Vol.004 - Pad, Paste, Tape or ? Choosing the Right TIM

 

How to Choose a TIM?

Start with Those “Last Few Degrees” Stuck at the Interface

In many thermal designs, the fans, heatsinks, and heat pipes are all in place, simulations look fine, and on paper “the heat should be dissipated.” Yet in practice, junction temperature still stops just a few degrees away from your target.

At that point, the problem is often not the heatsink itself, but the interface—that seemingly thin but crucial layer of Thermal Interface Material (TIM) between the chip and the heatsink.

What a TIM Really Does: Kicking the Air Out

The actual job of a TIM is very simple in principle. Even if the chip and heatsink surfaces look flat and tightly mated, only a small portion of the microscopic asperities are truly in contact. Most of the gaps are filled with air—and air is a very poor thermal conductor.

A TIM’s role is to displace the air in those gaps and replace it with a much more efficient heat transfer path, thereby reducing the thermal resistance at the contact interface. When we talk about Thermal Resistance or Thermal Impedance in engineering, we’re essentially describing: how much the interface is blocking the heat.

Why Do Contact Interfaces Trap Heat?

The value of a TIM is not just having an impressive thermal conductivity k (W/m·K) printed on the datasheet. What really matters is whether, under actual assembly conditions, it can drive down the extra impedance caused by air gaps to a sufficiently low level.

In practice, performance is usually governed by three aspects:

• Intrinsic thermal conductivity of the TIM material
• Its ability to fill the gaps caused by surface roughness and tolerance—i.e., thickness, BLT (Bond Line Thickness), and compressibility
• Interface performance under real contact pressure—i.e., thermal impedance vs. pressure

In other words: a TIM labeled as 10 W/m·K on the datasheet does not guarantee you “10 W/m·K performance” once the screws are tightened.If the TIM is not properly compressed or brought into the intended thickness range, heat can still get stuck at the contact interface.

Main TIM Types and How They Fit

In real applications, mainstream TIMs can be grouped into several major categories:

• Thermal Paste
• Phase Change Materials (PCM)
• Thermal Pads
• Thermal Tapes
• And extended solutions for in‑plane heat spreading, such as graphite‑sputtered copper thermal films
   

  

  
  

On the engineering and production side, TIM selection is usually a trade‑off along several axes:

• Reworkability / serviceability
• Process efficiency (dispensing, printing, placement, lamination…)
• Thickness and gap‑filling capability
• Whether it must also provide mechanical attachment
• Long‑term reliability (pump‑out, drying, creep, interface aging…)
• Whether the main problem is interface conduction only, or also in‑plane heat spreading

Very roughly, you can think about their roles like this:

• Pad (Thermal Pad):Good at handling height differences, mechanical tolerances, and warpage. Suitable when you need a relatively large bond line thickness and can’t tightly control assembly tolerances.
• Paste / PCM (Thermal Grease / Phase Change):Good at achieving very thin interfaces. If your mechanical design can provide sufficient pressure and flatness, these materials usually have a better chance of delivering lower interface resistance.
• Tape (Thermal Tape):Provides a compromise between thermal conduction and mechanical attachment, often allowing you to reduce hardware (brackets, screws) and simplifying assembly on the line.
• Graphite‑Sputtered Copper Thermal Film:Focuses more on what happens after heat enters the structure—using high in‑plane conductivity to spread heat quickly and reduce local hotspots. Well suited to thin form factors and applications where temperature uniformity is critical.

Reading the Datasheet: Beyond k – Impedance, Pressure, and Thickness

When choosing a TIM, thermal conductivity k is important but far from sufficient. What’s more critical are the supplier’s data under specified conditions:

• Thermal impedance / thermal resistance
• The corresponding pressure (psi)
• The actual thickness or BLT

Take the T‑work8000 thermal pad as an example. The product is available in 1.0 to 3.0 mm thicknesses. Its thermal conductivity is specified as:

• 15 W/m·K under ASTM D5470
• 8.5 W/m·K under ISO 22007‑2

This already tells us: looking at k alone is not enough to predict real, assembled thermal performance.

What’s more informative is its thermal impedance at different pressures. For the 1.0 mm data:

• At 10 psi, thermal impedance is about 0.185 °C‑in²/W
• At 20 psi, about 0.122 °C‑in²/W
• At 30 psi, about 0.074 °C‑in²/W
• At 40 psi, about 0.054 °C‑in²/W
• At 50 psi, down to about 0.046 °C‑in²/W

The spread clearly shows: whether the pad is properly compressed has a very significant impact on actual thermal performance.

If you also look across different thicknesses, the trend becomes even clearer. At the same pressure, 1.0 mm, 2.0 mm, and 3.0 mm pads do not have the same impedance, and their compression ratios change with both thickness and pressure. For example, at 50 psi:

• Thermal impedance for 1.0 / 2.0 / 3.0 mm is approx.
  
  
  0.046 / 0.059 / 0.064 °C‑in²/W
• Compression ratio is about
  
  
  79% / 86% / 90%

This implies:

• Thicker pads are more forgiving for tolerance and warpage, but both material and interface resistance usually go up.
• Thinner pads demand better flatness and assembly precision, but can achieve lower total thermal impedance.

So thickness is neither “the thicker the safer” nor “the thinner the better”. You have to bring it back to system constraints:

• Surface flatness, tolerance, and warpage
• Actual pressure range from screws / springs / clips
• Long‑term reliability under compression
• Assembly tolerance you can realistically hold

Let Application Drives Selection

Instead of starting with “Pad or Paste?”, it’s usually more effective to start with the actual pain point.

1. If the pain is height differences, tolerance, or warpage

Pads will typically be your first candidates.

In this situation, don’t just look at k. Map your available mechanical pressure range against the material’s thermal impedance vs. pressure curve, then look again at:

• Compression ratio
• Impedance vs. thickness

to find a thickness window that balances gap‑filling and thermal performance.

The most common pitfall here is:

Choosing a very high‑k, thick pad based only on conductivity,but with insufficient pressure, it never compresses properly,and in real use its thermal impedance is worse than a “medium‑k, thinner but well‑compressed” material.

2. If the pain is pushing interface resistance even lower

The precondition is that your mechanical design can provide stable, sufficient pressure and flatness.

In such cases, paste / PCM typically have a better chance of achieving a very thin BLT. In the grease example from your data:

• Post‑flash thermal conductivity is around 6.5 W/m·K
• Thermal resistance is 12.8 mm²·K/W
• Thin‑BLT is about 72 μm at 20 psi

This combination is exactly why it suits low interface resistance designs.

But in parallel, you must also consider:

• Process: how to dispense or print, risk of overflow, process windows
• Reliability: pump‑out, drying, contamination risks
• Service / rework: what happens to the interface after disassembly? How do you clean and re‑apply?

3. If you need both attachment and conduction

Thermal tapes are often the intuitive choice.

In the AT900A thermal tape data, beyond thickness and thermal conductivity, the datasheet also provides:

• Initial tack
• Shear strength
• Holding power
• Peel strength
• Adhesion performance after thermal aging, HAST, and thermal cycling

The value of this kind of material is less about hitting an extreme thermal number, and more about making design, process, and reliability much easier to implement in practice.

For thermal performance specifically:

• Thermal conductivity is 0.9 W/m·K
• Thermal impedance at 5 / 10 / 15 psi is about
  
  
  0.87 / 0.85 / 0.82 °C‑in²/W

It’s usually not the first choice if you’re chasing absolute minimum thermal resistance, but when you need to simplify assembly and combine attachment with conduction, it’s a very practical compromise.

4. If the pain includes in‑plane spreading, not just conduction away

In that case, it’s worth evaluating graphite‑sputtered copper thermal films.

Here the focus is not only on the contact interface, but on using very high in‑plane thermal conductivity to spread heat from a local hotspot across a limited area, reducing peak temperature.

From the specs you provided:

• Copper foil base, with graphite coating ratio below 1%
• Coating adhesion strength: 100%
• Temperature range: approx. ‑20 to +200 °C
• Thickness: about 70–160 μm
• In‑plane (x‑y) thermal conductivity: > 1200 W/m·K
• Through‑thickness (z) thermal conductivity: > 350 W/m·K
• Surface resistance: < 0.01 Ω
• Flame rating: UL‑94 V‑0

This points to applications where you need to balance spreading performance, thin form factor, and structural integration.

It’s also worth emphasizing: this type of material is not always meant to replace traditional TIMs. In many designs, it’s another piece in the thermal puzzle:TIMs handle interface contact, while graphite‑sputtered copper films enhance in‑plane spreading. They operate at different layers of the thermal problem.

TIMs Are Not Supporting Actors – They’re Critical Nodes in Your Thermal Path

TIMs may look like just a thin layer, but in many high power‑density, space‑constrained designs, they effectively determine:

• Whether the same thermal hardware can deliver a few more degrees of Tj reduction
• Whether module power or frequency can be pushed to the next level
• Whether your temperature margins are still safe after tolerance stack‑up, assembly variation, and long‑term aging

Instead of stacking k‑values on a spec sheet, it’s more practical to first clarify the basics of your interface:

• Surface flatness, tolerance, warpage
• Available pressure range and clamping method
• Whether you’re solving only interface resistance, or also planar spreading

Then you can go back and decide:

• Which type of material fits the problem
• Which segment of the thermal impedance vs. pressure / thickness curve you can actually reach in your design
• Whether your process, reliability plan, and service strategy can support that choice

TIMs chosen this way won’t just be another line item on the BOM. They’ll meaningfully help your thermal design unlock those “last few degrees” that were stuck at the interface.