For electronics procurement managers specifying interleaving paper for PCB board storage and transit, the most expensive paper-related mistake is using standard kraft paper — which contains sulphur compounds as a natural residue of the kraft pulping process — for direct contact with copper traces, silver contacts, or gold-plated connectors. Sulphur compounds react with copper and silver over time, forming copper sulphide and silver sulphide tarnish that discolours contact surfaces, increases contact resistance, and in severe cases causes intermittent electrical connection failures. The repair cost on a batch of tarnished gold-plated connectors that should have been protected by sulphur-free interleaving paper is typically $2,000–$15,000 per incident — versus the $0.08–$0.15 per sheet cost premium of sulphur-free paper over standard kraft.
Sulphur-free paper is not a niche specialty product. It is the industry standard for direct-contact interleaving in PCB manufacturing, semiconductor component packaging, precious metal part storage, and optical component protection. Understanding what sulphur-free certification means, how to verify it, and which applications require it versus which can use standard paper is the foundation of cost-effective electronics packaging specification.
What Is Sulphur-Free Paper and How Is It Different From Standard Paper?
Most industrial paper — including unbleached kraft paper, newsprint, and standard tissue paper — contains residual sulphur compounds from the kraft pulping process (sodium sulphide, Na2S, is used in kraft pulping to break down lignin). These compounds remain in the finished paper in concentrations typically measured at 50–500 ppm total sulphur by weight. Under ambient humidity and temperature conditions, these sulphur compounds off-gas slowly as hydrogen sulphide (H2S) or sulphur dioxide (SO2) — gases that react with reactive metals.
| Property | Standard Kraft Paper | Sulphur-Free Paper |
|---|---|---|
| Sulphur content | 50–500 ppm (natural kraft pulping residual) | < 10 ppm (typically < 5 ppm tested) |
| pH | 5.5–7.0 (moderately acidic to neutral) | 6.5–8.5 (pH-neutral to mildly alkaline — ‘acid-free’) |
| Tarnishing risk on copper | High — tarnish visible within 1–6 months | None — no reactive sulphur compounds present |
| Tarnishing risk on silver | High — tarnish within weeks | None |
| Tarnishing risk on gold | None (gold does not react with sulphur) | None |
| Tarnishing risk on tin/solder | Moderate — can affect solderability over time | None |
| Certification document | None specific | Sulphur-free test report (ROSH/XRF/ion chromatography) |
| Cost premium over std kraft | Baseline | 15–35% premium |
| Applications | Void fill, pallet wrap, non-contact packaging | PCB interleaving, precious metal parts, semiconductor packaging |
The Chemical Reaction: Why Sulphur Compounds Tarnish Copper and Silver
The tarnishing reaction is simple electrochemistry. Copper exposed to hydrogen sulphide (H2S) off-gassed from standard paper:
Cu + H2S → CuS (copper sulphide, black) + H2
Silver exposed to H2S: 4Ag + 2H2S + O2 → 2Ag2S (silver sulphide, black) + 2H2O
The reaction rate depends on temperature (doubles approximately every 10°C above ambient), humidity (accelerates above 50% RH), sulphur concentration in the paper, and exposure time. In a warehouse at 25°C and 60% RH, copper traces on PCBs in direct contact with standard kraft paper may show visible tarnish within 3–6 months. In a tropical storage environment at 35°C and 80% RH, tarnish can appear in 4–8 weeks.
Gold does not react with sulphur — gold contacts in direct contact with sulphur-containing paper are unaffected. However, the connector body (copper or copper alloy substrate under gold plating), exposed copper traces, solder joints (tin-lead or lead-free SAC alloy), and silver contacts are all vulnerable.
Applications Requiring Sulphur-Free Paper
| Application | Why Sulphur-Free Is Required | Consequence of Using Standard Paper |
|---|---|---|
| PCB interleaving (board-to-board) | Copper traces, solder pads directly exposed to paper surface | Copper tarnish, reduced solderability, contact resistance increase |
| Connector packaging (silver/Cu contacts) | Silver and copper contacts in direct contact with paper | Silver sulphide tarnish, increased contact resistance, intermittent failure |
| Semiconductor component wrapping | Lead frames and bond wires (Cu, Ag, Au) sensitive to sulphur | Cu/Ag tarnish on lead frames, solder adhesion failure |
| Precious metal parts (Ag, Cu, bronze) | Bare metal surfaces react directly with sulphur off-gas | Tarnish requiring costly cleaning/repolishing operations |
| Optical lens/mirror packaging | Silver mirror coatings react with H2S | Silver tarnish on optical coating — part destruction |
| Medical implant storage (titanium, cobalt-chrome) | Not a tarnishing risk but acid-free required for biocompatibility | Acid-induced surface etching if paper pH < 5.0 |
| Electroplating interleaving | Plated surfaces (Cu, Ni, Sn) sensitive to sulphur and acidic pH | Surface attack, adhesion loss, appearance defects |
Applications Where Standard Paper Is Acceptable (Sulphur-Free Not Required)
- Outer carton packing — paper does not contact electrical surfaces; sulphur off-gas dilutes too much at distance to affect enclosed parts
- Void fill crumpled paper in boxes where parts are individually sealed in barrier bags — barrier bag eliminates sulphur exposure
- Structural packaging where parts are not electrically or optically sensitive (steel fasteners, structural aluminium brackets)
- Pallet interleaving for products in protective packaging that creates a barrier between paper and part
How to Verify Sulphur-Free Certification
‘Sulphur-free’ is a chemical specification, not a visual one — standard paper and sulphur-free paper look identical. Verification requires a test report from a qualified laboratory:
- Ion chromatography (IC): measures total soluble sulphur ions (sulphate, sulphite, sulphide) in a water extract of the paper. Most sulphur-free specifications require total soluble sulphur < 10 ppm.
- X-ray fluorescence (XRF): measures total elemental sulphur content. Sensitive to 1–2 ppm; appropriate for total sulphur specification.
- ROSH test report: Chinese and Asian suppliers typically provide a ROSH (equivalent to RoHS screening) test report that includes total sulphur content analysis.
- Corrosion test (IEC 60068-2-43): exposes a copper coupon to the paper in a controlled environment (25°C, 75% RH, 10 days) and measures copper sulphide formation gravimetrically. Best functional test for tarnish resistance.
Kangchuang Papers provides sulphur-free paper with a documented test report for each production lot, with total sulphur < 5 ppm verified by ion chromatography. Our test reports are available on request with every order. Request samples and specifications at kangchuangpapers.com.
Frequently Asked Questions
What does sulphur-free paper mean?
Sulphur-free paper (also spelled sulfur-free) is paper manufactured from pulp that has been processed and washed to reduce residual sulphur compounds — primarily sodium sulphide (Na2S) and its breakdown products — to below 10 ppm total sulphur. Standard kraft paper contains 50–500 ppm total sulphur from the kraft pulping process. Sulphur-free paper eliminates the risk of copper sulphide and silver sulphide tarnish formation on reactive metal surfaces in direct contact with the paper. It is also typically pH-neutral to mildly alkaline (acid-free), further protecting sensitive metal and electronic components from acid-induced surface attack.
What happens if I use standard paper to package PCBs?
Standard kraft paper off-gasses hydrogen sulphide (H2S) slowly under ambient temperature and humidity. PCBs in direct contact with standard paper will develop copper sulphide tarnish on exposed copper traces, solder pads, and component leads over 3–12 months depending on storage temperature and humidity. Tarnished solder pads have reduced solderability — meaning the solder will not wet the pad correctly during assembly, producing cold or non-wetting solder joints. On high-reliability electronics, this can cause field failures that are difficult to trace back to packaging. The cost of rework or field failure far exceeds the $0.05–$0.15/sheet premium of sulphur-free paper.
Is acid-free paper the same as sulphur-free paper?
No — but they overlap. Acid-free paper has a pH ≥ 7.0 (neutral or mildly alkaline), preventing acid-induced degradation of organic materials and paper itself. Sulphur-free paper has < 10 ppm total sulphur, preventing metal tarnish from sulphur off-gassing. Most high-quality sulphur-free papers for electronics use are also acid-free — but acid-free alone does not guarantee sulphur-free, and sulphur-free paper can be slightly acidic. For electronics and PCB applications, specify both: acid-free (pH 7.0–8.5) AND sulphur-free (< 10 ppm total sulphur) with supporting test report.
How do I specify sulphur-free paper on a purchase order?
The correct specification for electronics interleaving sulphur-free paper on a purchase order or engineering specification: Sulphur content: < 10 ppm total sulphur (or < 5 ppm for highest-sensitivity applications). pH: 7.0–8.5 (acid-free, neutral to mildly alkaline). Basis weight: specify in GSM appropriate to application (40–60 GSM typical for PCB interleaving). Test documentation: ion chromatography or XRF sulphur test report for each production lot, dated within 12 months. Optional: corrosion test per IEC 60068-2-43 for critical applications.
Conclusion: The Paper Specification Is Part of the Quality System
- Sulphur-free paper (< 10 ppm total sulphur, pH 7.0–8.5) is required for any application where paper is in direct contact with copper, silver, tin, or solder surfaces
- Standard kraft paper (50–500 ppm sulphur) will cause copper sulphide and silver sulphide tarnish within months under typical warehouse storage conditions
- Verification requires a test report (ion chromatography or XRF) — ‘sulphur-free’ is not verifiable visually or by supplier claim alone
