{"id":5576,"date":"2026-05-18T06:58:25","date_gmt":"2026-05-18T06:58:25","guid":{"rendered":"https:\/\/kangchuangpapers.com\/?p=5576"},"modified":"2026-05-20T07:05:35","modified_gmt":"2026-05-20T07:05:35","slug":"sulphur-free-paper","status":"publish","type":"post","link":"https:\/\/kangchuangpapers.com\/zh\/sulphur-free-paper\/","title":{"rendered":"Sulphur-Free Paper: What It Is and Why Electronics Manufacturers Require It"},"content":{"rendered":"

For electronics procurement managers specifying interleaving paper for PCB board storage and transit, the most expensive paper-related mistake is using standard kraft paper<\/a> \u2014 which contains sulphur compounds as a natural residue of the kraft pulping process \u2014 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\u2013$15,000 per incident \u2014 versus the $0.08\u2013$0.15 per sheet cost premium of sulphur-free paper over standard kraft.<\/p>\n\n\n\n

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.
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What Is Sulphur-Free Paper and How Is It Different From Standard Paper?<\/strong><\/h2>\n\n\n\n

Most industrial paper \u2014 including unbleached kraft paper, newsprint, and standard tissue paper \u2014 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\u2013500 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) \u2014 gases that react with reactive metals.<\/p>\n\n\n\n

Property<\/strong><\/th>Standard Kraft Paper<\/strong><\/th>Sulphur-Free Paper<\/strong><\/th><\/tr><\/thead>
Sulphur content<\/td>50\u2013500 ppm (natural kraft pulping residual)<\/td>< 10 ppm (typically < 5 ppm tested)<\/td><\/tr>
pH<\/td>5.5\u20137.0 (moderately acidic to neutral)<\/td>6.5\u20138.5 (pH-neutral to mildly alkaline \u2014 ‘acid-free’)<\/td><\/tr>
Tarnishing risk on copper<\/td>High \u2014 tarnish visible within 1\u20136 months<\/td>None \u2014 no reactive sulphur compounds present<\/td><\/tr>
Tarnishing risk on silver<\/td>High \u2014 tarnish within weeks<\/td>None<\/td><\/tr>
Tarnishing risk on gold<\/td>None (gold does not react with sulphur)<\/td>None<\/td><\/tr>
Tarnishing risk on tin\/solder<\/td>Moderate \u2014 can affect solderability over time<\/td>None<\/td><\/tr>
Certification document<\/td>None specific<\/td>Sulphur-free test report (ROSH\/XRF\/ion chromatography)<\/td><\/tr>
Cost premium over std kraft<\/td>Baseline<\/td>15\u201335% premium<\/td><\/tr>
Applications<\/td>Void fill, pallet wrap, non-contact packaging<\/td>PCB interleaving, precious metal parts<\/a>, semiconductor packaging<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The Chemical Reaction: Why Sulphur Compounds Tarnish Copper and Silver<\/strong><\/h2>\n\n\n\n

The tarnishing reaction is simple electrochemistry. Copper exposed to hydrogen sulphide (H2S) off-gassed from standard paper:<\/p>\n\n\n\n

Cu + H2S \u2192 CuS (copper sulphide, black) + H2<\/p>\n\n\n\n

Silver exposed to H2S: 4Ag + 2H2S + O2 \u2192 2Ag2S (silver sulphide, black) + 2H2O<\/p>\n\n\n\n

The reaction rate depends on temperature (doubles approximately every 10\u00b0C above ambient), humidity (accelerates above 50% RH), sulphur concentration in the paper, and exposure time. In a warehouse at 25\u00b0C and 60% RH, copper traces on PCBs in direct contact with standard kraft paper may show visible tarnish within 3\u20136 months. In a tropical storage environment at 35\u00b0C and 80% RH, tarnish can appear in 4\u20138 weeks.<\/p>\n\n\n\n

Gold does not react with sulphur \u2014 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.<\/p>\n\n\n\n

Applications Requiring Sulphur-Free Paper<\/strong><\/h2>\n\n\n\n
Application<\/strong><\/th>Why Sulphur-Free Is Required<\/strong><\/th>Consequence of Using Standard Paper<\/strong><\/th><\/tr><\/thead>
PCB interleaving (board-to-board)<\/td>Copper traces, solder pads directly exposed to paper surface<\/td>Copper tarnish, reduced solderability, contact resistance increase<\/td><\/tr>
Connector packaging (silver\/Cu contacts)<\/td>Silver and copper contacts in direct contact with paper<\/td>Silver sulphide tarnish, increased contact resistance, intermittent failure<\/td><\/tr>
Semiconductor component wrapping<\/td>Lead frames and bond wires (Cu, Ag, Au) sensitive to sulphur<\/td>Cu\/Ag tarnish on lead frames, solder adhesion failure<\/td><\/tr>
Precious metal parts (Ag, Cu, bronze)<\/td>Bare metal surfaces react directly with sulphur off-gas<\/td>Tarnish requiring costly cleaning\/repolishing operations<\/td><\/tr>
Optical lens\/mirror packaging<\/td>Silver mirror coatings react with H2S<\/td>Silver tarnish on optical coating \u2014 part destruction<\/td><\/tr>
Medical implant storage (titanium, cobalt-chrome)<\/td>Not a tarnishing risk but acid-free required for biocompatibility<\/td>Acid-induced surface etching if paper pH < 5.0<\/td><\/tr>
Electroplating interleaving<\/td>Plated surfaces (Cu, Ni, Sn) sensitive to sulphur and acidic pH<\/td>Surface attack, adhesion loss, appearance defects<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Applications Where Standard Paper Is Acceptable (Sulphur-Free Not Required)<\/strong><\/h2>\n\n\n\n