1.0 vs 2.0 vs 3.0 vs 4.0 W/m·K: What You Actually Pay For in a Potting Compound
This is the most common misunderstanding in potting compound selection, and it costs real money in both directions: paying for thermal performance you cannot use, or buying a cheap grade that bridges a gap and traps air under a hot component.
Thermal conductivity is not a quality score. It is a trade-off against everything else in the compound.
All values in this article are drawn from the published FD503 series TDS documents. No test data is estimated or extrapolated.
01 What actually changes when the number goes up
A silicone potting compound conducts heat because it is filled with ceramic particles — aluminium oxide, aluminium nitride, boron nitride. The silicone resin itself is a poor conductor.
So the thermal grade is essentially a filler loading figure. Double the conductivity and you roughly double the filler content. That has consequences:
- Viscosity rises. More filler means a thicker mix. This is the consequence that causes failures.
- Density rises. More ceramic in the same volume means a heavier compound.
- Hardness rises, elasticity falls. Highly filled systems are stiffer and less forgiving of thermal cycling.
- Cost rises sharply. Filler and the processing to wet it out are the cost drivers — not the resin.
Nothing here is a defect. It is physics. The mistake is treating the conductivity number in isolation.
02 The four grades, side by side — measured values
The table below is the FD503 series as published in our TDS documents. Viscosity is mixed A+B at 25 °C.
| Grade | Thermal conductivity | Mixed viscosity | Hardness | Density | Typical use |
|---|---|---|---|---|---|
| TG101 / ZS-GF-5299G | ≥ 0.9 W/m·K | 2,500 ± 1,000 cps | 45–55 Shore A | 1.72 ± 0.05 | General PCBA, LED drivers |
| TG102 / TG15 | ≥ 1.5 W/m·K | 4,000–6,000 cps | 30–40 Shore A | 2.15 ± 0.1 | Power supply modules |
| TG20 | 2.0 ± 0.2 W/m·K | 3,000–5,000 cps | 20–30 Shore A | 2.73 ± 0.1 | Energy storage BMS |
| TG301 | 3.0 ± 0.3 W/m·K | 5,500–7,500 cps | 45–65 Shore 00 | 3.0 ± 0.1 | Automotive OBC / BMS |
| TG30 | ≥ 3.0 W/m·K | 8,000–15,000 cps | 40–60 Shore A | 3.0 ± 0.1 | Thick-layer, high heat |
| TG40 / TG40H | 4.0 ± 0.4 W/m·K | 10,000–15,000 cps | 60–70 Shore A | 3.0 ± 0.1 | High-power modules |
Read the viscosity column against the conductivity column. From 0.9 to 4.0 W/m·K — a 4.4× increase in conductivity — viscosity climbs by roughly 6×, from 2,500 to 15,000 cps.
03 Matching grade to gap, not to ambition
The engineering sequence is simple, and it runs in the opposite order to how most buyers shop:
Step 1 — Find the narrowest gap in the cavity. Not the average, not the main volume. The single tightest place the compound has to reach — usually between a component body and the housing wall, or under a transformer.
Step 2 — Pick the viscosity ceiling that gap allows. The narrower the gap, the lower the viscosity must be. Rough working bands from field experience:
| Narrowest gap | Viscosity ceiling (mixed, 25 °C) | Grades that fit |
|---|---|---|
| ≤ 1.5 mm | ≈ 1,500 cps | TG60S (600–1,200), TG60E (1,500–2,500 with assistance) |
| 1.5 – 3 mm | ≈ 3,000 cps | TG101, TG20 |
| 3 – 6 mm | ≈ 6,000 cps | TG102 / TG15, TG301 |
| 6 – 15 mm | ≈ 15,000 cps | TG30, TG40 / TG40H |
These bands are engineering rules of thumb for gap filling, not measured specifications. Confirm against your own cavity with a trial shot.
Step 3 — Only now, check whether that grade carries enough conductivity for your heat load. If the gap is 0.8 mm, your ceiling is around 1,500 cps. The highest conductivity available at that viscosity in our range is roughly 0.9 W/m·K, at the low end of TG101’s viscosity tolerance. If your heat load needs 2.0 W/m·K, the problem is not the material — it is the mechanical design of the cavity, and no potting compound will fix it.
kmin = 10 × 3 × 2 / (6 × 15) = 0.67 W/m·K.
A 1.0 W/m·K grade clears this with margin. The narrowest gap is 1.2 mm, which caps viscosity near 1,500 cps. TG101 at 2,500 ± 1,000 cps sits at the edge of that; the lower-viscosity grades in the range (TG60E at 1,500–2,500 cps) would fill more reliably. Paying for a 4.0 W/m·K grade here buys nothing and fills worse.
04 When the high grades are the right answer
They are not a marketing tier. They solve specific problems:
- 4.0 W/m·K (TG40 / TG40H) — large modules where the heat path is long and the compound is thick, and where the cavity is open enough to accept 10,000–15,000 cps. Traction inverters, high-power supplies. Note these are the stiffest grades, 60–70 Shore A: limited relief for thermal cycling stress on solder joints.
- 3.0 W/m·K (TG30, TG301) — the practical ceiling for most automotive work. TG301 is the lower-viscosity of the two at 5,500–7,500 cps and carries UL94 V-0, which is why it is the usual choice for on-board chargers and BMS.
- 2.0 W/m·K (TG20) — worth noting for its softness: 20–30 Shore A, the most elastic grade in the range. Where thermal cycling is severe and the compound must absorb movement rather than resist it, this is often better than a higher-conductivity, stiffer grade.
Note the pattern: TG20 has the second-lowest conductivity but the highest filler loading by density and the lowest hardness. Conductivity and stiffness are not the same axis, and treating them as one is where specifications go wrong.
05 The three questions that settle it
Before you compare grades, answer these. They take ten minutes and they remove most of the guesswork:
1. What is the narrowest gap the compound must reach? This sets your viscosity ceiling, and it is the one question that can rule a material out entirely.
2. What heat must cross the compound, through what area, with what temperature rise accepted? This sets your conductivity floor. Use loss power, not rated output.
3. What will the assembly see over its life? Thermal cycles, vibration, humidity, and whether the module ever needs rework. This decides hardness and cure system, not conductivity.
06 Downloads and further reading
Every grade above has a published TDS and MSDS. If you want the source documents rather than the summary table, ask and we will send the specific model numbers you are evaluating.
- How to choose a thermally conductive potting compound: a 4-question framework
- Inverter potting compound selection guide: thermal conductivity, processability and reliability
- Two-component or one-component potting? A practical guide to addition-cure silicone
- Potting compound application scenarios & selection approach for 2026
- NEV penetration past 50%: what it changes for potting compound demand
Send us your cavity dimensions
Narrowest gap, heat load and target temperature rise — three numbers. We will tell you which grade fits and which ones do not, including the ones we would rather not sell you.
