How outgassing in materials is tested

How Outgassing in Materials is Tested: Key Parameters and Procedures

What is outgassing?

Out­gassing occurs when gas is released from a sol­id or liq­uid mate­r­i­al due to increas­ing tem­per­a­ture, decreas­ing pres­sure, and cer­tain chem­i­cal reac­tions like decom­po­si­tion. Ultra-low pres­sure lev­els in vac­u­um envi­ron­ments result in out­gassing as gas mol­e­cules always tend to move from a high to a low pres­sure area. On the oth­er hand, increas­ing tem­per­a­ture increas­es the kinet­ic ener­gy of the mol­e­cules with­in the sol­id or liq­uid mate­r­i­al, mak­ing these mol­e­cules more volatile, lead­ing to a poten­tial phase change, and final­ly releas­ing gas.

Four Mechanisms of Outgassing in Solids and Liquids

Sur­face atoms, unlike those with­in the mate­r­i­al, have incom­plete bonds, mak­ing the sur­face an active site for the adsorp­tion of vapors and gasses. Need­less to say, this makes near­ly all mate­ri­als under­go out­gassing. Although some mate­ri­als, includ­ing poly­mers (plas­tics, elas­tomers, ther­mosets, and adhe­sives), and porous ceram­ics and met­als, have it worse. 

Four pri­ma­ry mech­a­nisms con­tribute to out­gassing– vapor­iza­tion, des­orp­tion, dif­fu­sion, and per­me­ation. Out­gassing rate is the sum of these four mechanisms.

Vapor­iza­tion. This includes the phase change from sol­id or liq­uid to gas of the mate­r­i­al sur­face itself. Vapor­iza­tion is neg­li­gi­ble for met­al sur­faces under nor­mal oper­at­ing con­di­tions (tem­per­a­ture and pressure). 

Des­orp­tion. This involves the release of gas mol­e­cules adsorb on the mate­r­i­al surface.

Dif­fu­sion. This refers to the move­ment of mol­e­cules from the material’s inner (bulk) to its surface.

Per­me­ation. Out­gassing through per­me­ation occurs when mol­e­cules from an exter­nal atmos­phere move through the material’s sur­face then across the bulk and final­ly released to the oth­er surface.

Why does outgassing matter?

Out­gassing pos­es a chal­lenge for elec­tron­ic devices designed for high-vac­u­um and/or high-tem­per­a­ture envi­ron­ments. The sub­stances released dur­ing out­gassing, which com­mon­ly are water vapor, volatile organ­ic com­pounds, and oil (or grease) can con­t­a­m­i­nate the vac­u­um envi­ron­ment, influ­enc­ing the per­for­mance of del­i­cate equip­ment or exper­i­ments. Take semi­con­duc­tor man­u­fac­tur­ing, for instance. It involves high­ly sen­si­tive process­es that demand ultra-clean envi­ron­ments with strin­gent stan­dards. The rea­son is sim­ple: even tiny amounts of con­t­a­m­i­nants can sig­nif­i­cant­ly affect the desired outcomes.

In the auto­mo­tive indus­try, out­gassing is also cru­cial. Mod­ern vehi­cles are equipped with a wide range of elec­tron­ic com­po­nents like sen­sors, con­trol units, and dis­plays, all con­tain­ing out­gassing-prone poly­mer­ic mate­ri­als, such as semi­con­duc­tor and opto­elec­tron­ic mold­ing com­pounds, along with die attach and ther­mal inter­face mate­ri­als. The out­gassing from these com­po­nents has the poten­tial to impact their long-term per­for­mance and reli­a­bil­i­ty. With­in elec­tron­ic sys­tems, the pres­ence of con­t­a­m­i­nants result­ing from out­gassing can lead to issues like cor­ro­sion, reduced con­duc­tiv­i­ty, and com­pro­mised functionality.

Out­gassing also holds vital sig­nif­i­cance in aero­space appli­ca­tions. First­ly, the typ­i­cal space­craft con­di­tions char­ac­ter­ized by low pres­sures and extreme tem­per­a­tures can trig­ger out­gassing. Sec­ond­ly, sim­i­lar to the auto­mo­tive indus­try, the par­ti­cles released dur­ing out­gassing can dis­rupt the prop­er func­tion­ing of essen­tial elec­tron­ic equip­ment cru­cial for space­craft oper­a­tion. Due to this sig­nif­i­cance, out­gassing tests are con­duct­ed on numer­ous aero­space com­po­nents to guar­an­tee opti­mal per­for­mance and reli­a­bil­i­ty. The Nation­al Aero­nau­tics and Space Admin­is­tra­tion (NASA) devel­oped a screen­ing tool to assess mate­r­i­al out­gassing per­for­mance, lat­er trans­lat­ed into ASTM E595 by the Amer­i­can Soci­ety for Test­ing and Mate­ri­als (ASTM).

How is outgassing tested?

The test method in ASTM E595-77/84/90 involves expos­ing the mate­ri­als to spe­cif­ic con­di­tions to assess their out­gassing behav­ior. First, the com­po­nents are held at 25 °C and 50% rel­a­tive humid­i­ty for 24 hours in a con­trolled envi­ron­ment. Then, they are weighed to mea­sure any ini­tial mass changes. To sim­u­late space con­di­tions, the mate­ri­als are then exposed to vac­u­um (7×10–3 Pa or 5×10–5 Torr) and held at 125 °C for anoth­er 24 hours. Dur­ing this phase, a cool­ing col­lec­tor plate at 25 °C col­lects gas con­den­sate for fur­ther analy­sis. Final­ly, the sam­ples and the col­lec­tor plate are weighed to deter­mine the mate­ri­al’s out­gassing performance. 

Outgassing Testing

Key para­me­ters, like total mass loss (TML) and col­lect­ed volatile con­dens­able mate­ri­als (CVCM), play a vital role in eval­u­a­tion. TML refers to the per­cent­age of a mate­ri­al’s ini­tial mass that is lost dur­ing the out­gassing test. It quan­ti­fies the total weight change of a spec­i­men as it under­goes expo­sure to spe­cif­ic envi­ron­men­tal con­di­tions, typ­i­cal­ly high tem­per­a­tures and vac­u­um. CVCM rep­re­sents the per­cent­age of volatile mate­ri­als released dur­ing out­gassing that con­dense and are col­lect­ed on a des­ig­nat­ed sur­face, often a cool­ing col­lec­tor plate. These are the sub­stances that tran­si­tion from a gaseous to a con­densed state dur­ing the test­ing process. Mon­i­tor­ing CVCM is cru­cial because it helps assess the poten­tial for con­t­a­m­i­nants released dur­ing out­gassing to con­dense on crit­i­cal sur­faces with­in a space­craft or aero­space sys­tem. NASA sets tar­get bench­marks for accept­able TML and CVCM at less than 1% and 0.1%, respec­tive­ly. You can find the NASA out­gassing data table here.

CAPLIN­Q’s low out­gassing ther­mal inter­face mate­ri­als, die attach solu­tions, assem­bly adhe­sive inks and films, and under­fills match NASA’s stan­dards, ensur­ing excel­lent per­for­mance and reli­a­bil­i­ty in aero­space appli­ca­tions. Among the dif­fer­ent types of ther­mal inter­face mate­ri­als, sil­i­cone-free phase change mate­ri­als, such as PTM 7000 and PTM 7900, stand out for their min­i­mal out­gassing, with TML lev­els as low as 0.030% and 0.021%, respec­tive­ly, after bak­ing at 160 °C for 48 hours. On the oth­er hand, the fol­low­ing table sum­ma­rizes the TML and CVCM val­ues of low out­gassing die attach mate­ri­als, under­fills, and assem­bly adhe­sives avail­able at CAPLINQ.

Prod­uctTML [%]CVCM [%]
Die Attach Pastes
Assem­bly Adhe­sive Inks and Films
LOCTITE ABLESTIK CE3103WLV Assem­bly Adhe­sive Ink0.140.02
LOCTITE ABLESTIK 5020K Assem­bly Film Adhesive0.240.02
LOCTITE ABLESTIK 563K Assem­bly Film Adhesive0.440.04

Under­stand­ing and test­ing for out­gassing are piv­otal in ensur­ing the opti­mal per­for­mance and reli­a­bil­i­ty of mate­ri­als in crit­i­cal indus­tries like elec­tron­ics, auto­mo­tive, and aero­space. The test­ing stan­dards, such as ASTM E595-77/84/90, pro­vide a rig­or­ous method­ol­o­gy to eval­u­ate a mate­ri­al’s out­gassing behav­ior. At CAPLINQ, we rec­og­nize the sig­nif­i­cance of mate­r­i­al reli­a­bil­i­ty in these indus­tries. Our range of mold­ing com­pounds, die-attach mate­ri­als, liq­uid encap­su­lants and under­fills, and ther­mal inter­face mate­ri­als adhere to strin­gent stan­dards. Take the next step toward mate­r­i­al excel­lence and reli­a­bil­i­ty– reach out to our team today. 

About Rose Anne Acedera

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