Adsorbed contamination on ceramic surfaces stored in industrial ambient conditions and its effect on epoxy bleed

Abstract

Maintaining cleanliness of substrates for assembly in optoelectronic modules is important where surfaces will be subjected to further processes in which the surface properties can affect performance. Cleanliness is counter productive when considering epoxy bleed, since the carbon based contamination of surfaces has been seen to reduce surface free energy and inhibit the spread of epoxy material. The origin of this contamination can be from a number of sources including atmosphere, handling, surface treatments and outgassing from storage media. Whilst allowing contamination to remain on the surface can be an effective means of controlling the epoxy bleed, it is not a reliable solution through lack of controllability. Identifying and quantifying this contamination will be a useful step towards the understanding and control of epoxy bleed, whilst its removal will homogenise all surfaces allowing controllable solutions to be implemented. The substrate materials of interest were aluminium oxide and aluminium nitride, which are commonly used in the optoelectronics industry. Storage methods used in industry were recreated for the purpose of this study with storage of samples in tin foil used for comparison. Samples were stored in commercial polymer waffle packs in a variety of atmospheres which they might experience in industry, on an industrial site, for one month. XPS measurements were made following storage to identify the composition of the contamination and its source. Both the degree of carbon contamination and the functional groups of any adsorbed species are known to affect surface energy and epoxy bleed. Therefore narrow band XPS spectra for carbon were analysed for all samples. Of the many methods which could be employed to remove the surface contamination, solvent, plasma cleaning and firing were chosen for their suitability and due to their availability to industry. XPS was performed on samples following cleaning. It was found that the composition of the contamination on the surfaces was not linked to their storage method but the quantity of contamination was. Storing ceramics in polymer waffle packs does not protect them from build up of carbon contamination regardless of storage atmosphere. The use of tin foil for storage can reduce the degree of contamination presence significantly, but not prevent build up entirely. A high degree of bleed was seen in both samples cleaned but not stored as well as in samples cleaned after storage, showing the effects of storage contamination are easily reversed. While storing ceramics for an extended period of time will allow build up of sufficient contamination to stop bleed occurring, samples fresh from suppliers will not have built up sufficient contamination to reduce the surface free energy to a degree such that bleed will not occur.