addition shows immediately as a change in wave height and in the sound of the pump. Bar lot numbers should be kept until the pot is next analysed, so that an unexpected analysis result can be explained.

Records and Verification

The record should carry the bar grade, the lot, the weight added, the level before and after, and the time of the addition. That set allows a level or analysis change to be traced to a specific bar lot. A pot whose level is never recorded cannot be reconciled with its alloy consumption, and the loss will be found as a defect.

Acceptance of the soldered assembly follows the published IPC documents. A pot topped up with an unrecorded bar is a pot whose analysis cannot be explained afterwards.

FAQ

How often should a solder pot be topped up? Often enough to stay inside the working level band, checked at the start of each shift and after long runs rather than once a week.

Can any solder bar be added to any pot? No. The grade and the minor alloy additions have to match what is already in the pot and what the process specification calls for.

Why add alloy rather than just topping up the level? Alloy addition replaces the metal that was consumed, while the level alone says nothing about the composition of what is left.