High-precision picosecond laser structuring on LTCC for silicon chip assembly with high electrical contact density

The paper presents a novel manufacturing process which offers a high structure resolution and a high density of electrical contacts on Low Temperature Co-Fried Ceramic (LTCC) for silicon chip assembly. To provide of the small pitch of contacts screen printed gold thick films are laser structured with a picosecond laser ablation machine. Laser structured thick film contact pads have dimensions of 59 μm by 59 μm and laser drilled 50 μm vias are used to provide vertical electrical contacts to laser structured thick film lines on the inner layers. The internal structures are laser structured to provide a high line density. To demonstrate the usability of the laser structured contact pads for the chip assembly a silicon test chip is manufactured in thin film technology having gold pads with dimension of 60 microns by 60 microns. Gold stud bumps were placed on the chip pads using a thermosonic ball-wedge bonder and a 25 μm gold wire. The test chip is mounted on the laser structured surface of the LTCC by means of thermo-compression flip chip bonding. A daisy chain structure was realized on the surface of the test chip and on the LTCC to measure the electrical interconnections between the test chip and the LTCC package. The geometrical shape is measured by laser scanning microscopy (LSM). To ensure the functionality and to exclude internal failures (e.g. shorts) the samples are scanned with X-ray inspection.