This method of interconnecting ceramic integrated circuits to organic printed circuit boards (PCBs) is designed to substantially increase the life of the interconnections. This is accomplished by providing a means of compensating for the shear stresses produced by thermal excursions as a result of the large mismatch of coefficients of thermal expansion between the integrated circuit and the printed circuit board.
The micro-coil spring is highly flexible and allows significant movement to counteract the thermal expansion forces between the ceramic and the organic PCB. Once a ceramic part with micro-coil springs is mounted onto an organic PCB, each micro-coil spring essentially forms two solder bumps: one at the ceramic surface and one at the organic PCB surface. Both solder bumps are encapsulated in the BeCu closed coils. These two solder bumps are interconnected with active coils of the BeCu micro-coil spring (see figure). These active coils provide the flexibility to compensate for the opposing forces.
Micro-coil springs provide a flexible interconnection. The spring is unique as an interconnection because it provides flexibility in the x, y, and z axes, and is constructed of a BeCu wire, which is 0.0034 in. (≈86 μm) in diameter. The BeCu is hard tempered, and has final tensile strength of approximately 216.5 kilopounds per square inch (≈1.4 GPa). The overall dimensions of the spring, which can be tailored for any application are 0.020×0.050 in. (≈0.5×1.3 mm). The spring consists of a total of 7.3 coils of which four are inactive (two top and two bottom). After the spring is formed, it is plated with solder. The top and bottom coils serve as near solid surfaces to allow a strong bond between the two substrates (ceramic and organic PCB).
The process for attachment of the micro-coil springs to the ceramic was developed in-house. First, the ceramic substrate is printed with solder paste using an automatic printing machine and a stainless steel stencil. After printing, the ceramic is inspected for coverage and solder paste thickness, then is transferred to a holding fixture. The holding fixture is in two pieces with the top piece serving as a jig and having an aperture pattern matching the electrical pads on the ceramic to allow insertion of the micro-coiled springs into the solder paste. After insertion, the fixture is reflowed using a typical solder paste reflow process to form the metallurgical bond between the spring and the electrical pads on the ceramic. The ceramic is then removed from the fixture, cleaned, and inspected.