Electronics plants: the lead is in the dust, not the solder smoke

The smoke you can see rising off a solder line is almost entirely flux, and the lead is somewhere else. That mismatch is where most air monitoring in electronics assembly goes wrong.

The mistake: sampling the smoke for lead

Assembly plants sample the visible smoke above a hand-solder station or at the exit of a reflow oven. The laboratory finds no lead, and the plant considers the question closed.

At soldering temperatures, lead barely vaporizes. A wave solder pot runs around 250 °C, and a reflow oven peaks near 240 °C. The white smoke is rosin flux (also called colophony) decomposing in the heat. It consists of abietic acid and related resin acids, plus carrier solvent.

The lead in an electronics plant is dust, and it is found in the places where nobody puts a sampling pump:

• Skimming dross off wave pots and hot air solder leveling (HASL) pots, which disturbs hot oxide and is the highest-exposure task in the building.
• Pot maintenance, solder bar handling and tinning
• Lead on surfaces such as benches, tools, keyboards and hands, where it is ingested, not inhaled.

OSHA's lead standard covers lead in the workplace in any form, with an action level of 30 micrograms per cubic meter of air (µg/m³) and a permissible exposure limit (PEL) of 50 µg/m³. But the standard's provisions on hygiene, housekeeping and not eating in work areas exist because ingestion is a real route into the body.

A shop that has shown airborne lead is low has not shown much about a route that air monitoring does not measure. Wipe sampling does measure it, but it is outside the four fields we work in.

If you run a lead-free process, the metal question changes but the flux question does not.

→ How we monitor lead exposure

Rosin flux fume has no OSHA limit, and that is the point

OSHA has no PEL for fume from rosin-based solder flux. ACGIH gives no number either: its entry says only that exposure should be kept as low as possible.

The number most industrial hygienists reach for is British. The UK Health and Safety Executive sets a workplace exposure limit of 0.05 milligrams per cubic meter of air (mg/m³) as an 8-hour time-weighted average (TWA), measured as total resin acids. It also sets 0.15 mg/m³ over 15 minutes. It is a foreign limit, used because it is written for the resin acids themselves.

NIOSH separately sets a recommended exposure limit (REL) of 0.1 mg/m³ as formaldehyde for the decomposition products of rosin core solder. Cal/OSHA, California's state OSHA program, has a 0.1 mg/m³ legal limit on the same basis. That is a completely different basis of measurement, so your report should say which one your result is on.

Rosin is a respiratory sensitizer, a substance that can make the airways react to it, and it is one of the most frequently reported causes of work-related asthma in national surveillance programs. Once a worker is sensitized, the dose no longer determines the reaction, and they react at levels that never bothered them before. OSHA has no medical removal provision to fall back on, because there is no OSHA standard for rosin.

In practice, the case for local exhaust ventilation at hand-solder benches rests on health, not on the law, and nobody will cite you for going without it.

The wet process side

The etch, strip, plate and desmear lines amount to a different building, even when they are in the same building.

• Sulfuric acid mist. The OSHA PEL is 1 mg/m³. ACGIH's threshold limit value (TLV) is 0.2 mg/m³ on the thoracic fraction, the part of the mist fine enough to get past the throat. ACGIH classifies strong inorganic acid mists as A2, a carcinogen category, and that classification applies to the mist, not the liquid.
• Hydrochloric acid (HCl). The OSHA PEL is a 5 parts per million (ppm) ceiling limit, a level that must not be exceeded at any time, and it is not a TWA. A reported 8-hour TWA for HCl corresponds to no limit at all.
• Hydrofluoric acid (HF). The OSHA PEL is 3 ppm, and the ACGIH TLV is 0.5 ppm as a TWA, with a 2 ppm ceiling. With HF, the emergency response plan and having calcium gluconate on hand matter more than the sampling result.
• Ammonia in alkaline etchants, and cupric chloride regeneration.

→ How we sample for acid mist · Air monitoring surveys

Solvents and coatings

Isopropanol makes up most of the cleaning solvent by volume. The OSHA PEL is 400 ppm, and the ACGIH TLV is 200 ppm as a TWA, with a 400 ppm short-term exposure limit (STEL). It is usually used alongside acetone, and flux removers increasingly contain modified alcohols and glycol ethers as well.

Conformal coating, the thin protective coating applied to finished boards, is where the mix changes:

• Solvent-based acrylics bring xylene and toluene.
• Urethane conformal coatings bring isocyanates.
• UV-cure coatings bring acrylate monomers that are skin sensitizers. That is a dermal exposure, and air sampling will not find it.

→ How we sample for solvents · How we sample for isocyanates

Two traps beyond air monitoring

The first is the Toxics Release Inventory (TRI), EPA's annual toxic chemical release reporting program. Lead is a persistent, bioaccumulative and toxic (PBT) chemical, so its reporting threshold is 100 pounds, not 25,000 or 10,000. The de minimis exemption does not apply to it either.

A plant that runs through a few tons of solder a year is well past that threshold. There is one narrow exception: lead contained in stainless steel, brass or bronze alloy keeps the ordinary thresholds, but that exception does not cover tin-lead solder.

The second is ISO 14001, the international standard for environmental management systems. Electronics customers increasingly ask suppliers to be certified to it as a condition of staying on an approved vendor list. That is a purchasing requirement, not a legal one, and it pays to read the actual supplier standard, not the summary you were sent.

→ Form R and TRI reporting · ISO 14001 consulting · Semiconductor plants

What we do for electronics plants

An air monitoring survey measures what your workers are breathing, and a noise survey measures the noise exposure of each job. We also handle Form R, the full annual TRI report, and help with ISO 14001.