Gold wire bonding onto flexible polymeric substrates

As part of a program to develop very thin, low cost packages using available technology, copper clad polymeric materials were examined as potential substrates for high temperature wire bonded chip-on-flex circuits. New thermoplastic flex and a PTFE/woven glass substrate were evaluated along with polyimides using high temperature copper laminate adhesives and adhesiveless constructions. Initial wire bond pull tests indicated all the materials were suitable for high speed gold wire bonding. Similarly, conventional glob top encapsulants were found to adhere to each of the 13 substrates tested; experimental UV curable glob top materials were found to adhere well to polyester and PTFE substrates, but not to polyimides or polyether imides. Electrical test vehicles were constructed of gold wire bonded daisy chain chips on low cost polyimide substrates. Temperature cycling and thermal shock data showed no increase in circuit resistance for packages using commercial encapsulant formulations as well as using a formulation with high CTE and high modulus.