phase change overview

Phase change materials: 101 — A quick overview

What are Phase change materials?

Phase change mate­ri­als (PCM) are sil­i­cone free pads and sten­cil print­able pastes that are typ­i­cal­ly used as matrix mate­ri­als for ther­mal inter­face appli­ca­tions.

The mag­ic of Phase change mate­ri­als is that they change phase (form) at 45°C from sol­id to gel like. In this state and with the help of some pres­sure, they can squeeze into any gap and severe­ly warped design and fill it in with their ther­mal­ly con­duc­tive good­ness. This flex­i­bil­i­ty also allows for very thin bond lines that after this one time appli­ca­tion step ensure a low Ther­mal imped­ance and amaz­ing ther­mal prop­er­ties.

They exhib­it effec­tive wet­ting prop­er­ties dur­ing typ­i­cal oper­at­ing tem­per­a­ture ranges, result­ing in very low sur­face con­tact resis­tance. With a break­down tem­per­a­ture of ~180°C they pro­vide supe­ri­or reli­a­bil­i­ty and main­tain low ther­mal imped­ance, mak­ing PCM desir­able for high-per­for­mance inte­grat­ed cir­cuit devices. Typ­i­cal oper­at­ing tem­per­a­ture range is ‑40 to 125°C that can go much high­er depend­ing on the spe­cif­ic prod­uct. PTM6000-HV for exam­ple that has been designed for IGBT Hon­ey­comb struc­tures offers great ther­mal sta­bil­i­ty after var­i­ous long-term reli­a­bil­i­ty tests includ­ing HAST 192hrs, T/C‑B 4000x and High Tem­per­a­ture Bak­ing at 3000hrs.

Ther­mal imped­ance remains sta­ble across accel­er­at­ed aging tests with­out any bleed out, pump out or slow out thanks to the steric hin­drance and the branch­ing chem­i­cal bond struc­ture of PCM.

Installing Phase change materials

PCM are TIM2 prod­ucts that require a set­up stage before they exhib­it their final prop­er­ties. Their ini­tial thick­ness­eses are 0.2, 0.25 or 0.3mm and these should cov­er even the most warped designs. 10–20psi are enough to achieve the 30–40um sweet spot. But how do we get to that?

These mate­ri­als need to be clamped down under pres­sure. Spring loaded screws are typ­i­cal­ly used to main­tain the pres­sure. Once the mate­r­i­al under­goes the phase change phase and spreads out, it needs sus­tained pres­sure to achieve the final prop­er­ties.

At time zero, Phase change mate­ri­als are sol­id. They start soft­en­ing above 45°C and our sug­gest­ed tem­per­a­ture is 60°C so that the mate­ri­als are soft enough.

At this point you clamp the mate­ri­als with 20–30psi pres­sure for around 30 min­utes. The soft mate­r­i­al melts, spreads and wets on the heat sink and on the heat com­po­nent, reduc­ing the inter­fa­cial resis­tance. The ini­tial 0.2mm slow­ly goes down to the sweet spot of 30–40um where the prod­uct sees the opti­mal and low­est Ther­mal imped­ance.

The set­up phase (change) needs to be done for at least one cycle. It can hap­pen after mount­ing or even as part of a func­tion­al test and even though it is rec­om­mend­ed (espe­cial­ly if you want to heat cycle imme­di­ate­ly), it does­n’t have to be a sep­a­rate step. For more infor­ma­tion about sten­cil print­ing and the instal­la­tion process, take a look at our Phase change Appli­ca­tion notes.

Typical Properties and limitations

We already men­tioned that steric hin­drance and the branch­ing chem­i­cal bond struc­ture help with elim­i­nat­ing bleed out and pump issues.

Slump­ing and drip­ping on the oth­er hand can be a prob­lem for ver­ti­cal appli­ca­tions that see high oper­at­ing tem­per­a­tures. Nor­mal­ly we do not expect such issues for 70–80°C but they are to be expect­ed when we use PCM in i.e. 120°C. In these high temp ver­ti­cal appli­ca­tions, ther­mal gap pads, ther­mal put­ty pads and two part hybrids are much more suit­able solu­tions. Let’s stress this a third time. Ver­ti­cal appli­ca­tions. This issue does not apply in hor­i­zon­tal appli­ca­tions.

Anoth­er com­mon mis­con­cep­tion is adhe­sion. Phase change mate­ri­als are not designed for struc­tur­al strength. Peri­od. They are there to pro­vide flex­i­bil­i­ty and ther­mal con­duc­tiv­i­ty. They are rework­able by design so it goes with­out say­ing that they don’t exhib­it good adhe­sion strength. If you run them at high bond line thick­ness­es for adhe­sion and reli­a­bil­i­ty, you are doing it wrong.


Why should I choose PCM instead of Thermal Grease?

Phase change mate­ri­als are hands down the best mate­ri­als you can use as ther­mal inter­face if the appli­ca­tion para­me­ters allow it. They are clean, efficient,can achieve the thinnest bond­lines out of any oth­er mate­ri­als and are sil­i­cone free. Pow­er cycling? Phase change can eas­i­ly han­dle it while ther­mal grease can be pushed under the chip after a few hun­dred cycles. Grease does­n’t work for high pow­er cycling and we have seen that 300–500W chips can’t work with Grease. At these pow­er ranges noth­ing works prop­er­ly except for Phase change mate­ri­als.

On this note we should say that Phase change mate­ri­als are designed for post reflow oper­a­tions. They are not meant to be used at 245–260°C.

Typ­i­cal phase change mate­r­i­al Appli­ca­tions include and are not lim­it­ed to:

  • Pow­er con­trol units, invert­ers, onboard elec­tron­ics, IGBT
  • Servers, super­com­put­ing, video graph­ic array (VGA) cards, AI, GPU/CPU/Desktop, sol­id state dri­ves (SSD)
  • Switch­es, routers, base sta­tions
  • Tablets, gam­ing, note­books, smart­phones, action cam­eras & light­ing

Com­pared to Sil­i­cone grease, PCM have longer mol­e­c­u­lar chains with high mol­e­c­u­lar weight and sur­face ten­sion ensur­ing a robust poly­mer struc­ture and a sta­ble filler-poly­mer matrix. The H steric hin­drance pro­vides a Rigid struc­ture that ensures low filler migra­tion and sep­a­ra­tion that at the same time lim­its the mate­r­i­al mobil­i­ty and pump out.

Does this make ther­mal grease bad? Def­i­nite­ly not. Grease is great for all the oth­er appli­ca­tion types that PCM can’t cov­er due to the tem­per­a­ture restric­tions. To be fair, TG 3000I boasts a 7 micron bond line thick­ness. That’s prob­a­bly the thinnest BLT you’ll be able to get out of any ther­mal inter­face mate­r­i­al.

Conclusions

To sum up, phase change mate­ri­als should be your go to choice if your appli­ca­tions allows it. Thin BLT, low ther­mal imped­ance, great wet­ting and flex­i­bil­i­ty, no bleed out, pump out or blow out. The works.

Con­tact us with your appli­ca­tion require­ments and we’ll work togeth­er to find the best ther­mal inter­face mate­r­i­al for your appli­ca­tion.

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About George Kountardas

George is a Jack of all trades with an unappeasable inquiring mind. Obsessed with new products and technologies, he is always pushing forward for better, faster and more efficient applications. Always learning something new.

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