influence of formulation chemistry on the locus of failure of adhesive joints

SURFACE AND INTERFACE ANALYSIS, VOL. 23, 342-348 (1995)

Influence of Formulation Chemistry on the Locus of Failure of Adhesive Joints

A. M. Taylor,' C. H. McLean,' M. Charltod and J. F. Watts'* Department of Materials Science and Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK ZENECA Specialties R & D Centre, PO Box 8, The Heath, Runcorn, Cheshire WA7 4QD, UK ZENECA Group plc Specialties Research Centre, Hexagon House, Blackley, Manchester M24 8ZS, UK

The durability of photocured resins adhered to ceramic substrates has been investigated. Monochromated XPS and time-of-flight SIMS were employed to determine the precise locus of failure in butt-joints comprised of a photocured resin adhered to either a 96% pure, debased alumina substrate or single-crystal silicon. Prolonged (>14 days) immersion in water at 50°C resulted in cohesive failure, with 1-2 nm of polymer remaining on one of the ceramic substrates. An absence in the spectrum of any components attributable to characteristic groups from a major constituent of the polymer within this overlayer suggested the possibility that one of the minor resin components was aggregating at the organic/inorganic interface to produce an in situ weak boundary layer. The interactions of some of the organic molecules present in the formulation with the a-alumina substrate were investigated using molecular modelling techniques to indicate the most favourable interactions with the substrate. Durability studies were also carried out on a reformulated photocured resin to investigate the effect of removal of the ethoxylated methacrylate monomer, suspected of being the principal organic component at the weak boundary layer, on both joint performance and locus of failure.

INTRODUCTION

Recent changes in electronics circuit construction technology have resulted in an increase in the use of ceramic substrates such as alumina. In turn, this has created a demand for new types of adhesives and encapsulants. Considerable interest has been shown in one such cate- gory, that of 'radiation-curables'. These materials are rapid-setting adhesive systems where polymerization reactions are induced by some form of electromagnetic radiation.',' The aim of this work was to study the adhesion and subsequent failure of a photocurable resin to 96% pure, debased alumina of a type frequently used in electronics applications, particularly surface mount technology (SMT). Special emphasis was placed on the environmental durability of the system.

Monochromated XPS and time-of-flight (ToF)-SIMS analyses were carried out on the failure surfaces of adhesive joints previously immersed in water at 50 "C for varying lengths of time (0-2 weeks). Previous studies indicated that the locus of failure occurred close to the inorganic surface, with 1-2 nm of polymer remaining on the s~ r face .~ X-ray photoelectron spectroscopy results also revealed subtle differences between this thin polymer overlayer and the bulk material, suggesting the interface segregation of a minor component of the resin. Time-of-flight SIMS spectra were obtained from the various components of the resin in addition to the fracture surfaces of joints following immersion in water, in order to provide a means of 'fingerprinting' the polymer overlayer remaining on the ceramic side of the failure. The interactions of some of the molecules present in the

Author to whom correspondence should be addressed.

photocured resin with the inorganic surface were also investigated using Sybyl, a molecular modelling

Photocured resins, typical of most adhesives, are multicomponent systems. Individual resin components may differ significantly in terms of both their chemical properties and molecular structure. In the formulation of a new resin there is a tacit assumption that the individual components will be distributed evenly through- out the adhesive, to produce a homogeneous product and, consequently, reproducible products. The aim of this work has been to investigate the possibility that in certain photocured resins, aggregation of specific components at the resin/substrate interface can lead to the production of an in situ weak boundary layer (WBL). Part of this work also compared the performance of a reformulated photocured resin with a standard resin following prolonged immersion in water (the reformulated resin contains no ethoxylated methacrylate monomer (EMM), the component from previous studies suspected of aggregating at the inorganic surface). The EMM is added as a relative diluent, to optimize the physical properties of the uncured resin.

EXPERIMENTAL PROCEDURES

Photocured resins

The commercial resin (LCR000) was supplied by Zeneca Specialties, and is based on aromatic methacrylate-type oligomers; an EMM is added as a reactive diluent. Full details of the formulation are not available. The modi- fied resin (ModLCR000) was specially formulated for

Received 14 April 1994 Accepted 13 January 1995

CCC 0142-2421/95/050342-07 0 1995 by John Wiley & Sons, Ltd

THE LOCUS OF FAILURE OF ADHESIVE JOINTS 343

this study by Zeneca Specialties, and was, essentially, the LCROOO material minus the EMM component.

Time-of-flight SIMS

Samples for ToF-SIMS analysis were prepared as follows : (1) Dilute solutions of LuxtrakTM LCROOO resin, Lux-

trakTM LCROOO resin minus the EMM component and the EMM were made up in toluene (3.3-4.3 mg ml- ’ toluene).

(2) Squares of silicon (1 cm x 1 cm) were cut with a diamond scribe and cleaned ultrasonically in isopro- panol.

(3) A drop of the solution was placed on a clean square of silicon and the excess solvent drained off onto filter paper. This produced a film that varied slightly in thickness across the sample. Moving the sample position then allowed the optimum thickness for analysis to be located. The silicon substrates were then placed in spring-loaded sample stubs.

(4) Both positive and negative ion spectra were recorded over a mass range m/z of 0-400 using a VG Scientific Type 23 (Fisons Instruments, East Grin- stead, UK) system. This system is equipped with a single-stage reflectron ToF analyser and a MIG300PB pulsed liquid-metal ion gun (gallium source). Static SIMS conditions were employed using a pulsed (20 kHz and 20 ns) 30 keV 69Ga+ primary ion beam (i.e. <lo’’ ions cm-2 for each analysis).

Monochromated XPS

A series of Coors AD96 alumina and silicon butt-joints were fabricated using the reformulated resin (i.e. the resin not containing the EMM) specifically for XPS analysis6 The joints were immersed in water at 50°C for 14 days and fractured in air 48 h before XPS analysis was carried out using a Scienta ESCA3OO spectrometer at the RUST1 SERC Daresbury Labor- atory. Both sides of the joints were analysed using a monochromated XPS A1 Ka source operated at 14 kV/2OO mA, coupled with an electron flood gun to neutralize ~ h a r g e . ~ In the case of the silicon samples covered by a thin overlayer of polymer, the electron flood gun proved to be unnecessary because electrons from the substrate were sufficient to neutralize charge in the insulating overlayer. A spectrometer pass energy of 150 eV was used, with slit widths of 1.9 and 0.5 mm for the survey and high-resolution spectra, respectively.

occur between the (102) planes and there is a correlation between the energetically most favourable faces, (which would tend to result when polishing) and the magnitude of d.’ The major interaction energy between the alumina and the organic components of the resin is likely to be electrostatic owing to the polarity of the inorganic. Assigning purely ionic charges of + 3 and -2 to the A1 and 0, respectively, would fail to account for the degree of covalency in the system, hence the charge equilibrium method within Cerius was used, with typical values of + 1.95 and - 1.3 assigned arbi- trarily (still maintaining the 2 : 3 ratio).

The organic molecules were optimized using the Tripos force field’’ within the Sybyl package and charges for the electrostatics came from a single-point calculation using the AMI’ semi-empirical method in MOPAC.” Interactions of the organic molecules with the substrate were investigated using the dock option within sybyl, which allowed the interaction energies to be minimized. It should also be noted that during all minimizations the alumina surface was held rigid (i.e. not allowed to relax).

Durability studies

A second series of simple butt-joints were also fabricated for mechanical testing, from LuxtrakTM LCROOO resin and from LuxtrakTM LCROOO resin minus the EMM component. The resins were sandwiched between as-received Coors AD96 alumina discs (10 mm in diam- eter and 1 mm thick) and squares of single-crystal silicon with 2.5 nm thickness of native silicon dioxide (1 cm x 1 cm).

The joints were immersed in water at 50°C for between 0 and 14 days. On removal from water, sample stubs (generally used for mounting samples for XPS analysis) were attached to either side of the ceramic/ photocured polymer ‘sandwich’ using standard Araldi- teTM. Samples were then located in specially designed grips (see Fig. 1) and the load to failure was recorded using a J. J. Lloyd tensile tensometer (1 kN load cell; crosshead speed 1 mm min- ’). The failure stress of the joints could not be recorded because, owing to the topographic nature of the alumina substrate, it was not possible to measure an accurate value for the surface area. Data were plotted as load to failure versus immersion time in water.

RESULTS

Time-of-flight SIMS Molecular modelling

Molecular modelling of the system was carried out at ZENECA Group plc Specialties Research Centre, Blackley, UK. The structure for a-alumina was obtained from the Inorganic Crystal Structure Database at Dare- sbury (ICSD), structure number 4685. The Cerius’ materials modelling package was used to build up the unit cell first and then a larger array of the solid (43 A x 37 A). The crystal was cut along the (102) direction. In alumina, the widest interplanar spacings (d spacing)

Figure 2 shows the positive ion mass spectra recorded from EMM dissolved in toluene and cast onto a silicon wafer (a), from the silicon fracture surface from a silicon/standard LuxtrakTM LCROOO joint fractured after 14 days immersion in water (b) and from Lux- trakTM LCROOO resin minus the EMM dissolved in toluene and cast onto a silicon wafer (c). The spectra have been plotted between 5 and 160 m/z, with inserts made of the regions of particular interest, together with the appropriate scaling factors. The spectrum from the

344 A. M. TAYLOR ET AL.

LuxtrakTM LCROOO resin minus the EMM (c) has also been plotted between 160 and 370 m/z to indicate the high-mass peaks that appear to be diagnostic of the standard (base) resin. Repeated analysis of type (a) and (b) samples failed to produce the characteristic high- mass peaks observed in type (c).

Monochromated XPS

The high-resolution C 1s spectra in Fig. 3 were obtained from the fracture surfaces of a silicon/ reformulated resin joint that had previously been immersed in water at 50 "C for 14 days and fractured in air prior to XPS analysis. The 71 -+ R* shake-up satellite, characteristic of aromatic groups, is visible in the high- resolution C 1s spectra recorded from both sides of the failure. The major component of the photocured resins used in this study is a highly aromatic methacrylate oligomer, accounting for the observation of the R-+TC* shake-up satellite. Analysis of a similar alumina joint also revealed the presence of polymer on the ceramic side of the failure; however, the R -+ .n* shake-up satellite is less clearly visible, owing to the presence of the two potassium 2p,,, and 2p,,, peaks at binding energies of 296 and 293 eV, respectively (see Fig. 4).

Molecular modelling

Figure 5 illustrates how an EMM molecule might 'dock with the alumina substrate, using the C - 0 oxygen atom within a terminal methacrylate group as the prin-

enable the maximum number of molecules to interact Figure 1. Grips used to fracture butt-joints in the tensile testing cipal mode Of binding to an A1 atom Of the to machine.

c

" t II

I 69 1149 ) ' 5 8

2000 - -

O L I * I '

20 40 60 80 100 120 140 160 Atomic Mass Units

m Y

" Y i_i 10

20

x 168 L 70 75

I x 168

20 - - X L 25

15

10 70 75 150 155

-

-

5 -

0 A ' -A .J*d1. ' ' I

20 40 60 80 100 120 140 160 Atomic Mass Units

I x 457 1 L 150 155

d 40 60 80 100 120 140 160

Atomic Mass Units

(c) 3000

u

2500

: 2000

: 1500

1000

500 5

0

Atomic Mass Units Atomic Mass Units

Figure 2. Positive ion ToF-SIMS spectra of: (a) EMM dissolved in toluene and cast onto a silicon wafer; (b) the silicon fracture surface from a silicon/standard LuxtrakTM LCROOO joint, fractured after 14 days immersion in water; (c) LuxtrakTM LCROOO resin minus the EMM, dissolved in toluene and cast onto a silicon wafer.


CI 0

X

m C 3 0 0

7

-

,, 0 7

X

m (I: 3 0 0

I

I (a) c-c I 12 -

10 -

8 -

6 -

21 1 2 9 5 290 285 280

binding energy (eV1

20 (b l c-c 1

10 I

15

10

5

0 296 2 9 2 2 8 8 284

5 I d \ U 0

296 2 9 2 2 8 8 284

binding energy ( e V l

Figure 3. High-resolution C Is spectra obtained from the fracture surfaces of a siIicon/reformulated resin joint: (a) resin side; (b) silicon side.

binding energy (eV1

12

10

- 8

E 6

CI 0

X

m

0 0

O> 300 295 290 285 280

binding energy (eV1 Figure 4. High-resolution C Is spectra obtained from the fracture surfaces of a Coors AD96 alumina/reformulated resin joint: (a) resin side; (b) alumina side.

f l CH,

Methauylate Groups F C H 3 Methauylate Groups

a -Alumina Substrate Figure 5. Schematic of the ethoxylated methacrylate monomer 'docking' with an u-alumina substrate.

with the substrate and hence optimize the total interaction energy, as indicated in Table 1. The EMM molecule can be seen approaching a section of an cr-alumina substrate at an angle of -40" to the substrate, with the result that the op osite end of the EMM molecule lies

ecule has also been studied at angles of 30-40" to the substrate, with energy values appearing to be relatively angle-independent at these values.

between 8 and 9 x above the surface. The EMM mol-

Durability studies

Figure 6 assesses the performance of the standard Lux- trakTM LCROOO resin and the resin that contains no EMM, following immersion in water at 50°C for varying lengths of time. Figure 6 depicts the data for as-received Coors AD96 alumina joints.

DISCUSSION

The positive ion ToF-SIMS spectra depicted in Fig. 2 support the idea originally presented in Ref. 3 that the EMM aggregates at the inorganic surface to produce a 'nanoscale interphase region' along which failure can occur. Although the formulation of such an interphase

X standard resin (average glue line thickness 150pm)

1000 A reformulated resin (average glue h e thickness 300pm)

In 6 900

F 700

5 800 . 1 o 600

2 500

400

300

. .- 73

A reformulated resin (average glue h e thickness 300pm)

6 900

.

3001 x

200-1 0 2 4 6 8 10 12

Immersion time in water at 323K/days Figure 6. Load to failure versus immersion time in water for a series of Coors AD96 alumina joints.


Table 1. Parameters associated with the molecular modelling procedure

Number LX per Footprint area of of NT,, molecule molecule Interaction density

molecules (kJ) (kJ) (A2) (kJ A-Z) Comments

1 -151 -151 87.7 -1.72 Molecule lying parallel to surface 1 -217 -217 87.7 -2.47 C-0 groups pointing to surface 1 -117 -117 38.2 -3.06 Molecule angled at 40" to surface

10 -1531 -153 381.8 -4.01 Molecules angled at 40" to surface

would generally be considered advantageous and would be expected to result in improved joint performance, it is clear that in this case it functions as a WBL that has developed in situ. The spectrum obtained from the silicon fracture surface is similar, though not identical, to the spectrum obtained from the EMM. There is a strong signal from the silicon substrate in all of the samples analysed, with peaks at 28 and 29 m/z. In each of the spectra (a)-(c) the usual hydrocarbon features are observed and the C3 cluster (39, 41 and 43 m/z) is clearly visible, in addition to the C,H9+ ion (69 m/z).13 The spectrum of Fig. 2(c) also contains ions at m/z 77+, 91+, 103+ and 115+, indicative of aromatic species. This is consistent with the major component, which is described as an aromatic methacrylate-type monomer. A characteristic peak at 149 m/z (C6HI3O4+) was also observed in both spectra (a) and (b), but not in (c), which suggests that it is a peak characteristic of the EMM. The alternative assignment, that of a phthalate, is ruled out because there is no peak at m/z 121- in the negative ion spectrum. An absence of certain peaks may also prove significant. High-mass peaks at 165, 281 and 365 m/z were observed in spectrum (c). The peaks at mlz 73+, 147' and 281' are diagnostic (although not confirmatory) of poly(dimethy1 siloxane) (DMS), which is often found as a contaminant in the SIMS analysis of organic polymers. The confirmatory peaks for DMS are to be found in the negative ion spectrum (m/z 75S, 119-, 149- and 223-) and these are absent from the negative ToF-SIMS spectrum of the reformulated resin (not shown). Thus the high-mass peaks of Fig. 2(c) are ions diagnostic of the reformulated resin. The peak at 281 m/z is believed to originate from the repeat unit of the base resin molecule (CI8H,,O3+) and the peak at 165 from the C,H,03+ ion. The absence of the peaks at 165, 281 and 365 m/z in the spectrum obtained from the fractured silicon joint again provides evidence that the polymer overlayer remaining on the failure surface is predominantly comprised of EMM, with some hydrocarbon contamination.

It was unlikely that a perfect fingerprint would be obtained for the silicon fracture surfaces by comparison with the standards, because the sample histories were different (i.e. samples (a) and (c) were produced by casting solutions of the polymers onto clean pieces of silicon wafer, while the silicon joints were immersed in water for 14 days and then fractured in air prior to ToF-SIMS analysis). The results discussed above were, however, reproducible.

Monochromated XPS results obtained from the ceramic sides of failures for silicon joints comprised of the reformulated resin indicate that the locus of failure following prolonged immersion in water is cohesive, with a thin layer of photocured polymer remaining on

the inorganic substrate. This is indicated by the presence of the rc -+ rc* shake-up satellite, diagnostic of the highly aromatic molecules of the base resin. The overlayer can be assumed to be thin (i.e. < 10 nm) because a signal from the substrate was still detectable. Scanning electron micrographs obtained from the fracture surfaces of silicon joints also revealed the presence of some thicker, angular polymer fragments [see Fig. 7(a)]. The presence of such debris, which is in contrast to previous work,3 precludes the use of a Beer-Lambert approach to estimate the overlayer thickness. This is in contrast to the standard resin, which, on prolonged immersion in water, appears to fail at the interface between a WBL of EMM and the bulk resin. This conclusion was drawn following previous studies, where the rc + rc* shake-up satellite characteristic of aromatic molecules was not observed in the C 1s spectrum recorded from the polymer overlayer present on the inorganic failure surface. Angular polymer fragments were also detected on the alumina side of a reformulated resin joint, again, this is debris that was not observed with joints comprised of the standard resin [see Fig. 7(b)].

Previous XPS and atomic force microscopy studies3 of the standard resin indicated that the thickness of the polymer overlayer remaining on the ceramic substrate lay in the region of 1-2 nm. The results of the molecular modelling correlated well with this data. In order to achieve the maximum possible interaction with the substrate, it is necessary to maximize the interaction energy per unit area. The greatest interaction energy was recorded for the EMM molecule oriented parallel to the substrate, as shown in Table 1. However, this does not give the largest value for interaction energy per unit area, owing to the large surface coverage of the molecule. It is possible to produce an increase in the interaction energy per molecule if the carbonyl oxygens in both of the methacrylate end-groups are angled towards the substrate, because the carbonyl oxygens are -1.3 times more negative than the ether oxygens. This is achieved by internal rotations within the molecule, with the molecule oriented parallel to the surface. Alterna- tively, the total interaction energy may be increased by angling one of the methacrylate end-groups of each EMM molecule towards the surface. The cross-sectional area of the end of an EMM molecule is only 38.2 A', which would then allow large numbers of molecules to interact with the surface, giving a higher total interaction energy. In this orientation, the end of the EMM molecule has been calculated to lie between 8 and 9 8, above the alumina surface, as indicated schematically in Fig. 5. The thickness of the overlayer remaining on the fracture surface also lies within this region. This suggests that the polymer layer observed may be a molecular layer of EMM remaining on the fracture surface. An


Figure 7. Scanning electron micrographs from: (a) silicon side of a reformulated resin/silicon joint, immersed in water for 14 days and fractured; (b) the equivalent alumina surface.

increase in interaction energy is also observed when going from one to ten EMM molecules. This suggests that if one EMM molecule interacts with the alumina, interaction of additional EMM molecules at neighbour- ing sites is favoured, i.e. cooperative effects may be present. This cooperative effect will also occur in the bulk solution.

Durability studies were carried out on simple butt- joints as a means of assessing the performance of the photocured resin both with and without the EMM component. In both cases dry alumina joints failed, with a characteristic conchoidal fracture surface morphology that has been described in detail elsewhere.6 This model proposes that failure is initiated at defects (cavities) that become trapped in the joints during fabrication. As similar sized defects are likely to be present in both

resins, it is perhaps not surprising to discover that, under dry conditions, similar loads to failure are recorded for both, between 0.85 and 1 kN. In all cases, a significant reduction in mechanical performance following prolonged immersion in water was observed. Joints comprised of photocured resin minus the EMM had more residual strength on prolonged immersion in water than the standard photocured resin. This may be explained by knowledge of the formulations. The EMM is a polar (hydrophilic) molecule, whereas the methacrylate-terminated oligomer, being highly aromatic, is hydrophobic. Water uptake by a joint would therefore be strongly influenced by the presence (or absence) of EMM. The differences in the performance of the two resins when adhered to alumina were less marked.


CONCLUSION

There are two important conclusions that can be drawn from this work: the first concerning the role that XPS and ToF-SIMS have to play in the examination of fracture surfaces and the subsequent reformulation of resin systems to improve joint durability; the second con- cerns the potential deleterious effects that some minor components can have on joint durability.

To our knowledge this is the first published work in which the adhesion failure of a commercial resin system has been fully characterized by XPS and ToF-SIMS and, in the light of these data, a reformulated resin has been produced that has improved durability. The com- plementarity of the two techniques must be emphasized. The ability of XPS to determine overlayer thickness and identify aromatic components quite simply via the n -+ n* transition is well known; however, when this is combined with the molecular specificity of ToF-SIMS a fully quantitative scheme can be produced, which has been supported with molecular modelling.

The key message that this work provides for formulation chemists is that when choosing components, con- sideration needs to be given to how the individual components in a formulation might interact with each other (i.e. the presence or absence of non-covalent inter- molecular interactions) as well as with the adherend. It is possible that components purposely ad-ded for adhesion promotion could not only bind strongly to the

adherend, as desired, but additionally set up a WBL. Adhesion to the substrate would therefore be improved, but if the locus of failure is simply moved to the interface between a WBL and the bulk polymer, then a reduction in joint strength is likely to be observed.

Of importance to adhesion scientists is not so much the recognition of a segregated zone, as has been report- ed in XPS studies several times over many years,14 but the recognition that it may reduce the environmental durability of a joint. The logical remedy is to reformu- late the resin with the component removed, as described in this paper. This is clearly an oversimplification because the physical properties of the uncured resin may be compromised by removal of the diluent (e.g. the viscosity would increase); however, it illustrates an important point. Studies of the type described in this paper provide a new and exciting route to the definition of adhesion failures and, perhaps, a more thorough understanding of the mass transport phenomena that occur prior to the curing of commercial resins used for adhesives, coatings or encapsulant applications.

Acknowledgements

The financial support of both 1C1 Chemicals & Polymers Ltd. and SERC is gratefully acknowledged by AMT for funding this CASE award. Special thanks to G. Beamson at the RUSTI, SERC Dare- sbury Laboratory for his invaluable assistance with the monochromated XPS analysis. Thanks also to A. M. Brown and S. R. Leadley for their technical advice and informative discussions.

REFERENCES

1. V. D. McGinniss, Adhesive Chemistry, Developments and Trends, Part 1, pp. 363-377. Plenum Press, New York (1 984).

2. K. C. Stueben, J. Radiat. Curing 9(2), 31 9 (1982). 3. A. M. Taylor, J. F. Watts, J. Bromley-Barratt and G. Beamson,

Surf. Interface Anal. 21, 697 (1 994). 4. M. Charlton, ZENECA Specialties Internal Report (January

1 994). 5. Sybyl modelling package. SYBYL v6.03, Tripos Associates,

Centennial House, Bracknell, RG12 1 NN. 6. A. M. Taylor, J. F. Watts, H. Duncan and I. W. Fletcher, J.

Adhes. 46, 145 (1 994). 7. G. Beamson and D. Briggs, in High Resolution XPS of

Organic Polymers-The Scienta ESCA300 Database, Chapt. 2, pp. 3-8. Wiley, Chichester (1992).

8. Cerius materials modelling, diffraction simulation, v3.2,

9. J. D. H. Donnay and D. Harker, Am. Miner. 22,463 (1 937). Molecular Simulations Inc., Cambridge, UK.

10. M. Clark, R . D. Cramer and N. van Opdenbosch, J. Comput.

11. M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P.

12. MOPAC ‘93, QCPE Program 455, University of Indiana,

13. D. Briggs, Surf. Interface Anal. 4(4), 151 (1 992). 14. J. F. Watts and J. E. Castle, J. Mater. Sci. 18, 2987 (1 983).

Chem. 10 (8 ) . 982 (1989).

Stewart, J.Am. Chem. SOC. 107,3902 (1985).

Bloomington, USA.

influence of formulation chemistry on the locus of failure of adhesive joints

Documents