Epoxy adhesive behaviour on ceramic surfaces in commercial
, C. Liu
, D.P. Webb
, P. Firth
Mechanical and Manufacturing Engineering, Loughborough University, Loughborough LE11 3TU, UK
Bookham Technology PLC, Brixham Road, Paignton, Devon TQ4 7BE, UK
Accepted 8 February 2010
Available online 16 February 2010
Chemical and physical variability in the as-received state of aluminium oxide and aluminium nitride
ceramic substrate materials used in optoelectronic modules currently leads to a process yield less than
100% when adhesives are used for assembly and interconnection. The phenomenon of epoxy bleed is a
contributing factor to this yield and steps are not yet taken in the industry to control or inhibit the
Standard surface texture measurement techniques, XPS and contact angle measurements were
implemented to characterise and compare commercial as-received samples. The quality controls
currently in place are assessed and additional analysis methods in the QC stage are suggested for
increasing yield. Commercially available conductive and thermally conductive adhesives, also used in
optoelectronic module manufacture, were studied along with the surfaces.
In this work the surface property measurements, wetting observations and shear failure modes are
compared and discussed. It was found that varying as-received surface properties did not affect
adhesion strength between the adhesives and ceramics enough to induce an undesirable failure mode,
although epoxy bleed distance varies dramatically with varying surface conditions.
& 2010 Elsevier Ltd. All rights reserved.
The components in many modern optoelectronic modules are
assembled using adhesives at greater spacing than optically
necessary. Given the current trend towards miniaturisation in
the optoelectronics industry , it is desirable to reduce the
component spacing to allow for further reduction in the size of
the module. This issue extends to many microelectronic applica-
tions in which adhesives are used for component assembly. One of
the factors limiting reduction in component spacing is lack of
control of the surface wetting behaviour of the epoxy adhesives
on the ceramic sleds used as the optical bench. Good wetting is
usually considered desirable to ensure strong and reliable
adhesive joints. However, good wetting also increases the area
of the joint ﬁllet and hence reduces the potential to pack
components more closely together. A further problem is varia-
bility in wetting behaviour and hence ﬁllet size from batch to
batch of the ceramic sleds. In extreme cases epoxy bleed occurs,
where one or more adhesive components separate out before
curing and spread widely across the ceramic surface, interfering
with wire bonding processes and the application of subsequent
The aims of the investigation described in this paper were
ﬁrstly to understand the origin of the variability in adhesive
wetting on ceramic sleds observed on the production line, and
then to suggest cost-effective methods of reducing or eliminating
it, particularly bleed. A subsidiary aim was to scope the possibility
of reducing adhesive joint ﬁllet sizes, without affecting the
reliability of the optoelectronic modules. To achieve these aims
the study was carried out on a representative range of different
types of ceramic sled obtained from the optoelectronic industry
supply chain, in the condition in which they would be received by
an optoelectronic module assembly plant. The results of surface
characterisation of the as-received samples, of bleed measure-
ments with epoxy adhesives used in optoelectronic assembly on
as-received and plasma cleaned samples, and of adhesion
strength tests with the epoxies and ceramics, are reported here.
Conclusions are drawn as to the alternatives for reducing
variability in wetting behaviour and eliminating bleed.
Aluminium oxide and aluminium nitride are popular substrate
materials used in optoelectronic modules. Their low thermal
expansion, dimensional stability and relatively high thermal
conductivity make them ideal for use in high power applications.
The similarity between the coefﬁcient of thermal expansion (CTE)
ARTICLE IN PRESS
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journal homepage: www.elsevier.com/locate/ijadhadh
International Journal of Adhesion & Adhesives
0143-7496/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
Corresponding author. Tel.: + 44 01509 227677.
E-mail address: O.Williams@Lboro.ac.uk (O. Williams).
International Journal of Adhesion & Adhesives 30 (2010) 225–235