Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-12-11
2001-04-24
Sough, Hyung-Sub (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S792000, C428S209000, C428S212000, C428S220000, C428S901000, C174S255000, C174S260000, C029S830000
Reexamination Certificate
active
06222740
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multilayer circuit board.
BACKGROUND INFORMATION
When populating multilayer circuit boards with electronic components, it is known to solder components without connecting leads, e.g. so-called LCCC (leadless ceramic chip carrier) components, directly onto corresponding connecting surfaces of the multilayer circuit board. When thermal shock loads occur, the different coefficients of thermal expansion of the ceramic chip carriers and the circuit board result in shear forces which must be absorbed by the soldered connections of the component. With severe temperature fluctuations, such as occur in particular in space technology, for example in a satellite orbiting the earth, the difference in thermal expansion between large-area ceramic components and the circuit board would lead to damage to the soldered connections and failure of the assembly. In the case of a satellite located in space, a soldered connection defect of this kind is extremely disastrous, since it generally leads to failure of the unit in question. To solve this problem, it is known to use multilayer circuit boards having a core substrate. Core substrates made of metal or composite carbon fiber materials are known.
U.S. Pat. No. 4,921,054, for example, discloses a thermomechanically adapted multilayer circuit board having two core substrates, made of copper-plated Invar, which control the expansion behavior of the circuit board. The layered composite material made up of the core substrates and circuit board material is produced by stacking and pressing. The coefficient of expansion of the overall arrangement comprising insulating material, conductor path layers, and core substrates are determined by the proportion of the total thickness of the layered composite accounted for by the core substrate. Since the core substrate has a much lower coefficient of thermal expansion than the circuit board material, which as a rule is made of an epoxy/glass material, it is possible for the resulting coefficient of thermal expansion of the multilayer circuit board equipped with the core to be adapted to the coefficients of expansion of the ceramic components. Disadvantageous shear forces which might damage the soldered connection therefore do not occur in the coverage area of the components.
Despite the advantages of the known multilayer circuit boards, producing the through-plated holes guided through the core substrate is extremely complex. A further disadvantage is that the core substrates extending over the entire surface extent of the circuit board greatly increase the weight of the circuit board, which is extremely undesirable in space technology. It is also disadvantageous that the known core substrates increase the size of the multilayer circuit board. Since conductor paths cannot be laid out in the plane of the core substrate, either the thickness of the circuit board must be increased or the circuit board must be enlarged laterally. Both are extremely undesirable, since the space available, especially in space technology, is extremely limited.
SUMMARY OF THE INVENTION
The multilayer circuit board according to the present invention eliminates the drawbacks which occur with the existing art. It is particularly advantageous that in the case of the multilayer circuit board according to the present invention, the at least one core substrate is provided only in the coverage area of the at least one component, and does not cover the entire lateral extent of the circuit board. Because the at least one core substrate is provided only at points which later will actually be populated with ceramic components on the upper or lower side of the circuit board, the weight of the circuit board is greatly reduced as compared to the known multilayer circuit boards having a core substrate which covers the entire extent of the circuit board. This constitutes a substantial advantage especially in space technology. When the multilayer circuit board is produced, the at least one core substrate is simply laid as an insert into a recess provided for it in an inner layer portion of the multilayer circuit board. Further insulating material layers, equipped with conductor paths, are laid onto the inner layer portion. The entire stack is then further processed, in known fashion, to produce a multilayer circuit board. Large-area leadless ceramic chip carriers are soldered on only at those positions on the outer sides of the multilayer circuit board which are located opposite at least one core substrate arranged in the interior of the circuit board. This ensures that the expansion properties of the multilayer circuit board in the coverage area of the ceramic chip carriers are adapted to the thermal expansion properties of those components, so that any damage to the soldered connections in the event of thermal shock loads is avoided.
It is particularly advantageous if the inner layer portion has multiple conductor path planes separated by insulating material layers. Since the at least one core substrate is laid into a recess of the inner layer portion, sufficient room still remains in the frames surrounding this recess to provide further conductor path planes there. The size of the multilayer circuit board can be further reduced as a result, since the conductor paths are at least partially also laid out in the inner layer portion in which the at least one core substrate is arranged. It is furthermore advantageous that through-plated holes which are intended to be guided through the multilayer circuit board do not need to be guided through the core substrate, but rather are simply provided in the portion of the inner layer portion which laterally adjoins the at least one core substrate. The technological outlay for producing the through-plated holes can thereby be greatly reduced as compared to the existing art.
It is furthermore advantageous to introduce the core substrate as an insert, into the recess in the inner layer portion provided therefor, in as accurately fitted a manner as possible.
It is advantageous if the inner layer portion having the at least one core substrate is arranged centeredly between the two outer sides of the multilayer circuit board, so that a symmetrical layer structure results and the thermal expansion coefficients on the upper side and lower side of the circuit board are uniformly adapted to the expansion behavior of the ceramic chip carriers. This prevents different shear forces from acting on the upper side and lower side of the core substrate in response to thermal shock loads, which would result in bending of the multilayer circuit board. A centered arrangement of the inner layer portion also makes possible, in particular with circuit boards populated on both sides, a uniformly good thermomechanical adaptation of the circuit board in the coverage area of components placed on the upper side and on the lower side.
It is also particularly advantageous if the core substrate is made of molybdenum. Molybdenum offers the advantage of possessing a low coefficient of thermal expansion and at the same time a high modulus of elasticity. With the combination of these two properties, on the one hand the thermal expansion behavior of the circuit board is adapted to the coefficients of expansion of the ceramic chip carriers, and on the other hand the molybdenum core is well suited, because of its rigidity, for counteracting shear stresses which occur in the multilayer circuit board as a result of the different expansion behaviors of the circuit board material and core substrate.
It is also advantageous to use a carbon fiber core which includes multiple unidirectional carbon fiber plies arranged in layered fashion above one another. This core substrate advantageously has a lower weight than the cores made of metal. In this context, the carbon fibers contained in the carbon fiber plies and running parallel to one another combine the advantage of a high modulus of elasticity with the advantage of a low coefficient of thermal expansion. It is particularly advantageous if the carbo
Bovensiepen Kurt
Messarosch Gerhard
Ulmer Helmut
Kenyon & Kenyon
Robert & Bosch GmbH
Sough Hyung-Sub
Vigushin John B.
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