Conductivity gets the marketing, but hardness decides whether the pad works in your stack. A pad that is too hard never conforms to surface roughness; one that is too soft squeezes out of narrow gaps or creeps under load. This guide explains Shore 00 and Shore A scales, deflection force, compression set and how to pick hardness from the gap - not from a brochure.
The hardness scales that matter
| Scale | Typical range for TIMs | Where you meet it |
|---|---|---|
| Shore 00 | 10-70 | Soft gap fillers 0.5-2.0 mm; most silicone pads sit between 20 and 60 Shore 00 |
| Shore A | 5-40 | Firmer pads, reinforced and fibrous pads, thin dielectric pads |
| Shore D | - | Rigid plates and graphite sheets - not pads |
The scales overlap and do not convert cleanly. A 40 Shore 00 pad is roughly comparable to a very low Shore A value, but the equivalence depends on formulation. When comparing two suppliers, convert nothing - compare like with like on the same scale, or request samples and measure.
Deflection force: the number nobody quotes
Hardness tells you how the material feels; deflection force (or force-to-compress, often given as a curve of pressure vs % deflection) tells you what the pad does to your assembly. Two practical rules:
- For fragile packages (bare dies, glass, ceramic substrates), keep assembly pressure under roughly 50 psi at the target compression - check the deflection curve, not the hardness number.
- For large-area pads (hundreds of cm²), even a moderate hardness produces large total clamping loads. A pad that deflects 30% at 25 psi over 300 cm² needs 470 N just for the interface. Size the screws for the pad.
Compression set and stress relaxation
Compression set is the permanent thickness loss after long compression at temperature - the pad thins over its first months and the joint loosens. Stress relaxation is the fall in contact pressure over time even at constant deflection. Both matter more than initial performance for anything that must survive years in the field:
- High set = falling contact pressure = rising contact resistance, especially through thermal cycling.
- Good pads hold the joint closed after 1000 h at 150 °C with minimal set; our 180 W/m·K pad is verified over 1500 h at 150 °C with stable contact resistance.
- Fibrous or plate-reinforced pads trade some conformity for far better set and pump-out resistance - usually the right trade above 1 mm gaps in sealed hardware.
Choosing hardness from the gap
| Gap (assembled) | Suggested pad | Hardness guide | Why |
|---|---|---|---|
| < 0.3 mm | 0.3 mm pad | 40-70 Shore 00 or low Shore A | Thin pad; needs enough hardness to survive handling and avoid extrusion |
| 0.3-0.7 mm | 0.5 mm pad | 25-45 Shore 00 | Conforms to roughness while holding tolerance |
| 0.7-1.5 mm | 1.0 mm pad | 20-35 Shore 00 | Large tolerance absorption; soft enough to close at moderate screw load |
| > 1.5 mm, big stack spread | 1.0-3.0 mm | Soft core 15-30 Shore 00, or reinforced pad | Watch total clamp load and creep; consider pad + spreader stack |
Rule of thumb from our selection guide: pad thickness ≈ gap / 0.7, and thickness should not exceed gap by more than about 40%. Then confirm with the deflection curve that the assembly pressure lands in the pad's sweet spot - typically 20-50% compression for soft pads.
The three-question shortcut
Before you commit a hardness choice, answer: (1) What is the gap range, including tolerances and board bow? (2) What pressure can the assembly actually apply, at the corners as well as the centre? (3) What does the interface need to survive - cycling, shock, years of soak? We cut samples to your footprint in 0.3 / 0.5 / 0.7 mm as standard (custom to 3 mm), so the hardness decision can be validated on real hardware instead of in a catalogue.