Mid Sweden University

The integration of more and more functionality into smaller and smaller form factor electronic products, drives the need for denser chip to substrate interconnect systems. As the number of I/O pins increases the use of area array chips or packages becomes inevitable. Metal patterned elastomer chip sockets have now been improved to work with contact densities as high as 80 000 contacts/cm2 corresponding to a pitch of 36 µm. Sockets with 10 000 contacts and a 72 µm pitch have survived more than 400 cycles in air to air thermal cycling chambers as well as freezing shocks caused by dipping into liquid nitrogen. Although the daisy chain test circuits breaks for temperatures lower than -50 C and higher than 90 C, they always return to the initial resistance values when entering the normal temperature range. The combination of a gold to gold contact interface and the elastic features of the contact bumps, makes this socket an ideal compliance layer between bare chips and different types of carrier substrates, reducing the problems caused by thermomechanical mismatch between the substrate and the chip. Bad dies can easily be replaced, since the chip is not soldered or glued to the socket. The size and the possibility to control the geometry of the contacts, provides means to maintain a good high frequency characteristic impedance matching all the way to the chip pad.