NHML Resources - Is Your Solder Bath... All Wet?

Maintaining the right solder chemistry is required for proper electronic assembly and long product life. Grittiness, sluggishness, and bridging are all symptoms of contaminated solder. Contaminants can be introduced from production tools that seem innocuous: fixtures, solder tanks and the electronic assemblies themselves.

The following discusses some of the "foreign" metals found in tin/lead solder baths and their impact on product quality.

Aluminum

Aluminum is a popular choice in the design of manufacturing fixtures and heat sinks. However, repeated immersion of non-anodized aluminum introduces contaminants to the solder bath.

Aluminum contamination can cause poor adhesion, grittiness, and hot-short cracking of the solder even in levels as low as 0.001%, although type specifications permit a maximum of 0.005% in virgin material.

To assure quality performance from your soldering process, avoid aluminum fixturing. If you have an aluminum solder tank, replace it. If aluminum fixtures are required, be sure to anodize them. This surface treatment will delay the erosion of the metal, thereby postponing the onset of contamination.

Antimony

Antimony is added to high tin solder alloys to reduce the problem of "Tin Pest" (a spontaneous transformation of white, metallic tin into a gray, amorphous powder). The addition of antimony is regulated by a federal specification.

In small quantities the addition of antimony improves the wetting of the solder. In larger quantities it is harmful.

Copper

The potential for copper contamination should come as no surprise. It is found routinely in the solders of both wave and dip systems. When we consider that copper is highly soluble (and liquid solder is a strong solvent), it's hard to avoid copper contamination. A freshly charged solder reservoir can be highly aggressive. You may find that small copper wires are lost or square pins become rounded. This will continue until the bath reaches a "steady state" of composition.

The copper contaminants form an intermetallic compound with the tin. This compound causes the solder to become gritty and sluggish. This can result in uneven deposits and bridging.

Lowering the bath temperature to just above the melting point causes the intermetallic compounds to become visible as needles on the surface. Once visible, they can be scooped out. Although this does not capture all the contaminants, it can help reduce the problem. Remember to make up the tin you have lost.

Iron

Problems with iron contamination are generally found in dip solder tanks and they are made worse by "hot spots" in the tanks that cause the iron to dissolve more rapidly. This is a common problem in systems that use immersed sheath heaters and iron/steel tanks.

Iron levels exceeding 0.1% cause the solder to become gritty. To reduce the problem, avoid heating systems that concentrate the heat.

Silver&Gold

Silver and gold are common bath contaminants in wave solder tanks. They are often recovered at low concentrations due to their high commercial value. Excessive gold will make the solder sluggish and the surface dull. Excessive silver will cause grittiness.

Zinc

Like iron, zinc is usually found in dip solder systems and it is extremely damaging to the solder. Unlike iron, it does not form intermetallic compounds with either tin or lead. Zinc contamination can cause poor solder adhesion and grittiness. Zinc contaminated solder is predisposed to failures during solidification.

The maximum level of zinc allowed in virgin metal is typically 0.005%. We have found that many operators change their baths if that level is exceeded. Sources of zinc include dissolved brass, terminal pins, and zinc stearate contamination in fluxes.

The Others

Bismuth improves wetting, but can cause delayed dimensional changes in solidified solder. Growth may occur over several years. Usually bismuth does not cause trouble.

Cadmium is avoided by many manufacturers due to the toxicity of the vapors. Cadmium contamination results in sludge in the bottom of the bath.

Magnesium and Nickel are seldom found in solder baths.

See our Industry Definitions for further insight.

 

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