Conductive adhesives
Electrical versus thermal conduction, isotropic and anisotropic joints, resistivity and contact resistance, cure and handling, and interface reliability.
Tags: consumables, adhesives, electronics, conductive-bonding
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EPO-TEK H20E lists volume resistivity ≤0.0004 Ω·cm at 23°C after the reference cure of 1 hour at 150°C. Using the listed limit, a hypothetical uniform 50 µm bondline and 1 mm² conducting area give 0.0002 Ω bulk resistance. The model excludes interface resistance and voids. Measure the assembled interconnect after the actual cure and aging. EPO-TEK H20E TDS, pp. 1–2
Specify electrical interconnection, grounding or heat transfer as the required function. Verify the assembled interface after processing and aging.
Conduction direction and material construction
Isotropic electrically conductive adhesives (ICAs) conduct in all directions; anisotropic conductive adhesives (ACAs) conduct preferentially in one direction, commonly through the bond thickness. This distinction matters when neighboring electrical pads must remain isolated. Neither name specifies the resin, package format, cure schedule, or current rating. Permabond conductive-adhesive guidance.
For example, 3M 9703 is a filled acrylic pressure-sensitive tape with Z-axis conduction and in-plane insulation. Its particles connect opposing surfaces without forming an in-plane path. It requires no thermal cure. By contrast, 3M 9712 is an XYZ-conductive tape with a carbon-fiber scrim. Substituting one for the other changes the electrical topology of the joint. 3M 9703 technical data; 3M 9712 product description.
In particle-filled systems, filler contacts create the conductive paths while the polymer holds the joint together. Hoenle's Elecolit guidance explains that filler loading, particle size/shape, and cure influence conductivity; insufficient cure can leave a weakly crosslinked matrix in which mechanical loads displace filler. Adding metal to an arbitrary glue is therefore not equivalent to a formulated conductive adhesive. Hoenle Elecolit material and cure guidance.
Electrical and thermal conductivity are separate requirements. Master Bond EP48TC is a heat-transfer adhesive with electrical volume resistivity above 10¹⁴ Ω·cm; it is not an electrical interconnect. Always read both properties when bonding a live component to a heat sink. Master Bond EP48TC product information.
Bulk resistivity is not assembled contact resistance
Volume resistivity ρ, usually reported in Ω·cm or Ω·m, describes the material with geometry accounted for. Resistance is measured in Ω. For a uniform, approximately ohmic bulk path, Rbulk = ρL/A, where L is current-path length and A its cross-section; use consistent units. The assembled path also includes both adhesive-to-terminal interfaces. Surface condition, contact pressure, geometry, and measurement arrangement affect the result. Parker's probe discussion specifically warns that contact resistance and dimensional errors can alter measurements. Parker Chomerics, Surface vs. Volume Resistivity, July 2018.
Set an assembly resistance limit and verify it after cure and relevant aging. Assess current, voltage drop, heat rise and insulation between adjacent contacts.
Bounded example: EPO-TEK H20E
The manufacturer's August 2024, revision XIX sheet describes a silver-filled epoxy for chip bonding:
| Property | Stated value and condition |
|---|---|
| Components and ratio | Two components; 1:1 by weight |
| Reference cure | 150 °C for 1 hour |
| Volume resistivity | ≤0.0004 Ω·cm at 23 °C, for the stated reference cure |
| Particle size | ≤45 µm |
| Bulk/syringe shelf life | One year at room temperature / one year at −40 °C |
The sheet lists shorter or cooler alternatives but explicitly warns that they may not achieve its reported properties. Its typical data are guidance, not a guaranteed joint specification. A frozen syringe is a distinct supplied format, not permission to treat the two bulk components as a room-temperature one-part adhesive. EPO-TEK H20E TDS, pp. 1–2.
Illustrative bulk-only calculation: Rbulk = 0.0004 × 0.005 / 0.01 = 0.0002 Ω, using centimeters for thickness and square centimeters for area. The maximum listed particle size is close to the assumed film thickness. Verify actual geometry and measure the interfaces separately.
Process and mechanical design
For H20E, the manufacturer requires filled components to be stirred thoroughly before mixing and before use and containers kept closed. It also cautions that repackaging or post-processing can change rheology and conductivity. Control the supplied format, weighing, mixing, dispense quantity, and cure rather than borrowing instructions from another silver epoxy. H20E processing notes.
Choose rheology and particle size for the dispensing method, pad geometry, and gap. Evaluate mechanical loading, differential thermal expansion, flexibility, and the complete component temperature budget. Conductive epoxies, silicones, and elastomers have different combinations of stiffness, cure, storage, and electrical properties. Master Bond conductive-material selection; Permabond selection considerations.
For a pressure-sensitive interconnect, maintaining electrical contact may require a mechanical load path. The 9703 sheet calls for clean, dry surfaces and application pressure and recommends considering a clamp or other sustained compression in flexible-circuit connections. A joint can remain attached mechanically while its electrical performance deteriorates. Its typical <0.3 Ω contact result applies to gold polyimide flex on gold PCB, initial room-temperature resistance, 6 mm² contact area; it is not the resistance of every 9703 joint. 3M 9703 application and electrical data.
Metallization and environmental reliability
Identify the exposed terminal finish, not merely the metal beneath it. EPO-TEK's January 2026 metallization guidance explains that silver combined with active metals such as tin or aluminum in moisture can promote galvanic corrosion and increasing resistance. It recommends noble finishes and treats copper/nickel preparation as consequential. These recommendations concern silver-epoxy interfaces; they are not a prohibition on every conductive product used with aluminum. EPO-TEK Tech Tip 12.
The same guidance discusses sulfur-containing atmospheres attacking silver conduction paths. Initial continuity therefore does not demonstrate resistance stability. Check the intended humidity, thermal cycling, chemicals, and mechanical loading using representative finished joints, with both continuity/resistance and adhesion criteria. Avoid extrapolating a resin's temperature rating to every terminal finish or environment. EPO-TEK interface/environment guidance.
Solder replacement and safe handling
Conductive adhesives can avoid solder-reflow exposure for temperature-sensitive parts. That benefit does not make a grounding adhesive suitable for every signal or power connection: conductivity, joint geometry, cure, strength, and service conditions still govern selection. Permabond applications and limitations.
Consult the current regional SDS for each component and supplied format. The manufacturer's retrieved H20E A/B sheets, revised 30 January 2023, identify skin irritation/sensitization for A and serious eye damage for B, with aquatic hazards for both. Follow their protective-equipment, exposure-control, spill, and disposal instructions. H20E US component SDS set.