The x- and y-shrinkages of free-sintered LTCC circuits have been variously reported in the range of 8 to 20%. For well-characterized LTCC systems these numbers are predictable and reproducible and can be compensated for by appropriate resizing of the phototools. What is not predictable is the tolerance associated with these numbers. The shrinkage tolerance of a free-sintered LTCC substrate is at best ±0.15%. Thus, the uncertainty in the true position of a point 0.5cm along the diagonal from a corner of a 20cm × 20cm fired laminate is a circle of 409μm (16.1mil) diameter. This is a significant number and the only way to compensate for it is by making circuit features large enough to overpower it or by making substrates small enough to render it insignificant. Thus it represents an upper limit on the usable substrate size and a lower limit on the size of the components used to populate it. Through the use of constrained sintering (zero-shrink) processes the overall x- and y-shrinkage of a green ceramic laminate can be reduced to almost zero and, more importantly, the dimensional tolerance of the structure can be improved by at least 5 times. Under the same criterion as above this represents a circle of uncertainty of ～80μm in diameter - something which is much more controllable than in the free sintering case. Both pressureless and pressure-assisted versions of constrained sintering are available. Two pressureless methods are discussed and compared in this paper. The first is release tape based (PLAS) and the second is self-constraining tape based (SCPLAS).
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