Printed circuit board having conductors interconnected by foamed electroconductive paste

A printed circuit board has a vibration-absorbing electroconductive body disposed in a through-hole to electrically interconnect electrical conductors disposed on opposed sides of the board. The electroconductive body is formed from the expansion of a decomposed, foamed electroconductive paste which is heated and hardened to form the electroconductive body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a printed circuit board, and, more 
particularly, to electroconductive paste to be enclosed in a through hole 
connecting circuits disposed on the two sides of a printed circuit board. 
2. Description of the Related Art 
Hitherto, double-side printed circuit boards have been designed in such a 
manner that predetermined patterns of electroconductive bodies are formed 
by two sheets of copper foil laminated on the two sides of an insulating 
substrate of a copper laminated board, through holes being formed at 
predetermined positions in the substrate, and electroconductive paste 
which is normally made of copper paste being enclosed in predetermined 
through holes so that electrical connection between two circuits on the 
two sides of the insulating substrate is established. 
The other manufacturing processes such as mounting of the electronic parts, 
soldering, and forming insulating coating are performed in known ways. 
Thus detailed description of these factors will be omitted here. 
There has, however, been a demand for the size of such printed circuit 
boards to be reduced in accordance with the trend for reducing the size of 
electronic parts. Furthermore, in order to reduce the number of 
manufacturing processes, a significant degree of automation has been 
achieved. This raises the problem that when a copper paste is introduced 
from one side into the through holes by means of silk screen printing, the 
copper paste cannot be properly adhered to the conductive body of the 
circuit disposed on the reverse side, leading to a condition of defective 
conduction. 
If an excessive amount of copper paste is supplied on the silk screen when 
silk screen printing with copper paste is performed, the copper paste 
overflows the target regions, causing short circuits or the like between 
the circuits on the two sides. There is, therefore, a certain limitation 
to the amount of paste that can be supplied on the silk screen, but this 
restricted amount of paste is insufficient to overcome the problem of 
defective conduction. 
To this end, an object of the present invention is to provide a printed 
circuit board capable of overcoming the above-described problems and of 
displaying a high level of reliability and in which the generation of 
defective conduction can be prevented without comprising the 
conventionally achieved level of facility for manufacture and maximized 
economic advantage. 
SUMMARY OF THE INVENTION 
A printed circuit board according to the present invention in which a 
printed circuit is formed on either one side or both sides of an 
insulating substrate, and a through hole penetrating the insulating 
substrate is formed, wherein circuits on the two sides are electrically 
connected by an electroconductive paste enclosed in the through hold. The 
printed circuit board comprises foaming electroconductive paste as the 
electroconductive paste. 
A printed circuit board according to the present invention can overcome 
defective conduction by exploring foaming electroconductive paste. The 
foaming electroconductive paste is enclosed in the through holes where it 
expands in the through holes bringing it into complete contact with the 
conductors disposed on the two sides so that the two sides are connected 
to each other. Furthermore, such a foaming electroconductive paste 
exhibits vibration-absorption characteristics, and formation of cracks or 
the like can be prevented therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will now be described with reference 
to the drawings. 
The accompanying drawing is a partial enlarged cross-sectional view which 
illustrates an embodiment of a printed circuit board according to the 
present invention. 
In FIG. 1 reference numeral 1 represents an insulating substrate, 2 and 3 
represent conductors formed by predetermined patterns and disposed on two 
sides of the insulating substrate 1, reference numerals 4, and 5 represent 
predetermined solder resist films, and 6 represents a through hole formed 
in the insulating substrate 1. 
The foaming electroconductive paste according to the present invention is 
formed by enclosing foaming electroconductive paste 7 from one side in the 
through hole 6 by means of silk screen printing or the like, and the 
thus-enclosed foaming electroconductive paste 7 is heated up to 
100.degree. to 200.degree. C. at which the foaming material therein can be 
decomposed, the thus-decomposed foaming material thereby being foamed. In 
addition, the thus-foamed foaming material is then subjected to a 
predetermined means such as an application of ultraviolet rays as to be 
hardened. 
The foaming electroconductive paste according to the present invention can 
be prepared by, for example, mixing several % of a known foaming material 
with usual copper paste, the quantity of the foaming material being 
optionally determined on the basis of the post-treatment and the desired 
degree of expansion. It is preferable to arrange the conditions as to make 
the magnification of expansion of the volume twice due to the 
above-described heating. 
The accompanying drawing is a view which illustrates a state of the 
electroconductive paste 7 after it has been subjected to the heating. The 
central portions of the two sides project as a result of expansion due to 
foaming of the foaming material as an alternative to the pits formed in 
the central portions of the two sides in the conventional manner. 
Therefore, contact with the conductors 2, 3 disposed on the two sides can 
be established, causing defective conduction to be prevented. A great 
volume of electroconductive paste 7 cannot conventionally be supplied 
using silk screen printing. However, according to the present invention in 
which the foaming material is contained therein, the volume of the 
electroconductive paste can be increased in the through hole at the time 
of performing the silk screen printing. 
Since the other treatments to be performed in order to manufacture the 
printed circuit board, such as mounting electronic parts, soldering, 
forming the insulating film, and forming the protective film do not relate 
to the present invention. Thus, their descriptions are omitted here. 
As described above, a complete connection can be established between the 
conductors disposed on the two sides by increasing the volume of the 
electroconductive paste in the through holes by using electroconductive 
paste in which foaming material is contained according to the present 
invention. In addition, there is not any necessity to increase the 
quantity of foaming material at the time of performing silk screen 
printing when the electroconductive paste is enclosed in the through 
holes. 
In addition, the generation of cracks in the conductor or in a through hole 
can be overcome due to the shock absorption capability and vibration 
absorbing performance exhibited by the foaming body.