Absorbent Kraft Paper for PCB – Moisture Control & Resistance

Absorbent kraft paper plays a vital role in PCB processing by using moderate water absorption to capture drilling dust, excess resin, and surface moisture. However, it must also resist over-absorption, since too much water can cause swelling, breakage, or contamination of the board. Therefore, we strengthen water resistance through careful raw material selection, precise forming control, optimized drying, and additional resin coating treatment. With these measures, the paper achieves balanced absorbency and durability. Moreover, its performance is verified by Cobb water absorption tests and 24-hour immersion tests, which provide clear standards for stability and reliability in actual PCB applications.

Absorbent PCB Kraft Paper: Intro & Moisture Control Role

Absorbent Kraft paper is a specialty grade developed through targeted processing to greatly enhance resistance to moisture penetration. Unlike regular kraft paper, which easily absorbs water and swells, this paper maintains its original fiber strength while offering controlled absorbency and low expansion. By combining optimized raw materials with water-resistance modification, it delivers reliable performance in industrial applications that require contact with water or humid environments.

Why It Matters: Moisture Control in PCB Drilling & Lamination

Absorbing Drilling Dust: Moisture Balance for Effective Capture

During PCB drilling, dust particles with diameters of 5–50 μm are generated. Kraft paper moisture resistance directly determines whether these particles are properly captured.

Too absorbent: the paper softens rapidly, losing structure and failing to hold the dust.
Too resistant: no absorption occurs, dust disperses into the air and settles on the copper foil, causing etching shorts and lowering yield by over 15%.

👉 Moisture control target: kraft paper should slowly absorb cutting fluid along with dust (20%–30% water content), fixing particles on its surface without penetration or shedding.

Absorbing Excess Resin: Controlling Resin Flow with Moisture Resistance

During PCB lamination, liquid resins such as epoxy require precise moisture management.

If uncontrolled: excess resin overflows, sticking the steel plate to the PCB and tearing copper foil during demolding.
With balanced resistance: kraft paper absorbs just enough water from the resin (5%–8%), guiding even distribution within fiber gaps.

👉 Moisture control target: resist penetration to the backside while preventing paper expansion, ensuring resin remains evenly settled.

Blocking Ambient Moisture: Safeguarding PCB Insulation

PCB workshops typically maintain 40%–60% humidity. Poorly balanced kraft paper absorbs excess moisture, swells (>2%), and breaks, allowing water to seep into the PCB. This reduces insulation resistance from 10¹² Ω to below 10⁸ Ω, leading to leakage failures.

👉 Moisture control target: under 60% RH for 48 hours, absorption should remain ≤5%, with no swelling or deformation.

Controllable Moisture: How We Balance Absorption & Resistance

We balance kraft paper’s absorption and water resistance via precise process control. We use ≥80% long-fiber pine pulp (2.8–3.5 mm), which weaves into narrow pores for small-moisture absorption (e.g., cutting fluid) while blocking bulk water—avoiding short-fiber hardwood pulp’s large pores (>10 μm) and fast absorption. In forming, we adjust vacuum (-0.06 to -0.08 MPa) and pressure (180–220 kPa) to get 3–5 μm pores, preventing blockage (lost absorbency) or oversizing (excess absorption).

For drying, we use a low-to-high gradient: 80–90℃ (10min), 100–110℃ (8min), 115–120℃ (5min) to stabilize fibers, controlling moisture at 6%–8% (vs. industry 10%–12%). Finally, we coat one side with modified acrylic resin (3–5g/㎡ dry, 2–3μm film) cured at 120–130℃, blocking bulk moisture but allowing small-moisture/impurity absorption.

Moisture Testing: Methods for PCB Kraft Paper Qualification

Cobb Absorbency Test (Quick Evaluation of Surface Water Uptake)

The water absorption of PCB kraft paper is tested according to GB/T 461.3-2005 “Paper and Board—Determination of Absorbency—Part 3: Cobb Method”, a method widely adopted in the PCB industry. The test uses a Cobb tester equipped with a pressurizing device and a precision balance (accuracy 0.001 g).

24h Immersion Water Absorption Test (Long-Term Stability Evaluation)

To simulate prolonged exposure to moisture or resin during PCB processing, immersion testing is conducted. The setup includes a constant-temperature water bath (±1 ℃ accuracy), micrometer for thickness measurement, and a precision balance.

Test Assurance: Batch-by-Batch Control with Traceabilit

Sampling Rule: From each batch, 5 rolls are randomly selected, with 3 samples taken per roll;

PCB Applications: Where Moisture Control Is Critical

Moisture resistant kraft paper is a specially treated kraft paper that can effectively resist the erosion of moisture. It is usually coated with wax or plastic film coating, which greatly improves the water resistance of the kraft paper itself. This moisture resistant kraft paper is very suitable for food or other applications that require moisture resistant packaging, which can ensure that the packaging material will not be damaged due to moisture.

Practice Areas

Food Packaging

food industry use it to package and wrap food products such as meat, cheese, and baked goods

Medical Supplies Packaging

use in the medical industry for packaging sensitive equipment and products such as surgical instruments, medical implants, and diagnostic kits

Construction and Building Materials

be used as a vapor retarder for insulation materials, such as fiberglass batts or rigid foam boards, to protect the building from moisture

Electronics Packaging

protect electronic components from moisture and humidity damage during transport and storage

Moisture-resistant kraft paper is a versatile material that can be used in a variety of applications where protection against moisture is necessary. It provides excellent protection against water damage, making it a valuable product in many different industries.