Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2001-02-07
2003-11-25
Talbott, David L. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C361S719000
Reexamination Certificate
active
06653572
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multilayer circuit board well suited for high density mounting of electronic devices such as semiconductor devices and more particularly to a structure of a multilayer circuit board formed by laminating together a plurality of printed wiring boards.
2. Related Background Art
As the result of a recent trend towards realizing a greater degree of integration of semiconductor devices, the density of wiring patterns on printed wiring boards (PWB) for carrying such semiconductor devices has become increasingly higher and higher. For instance, lead less chip-types have become the main current in the actual mounting forms of various electronic devices including semiconductor devices as well as resisters and capacitors, and high density surface mounting techniques for directly soldering chip-type devices on the printed wiring of a PWB to realize a high density mounting on the PWB have already been developed.
With the advent of higher levels of mounting forms, there has been an increase in the thermal stress at the soldered bonded joints between electronic devices, particularly semiconductor chips and a PWB thus giving rise to a danger of increasing the rate of occurrence of cracking at the bonded joints. To overcome this danger, measures have been proposed to reduce the coefficient of linear expansion of a substrate constituting a PWB so as to be close to that of the semiconductor chips. For instance, a glass epoxy multilayer substrate FR-5, formed by laminating a plurality of epoxy resin impregnated glass fabrics made from T-glass fibers having a low coefficient of linear expansion and used as reinforcement material, has been commercially available and PWBs formed from these substrates have already been put in practical applications.
However, the PWB formed from such a substrate reinforced with T-glass fibers has a coefficient of linear expansion in the range from 7 to 10 ppm and this rate is still large as compared with the coefficient of linear expansion, i.e., 3 ppm, of silicon semiconductor chips, for example. In other words, even if such a PWB is used, a thermal stress caused within the PWB due to temperature variations suffering during the mounting of electronic devices by soldering or during the use cannot be reduced so much, and the danger of causing cracking in the soldered layer at the bonded joints between the silicon chips and the wiring on the PWB still remains unsolved. In this case, in order to prevent the occurrence of cracking in the soldered layer, it is rather necessary to effect an additional procedure of filling the gaps between the silicon chips mounted on the PWB and its surface with a resin to reduce the thermal stress.
Also, the effect of the glass fibers or the reinforcement material of the substrate does not practically act on the through-holes formed in the PWB and thus the local coefficients of linear expansion in the axial direction of the through-holes show large values ranging from 50 to 150 ppm. Thus, due to the thermal stress caused by variations in the ambient temperature, there is the danger of causing a damage leading to any electrical break in the copper plating applied into the through-holes or the conductive paste (e.g., copper paste) filling the through-holes.
On the other hand, the formation of through-holes for providing electric connections between the respective layers is essential in the case of a multilayer printed wiring board (MPWB). Conventionally, a drill is used to form holes through the substrates for the formation of the required through-hole portions, and this method not only increases the number of drilling operations with an increase in the number of holes thus requiring much labour and time, but also has the danger of causing a gap due to separation at the substrate resin/glass fiber interface inside any through-hole due to the shock of drilling operation and causing the migration of copper ions from the copper plating layer formed on the through-hole inner surface into the gap, i.e., a so-called migration phenomenon. Such migration must be avoided by all means since it results in the deterioration of the electrical insulation performance between the respective through-holes or between the through-holes and other printed wiring.
According to the series of processing steps hitherto used for the fabrication of an MPWB, a plurality of double-sided printed wiring boards are first prepared and, after the double-sided printed wiring boards have been laminated together and subjected to the application of pressure at once, one or more through-holes are formed in this multilayer structure and a plating layer is formed on the inner surface of the through-holes. In this case, while the laminating process can be effected efficiently by pressing the plurality of double-sided printed wiring boards as a whole, the following through-hole forming procedure is inevitably required to collectively form the desired through-holes through the whole layers and the positions of the through-holes at the respective layers cannot be arbitrarily selected independently. Thus, there is the disadvantage of deteriorating the degree of freedom in wiring designing.
Also, in order to ensure improved electromagnetic noise shielding function and intercicuit crosstalk withstanding performance, it is desirable as one countermeasure to provide shielding layers between the respective layers of an MPWB. However, simply adding shielding layers between the respective layers only results in a considerable increase in the number of constituting layers as a whole and the resulting increase in cost cannot be avoided.
In consideration of applications in the high frequency range, the wiring formed on a PWB must be designed as transmission lines. In the case of the conventional PWB or MPWB, however, the provision of ground layers between the respective layers tends to considerably increase the number of wiring layers and this is disadvantageous from the standpoint of economy. Despite this fact, the absence of such ground layers tends to make it difficult to provide the desired impedance matching of the transmission lines and this eventually makes it extremely difficult to supply PWBs or MPWBs which are well suited for use in high frequency applications and are low in cost.
Attempts have already been made for solving such problems. As for example, there has been proposed a multilayer structure including a plurality of PWBs in which the substrates constituting the adjacent layers are bonded together through local mounds such as raised bumps formed from insulating resin, e.g., polyimide, solder balls or raising of conductive patterns so as to form an air gap between the two substrates (See, for example, U.S. Pat. No. 5,786,986). By means of the air gaps formed between the substrates, not only the dielectric loss of the high frequency circuits can be decreased, but also the occurrence of thermal stress due to a difference in linear expansion can be greatly decreased as compared with the case where shielding material having a high coefficient of linear expansion e.g., organic material is present around the bonded joints between the electrodes of the wirings, thereby ensuring improved reliability of the bonded joints as well as the improved heat dissipating properties and reduced weight of the whole assembly.
In this case, the size of the air gaps between the adjacent substrates is determined depending on the thickness of the local mounds such as the bumps, balls or raised portions. Where solder balls are used for the formation of local mounds, essentially it is difficult to make nonuniform the amount of application of solder on the same substrate and also, in the case of soldering for bonding by passing through a reflow furnace, there is the danger of the solder balls between the substrates being collapsed excessively due to the load applied to the substrates, thus making it extremely difficult from the production technical point of view to uniformly control the size of air gaps between the adja
Iizuka Kenji
Ishiwa Masayuki
Nishimura Takeshi
Knobbe Martens Olson & Bear LLP
Norris Jeremy
Talbott David L.
The Furukawa Electric Co. Ltd.
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