Heat dissipation and cooling solutions — Hinglish notes

6.4.5 · Hardware › Power, Thermal & Reliability

Heat transfer ki physics

Heat teen mechanisms ke through hot se cold ki taraf move karti hai:

Conduction: heat solid materials ke through flow karti hai (chip → heatsink)
Convection: moving fluid (air/liquid) heat ko carry karke le jaata hai (heatsink → environment)
Radiation: electromagnetic waves heat carry karti hain (<100°C par minor, space mein dominate karti hai)

Electronics ke liye, conduction + convection dominate karte hain. Radiation typical operating temperatures par <5% contribute karti hai.

Formula Fourier's Law (conduction)

Kisi material ke through heat flow: $Q = \frac{k \cdot A \cdot Δ T}{d}$

$Q$ = heat flow rate (watts)
$k$ = thermal conductivity (W/m·K) — material property
$A$ = cross-sectional area (m²)
$d$ = thickness (m)
$Δ T$ = material ke across temperature difference

First principles se derivation: Heat temperature gradients ke neeche diffuse karti hai. 1D steady state mein, heat flux (W/m²) temperature gradient ke proportional hoti hai: $q = - k \frac{d T}{d x}$ . Constant $k$ wale thickness $d$ ke slab ke liye, integrate karo: $q = - k \frac{T _{2} - T _{1}}{d} = k \frac{Δ T}{d}$ Total heat flow flux times area hoti hai: $Q = q \cdot A = \frac{k \cdot A \cdot Δ T}{d}$ .

Yeh humein kya batata hai:

Area double karo → heat flow double hogi (isliye heatsinks mein fins hote hain)
Thickness double karo → heat flow aadhi hogi (isliye thermal paste얇얇얇얇 얇thin hota hai)
High $k$ materials (copper: 400 W/m·K, aluminum: 205 W/m·K) heat zyada fast move karte hain

Fourier's law se thermal resistance: $R_{t h, co n d} = \frac{d}{k \cdot A}$

Convection se thermal resistance: $R_{t h, co n v} = \frac{1}{h \cdot A}$

Total thermal resistance: series mein resistances add hote hain. $R_{t h, t o t a l} = R_{j u n c t i o n - c a se} + R_{c a se - h e a t s ink} + R_{h e a t s ink - ambi e n t}$

Figure — Heat dissipation and cooling solutions

Cooling solution categories

1. Passive cooling (no moving parts)

Heatsinks component se heat ko fins mein conduct karte hain jo natural convection ke liye surface area badhate hain.

Design elements:

Material: Copper ( $k = 400$ W/m·K) ya aluminum ( $k = 205$ W/m·K). Copper thermally 2× behtar hai lekin 3× heavy aur costlier hai. Aluminum standard hai; extreme cases ke liye copper.
Fin geometry: Thin, tall fins area maximize karte hain lekin airflow restrict karte hain. Natural convection ke liye optimal spacing ~2–3 mm, forced ke liye ~1–2 mm hai.
Surface finish: Black anodizing radiation badhata hai (abhi bhi minor), oxidation rokta hai

Pros: Silent, reliable (fail hone ke liye koi fans nahi), low cost Cons: Airflow ke bina ~30–50 W tak limited, zyada power ke liye large/heavy

2. Active air cooling

Fans add karta hai airflow force karne ke liye, $h$ ko ~8 se ~60–100 W/m²·K tak badhata hai.

Fan types:

Axial: standard case fans, air ko shaft ke parallel move karte hain, high flow rate
Centrifugal (blower): air ko radially move karte hain, restricted spaces (laptops) ke liye high static pressure

Performance factors:

CFM (cubic feet per minute): volumetric flow rate. Higher CFM = zyada air = zyada heat removal (agar heatsink surface saturate nahi hui hai)
Static pressure: resistance ke through push karne ki ability (dense heatsink fins). mmH₂O ya Pa mein measure kiya jaata hai.
RPM vs noise: fan noise $\propto$ (RPM)⁵ approximately. Speed double karne se noise ~32 dB badhti hai. PWM (pulse width modulation) control variable speed allow karta hai.

Thermal interface materials (TIM): Chip aur heatsink ke beech, microscopic air gaps (terrible conductor: $k \approx 0.025$ W/m·K) resistance dominate karte hain. Thermal paste gaps fill karta hai:

Stock paste: $k \approx 3 - 5$ W/m·K, $R_{t h} \approx 0.15$ °C/W (thin layer)
High-end paste: $k \approx 8 - 12$ W/m·K, $R_{t h} \approx 0.05$ °C/W
Liquid metal: $k \approx 70$ W/m·K, $R_{t h} \approx 0.01$ °C/W (conductive, girne par risky)

Application: Thin layer (matar ke daane jitni drop ya thin spread). Excess paste increase karta hai resistance thickness $d$ badhane se, gaps fill karne se zyada.

3. Liquid cooling

Fluid (water, glycol mix) chip par cold plate ke through circulate karta hai, heat ko radiator tak carry karta hai jise fans cool karte hain.

Water kyun? Specific heat capacity $c_{p} = 4.18$ kJ/kg·K (air se 4×), density 1000 kg/m³ (air se 800×). Volume ke per bahut zyada heat carry kar sakta hai.

Components:

Cold plate: channels wala metal block, heatsink ki tarah mount hota hai. Water contact deta hai $h \approx 1000 - 10000$ W/m²·K vs air ka 60.
Pump: fluid circulate karta hai. Flow rate ~1–2 L/min typical.
Radiator: liquid ke liye heatsink. Multiple fans, large area.
Reservoir: expansion tank, loop fill karta hai

AIO (all-in-one): sealed unit, pre-filled, no maintenance. Custom loops behtar performance allow karte hain lekin filling chahiye, leak risk hota hai.

Thermal performance: Liquid cooling ka advantage transport capacity hai, surface heat transfer nahi. Radiator abhi bhi air convection use karta hai (same $h$ ), lekin chip se door bade area mein distributed hota hai.

Total resistance: $R_{t h} = R_{c hi p - w a t er} + R_{w a t er - r a d ia t or} + R_{r a d ia t or - ai r}$

Typically $R_{c hi p - w a t er} ≪ R_{r a d ia t or - ai r}$ cold plate par high $h$ ki wajah se. Radiator bottleneck ban jaata hai, isliye bada radiator area matter karta hai.

Pros: 200–400+ W handle karta hai, quieter (bade, slower fans), flexible placement Cons: Cost ( $100-$ 400), complexity, pump/leak failure modes, <150 W loads ke liye top-tier air se thoda worse

4. Exotic solutions

Phase-change cooling: Refrigerant cold plate par evaporate karta hai (latent heat absorb karke), radiator par condense karta hai. Refrigerator ki tarah. Sub-ambient temperatures tak pahunch sakta hai (condensation risk). Extreme overclocking mein use hota hai.

Direct-die cooling: Heatspreader remove karo, cooler directly silicon par apply karo. Ek thermal resistance layer eliminate hoti hai, ~10–15°C gain milta hai. Warranty void hoti hai.

Immersion cooling: Poora system dielectric fluid mein submerge karo. Datacenters mein density aur efficiency ke liye use hota hai. Consumer ke liye practical nahi.

Recall Ek 12 saal ke bachche ko explain karo

Tumhare computer ka CPU ek tiny oven ki tarah hai jo kabhi band nahi hota. Yeh jab bhi kaam karta hai heat banata hai, aur yeh hamesha kaam karta rehta hai. Agar hum woh heat nahi hatate, chip garm se garmtar hoti jaati hai jab tak toot na jaye (ya khud ko tootne se bachane ke liye band na ho jaye).

Toh heat se kaise chhutkaara paate hain? Teen tarike:

Conduction (jaise hot soup mein dhatu ka chammach): heat solid cheezoon se travel karti hai. Hum CPU par dhatu ka bada tukda (heatsink) chipkate hain. Heat hot chip se cooler metal mein flow karti hai.
Convection (jaise hot cocoa par phoonkna): moving air heat ko carry karke le jaati hai. Heatsink mein bahut saari thin metal "fins" hoti hain taaki air ko heat churane ke liye zyada jagah mile. Ek fan in fins ke past air blow karta hai, isse speed up karta hai.
Radiation (jaise sun ka tumhara chehra garam karna): hot cheezein invisible light emit karti hain jo heat carry karti hai. Computer chips ke liye yeh barely matter karta hai kyunki woh itni hot nahi hoti (maan lo, stove burner ki tulna mein).

Aise socho: chip ek nala (faucet) hai jo ek bucket (chip khud) mein paani (heat) tapka raha hai. Agar hum bucket drain nahi karte, woh overflow ho jaati hai (overheats). Heatsink aur fan drain hain — woh paani (heat) ko utni hi tezi se escape karne dete hain jitni tezi se aata hai. Bada drain (behtar cooler) ka matlab hai bucket kam bhari rehti hai (chip cooler rehti hai).

Kuch coolers air ki jagah water use karte hain! Woh chip par metal plate ke through water pump karte hain, water heat soak up karti hai, phir woh ek bade radiator (car radiator ki tarah) par flow karti hai jisme fans lage hote hain. Water air se heat zyada better carry karta hai, isliye yeh bahut hot chips ke liye kaam karta hai.

Key formulas summary

Quantity	Formula	Units
Thermal resistance	$R_{t h} = \frac{Δ T}{P}$	°C/W or K/W
Conduction	$Q = \frac{k \cdot A \cdot Δ T}{d}$	W
Conduction resistance	$R_{t h} = \frac{d}{k \cdot A}$	°C/W
Convection	$Q = h \cdot A \cdot Δ T$	W
Convection resistance	$R_{t h} = \frac{1}{h \cdot A}$	°C/W
Junction temperature	$T_{j} = T_{amb} + P \cdot R_{t h, t o t a l}$	°C

Connections

Thermal Design Power (TDP) — worst-case cooling requirements define karta hai
CPU architecture and performance — higher clock = zyada power = zyada heat
Power supply efficiency — inefficiency PSU mein heat ban jaati hai, cooling chahiye
Case airflow and positive/negative pressure — system-level cooling strategy
Thermal throttling — cooling fail hone par automatic slowdown
Overclocking — power aur heat ko stock se zyada push karta hai
Heat pipes — phase change use karne wali advanced heatsink tech
PCB thermal vias — surface-mount components se heat conduct karke dur le jaana
Reliability and MTBF — high temperatures failure accelerate karte hain (Arrhenius equation)

#flashcards/hardware

Teen heat transfer mechanisms kya hain? :: Conduction (solids ke through), convection (moving fluids ke zariye), aur radiation (electromagnetic waves). Electronics ke liye, conduction + convection dominate karte hain; radiation 100°C se neeche <5% hai.

Thermal resistance define karo aur uski units batao :: Thermal resistance $R_{t h}$ heat flow ke resistance ko measure karta hai, $R_{t h} = Δ T / P$ se calculate hota hai, jahan $Δ T$ temperature difference hai aur $P$ power hai. Units: °C/W ya K/W. Kam ho toh behtar.

Fourier's Law of conduction state karo

Q = \frac{k \cdot A \cdot Δ T}{d}

, jahan

Q

heat flow (W) hai,

k

thermal conductivity hai,

A

area hai,

d

thickness hai,

Δ T

temperature difference hai. Heat flow area aur conductivity ke saath badhti hai, thickness ke saath ghatati hai.

Copper vs aluminum ki thermal conductivity kya hai?

Copper: ~400 W/m·K, Aluminum: ~205 W/m·K. Copper heat 2× better conduct karta hai lekin heavy aur zyada expensive hai.

Newton's Law of Cooling state karo

Q = h \cdot A \cdot Δ T

, jahan

h

heat transfer coefficient (W/m²·K) hai,

A

surface area hai,

Δ T

temperature difference hai.

h

fluid type aur flow velocity par depend karta hai.

Air mein natural convection ke liye typical heat transfer coefficient kya hai?

h \approx 5-10

W/m²·K (still air, koi fan nahi)

Forced air convection ke liye typical heat transfer coefficient kya hai?

h \approx 50-100

W/m²·K (fan ke saath, ~2 m/s airflow)

Water cooling ke liye typical heat transfer coefficient kya hai?

h \approx 500-10000

W/m²·K (liquid ka

h

air se bahut zyada hota hai)

Series mein thermal resistances kaise combine hote hain?

Woh add hote hain:

R_{t h, t o t a l} = R_{1} + R_{2} + R_{3} + ..

. Heat sequentially har resistance se flow karti hai, jaise electrical resistors series mein.

Conduction se thermal resistance ka formula

R_{t h, co n d} = \frac{d}{k \cdot A}

, jahan

d

thickness hai,

k

thermal conductivity hai,

A

area hai. Thinner aur bada area resistance reduce karta hai.

Convection se thermal resistance ka formula

R_{t h, co n v} = \frac{1}{h \cdot A}

, jahan

h

heat transfer coefficient hai,

A

surface area hai. Bada area ya higher

h

(faster flow) resistance reduce karta hai.

Heatsinks fins ke saath kyun banaye jaate hain?

Fins surface area

A

badhate hain, jo convection thermal resistance

R_{t h} = 1/ (h \cdot A)

reduce karta hai. Zyada area air ko zyada heat transfer allow karta hai.

Thermal paste kyun zaroori hai?

Chip aur heatsink ke beech microscopic air gaps ki thermal conductivity bahut kharab hoti hai (

k \approx 0.025

W/m·K). Thermal paste (

k \approx 3-12

W/m·K) gaps fill karta hai, contact resistance kam karta hai.

Bahut zyada thermal paste use karne par kya hota hai?

Excess paste thickness

d

badhata hai. Kyunki

R_{t h} = d / (k \cdot A)

aur paste ki

k

metal se kam hai, mota paste layer resistance add karta hai. Ideal hai thin layer (~0.1 mm).

Fan cooling ko itna dramatically kyun improve karta hai?

Fan airflow velocity badhata hai, jo heat transfer coefficient

h

ko ~8 (natural convection) se ~60–100 (forced) tak badhata hai. Isse thermal resistance mein ~7–8× reduction milta hai.

Air vs thermal paste vs copper ki thermal conductivity compare karo :: Air: ~0.025 W/m·K (terrible insulator), thermal paste: 3–12 W/m·K (gap filler), copper: 400 W/m·K (excellent conductor). Air aur copper ke beech 16000× ka difference!

Junction temperature formula calculate karo

T_{j} = T_{ambi e n t} + P \cdot R_{t h, t o t a l}

, jahan

P

dissipated power hai,

R_{t h, t o t a l}

junction se ambient tak total thermal resistance hai.

AIO liquid cooler kya hota hai?

All-In-One: cold plate, pump, radiator, aur pre-filled coolant wala sealed liquid cooling system. Koi maintenance nahi, custom loop se aasaan, air se zyada expensive.

Liquid cooling air se zyada power kyun handle karta hai?

Water ki high specific heat (4.18 kJ/kg·K) aur density (1000 kg/m³) use heat ko chip se bade radiator area tak efficiently transport karne deti hai. Cold plate ka bhi bahut high

h

hota hai (~1000–10000 vs 60 W/m²·K).

PWM fan control kya hai?

Pulse Width Modulation: power signal ka duty cycle change karke fan speed vary karta hai. Noise/cooling balance ke liye dynamic speed adjustment allow karta hai. Kam speed = quieter lekin kam cooling.

Heat pipe kya hai aur yeh kaise kaam karta hai?

Wick aur working fluid (water) wali sealed copper tube. Fluid hot end par evaporate karta hai (latent heat absorb karke), vapor cold end par flow karta hai, condense hota hai (heat release karke), liquid wick ke zariye return karta hai. Bahut high effective thermal conductivity (~5000–20000 W/m·K).