Epoxy molding compounds how to improve adhesion to substrates

Improving Epoxy Molding Compound Adhesion to Substrates

The adhe­sion strength between mate­ri­als can direct­ly impact the per­for­mance and dura­bil­i­ty of the inter­face. In the con­text of semi­con­duc­tor pack­ages that are com­posed of dif­fer­ent lay­ers with dif­fer­ent mate­r­i­al prop­er­ties, ensur­ing that the bond­ed sur­faces are kept togeth­er all through­out the man­u­fac­tur­ing process, assem­bly, and oper­a­tion is cru­cial for main­tain­ing the integri­ty of the pack­age, ensur­ing the device’s long-term reli­a­bil­i­ty in chal­leng­ing oper­at­ing con­di­tions. How­ev­er, delam­i­na­tion, some­times referred to as dead­he­sion, may occur where­in the encap­su­lat­ing mate­r­i­al becomes sep­a­rat­ed from an adja­cent mate­r­i­al at their interface. 

This sep­a­ra­tion can occur at var­i­ous crit­i­cal points with­in the pack­age, includ­ing the inter­faces between the encap­su­lant and the semi­con­duc­tor die (Type I), between the encap­su­lant and the lead­frame (or sub­strate) (Type III), and between the encap­su­lant and the die pad­dle (Type IV). Among these delam­i­na­tion types, the loss of adhe­sion between the mold­ing com­pound and the sub­strate is the most com­mon mode of fail­ure encoun­tered dur­ing the pro­cess­ing and qual­i­fi­ca­tion of semi­con­duc­tor packages. 

Types of Delamination in Semiconductor Packages

Pre­vent­ing Type III delam­i­na­tion has become sig­nif­i­cant­ly impor­tant over the recent years because of the more strin­gent reli­a­bil­i­ty stan­dards imposed to meet the require­ments of the boom­ing auto­mo­tive industry. 

Strate­gies to improve the adhe­sion strength of epoxy mold­ing com­pounds on substrates

Improv­ing the adhe­sion strength of epoxy mold­ing com­pounds on sub­strates, which include lead­frames, lam­i­nates, and even wafers, can be done using the recent advance­ments in mate­ri­als engi­neer­ing, man­u­fac­tur­ing, and qual­i­ty con­trol and assur­ance. From a mate­ri­als per­spec­tive, address­ing Type III delam­i­na­tions involves two pri­ma­ry strate­gies: (1) improv­ing the epoxy mold­ing com­pound for­mu­la­tion and (2) enhanc­ing the sub­strate design. In this arti­cle, we will delve into these strate­gies, explor­ing the lat­est inno­va­tions and tech­niques that have emerged in epoxy mold­ing com­pound for­mu­la­tion and sub­strate design to com­bat Type III delaminations. 

Improv­ing the EMC formulation

  • Increas­ing the filler con­cen­tra­tion to decrease the coef­fi­cient of ther­mal expan­sion (CTE)

Coef­fi­cient of ther­mal expan­sion (CTE) is a prop­er­ty which mea­sures the changes in the dimen­sions of a mate­r­i­al in response to changes in the tem­per­a­ture. The CTEs of the most com­mon­ly used sub­strates in semi­con­duc­tor man­u­fac­tur­ing range from 17 ppm/ °C (Cu) to as low as 2 ppm/ °C (Si). We would want the CTE of the EMC to be as low as pos­si­ble to avoid high degrees of mis­match between those of the substrate.

When mate­ri­als with sig­nif­i­cant­ly dif­fer­ent CTE val­ues are heat­ed, they expand at dif­fer­ent rates. This induces ther­mal stress at the inter­face of the mate­ri­als, which increas­es the risk of ther­mal-stress induced delam­i­na­tion between the EMC and the sub­strate. To pro­duce EMCs with low CTEs, epoxy resins with low CTEs can be used. Nev­er­the­less, the most com­mon strat­e­gy is to add fillers, such as sil­i­ca and alu­mi­na, to reduce the over­all CTE of the formulation.

  • Mod­i­fy­ing the EMC for­mu­la­tion to decrease its mois­ture sen­si­tiv­i­ty and absorption

Aside from ther­mal stress caused by CTE mis­match, volatil­i­ty stress relat­ed to the mois­ture absorp­tion prop­er­ties of EMCs also affect their adhe­sion to sub­strates. This is par­tic­u­lar­ly sig­nif­i­cant for semi­con­duc­tor devices that need to under­go reflow at 260 °C. Dur­ing reflow sol­der­ing, entrapped mois­ture with­in the semi­con­duc­tor pack­age evap­o­rates, exert­ing stress and form­ing cracks with­in the package. 

As crack prop­a­ga­tion occurs, delam­i­na­tion between the pack­age com­po­nents may occur, caus­ing the dead­he­sion between the EMC and the sub­strate. This fail­ure mech­a­nism is more com­mon­ly known as the “pop­corn effect” and is con­sid­ered to be one of the most com­mon fail­ure modes in semi­con­duc­tor packages. 

To avoid such from hap­pen­ing, EMCs may be for­mu­lat­ed using epoxy resins that have less affin­i­ty to water, includ­ing but not lim­it­ed to bi-phenyl, mul­ti-aro­mat­ic, and dicy­clopen­ta­di­ene (DCPD) resins. Match­ing these epoxy resins to appro­pri­ate hard­en­ers can help improve the hydropho­bic­i­ty of the EMC. To illus­trate this point, we can com­pare the mois­ture absorp­tion of Hysol GR600-P1 with that of Hysol GR600-SL2. Although both mold­ing com­pounds exhibits low mois­ture absorp­tion, GR600-P1 (0.20%) absorbs less­er mois­ture than GR600-SL2. This is expect­ed as GR600-P1 con­tains a mul­ti-aro­mat­ic resin, where­as GR600-SL2 is based on a phe­no­lic novolac resin. The aro­mat­ic com­pounds in GR600-P1 can con­tribute to hydropho­bic­i­ty, reduc­ing mois­ture absorption.

Moisture Absorption of Different Epoxy Resins

Here are some water absorp­tion data for some of epoxy mold­ing com­pounds avail­able at CAPLINQ, along with their epoxy and hard­en­er chemistry.

Epoxy Mold­ing CompoundWater Absorp­tionEpoxy/ Hard­en­er Chemistry
Lin­q­sol EMC-75600.4Mul­ti-func­tion­al (MFN) resin + hydropho­bic hardener
Lin­q­sol EMC-90700.33Biphenyl (BP) resin + hydropho­bic hardener
Lin­q­sol EMC-75350.25Mul­ti-aro­mat­ic resin (MAR)
Hysol GR7200.4Epoxy O‑cresol Novolac (EOCN) resin
Hysol GR600.34EOCN resin + phenalka­mine (PN)
Hysol GR730HT0.24
Hysol GR300.31EOCN + PN
Hysol GR3000.33EOCN + PN
Hysol GR5100.21MAR + EOCN resin + MAR hardener
Hysol GR510-HP0.24MAR + EOCN resin + MAR hardener
Hysol GR640HV0.52EOCN resin
Hysol GR640HV-L10.42EOCN resin
Hysol GR7500.35MFN resin
Hysol GR700 C3D0.28MAR + low mol­e­c­u­lar weight (LMW) resin + MAR hardener
Hysol GR910‑C0.3MAR + BP resin + MAR + MFN hardener
Hysol GR9810-1PF0.4BP + MFN resin
Hysol KL-G100S0.4
Epoxy/hardener chem­istry and water absorp­tion of some epoxy mold­ing com­pounds avail­able at CAPLINQ

  • Adding adhe­sion-pro­mot­ing additives

Epoxy mold­ing com­pounds are com­pos­ite mate­ri­als com­posed of organ­ic and inor­gan­ic com­po­nents. The inter­face between the organ­ic resin and the inor­gan­ic fillers are often not load bear­ing and show poor mechan­i­cal prop­er­ties. To address this issue and make sure that there is suf­fi­cient adhe­sion in the for­mu­la­tion, cou­pling agents, such as silanes, titanates, alu­minum chelates, and zir­coa­lu­mi­nates, are added. 

Cou­pling agents work by cre­at­ing chem­i­cal bonds between the inor­gan­ic and organ­ic com­po­nents, and they work even in harsh con­di­tions. These addi­tives have a spe­cial struc­ture con­sist­ing of one part that can react with the “inclu­sion sur­face” or the inor­gan­ic fillers and anoth­er part that bonds with the poly­mer resin. In sim­pler terms, the struc­ture of cou­pling agents can be sim­pli­fied as A–B–C, Anchor–Bridge/Buffer–Couplant.

Coupling agents in epoxy molding compound formulation

For exam­ple, silane cou­pling agents have a struc­ture R–Si–(OR’)3. R reac­tive groups can form chem­i­cal bonds with organ­ic resins, and each (OR’) group binds with the inor­gan­ic fillers. Aside from cou­pling the com­po­nents in EMC for­mu­la­tions, cou­pling agents are also known to improve the hydropho­bic­i­ty of the com­pound, reduc­ing its mois­ture absorp­tion that pos­si­bly improves the adhe­sion strength of the encap­su­lant to sub­strates. There­fore, the improve­ment in adhe­sion between the fillers and the organ­ic matrix extends to the adhe­sion of the EMC to the device com­po­nents, includ­ing the lead frame or sub­strate, thus decreas­ing the like­li­hood of delam­i­na­tion failure.

These are some of the most com­mon EMC chem­istry mod­i­fi­ca­tion strate­gies employed to improve the adhe­sion strength of the plas­tic encap­su­la­tion on sub­strates. While these do improve adhe­sion and reduce the risk of Type III delam­i­na­tion, some prop­er­ty trade-offs exist. For exam­ple, EMCs with low mois­ture absorp­tion and good adhe­sion strength often have medi­um to low glass tran­si­tion tem­per­a­tures. Also, for opti­cal­ly clear epoxy mold­ing com­pounds, improv­ing the adhe­sion strength may affect its anti-blue ray decay per­for­mance. As in all oth­er things in life, strik­ing the right bal­ance between these prop­er­ties is essential. 

Sub­strate Prepa­ra­tion and Activation

  • Sur­face preparation 

As defined ear­li­er, adhe­sion occurs either through mechan­i­cal inter­lock­ing or mol­e­c­u­lar and chem­i­cal inter­ac­tions between the two sur­faces. In large part, mechan­i­cal adhe­sion depends on the area avail­able for bond­ing. That is to say that sur­faces with high­er sur­face areas would yield more sites at which mechan­i­cal inter­lock­ing may occur. Mate­ri­als may also bond chem­i­cal­ly, which requires the sur­faces to be clean and con­t­a­m­i­na­tion-free and to have high sur­face ener­gies for eas­i­er bond­ing. There­fore, improv­ing the adhe­sion between the EMC and the sub­strate is a two-way street. It does not only depend on hav­ing the right EMC for­mu­la­tion but also on hav­ing good sub­strate (sur­face) preparation. 

Pri­or to mold­ing, met­al lead frames can be cleaned through degreas­ing and sand­ing (abra­sion). Degreas­ing is per­formed to remove loose­ly attached dirt and con­t­a­m­i­nants on the sur­face often using volatile sol­vents, such as ace­tone, toluene, alco­hols, and some ketones. In this tech­nique, the sur­face is wiped or rinsed repeat­ed­ly using the sol­vent and then dried to remove resid­ual volatiles. Mean­while, abra­sion is done to remove heavy con­t­a­m­i­nants in the sur­face, which may include dirt and oxide lay­ers. Aside from that, abra­sion rough­ens the sur­face and there­fore, increas­es the sur­face area avail­able for bond­ing. 
Chem­i­cal treat­ments can also be per­formed not only to improve the chem­i­cal prop­er­ties (i.e., sur­face ener­gy) of the sur­face but also phys­i­cal prop­er­ties. A well-known tech­nique is chem­i­cal etch­ing, where the sub­strate is immersed in an etchant. The etch­ing solu­tion will vary on the type of sub­strate. Appro­pri­ate clean­ing pro­ce­dures are done after etch­ing to ensure that the sur­face is free from resid­ual chem­i­cals. In cas­es where degreas­ing, abra­sion, and/or chem­i­cal treat­ments are not enough, phys­i­cal meth­ods are also per­formed. Sim­i­lar to chem­i­cal meth­ods, phys­i­cal tech­niques are per­formed to mod­i­fy sur­face chem­istry. There are a lot of phys­i­cal meth­ods, includ­ing flame treat­ment, coro­na dis­charge, and plas­ma treatment.

For chem­i­cal and phys­i­cal acti­va­tion, the effec­tive time peri­od of suf­fi­cient adhe­sion high­ly depends on the sub­strate and the treat­ment employed. Deter­min­ing this “win­dow for bond­ing” is impor­tant to make sure that the treat­ment does not lose its effectiveness.

  • Use of adhe­sion promoters

Sim­i­lar to cou­pling agents added in the EMC for­mu­la­tions, adhe­sion pro­mot­ers may also be added on the sub­strate to facil­i­tate its bond­ing with the EMC. Adhe­sion pro­mot­ers work in a sim­i­lar fash­ion to cou­pling agents. One “tail” (end of the struc­ture) forms a bond with the sub­strate, and the oth­er tail links to the EMC. Dif­fer­ent adhe­sion pro­mot­ers have dif­fer­ent base chemistries but all of them work on a mol­e­c­u­lar lev­el. For exam­ple, Aculon’s primers have self-assem­bled mono­lay­ers of phos­pho­nates (SAMPs) that can improve the adhe­sion of met­als to poly­mers. For EMCs, hydrox­yl func­tion­al adhe­sion pro­mot­ers can be used, espe­cial­ly if the EMC is based on phe­no­lic epoxy or if it requires a ther­mal cure above 60–70 °C. Hydrox­yl primers also work for iso-cyanates and melamine formalde­hy­des. Aculon’s adhe­sion pro­mot­ers have exhib­it­ed supe­ri­or per­for­mance in improv­ing adhe­sion of die attach adhe­sives and EMCs to dif­fer­ent lead frames, includ­ing Au, Pd, Cu, Ag, and Ni. 

Thiol‑, acrylate‑, and hydrox­yl-based adhe­sion pro­mot­ers work best with dif­fer­ent types of materials.

Adhe­sion ExamplesEpoxy (Meth)acrylics(Meth)acrylates, amines, thi­ol basedPolyurethanes and iso-cyanates, melamine formalde­hy­des, phe­no­lic epoxy based
Appli­ca­tionSpray / Dip / WipeSpray / Dip / WipeSpray / Dip / Wipe
Vis­i­ble to Naked EyeNoNoNo
Thick­ness (nm)2+2+2+
Dry Time (sec)< 20< 30< 30
Cure RequiredFor <5 min cure, only Ti, Al and epoxy need a cureFor <5 min cure, only Ti, Al and epoxy need a cureFor <5 min cure, only Ti, Al and epoxy need a cure
Prop­er­ties of thiol‑, acrylate‑, and hydrox­yl-based adhe­sion promoters.

Aside from that, adhe­sion pro­mot­ers are easy to apply. Once the intend­ed part or sur­face is suf­fi­cient­ly clean, adhe­sion pro­mot­ers or primers can be applied through dip­ping, wip­ing, or spray­ing. For each appli­ca­tion method, post-appli­ca­tion cur­ing is rec­om­mend­ed to achieve the best performance.

Strong adhe­sion is the cor­ner­stone of semi­con­duc­tor pack­ag­ing reli­a­bil­i­ty. CAPLINQ offers epoxy mold­ing com­pounds with proven excel­lent adhe­sion to sub­strates, and we also offer effec­tive adhe­sion pro­mot­ers. Reach out to us to explore our com­pre­hen­sive solu­tions for your semi­con­duc­tor pack­ag­ing needs.

About Rose Anne Acedera

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