Conductive Adhesive
How Conductive Adhesives Join and Connect RF Components
A conductive adhesive is a two-phase composite: an organic binder (epoxy, silicone, or polyimide) carries a high loading of conductive filler, typically silver flake but sometimes silver-coated copper, nickel, graphite, or gold. The binder supplies mechanical strength and adhesion, while the filler forms a percolating network of touching metal particles that carries current and heat. Conduction only begins once the filler concentration exceeds the percolation threshold, which is why isotropic pastes are loaded to roughly 70 to 80 percent silver by weight. Below that point the joint is an insulator; above it the resistance drops sharply and then levels off.
In RF and millimeter-wave assemblies the adhesive does several jobs at once. It attaches bare MMIC and GaAs die to a carrier or package floor, it tacks down ground straps and shield walls, and it bonds absorber and gasket material into housings. Because the cure happens near 100°C rather than at solder reflow temperature, the adhesive protects temperature-sensitive die and accommodates the differential expansion between a silicon or GaAs chip and an aluminum or copper-tungsten carrier. The compliant polymer absorbs shear strain that a rigid solder joint would transfer into the die as cracking stress.
The cost of that compliance is electrical and thermal performance. A filled epoxy bondline conducts through particle-to-particle contacts, so its bulk resistivity is roughly ten times that of solder and its thermal conductivity is far lower, on the order of single-digit W/m-K against tens for AuSn. For high-power GaN stages the adhesive thermal path is often the limiting resistance, which is why power devices are still frequently eutectic-attached while conductive adhesive handles low-dissipation control, bias, and passive die.
Bondline Resistance and Thermal Path
Both the electrical and the thermal behavior of an adhesive joint follow the same geometry. Joint resistance scales with bondline thickness and inversely with bond area, so a thin, void-free layer is the design goal for any ground, power, or heat path. Voids raise both resistance and thermal impedance and are usually screened by acoustic microscopy or X-ray, with power joints specified below about 10 percent void area.
Rjoint = ρ × t / A
Thermal resistance of the bondline:
Rth = t / (k × A)
Example (isotropic silver epoxy):
ρ ≈ 2 × 10-4 Ω-cm, t = 25 μm, A = 1 mm2 → Rjoint ≈ 50 μΩ
k ≈ 4 W/m-K, same t and A → Rth ≈ 6.3 K/W
Where ρ = bulk resistivity, t = bondline thickness, A = bond area, k = thermal conductivity. Tripling t to 75 μm triples both Rjoint and Rth.
Adhesive Type Comparison
| Material | Cure / Reflow Temp | Bulk Resistivity | Thermal Cond. | Rework | Best Application |
|---|---|---|---|---|---|
| Isotropic Ag epoxy (ICA) | 80 to 150°C | 1 to 5 × 10-4 Ω-cm | 3 to 6 W/m-K | Good | Die attach, grounding |
| Ag-filled silicone | RT to 150°C | 5 to 20 × 10-4 Ω-cm | 1 to 3 W/m-K | Excellent | EMI gaskets, shielding |
| Anisotropic (ACA/ACF) | 130 to 200°C | z-axis only | Low | Limited | Fine-pitch flex, display |
| AuSn (80/20) eutectic | 280°C reflow | 1.6 × 10-5 Ω-cm | ~57 W/m-K | Difficult | High-power GaN die |
| SnPb / SAC solder | 183 to 245°C | 1 to 1.5 × 10-5 Ω-cm | 30 to 60 W/m-K | Good | SMT, board assembly |
Frequently Asked Questions
How does silver-filled conductive epoxy compare to AuSn solder for RF die attach?
Silver-filled ICA cures at 80 to 150°C with bulk resistivity near 1 to 5 × 10-4 Ω-cm, giving roughly 50 μΩ of bondline resistance for a 25 μm layer under a 1 mm2 die. AuSn (80/20) reflows near 280°C with resistivity around 1.6 × 10-5 Ω-cm and thermal conductivity of ~57 W/m-K against 3 to 6 W/m-K for filled epoxy. The adhesive wins on low-temperature, stress-relieving, reworkable assembly; AuSn wins on thermal path and current handling for high-power GaN.
What is the difference between isotropic and anisotropic conductive adhesive?
Isotropic conductive adhesive (ICA) is loaded with about 70 to 80 percent silver flake so particles touch in all directions and it conducts equally in x, y, and z, acting as a paste solder substitute. Anisotropic adhesive or film (ACA/ACF) uses sparse particles that only bridge vertically when compressed between a bump and pad, so it conducts in the z-axis but stays insulating laterally, which suits fine-pitch flex and display interconnect.
Why does bondline thickness matter for RF grounding with conductive adhesive?
Joint resistance follows R = ρt/A, so it grows directly with bondline thickness. A 25 μm bondline at 2 × 10-4 Ω-cm over 1 mm2 is about 50 μΩ; growing it to 75 μm triples that and adds series inductance that degrades grounding at mmWave. Thin, void-free joints also minimize thermal resistance, so designers control thickness via stencil aperture, dispense volume, and bond force and cap void content near 10 percent.