Die attach — bonding a semiconductor die to its package substrate, lead frame, or module base — is a critical assembly step that determines both the electrical performance and thermal management of the finished device. In power packages where the die back contact must be grounded or biased through the substrate, the die attach material must conduct electricity as well as provide mechanical bonding and thermal conduction. Electrically conductive epoxy is the die attach material of choice where eutectic solder and silver-sintered attach are unavailable, uneconomical, or incompatible with the die or substrate, and understanding its performance envelope is essential for qualifying it for specific power levels and operating temperatures.
Die Attach Functions in Power Packages
In a power semiconductor package — a power MOSFET, IGBT, power diode, or SiC/GaN device — the die attach material simultaneously accomplishes three independent functions that any candidate material must satisfy.
Electrical conduction through the die attach connects the die back metal (typically source or collector contact in vertical-conduction devices) to the package lead frame or substrate ground plane. The electrical resistance of the die attach contributes to the total on-resistance of the device (RDS(on) for MOSFETs), and minimizing this contribution requires low bulk resistivity in the die attach material.
Thermal conduction from the die to the package base is the primary determinant of the die junction-to-case thermal resistance (θjc). The die attach sits in series with the substrate and base plate in the thermal path; its conductivity and thickness together determine its contribution to total thermal resistance. For a 5 × 5 mm die with 0.05 mm bondline, silver-filled epoxy at 5 W/m·K gives a die attach thermal resistance of approximately 0.2°C/W — comparable to the substrate contribution in many designs, and directly relevant when bonding components to substrates with thermally conductive adhesive elsewhere in the same package.
Mechanical bonding retains the die on the substrate against handling, thermal cycling, and vibration loads throughout the device service life — typically 100,000 to 1,000,000 thermal cycles for automotive and industrial applications — without delamination, cracking, or void growth that would increase thermal or electrical resistance.
Silver-Filled Epoxy Die Attach: Properties and Performance
Silver-filled conductive epoxy for die attach is a two-phase system: an epoxy resin matrix at 15 to 25 percent by weight, and silver filler — typically silver flakes, silver spheres, or a combination — at 75 to 85 percent by weight. At these loading levels, the silver filler particles are in intimate contact throughout the matrix, providing percolating conduction paths for both electrical and thermal transport.
Electrical resistivity of fully cured silver-filled die attach epoxy ranges from 5 × 10⁻⁵ to 5 × 10⁻⁴ Ω·cm depending on formulation and filler morphology — approximately 50 to 500 times higher than bulk silver. For typical die attach geometries, this translates to milliohm-range joint resistances, acceptable for most power device applications. For wide-bandgap devices (SiC, GaN) at high switching frequencies, the contact resistance at the die-to-adhesive and adhesive-to-substrate interfaces changes with cure and aging and can dominate over bulk resistance for thin bondlines.
Thermal conductivity of silver epoxy die attach is typically 3 to 10 W/m·K for commercial formulations — adequate for moderate power densities but well below solder (50 W/m·K) or silver sinter (200 W/m·K). Silver epoxy die attach is most competitive at moderate power dissipation, where the added thermal resistance compared to solder is acceptable within the device thermal budget.
For die attach conductivity and resistivity specifications matched to your power device design, Email Us — Incure can provide formulation data for both electrical and thermal performance parameters.
Die Attach Process for Conductive Epoxy
The die attach process with conductive epoxy follows the sequence: substrate preparation, adhesive dispense, die placement, and cure. Each step affects the final bond quality, void content, and electrical and thermal performance.
Substrate surface condition at the die attach pad determines adhesion quality. Gold-finished lead frames provide an ideal bonding surface with high surface energy and no oxide layer; adhesion to gold is reliable without additional preparation. Silver and copper lead frames develop oxide layers that reduce adhesion, so light plasma treatment or N₂-formic acid vapor treatment before attach improves adhesion. Palladium-silver and OSP finishes have variable surface chemistry that requires process qualification for each finish type.
Adhesive dispense is typically a dot dispense or print process. For die sizes from 1 × 1 mm to 10 × 10 mm, a single dot or grid of dots of controlled volume is dispensed on the pad; the die is placed with controlled force to spread the adhesive under the die. Placement force and tool flatness affect final bondline thickness and void content — automated equipment with force control achieves more consistent bondlines than manual placement.
Void content in the bondline — measured by scanning acoustic microscopy after cure — affects both thermal resistance (voids are thermal insulators) and long-term reliability (voids initiate fatigue cracking). Target void content is typically below 5 percent of bond area; above 10 to 15 percent is associated with increased thermal resistance and reduced fatigue life.
Cure schedule is typically 150°C to 175°C for 30 to 60 minutes in a belt or batch oven, developing full crosslink conversion and the final electrical and thermal properties. Partial cure leaves unreacted chemistry that may outgas in later assembly steps and reduces final property development.
Reliability Qualification for Power Die Attach
Die attach reliability for power electronics is characterized by thermal cycling testing per JEDEC or automotive standards, including AEC-Q101 for discrete semiconductors, or customer-specific cycling profiles for module applications. The cycling profile specifies temperature amplitude (typically -40°C to +125°C for automotive, or -55°C to +150°C for higher-rated parts), cycle time, and number of cycles to evaluation.
Failure modes in die attach thermal cycling include delamination (adhesive separates from die or substrate), cohesive cracking (adhesive fractures internally), and die cracking from stress transmitted through the attach layer. The dominant mode depends on die attach modulus, die size, and the CTE mismatch between die, adhesive, and substrate — considerations that also drive selection between isotropic and anisotropic conductive epoxy when the package uses fine-pitch attach elsewhere.
Thermal resistance shift — increase in θjc measured during cycling — is the primary functional indicator of die attach degradation before catastrophic failure. Resistance shift above 10 to 20 percent of initial value is typically defined as end-of-life for thermal fatigue degradation purposes.
Contact Our Team to discuss silver-filled die attach epoxy selection, cure process optimization, void content control, and thermal cycling qualification data for your power electronics package design.
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