Development of epoxy adhesive for screen printing on aluminum of flexible printed circuit

The aim of this research is to develop epoxy based adhesive for screen printing on aluminum (Al) and polyimide (PI) substrates in flexible printed circuit (FPC) manufacturing. Two types of aromatic epoxy resins are evaluated in this work including diglycidyl ether of bisphenol-A (DGEBA) and epoxy phenolic novolacs (EPN). The two epoxy resins will also be blended with aliphatic epoxy (PPGDE) for adhesive property enhancement purpose. These epoxy systems can be cured at 170oC for 15 min using metaphenylene diamine (m-PDA) as a curing agent. The compositions of the aromatics epoxies were varied in a range of 50 to 90wt%. From the results, it was found that peel strength of EPN/PPGDE blends was higher than peel strength obtained from DGEBA/PPGDE systems. The maximum peel strength of EPN/PPGDE blends was measured to be 0.561 N/mm at 50/50 mass ratio. These blends were also found to show outstanding adhesive bonding with the aluminum substrate. In addition, it was also found that when the EPN/PPGDE blend at 50/50 mass ratio was modified with 5 phr of CTBN rubber, the resulting adhesive exhibited a good bonding with polyimide substrate providing maximum peel strength of about 0.706 N/mm. The above results also suggest difference in bonding mechanisms of the two adhesive layers and the Al or PI substrates. Consequently, two adhesive layers of epoxy systems with the first layer of EPN/PPGDE blend was applied on Al substrate whereas the CTBN modified EPN/PPGDE blend, a second adhesive layer, which provides a good adhesion on PI film was casted on top of the EPN/PPGDE layer. The obtained peel strength of this two-layered epoxy blends was found to be as high as 0.813 N/mm meeting the industrial requirement. These two-layered epoxy adhesive systems are a good candidate for adhesive bonding of Al and PI substrates through screening printing process.