Reliability Tests Of Thermal Interface Materials

High power/voltage IGBT mod­ules with SiC and GaN dies are wide­ly adopt­ed by pow­er invert­er man­u­fac­tur­ers, allow­ing for high­er switch­ing speeds to improve ener­gy effi­cien­cy and bat­tery range for EVs. The increas­ing pow­er den­si­ty in the design neces­si­tates supe­ri­or ther­mal man­age­ment solu­tions, where ther­mal inter­face mate­ri­als (TIM) play a key role in ensur­ing long-term reli­a­bil­i­ty and sta­ble performance.

In invert­ers, 50% of the total ther­mal resis­tance is com­ing from the inter­face between IGBT mod­ules and heatsink, where ther­mal inter­face mate­r­i­al is applied. Hence It is essen­tial to min­i­mize the TIM resis­tance while main­tain­ing long-term reli­a­bil­i­ty under high pow­er and switch fre­quen­cy conditions. 

The key con­sid­er­a­tions of Ther­mal inter­face mate­r­i­al selec­tion can be rough­ly out­lined as follows:

Excel­lent Ther­mal Performance

• Low ther­mal resis­tance down to 0.04 ˚C·cm2/W
• High ther­mal con­duc­tiv­i­ty (up to 8 W/mK)
• Opti­mal Sur­face Wet­ting & Low Con­tact resistance
• Great con­forma­bil­i­ty to fill up micro­scop­ic voids and cavities

High Reli­a­bil­i­ty and Long-life

• No bleed-out, pump-out, and flow-out issues
• No aging effects includ­ing dry-out over time and degrad­ed ther­mal performance 
• Sta­ble ther­mal imped­ance across accel­er­at­ed aging test

Man­u­fac­tura­bil­i­ty

• Easy assem­bly & broad process window
• High yield with low/zero mate­r­i­al waste
• Pad and paste form factor

Phase change mate­ri­als, specif­i­cal­ly, excel at those require­ments and become the most pop­u­lar TIM prod­uct for the pow­er mod­ules appli­ca­tion. They adopt a non-sil­i­cone-based poly­mer matrix with steric struc­ture and chem­i­cal bonds with fillers, which lim­its filler migra­tion and large­ly reduces the risk of mate­r­i­al mobil­i­ty issues like pump out. Addi­tion­al­ly, they go through phase change with a dra­mat­ic vis­cos­i­ty drop at tem­per­a­tures above 45°C. This helps to wet the con­tact sur­face, fill the mini gaps, squeeze out the air voids between the heatsink and heat source, and ulti­mate­ly achieve a very low ther­mal impedance. 

Reliability test types

Besides ther­mal imped­ance and bond line thick­ness, the per­for­mance of those reli­a­bil­i­ty tests is the key cri­te­ri­on when select­ing PCM prod­ucts for invert­er man­u­fac­tur­ers. The typ­i­cal reli­a­bil­i­ty tests that PCM needs to go through for pow­er mod­ule appli­ca­tions are:

(1) High-tem­per­a­ture baking 

• Test stan­dard: JESD22-A103 
• Test con­di­tions: 1000hrs @150°C

Objec­tive: Accel­er­ate changes in TIM’s mate­r­i­al and per­for­mance char­ac­ter­is­tics rel­a­tive to pro­longed and ele­vat­ed temperature

(2) 85°C & 85%RH dou­ble 85 tests 

• Test Stan­dard: IEC 61215
• Test con­di­tions: 85°C, 85%RH, 1000 hrs

Objec­tive: Accel­er­ate the cor­ro­sive impact of high humid­i­ty and tem­per­a­ture on the ther­mal per­for­mance of the test structure

(3) High­ly-accel­er­at­ed stress test 

• Test stan­dard: JESD22-A110‑B
• Test con­di­tions: 130°C, 85%RH, 96 hrs

Objec­tive: Accel­er­ate the cor­ro­sive impact of high humid­i­ty and tem­per­a­ture on the ther­mal per­for­mance of the test structure

(4) Tem­per­a­ture cycling test 

• Test Stan­dard: JESD22-A104C
• Test con­di­tion: ‑55 °C to 125 °C (TCB), 1000 cycles

Objec­tive: Deter­mine the resis­tance of TIM to extremes of high and low tem­per­a­tures, and its abil­i­ty to with­stand cycli­cal stresses 

Ther­mal cycling and stor­age tests are more of a tem­per­a­ture and mechan­i­cal stress con­cern. High-tem­per­a­ture stor­age tests pro­vide infor­ma­tion about how the com­po­nents will per­form under long-term high-tem­per­a­ture con­di­tions. Ther­mal cycling tests pro­vide infor­ma­tion about how the com­po­nents will per­form under con­di­tions of repeat­ed tem­per­a­ture changes.

HAST and dou­ble 85 also include humid­i­ty fac­tors. Dou­ble 85 tests are typ­i­cal­ly used to eval­u­ate the long-term reli­a­bil­i­ty of elec­tron­ic com­po­nents under nor­mal oper­at­ing con­di­tions. HAST tests, on the oth­er hand, are used to eval­u­ate the reli­a­bil­i­ty of elec­tron­ic com­po­nents under extreme con­di­tions, such as those that may be encoun­tered in extreme envi­ron­ments or dur­ing trans­porta­tion. The stress lev­el on the com­po­nent dur­ing the HAST test is high­er than dur­ing the Dou­ble 85 tests.

Although the test con­di­tions dif­fer from each oth­er to sim­u­late var­i­ous oper­at­ing and envi­ron­men­tal stress for devices, the mech­a­nism of PCM fail­ure is sim­i­lar. Please see our blog arti­cle Phase Change Mate­r­i­al Fail­ure Mech­a­nism In Reli­a­bil­i­ty Tests

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 com­pe­ti­tion as can be seen in the PTM7950 Reli­a­bil­i­ty Report.

Hon­ey­well PTM7XXX series also offers sta­bil­i­ty among ver­ti­cal appli­ca­tions, pass­es auto­mo­tive stan­dard vibra­tion tests, and is cur­rent­ly wide­ly used in auto­mo­tive appli­ca­tions such as pow­er invert­ers.

Are you inter­est­ed in learn­ing more about Pow­er mod­ule ther­mal inter­face mate­ri­als? Con­tact us and one of our Sales engi­neers will get back to you as soon as possible.