phase change materials. failure mechanisms after reliability testing

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.

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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