Solder Bar Purity: 5 Checks Before the Pot

Solder bar purity is the freedom of the incoming alloy from elements that raise its melting range, thicken the dross layer or embrittle the joint. A bar can match its nominal composition and still carry enough copper, iron or antimony to change the way the bath behaves after a few weeks of topping up. Purity is therefore a trend to be managed, not a certificate to be filed.

Every top-up adds metal and every board removes some, and the bath composition moves between the two. Impurities that arrive with the bar, or that dissolve from fixtures, nozzles and plated surfaces, have no route out except drag-out on the boards. This note covers what to check before a bar goes into the pot.

What Solder Bar Purity Means

A solder bar specification defines a nominal alloy and a maximum for each impurity, and the two together define the grade. A tin copper alloy for wave soldering is typically held to limits on iron, arsenic, antimony, bismuth, cadmium and zinc, with different ceilings for each. The limits exist because each element has a different effect on the process.

Grade names in the industry reflect how tightly those limits are held. A high purity bar costs more and is usually reserved for fine work or for processes where the bath will run for a long time between changes. Where a general purpose bar is used, the impurity load has to be monitored at the pot rather than trusted at the door.

Reading the Certificate of Analysis

Each batch should arrive with a certificate that lists the actual analysis rather than only the specification limits. Look for the deliberate elements first, then for the contaminants: iron, zinc, cadmium, aluminium and arsenic in particular. A certificate that reports only tin and copper has told you nothing about the elements that cause most trouble.

Compare the certificate with the previous batch rather than only with the specification. A drift inside the limits is still a drift, and three consecutive batches that each sit at the top of the iron limit will lift the bath past a usable level. The methods used for the analysis should follow the standards published by IPC so the numbers are comparable between suppliers.

Impurities That Matter Most

Copper is a deliberate element in some alloys and a contaminant in others. In a tin silver bath it raises the melting range and makes the alloy sluggish, while in a tin copper bath it is part of the specification. Iron clouds the alloy, raises viscosity and accelerates dross; it usually arrives from steel fixtures, pump parts and stencil hardware rather than from the bar.

Zinc and aluminium are the most damaging at low levels, because both oxidise readily and produce a thick, sticky dross that also lifts the alloy surface tension. Antimony and bismuth harden the joint and reduce fatigue life. Arsenic is a concern for operator exposure as well as for joint properties, which is why its limit is set very low in every solder alloy specification.

Sampling the Bath Before a Top-Up

Take the sample while the bath is molten and circulating, from the same depth and position each time, and pour it into a clean mould. A sample taken from the surface carries dross and reads high; one taken from a cold corner may not represent the bulk. The time of sampling should be recorded with the result, because composition moves during a shift. The full procedure is described in our notes on solder pot analysis.

A pot analysis is most useful when it is plotted rather than read. A single result inside the limits says little. Five results over two months show whether a contaminant is climbing and how fast, which is the information needed to decide between a partial change of alloy and a full rebuild of the bath.

Visual and Physical Checks on the Bar

Inspect the bar for a uniform surface, a clean fracture and the absence of oxide scale, cavities or embedded material. Bars that have been stored damp show white corrosion products, and bars that have been dropped are often cracked. Any of these can carry moisture into the pot, which turns into violent spitting at the surface.

Weigh or count what goes in. A shop that tracks alloy consumption against board area will notice contamination faster than one that tops up by eye, because a sudden rise in consumption usually means dross is forming more quickly than it used to. Our guide to dross reduction in the wave covers the settings that limit the loss.

Storage and Handling of Bars

Store bars dry, off the floor and away from anything that can shed metal particles. A wet bar spits when it enters the pot, and a bar stored beside grinding or drilling equipment can pick up swarf that dissolves into the alloy. Keep the packaging until the bar is needed, and never use a bar as a hammer or a wedge.

Handle with clean gloves. Skin oils and salts are small contributors to contamination but they are avoidable, and the same discipline that protects the bar protects the boards. Where a bar has been dropped on a dirty floor, wipe it and inspect it before it goes anywhere near the bath.

Contamination Paths on the Line

Most contamination does not come in through the door. Copper dissolves from brass fittings, iron comes from steel tools and pump parts left in the bath, and zinc appears when galvanised racks or baskets are used near molten alloy. Solder resist, board finishes and component plating also contribute, especially on high volume lines with heavy drag-out.

Keeping non alloy items out of the pot is the cheapest control available. A dedicated set of tools, stainless rather than galvanised handling hardware and a rule against leaving anything in the bath between runs remove most of the routes. The remaining load is why periodic testing matters, and the chemistry of a working bath is covered in our notes on bath contamination.

Building a Top-Up Routine

A written routine removes the judgement from the task. Specify the source of the metal, the brand and grade, the quantity added at each top-up, the level in the pot after addition and the recovery time before production resumes. Add the required pot analysis interval and the action taken when a result approaches a limit.

Keep the routine beside the pot and record every addition against it. The record explains a composition change far more reliably than memory does, and it also shows when the consumption rate has moved. Cleaning, dross removal and heater checks belong in the same document, and the sequence that works for most lines is set out in our solder pot maintenance guide.

FAQ

Can a contaminated pot be cleaned without emptying it? Partial treatment with a proprietary alloy conditioner can pull some metals out of solution, and skimming removes what floats. Where the contaminant is iron, zinc or aluminium, only a full change of alloy reliably resets the bath, and pretending otherwise risks months of marginal joints.

How often should the bath be analysed? Monthly is a reasonable baseline for a single shift line, and fortnightly where boards are dense, drag-out is heavy or the pot runs hot. Any change in dross rate, wetting or joint appearance should trigger a sample regardless of the calendar.

Does a high purity bar remove the need for testing? No. A pure bar only means the metal that goes in is clean. Copper, iron and zinc still accumulate from the boards, the fixtures and the tools, so the trend at the pot remains the control that matters.