Heat dissipation and cooling solutions: Visual walkthrough

6.4.5 · D2 · Hardware › Power, Thermal & Reliability › Heat dissipation and cooling solutions

Step 1 — Heat ek flow hai, aur temperature ek height hai

KYA. Kisi bhi symbol se pehle, picture dekho. Left side par, chip ek tap hai jo paani daal rahi hai. Tank mein paani ka level dikhata hai ki chip kitni hot hai. Neeche ek pipe hai jo paani bahar nikaalti hai — woh pipe cooling path hai.

YEH PICTURE KYUN. Heat aur paani ek hi bookkeeping rule follow karte hain: jo flow in hota hai woh flow out hona chahiye, warna level rise karega. Kuch accumulate nahi hota jab tak inflow outflow se zyada na ho. Yeh woh ek idea hai jis par poora page tika hai, isliye hum koi equation likhne se pehle ise draw karte hain.

PICTURE.

Figure — Heat dissipation and cooling solutions

Picture padho aur teen cheezein naam do:

Paani tank se bahar kyun nikalta hai? Kyunki tank ka level usse zyada upar hai jahan woh drain ho raha hai. Height ka difference flow drive karta hai. Yeh yaad rakho — yeh Step 2 hai.

Step 2 — Flow ke liye difference chahiye, absolute value nahi

KYA. Do tanks, same drainpipe. Tank A drain se 90 units upar hai; Tank B 20 units upar hai. A tez drain hota hai. Jo number matter karta hai woh hai gap, na ki absolute level.

KYUN. Heat sirf hot se cold ki taraf move karti hai. Agar chip aur air same temperature par hote, koi heat flow nahi hoti chahe dono kitne bhi hot kyun na hon. Toh driver ek difference hai. Hum ise ek naam dete hain.

PICTURE.

Isliye hi parent note temperatures ko hamesha ambient ke relative measure karta hai. Ground level (examples mein 25 °C ya 30 °C) woh jagah hai jahan se hum height count karna shuru karte hain.

Step 3 — Pipe ki narrowness ko naam do: thermal resistance

KYA. Same gap $Δ T$ , do alag pipes. Moti pipe bahut paani drain karti hai; patli pipe trickle drain karti hai. Hum ek number invent karte hain jo capture kare "yeh pipe flow ko kitna rokti hai."

YEH TOOL KYUN, KUCH AUR KYUN NAHI. Hum pipe ko uski width, length, aur roughness se alag-alag describe kar sakte the — lekin tank ke behaviour ke liye sirf ek combined cheez matter karti hai: ek given flow push karne ke liye kitna height-gap chahiye. Woh single number hai thermal resistance. Hum ise choose karte hain (rather than "conductance") kyunki engineers spec sheets ko °C-per-watt mein quote karte hain, aur kyunki chained resistances simply add hoti hain, jo hum Step 5 mein prove karte hain.

PICTURE.

Step 4 — Real cooling mein kai pipes ek row mein hoti hain

KYA. CPU se nikalne wali heat ek hop nahi leti. Woh silicon se package lid tak crawl karti hai (ek pipe), phir thermal paste ki smear se (doosri pipe), phir heatsink se air mein (teesri pipe). Inhe end to end draw karo.

KYUN. Har material ya gap ek alag obstacle hai, aur heat ko sab se ek-ek karke survive karna hai escape karne ke liye. "Ek-ek karke" key words hain — yeh Step 5 mein ek specific rule force karta hai.

PICTURE.

Same heat flow $P$ har link se guzarti hai — sides se kuch nahi nikalta. Yeh ek hi fact hai jo humein unhe add karne deta hai.

Step 5 — Resistances add kyun hoti hain

KYA. Pehle do-link case lo (case + paste), phir generalize karo. Kyunki same $P$ dono se guzarta hai, har link temperature ko apne $Δ T$ se raise karta hai, aur total rise sirf rises ka sum hai.

KYUN. Yeh Step 3 mein resistance choose karne ka payoff hai. Dekho har link ka rise staircase ki steps ki tarah stack karta hai.

PICTURE.

Link by link, Step 3 ka $Δ T = P R_{t h}$ use karke:

$Δ T_{1} = P \cdot R_{j c} Δ T_{2} = P \cdot R_{cs} Δ T_{3} = P \cdot R_{s a}$

$Δ T_{1}$ — silicon-to-lid link par drop.
$Δ T_{2}$ — paste par drop.
$Δ T_{3}$ — heatsink par drop.

Staircase add karo, kyunki air se junction tak total climb har step ka sum hai:

$Δ T_{t o t a l} = Δ T_{1} + Δ T_{2} + Δ T_{3} = P (R_{j c} + R_{cs} + R_{s a})$

Shared $P$ factor out karo (yeh tabhi legal hai jab yeh same flow har ek se guzre — Step 4):

Step 6 — Junction temperature ko ek saath put karo

KYA. Humhare paas total pipe hai. Ab pata karo ki transistors actually kitne hot hote hain.

KYUN. Yeh number decide karta hai ki chip khush hai, throttle karti hai, ya marr jaati hai.

PICTURE.

Ground (ambient) se shuru karo, total rise climb karo:

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

$T_{ambi e n t}$ — drain height, jahan se hum count karna shuru karte hain (e.g. 25 °C).
$P$ — heat pour in ki gayi ( = heat flow out, Step 1).
$R_{t h, t o t a l}$ — poori chain ki summed stubbornness (Step 5).
$T_{j}$ — junction temperature, answer.

Step 7 — Edge cases: jahan picture bend hoti hai lekin tooti nahi

KYA. Chaar scenarios jo logon ko trip karte hain. Har ek wahi tank hai jिसका ek knob extreme par turn kiya gaya hai.

PICTURE.

Definition Chaar extremes

Zero power ( $P = 0$ ): tap band. $Δ T = 0$ , isliye $T_{j} = T_{ambi e n t}$ . Chip room temperature par baithti hai. Koi flow nahi toh koi gap ki zaroorat nahi.
Zero resistance ( $R_{t h, t o t a l} \to 0$ ): perfect pipe. Koi bhi $P$ bina kisi height gap ke drain hoti hai, isliye $T_{j} = T_{ambi e n t}$ hamesha. Reality mein impossible, lekin yeh ideal cooler hai.
Infinite resistance ( $R_{t h, t o t a l} \to \infty$ ): pipe blocked (fan mar gaya, dried-out paste). Koi bhi $P > 0$ $Δ T \to \infty$ bana deta hai — level bina limit ke climb karta hai jab tak chip cook na ho jaaye. Isliye hi ek ruka hua fan hardware destroy karta hai (Reliability and MTBF).
Hotter ambient ( $T_{ambi e n t}$ upar): ground raise karo, poora tank iske saath rise karta hai. Same $Δ T$ , zyada $T_{j}$ . 40 °C par datacentre 20 °C wale se kam headroom deta hai — case airflow exactly isi se ladhti hai.

Ek-picture summary

Is page par har cheez ek water tank hai jo ek row mein pipes se drain hota hai: flow tap se set hoti hai ( $P$ ), height-gap se bahar push hoti hai ( $Δ T$ ), aisi pipes se jिनकी stubbornness ( $R_{t h}$ ) add hoti hai kyunki same flow har ek se ek-ek karke guzarti hai. Tank ka ground se upar level junction temperature hai.

Recall Feynman retelling — ise apne plain words mein wapas bolo

Socho ek bucket hai jise chip paani se bhar rahi hai. Kitna tez bhar raha hai woh power hai. Paani ka level temperature hai. Neeche ek drainpipe hai jo floor tak jaati hai — floor room ka temperature hai. Paani sirf isliye drain hota hai kyunki level floor se zyada upar hai, isliye jo actually matter karta hai woh level aur floor ke beech ka gap hai, level akela nahi. Patli, stubborn pipe ko tez drain karne ke liye bada gap chahiye; moti pipe ko kum hi chahiye. Woh stubbornness thermal resistance hai. Real cooling kai pipes hai jo end to end judi hain — chip, paste, heatsink — aur kyunki same paani har ek se guzarna chahiye, unki stubbornness simply add ho jaati hai. Total stubbornness ko kitni tez tum bhar rahe ho se multiply karo, aur wahi hai kitna upar floor se paani baitha hai — chip ka temperature. Tap band karo aur woh floor par baitha hai. Pipe block karo aur woh overflow ho jaata hai. Poora floor raise karo (hot room) aur sab kuch iske saath rise karta hai. Ek bucket mein yahi poora chapter hai.

Recall Quick self-test

$Δ T$ ko words mein define karo. ::: Hot surface aur cool surroundings ke beech ka temperature gap; woh cheez jo actually heat flow drive karti hai. Series thermal resistances kyun add hoti hain? ::: Kyunki same heat flow har link se ek-ek karke guzarti hai, isliye har ek apna rise $P R_{t h}$ contribute karta hai aur rises stack ho jaate hain. Ek CPU 100 W dissipate karta hai total $R_{t h} = 0.6$ °C/W se 25 °C air mein. $T_{j}$ kya hai? ::: $T_{j} = 25 + 100 \times 0.6 = 85$ °C. Fan stop ho jaaye toh $T_{j}$ ka kya hoga ( $R_{s a} \to \infty$ )? ::: $Δ T \to \infty$ ; junction temperature bina bound ke climb karta hai jab tak throttling ya failure na ho. Kya hotter room $Δ T$ change karta hai? ::: Nahi — yeh $T_{j}$ ko same amount se raise karta hai lekin gap fixed rakhta hai (gap $P$ aur $R_{t h}$ par depend karta hai).

Related: Thermal Design Power (TDP) sets the $P$ you must plan for · Heat pipes aur copper $R_{s a}$ lower karte hain · Overclocking $P$ raise karta hai aur $T_{j}$ ko throttle line ki taraf push karta hai · Power supply efficiency aur CPU architecture and performance decide karte hain ki pehli jagah kitni heat banti hai.