Thermal pad compression is enough when the pad fully contacts both surfaces, fills the gap across tolerance variation, reduces contact resistance, and does not create excessive mechanical stress on the PCB, component, solder joints, or housing. There is no universal compression ratio that fits every thermal pad. Many designs target a moderate compression range, but the correct value depends on pad thickness, hardness, gap tolerance, available pressure, surface flatness, dielectric requirements, and reliability testing.
わかりやすく言うと： enough compression means full thermal contact without overloading the assembly.
Too little compression can leave air gaps and high thermal resistance. Too much compression can bend boards, crack components, deform housings, squeeze the pad, reduce dielectric margin, or cause long-term compression set. Engineers should calculate compression at minimum, nominal, and maximum gap conditions, then validate thermal impedance and mechanical reliability in the real product.
What Is Thermal Pad Compression Ratio?
Thermal pad compression ratio is the percentage reduction in pad thickness after the pad is assembled between a heat source and a cooling surface.
The basic formula is:
Compression ratio = (original パッド thickness – compressed thickness) / original pad thickness × 100%
For example, if a 2.0 mm thermal pad is compressed to 1.6 mm after assembly:
(2.0 – 1.6) / 2.0 × 100% = 20% compression
The original pad thickness is the supplied thickness. The compressed thickness is the final bond line thickness after the device is assembled. This final thickness is what heat actually passes through during operation.
Compression ratio matters because thermal pads are designed to work under pressure. Proper compression helps the pad conform to surface roughness, fill small air gaps, and create a more complete heat path.
Why Thermal Pads Need Compression
Thermal pads are used between electronic components and cooling surfaces such as heat sinks, metal housings, cold plates, chassis, or enclosures. These surfaces are never perfectly flat. Even machined metal and molded component packages contain microscopic peaks and valleys.
Without compression, a thermal pad may touch only the highest points. Air remains in the gaps. Since air has very low thermal conductivity, those voids increase contact resistance and raise component temperature.
Compression helps the pad:
- Increase real contact area
- Fill surface roughness
- Reduce air voids
- Lower contact resistance
- Control final bond line thickness
- Compensate for small tolerance variation
- Improve unit-to-unit repeatability
HakTakの記事 圧縮がサーマルパッドの性能に与える影響 この関係について詳しく説明します。.
Is There a Standard Compression Ratio?
There is no single universal compression ratio for all thermal pads. A suitable compression ratio depends on material formulation and product design.
Some pads are designed for lower compression. Others need more compression to reach full contact. Softer pads may compress easily at low pressure. Harder pads may need higher force.
The correct compression ratio depends on:
- パッドの厚さ
- パッドの硬さ
- 隙間のサイズ
- 隙間公差
- 接触面
- 表面平坦度
- 組立圧力
- コンポーネントの脆弱性
- PCB stiffness
- Electrical insulation requirement
- Thermal impedance target
- 長期圧縮永久ひずみ
- 動作温度範囲
This is why supplier datasheets often provide recommended compression ranges, compression-deflection curves, or thermal impedance values at different pressures. Engineers should use those data points as a starting point, then test the real assembly.
What Happens with Too Little Compression?
Too little compression usually means poor contact.
If the thermal pad is not compressed enough, it may fail to fill surface roughness or may not touch both surfaces across the full interface area. This can create hot spots and unstable thermal performance.
Symptoms of Under-Compression
Common signs include:
- Higher component temperature
- Large variation between units
- 地元の人気スポット
- Inconsistent thermal test results
- Poor contact marks after disassembly
- Pad not visibly compressed
- Air gaps at maximum tolerance condition
Why Under-Compression Happens
Under-compression usually happens when:
- The pad is too thin for the maximum gap
- The pad is too hard for the available force
- Screw torque is too low
- The housing is not stiff enough
- The surfaces are not parallel
- Component height varies more than expected
- The pad area is too large for the available pressure
Under-compression is a common reason a high-W/mK thermal pad performs poorly in real devices.
What Happens with Too Much Compression?
Too much compression can improve contact in the short term, but it creates mechanical and reliability risks.
Symptoms of Over-Compression
Possible signs include:
- 基板の反り
- コンポーネントの割れ
- はんだ接合部の応力
- ハウジングの変形
- パッド押し出し
- Pad tearing
- Excessive assembly force
- Screw or clip stress
- 誘電体間隔の縮小
- Compression set after aging
Over-compression can be especially dangerous when the pad is thick, hard, or placed over fragile components.
Why Over-Compression Happens
Over-compression usually happens when:
- The pad is too thick for the minimum gap
- The tolerance stack-up was not considered
- Screw torque is too high
- The pad hardness is too high
- Mechanical stops are missing
- The assembly tries to use the pad as a structural spacer
The result may look acceptable at first, but long-term reliability can suffer.
Compression Ratio and Gap Tolerance
Engineers should calculate compression ratio at three gap conditions:
- 最小ギャップ
- 公称ギャップ
- 最大ギャップ
The same pad can be over-compressed at the minimum gap and under-compressed at the maximum gap.
Example Calculation
Assume:
- Pad thickness: 2.0 mm
- Minimum gap: 1.3 mm
- Nominal gap: 1.5 mm
- Maximum gap: 1.8 mm
| ギャップ条件 | Final Thickness | Compression Ratio | Risk |
| 最小ギャップ | 1.3 mm | 35% | Possible over-compression |
| 公称ギャップ | 1.5 mm | 25% | Likely acceptable if recommended |
| 最大ギャップ | 1.8 mm | 10% | May be low depending on pad |
This table shows why selecting a pad based only on nominal gap can be risky. The pad must work across the full tolerance range.
ハクタックのガイド 電子機器向けサーマルパッドの厚さの選び方 gives a practical thickness selection method based on gap range and compression.
Compression Ratio and Bond Line Thickness
Bond line thickness, or BLT, is the final compressed thickness of the thermal pad. It directly affects thermal resistance.
熱伝達の経路を簡略化すると：
R = t / (k × A)
どこ：
- R は熱抵抗である
- t is final bond line thickness
- k は熱伝導率である
- A は接触面積です
As compression increases, final thickness decreases. This can reduce bulk thermal resistance. But compression also needs to improve contact and avoid damage.
The best thermal performance comes from a pad that is compressed enough to make full contact and thin enough to reduce resistance, while still mechanically safe.
HakTakの記事 ボンドラインの厚さが熱性能に与える影響 explains why BLT is one of the most important variables in TIM selection.
Compression Ratio and Thermal Impedance
Thermal impedance is often more useful than thermal conductivity when evaluating thermal pad performance. It reflects the real interface under defined pressure and thickness.
As compression increases, thermal impedance often decreases because:
- Contact area improves
- Air gaps are reduced
- Final thickness decreases
- Surface wetting improves
However, after good contact is achieved, additional compression may provide only small thermal benefit while increasing mechanical risk.
This creates a practical engineering target:
Use enough compression to reach stable low thermal impedance, but avoid extra compression that does not significantly improve cooling.
ASTM D5470 is commonly referenced for thermal transmission properties of thermally conductive electrical insulation materials and is relevant for TIM thermal impedance and apparent conductivity testing.
HakTakの記事 TIM選定における熱伝導率と熱インピーダンス explains why thermal impedance is often more useful than W/mK alone.
Compression Ratio and Thermal Pad Hardness
Thermal pad hardness strongly affects the compression force required to reach a certain compression ratio.
A soft pad may reach 20% compression under low pressure. A harder pad may need much higher force to reach the same compression.
Hardness is commonly measured using durometer or Shore scales. ASTM D2240 is a key reference for rubber property durometer hardness. The standard explains that indentation hardness depends on material behavior and test conditions, and that different durometer types should not be treated as directly interchangeable.
Soft Thermal Pads
柔らかいパッドが役立つのは、次のような場合です：
- 圧力が制限されている
- 部品は壊れやすいです
- 表面が平らではありません
- PCBの曲げは最小限に抑える必要があります
- Multiple component heights exist
Soft pads may reach enough compression more easily, but they may also be harder to handle or more likely to deform.
Hard Thermal Pads
硬めのパッドが役立つのは、次のような場合です：
- Gap tolerance is controlled
- Assembly pressure is available
- ダイカットの取り扱いには注意が必要です
- Dimensional stability is important
- リワークはよりクリーンなものになる必要がある
Hard pads can perform well, but only if the assembly can compress them enough to make full contact.
HakTakの記事 ソフト対ハードサーマルパッド：どちらが良い？ gives a more complete comparison.
Compression Ratio and Electrical Insulation
Many thermal pads are electrically insulating. In power electronics, the pad may need to transfer heat while preventing current flow between a component and a metal heat sink or housing.
Compression changes final thickness, and final thickness affects dielectric spacing.
If a pad is over-compressed, the final thickness may be lower than expected. This can reduce electrical insulation margin. If the pad is under-compressed, thermal contact may be poor.
Engineers should check:
- 絶縁耐力
- 絶縁破壊電圧
- 最終圧縮厚さ
- Compression after aging
- Electrical testing after assembly
- Electrical testing after thermal cycling
HakTakの記事 電気絶縁性サーマルパッド：いつそれらが必要なのか？ この熱電トレードオフについて説明します。.
How Much Compression Is Enough by Application?
The table below provides practical guidance. It is not a universal specification. Final values must follow supplier recommendations and product testing.
| アプリケーション | Typical Design Concern | Compression Target Logic |
| パワーモジュール | Heat transfer plus insulation | Enough contact without reducing dielectric margin |
| MOSFETs and IGBTs | Component stress and heat sink isolation | Moderate compression with controlled torque |
| LED modules | Uniform contact and long life | Even compression across the module |
| 通信機器 | Long-term thermal cycling | Compression that remains stable after aging |
| EV用電子機器 | Vibration and high voltage | Validate after cycling and shock |
| AIサーバー | High heat flux and serviceability | Low impedance with repeatable assembly |
| バッテリー管理システム | 複数のコンポーネントの高さ | Softer pad or gap filler may be needed |
| 産業用制御 | Variable housings and long service | Stable compression across tolerance range |
How to Calculate Thermal Pad Compression Ratio
Use this workflow.
ステップ1:ギャップの測定
測定または計算する：
- 最小ギャップ
- 公称ギャップ
- 最大ギャップ
Include component, PCB, solder, housing, and heat sink tolerances.
Step 2: Choose Candidate Pad Thickness
The pad should be thick enough to contact both surfaces at the maximum gap.
Step 3: Calculate Compression at Each Gap
使用：
Compression ratio = (パッド thickness – gap) / pad thickness × 100%
This assumes the gap becomes the final pad thickness after assembly.
Step 4: Compare with Recommended Range
Check whether compression is acceptable at:
- 最小ギャップ
- 公称ギャップ
- 最大ギャップ
Step 5: Check Force and Stress
A pad may mathematically fit, but still require too much force. Review compression-deflection data and test the assembly.
Step 6: Validate Thermal and Mechanical Performance
Measure component temperature, thermal impedance, board deformation, and reliability after cycling.
Example: Choosing Between Two Pad Thicknesses
Assume:
- Gap range: 1.1 mm to 1.5 mm
- Candidate A: 1.5 mm pad
- Candidate B: 2.0 mm pad
| Candidate | Compression at 1.1 mm Gap | Compression at 1.5 mm Gap | Interpretation |
| 1.5 mm pad | 27% | 0% | Good at min gap, no compression at max gap |
| 2.0 mm pad | 45% | 25% | Good at max gap, possible over-compression at min gap |
Neither option is automatically perfect. A soft 2.0 mm pad might work if the assembly tolerates 45% compression. A 1.5 mm pad might fail at the maximum gap. Another thickness, softer formulation, or thermal putty may be better.
This is why compression ratio must be evaluated with force, hardness, and tolerance.
Compression Set and Long-Term Performance
Compression set is the permanent deformation that remains after a material has been compressed for a period of time and then released. For thermal pads, compression set matters because the pad must maintain contact over the product life.
High compression set can lead to:
- Loss of recovery
- 接触圧の低減
- より高い熱抵抗
- 手抜きのやり直し作業
- Long-term hot spots
Thermal cycling and high temperature can accelerate compression set. A pad that performs well at initial assembly may lose contact after months or years if its compression recovery is poor.
Engineers should test thermal performance after:
- 高温時効
- 熱サイクル
- 振動
- 電源の入れ直し
- Long-term compression
- 作り直す
Testing Thermal Pad Compression Ratio
Compression ratio should be tested with realistic assembly conditions.
Useful tests include:
- Compression-deflection testing
- Thermal impedance testing at different pressures
- Final bond line thickness measurement
- Component temperature testing
- Compression set testing
- Dielectric testing after compression
- 熱サイクル
- 振動試験
For broader thermal conductivity and diffusivity testing of polymer materials, ISO 22007-2 covers the transient plane heat source method.
For TIM selection, engineers should not compare thermal conductivity values without knowing pressure and thickness. Test conditions matter.
エンジニアが避けるべき一般的な間違い
The first mistake is using a fixed compression percentage for every pad. Different materials require different ranges.
The second mistake is calculating compression only at nominal gap. Minimum and maximum gap conditions are often where failures occur.
The third mistake is choosing a pad that is too thick “just to be safe.” This can increase thermal resistance and stress the assembly.
The fourth mistake is ignoring pad hardness. A hard pad may not compress enough under real pressure.
The fifth mistake is ignoring dielectric margin after compression. Final thickness matters for electrical insulation.
The sixth mistake is testing only initial performance. Compression set and aging can change long-term thermal contact.
The seventh mistake is comparing W/mK without thermal impedance. A high-W/mK pad can perform poorly if it is under-compressed or over-thick.
ハクタック・パースペクティブ
At HakTak, thermal pad compression ratio is treated as a design variable, not a fixed rule. The correct compression range depends on the pad material and the final assembly.
正確な材料選択のために、技術者は以下の情報を提供する必要があります：
- Original gap range
- 最小、定格、および最大ギャップ
- 接触面
- Heat source power
- Required thermal impedance or temperature target
- 使用圧力または締め付けトルク
- コンポーネントの脆弱性
- PCB stiffness
- 電気絶縁の必要性
- 動作温度範囲
- Vibration and thermal cycling conditions
- Product lifetime expectations
With this information, a supplier can recommend pad thickness, hardness, conductivity grade, compression range, and material type more accurately.
The goal is not maximum compression. The goal is stable contact, low thermal resistance, safe mechanical loading, and reliable long-term performance.
結論
Thermal pad compression ratio is enough when the pad fully fills the gap and makes reliable contact without overloading the mechanical or electrical design.
Too little compression leaves air gaps and increases thermal resistance. Too much compression can damage components, bend PCBs, deform housings, reduce dielectric margin, or cause long-term compression set.
Engineers should calculate compression at minimum, nominal, and maximum gap conditions. They should then validate the material using realistic pressure, final bond line thickness, thermal impedance, dielectric requirements, and reliability testing.
There is no universal compression percentage for every thermal pad. The right compression ratio is the one that works safely and repeatably in the final product.
よくある質問
What is thermal pad compression ratio?
Thermal pad compression ratio is the percentage reduction in pad thickness after assembly between a heat source and cooling surface.
How do you calculate thermal pad compression ratio?
Use the formula: compression ratio = (original pad thickness – compressed thickness) / original pad thickness × 100%.
How much thermal pad compression is enough?
Enough compression means the pad fully contacts both surfaces and reduces contact resistance without causing excessive mechanical stress.
Is more thermal pad compression always better?
No. More compression may improve contact, but excessive compression can bend PCBs, crack components, deform housings, or reduce dielectric spacing.
What happens if a thermal pad is not compressed enough?
The pad may leave air gaps, increasing contact resistance and component temperature.
What happens if a thermal pad is over-compressed?
Over-compression can cause mechanical stress, pad extrusion, compression set, and reduced electrical insulation margin.
Does thermal pad hardness affect compression ratio?
Yes. Softer pads compress more easily, while harder pads require more force to reach the same compression ratio.
Should compression be calculated at nominal gap only?
No. Engineers should calculate compression at minimum, nominal, and maximum gap conditions.
Does compression affect thermal impedance?
Yes. Proper compression usually reduces thermal impedance by improving contact and reducing bond line thickness.
What data should I provide to select the right compression ratio?
Provide gap range, pad thickness target, contact area, pressure limit, heat load, component fragility, electrical insulation requirements, and reliability conditions.
