Phase Change Material Failure Mechanisms

As dis­cussed in our pre­vi­ous blog arti­cle, Reli­a­bil­i­ty Tests Of Ther­mal Inter­face Mate­r­i­al In Pow­er Mod­ule Appli­ca­tions, there are sev­er­al typ­i­cal reli­a­bil­i­ty tests that PCM needs to pass in pow­er mod­ule appli­ca­tions, such as ther­mal cycling test (TCT), high-tem­per­a­ture stor­age (HTS), HAST or dou­ble 85 tests, etc. Although the test con­di­tions dif­fer from each oth­er to sim­u­late var­i­ous oper­at­ing and envi­ron­men­tal stress for devices, the mech­a­nisms of Phase change mate­r­i­al fail­ure are similar. 

There are sev­er­al fac­tors that could cause PCM to fail in pow­er inverters:

High Temperature

PCM is designed to oper­ate with­in a spe­cif­ic tem­per­a­ture range, and if the pow­er invert­er exceeds this range, the PCM may become inef­fec­tive or even fail. At high tem­per­a­tures, the chem­i­cal struc­ture of the PCM can break down. Anoth­er effect of high-tem­per­a­ture bake tests on PCM is that they can cause mechan­i­cal stress on the PCM. It’s worth not­ing that the spe­cif­ic effects of high-tem­per­a­ture bake tests on PCM will depend on the type of PCM being used and the con­di­tions of the test. For exam­ple, some paraf­fin-based PCMs can with­stand tem­per­a­tures up to 150°C and some salt hydrates-based PCMs can with­stand tem­per­a­tures up to 180°C.

Mechanical Stress

The repeat­ed expan­sion and con­trac­tion of the PCM due to tem­per­a­ture changes can cause mechan­i­cal stress on the PCM, which can lead to crack­ing and fail­ure. Addi­tion­al­ly repeat­ed ther­mal cycling can cause mechan­i­cal stress on the PCM lay­er, which can lead to leak­age, pump out, bleed out, or oth­er fail­ures. This can cause the PCM to no longer be in con­tact with the heat-gen­er­at­ing com­po­nents or the heat sink. To pre­vent mechan­i­cal stress, it’s impor­tant to choose PCMs that have a high ther­mal expan­sion coef­fi­cient and low mod­u­lus of elas­tic­i­ty, which will help to reduce the stress caused by tem­per­a­ture changes. 

It’s also impor­tant to design and test the pow­er invert­er to ensure that the PCM is prop­er­ly assem­bled and that it is com­pressed cor­rect­ly to make good con­tact with the heat-gen­er­at­ing com­po­nents and the heat sink. 

Chemical Degradation

While PCM is being applied to the com­po­nents, there are many envi­ron­men­tal fac­tors play­ing a role in PCM chem­i­cal degra­da­tion and fur­ther result­ing in failures. 

• Cor­ro­sion: PCM can be sen­si­tive to cer­tain types of cor­ro­sion, such as oxi­da­tion, which can occur when the PCM comes into con­tact with cer­tain met­als, such as alu­minum or cop­per. This can result in the for­ma­tion of an oxide lay­er on the sur­face of the PCM, caus­ing the PCM to degrade over time.
• Con­t­a­m­i­na­tion: Dust, oil, or oth­er chem­i­cals, can reduce PCM’s ther­mal con­duc­tiv­i­ty and effectiveness.
• Com­pat­i­bil­i­ty: PCM can be sen­si­tive to cer­tain types of mate­ri­als, such as cer­tain types of ther­mal grease or oth­er ther­mal inter­face mate­ri­als, which can cause chem­i­cal reac­tions that can degrade the PCM over time.
• Hydra­tion: Hydra­tion of PCM refers to the absorp­tion of water by the PCM, which can occur if the pow­er invert­er is oper­at­ed in humid envi­ron­ments or dur­ing the ship­ment and stor­age of PCM. It can cause the chem­i­cal struc­ture of the PCM to change, lead­ing to a reduc­tion in its ther­mal con­duc­tiv­i­ty. Besides, the absorbed water can cause the PCM to swell, which can cause mechan­i­cal stress on the PCM. Fur­ther­more, the water can cause the PCM to freeze at low­er tem­per­a­tures, which can cause the PCM to crack and fail. If the PCM is stored under a humid envi­ron­ment (>65%RH), the water can cause the PCM to increase its vis­cos­i­ty, which can make it more dif­fi­cult for the PCM to flow. It’s also worth not­ing that hydra­tion can cause the PCM to cor­rode and react with oth­er mate­ri­als in the invert­er, such as the met­al of the heat sink.

To pre­vent the hydra­tion of PCM, it’s impor­tant to choose PCMs that are hydropho­bic and keep the pow­er invert­er in a dry envi­ron­ment, or use pro­tec­tive coat­ings on the PCM to reduce the effects of water. Addi­tion­al­ly, it’s impor­tant to mon­i­tor the per­for­mance of the PCM over time and replace it if necessary.

Cyclic loading

Repeat­ed expo­sure to tem­per­a­ture changes can cause the PCM to degrade over time. As the PCM is repeat­ed­ly heat­ed and cooled, its chem­i­cal struc­ture can begin to break down, reduc­ing its ther­mal con­duc­tiv­i­ty and effectiveness.

Time Aging

PCM has a lim­it­ed life span, and if it is used beyond its rec­om­mend­ed life­time, it may fail. All of the above are aging factors.

Setup Issues

There are also some set­up issues that could also result in failure:

1. Leak­age: PCM is a liq­uid at cer­tain tem­per­a­tures, and if it leaks out of its con­tain­er or encap­su­la­tion, it may not be able to func­tion prop­er­ly. Thus, PCM is more suit­able for a hor­i­zon­tal installed position.
2. Com­pres­sion: PCM needs enough pres­sure (40 psi) to make good con­tact with the heat-gen­er­at­ing com­po­nents and the heat sink. This pres­sure also helps PCM to spread well and min­i­mize the air gap dur­ing the phase change stage. 
3. Over­heat­ing Spots: If the pow­er invert­er is not designed prop­er­ly, or if there are oth­er issues that cause over­heat­ing spots, the PCM may not be able to dis­si­pate the heat effectively.
4. Air bub­bles: Pow­er cycling can cause the for­ma­tion of air bub­bles in the PCM, which can cause the PCM to fail.

It’s worth not­ing that many of these fac­tors can be mit­i­gat­ed with prop­er design, test­ing, and main­te­nance of the pow­er invert­er. Addi­tion­al­ly, choos­ing the right PCM for the spe­cif­ic require­ments of the pow­er invert­er, as well as reg­u­lar mon­i­tor­ing of the sys­tem’s tem­per­a­ture and per­for­mance, can help to ensure the reli­a­bil­i­ty of the PCM.

Hon­ey­well PTM7950 is a super high­ly ther­mal­ly con­duc­tive Phase Change Mate­r­i­al (PCM). It is designed to min­i­mize ther­mal resis­tance at inter­faces, main­tain excel­lent per­for­mance through reli­a­bil­i­ty test­ing, and pro­vide scal­able appli­ca­tions at a com­pet­i­tive cost. Based on a nov­el poly­mer PCM sys­tem, PTM7950 exhibits excel­lent inter­face wet­ta­bil­i­ty dur­ing typ­i­cal oper­at­ing tem­per­a­ture ranges, result­ing in extreme­ly low sur­face con­tact resistance. 

Hon­ey­well PTM7950 is one of the most suc­cess­ful TIM prod­ucts used in high-pow­er invert­ers and high-per­for­mance com­put­ing devices. Ultra-low ther­mal imped­ance with min­i­mum achiev­able BLT and supe­ri­or long-term reli­a­bil­i­ty make PTM7950 stand out from the com­peti­tors (Check the PTM7950 Reli­a­bil­i­ty Report). PTM7XXX series also presents sta­bil­i­ty among ver­ti­cal appli­ca­tions and pass auto­mo­tive stan­dard vibra­tion test (Check the Vibra­tion & Shock Test Report), thus PTM7XXX is cur­rent­ly wide­ly used in auto­mo­tive appli­ca­tions such as pow­er invert­ers.

If you are in the process of select­ing a Phase change mate­r­i­al or you are hav­ing reli­a­bil­i­ty issues with your exist­ing TIM, Con­tact us and one of our Sales engi­neers will get back to you as soon as possible.