Thick film, multi-layer, ceramic interconnected circuit board

A thick film, multi-layer interconnected circuit board includes a substrate of an electrical insulating material which will withstand high temperatures, such as alumina, having over a surface thereof a plurality of stacked layers of a glass dielectric. Between each pair of dielectric layers is a thick film conductor layer. The dielectric layers have openings or vias therethrough which are filled with a conductive material which makes electrical contact with the adjacent conductive layers. A contact pad is over the topmost dielectric layer and is electrically connected to a via conductor material in the topmost dielectric layer. The contact pad is of a thick film conductive material which is a layer of glass having particles of a conductive material, such as a noble metal or mixture of noble metals, dispersed therein. A bonding layer is between the contact pad and the topmost dielectric layer and is bonded to both. The bonding layer is of a mixture of the glass of the dielectric layers and the thick film conductive material of the contact pad.

FIELD OF THE INVENTION 
The present invention relates to a hybrid thick film, multi-layer, ceramic 
interconnected circuit board, and, more particularly, to a contact pad 
structure for such a multi-layer board to which electrical components can 
be attached. 
BACKGROUND OF THE INVENTION 
Thick film, multi-layer ceramic interconnected circuit boards, in general, 
include a substrate of an insulating material, such as alumina, which can 
withstand high temperatures having on a surface thereof a plurality of 
thick film conductor layers separated by layers of a dielectric material. 
The thick film conductor layers are generally formed of a mixture of a 
glass frit and particles of a conductive material which is fired on the 
substrate to form a layer of glass having the conductive particles 
dispersed therethrough. The dielectric layers are generally of a glass 
fired on the substrate. The dielectric layers have openings or "vias" 
therethrough which are filled with a conductive material to connect the 
conductive layers in a desired circuit arrangement. The conductive layers 
are in the form of a circuit pattern, and can include areas forming 
resistors and capacitors. On the outermost dielectric layer are contact 
pads of a conductive material which are connected to the underlying 
conductive layers by the conductive material in the vias. Electrical 
components, such as integrated circuits, transistors, diodes, capacitors, 
resistors and the like, are mounted on and soldered to the contact pads. 
The contact pads are also generally made of a thick film material which is 
not only electrically conductive, but is also easily soldered to. 
There are a number of concerns with regard to this type of interconnected 
circuit board. One concern is that the contact pads be strongly bonded to 
the board. Since the thick film conductive material used for the contact 
pads does not adhere well to the glass generally used for the dielectric 
layer, it has been the practice to have the contact pads completely cover 
the via conductive material which extends completely through one or more 
of the layers of the circuit board and perhaps even to the substrate. 
Thus, the area of the via conductive material is generally the same as the 
area of the contact pads. In order to bond the electrical components to 
the contact pads, the pads must be relatively large. Therefore, the area 
of the via conductive material is relatively large so as to take up 
considerable room on the board. This undesirably makes more difficult the 
layout of the circuit conductors, and increases the cost of the board 
because large amounts of the conductive material, which is very expensive, 
are required. Another problem is the inability to remove and replace 
components on the contact pads so as to repair or otherwise change the 
circuit on the board. I have discovered that when a component is 
unsoldered and replaced on a thick film contact pad about two or three 
times, the thick film conductive material of the contact pad becomes 
unbonded from the underlying dielectric layers rendering the circuit board 
unusable. 
SUMMARY OF THE INVENTION 
A thick film, interconnected circuit board includes a substrate of an 
insulating material which can withstand high temperatures having over a 
surface thereof at least one layer of a conductive material and a layer of 
a glass dielectric over the conductive layer. The dielectric layer has an 
opening therethrough containing a conductive material which contacts the 
conductive layer. A contact pad is over the dielectric layer and 
electrically contacts the conductive layer in the opening in the 
dielectric layer. The contact pad is a layer of a glass having particles 
of a conductive material dispersed therethrough. A bonding layer is 
between the contact pad and the dielectric layer. The bonding layer is a 
mixture of the glass of the dielectric layer and the material of the 
contact pad.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to the drawing, a multi-layer, ceramic interconnected circuit 
board which incorporates the present invention is generally designated as 
10. Interconnected circuit board 10 includes a substrate 12 of an 
insulating material which will withstand high temperatures, such as 
alumina. On a surface 14 of the substrate 12 is a first conductive layer 
18. The first conductive layer 18 is covered with a first dielectric layer 
20. A second conductive layer 22 is on the first dielectric layer 20 and 
is covered with a second dielectric layer 24. The dielectric layers 20 and 
24 are of a glass, such as a barium borosilicate glass, which is fired on 
the board 10. One dielectric material suitable for this purpose is sold by 
the Electronic Materials Division of Ferro Corporation of Santa Barbara, 
Calif. under their trademark ISO-OHM series TC and RC multilayer-crossover 
pastes, such as TC1007. The conductive layers 18 and 22 are of a thick 
film paste which is a mixture of a glass frit and conductive particles. 
The paste is coated on the board and fired to form a layer of glass having 
the conductive particles dispersed therethroughout. The glass frit is 
generally a borosilicate glass. The conductive particles are generally a 
noble metal, such as gold, platinum, palladium or silver or mixtures 
thereof. Preferably the conductive particles are a mixture of gold and 
either platinum or palladium. Some conductive pastes which are suitable 
for this purpose are sold by the Electronic Materials Division of Ferro 
Corporation of Santa Barbara, Calif. under the trademarks CONDUCTROX and 
MULTIFIRE thick film conductor pastes, such as formula numbers 3127 and 
3141. The conductive layers 18 and 22 are in the form of conductive lines 
which form the conductors of a particular circuit. 
The dielectric layer 20 has openings or vias 26 therethrough which extend 
between the first and second conductor layers 18 and 22. The vias 26 are 
filled with a conductive material 28 which electrically connects the 
conductive lines of the first and second conductive layers 18 and 22. The 
second dielectric layer 24 has openings or vias 30 therethrough which 
extend to the second conductive layer 22. The vias 30 are also filled with 
a conductive material 32. The conductive material 32 in the vias 30 is 
electrically connected to the conductive lines of the second conductive 
layer 22. The via conductive material 28 and 30 may be of the same thick 
film conductive material as the conductive layers 18 and 22. 
On the second dielectric layer 24 and over the via conductive material 32 
are bonding layers 34. Each of the bonding layers 34 has an opening or via 
35 therethrough which is filled with a conductive material 37. Over the 
bonding layers 34 are contact pads 36 which electrically contact the via 
conductive material 37. The bonding layers 34 are of the same area as the 
contact pads 36. The contact pads 36 are of a thick film conductive 
material, which can be of the same material as the conductive layers 18 
and 22. Preferably, the contact pads 36 are of a first portion 38 directly 
on the bonding layer 34 which contains sufficient conductive particles so 
as to be highly conductive, such as a Ferro Corporation MULTIFIRE 3127 
conductive paste, and a second portion 40 over the first portion 38 which 
contains sufficient conductive particles so as to be easily soldered to, 
such as a Ferro Corporation MULTIFIRE 3141 conductive paste. 
The bonding layers 34 are of a mixture of the same glass used for the 
dielectric layers 20 and 24, and the thick film conductive material used 
in the first portion 38 of the contact pad 36. More particularly, the 
bonding layers 34 are made from a mixture by weight of about 30% to 60% of 
the thick film conductive material and about 70% to 40% of the dielectric 
glass. Preferably, the bonding layers 34 are made by weight of about 40% 
of the thick film conductive material and 60% of the dielectric glass. 
Thus, if the dielectric layers are made of a barium borosilicate glass, 
such as the Ferro Corporation ISO-OHM Series TC-1007 dielectric paste, and 
the contact pad 36 is made of a mixture of a borosilicate glass frit and 
particles of platinum and gold, such as the Ferro Corporation MULTIFIRE 
3127 thick film conductor paste, the bonding layer 34 would be made of a 
mixture by weight of about 30% to 60% of the MULTIFIRE 3127 and about 70% 
to 40% of the ISO-OHM TC1007. 
The multi-layer, interconnected circuit board 10 is made by first applying 
the first conductive layer 18 onto the substrate surface 14. This can be 
achieved by using a thick film conductive paste which is a mixture of the 
glass frit and the conductive particles in a carrier suitable for screen 
printing. The thick film conductive paste is screen printed over the first 
substrate surface 14 and fired at a temperature at which the glass frit 
fuses to form a layer of glass having the conductive particles embedded 
therein. For a thick film paste having a borosilicate glass frit and 
conductive particles of gold and platinum, the firing temperature is 
generally between 800.degree. C. and 950.degree. C. 
The first dielectric layer 20 is then screen printed over the first 
conductive layer 18. This can be achieved by using a dielectric glass frit 
in a carrier suitable for screen printing and screen printing the material 
onto the first conductive layer 18 and any exposed area of the substrate 
surface 14 not covered by the first conductive layer 18. The layer is then 
fired at a temperature at which the glass frit fuses and forms a layer of 
the dielectric glass. For a barium borosilicate glass, the firing 
temperature in generally between 850.degree. C. and 1000.degree. C. 
However, the pattern in the screen used to apply the first dielectric 
layer 20 is such that it leaves blank areas in the first dielectric layer 
20 where the vias 26 are to be provided. After the first dielectric layer 
20 is fired, a conductive material is screen printed in the via areas 26 
and fired to form the via conductors 28. The second conductive layer 22 is 
then applied over the first dielectric layer 20 in the same manner as the 
first conductive layer 18. After the second conductive layer 22 is fired, 
the second dielectric layer 24 is applied over the second conductive layer 
22 in the same manner as the other dielectric layer 20. The second 
dielectric layer 24 is provided with blank areas forming the vias 30 which 
are then filled with the conductive material and fired to form the via 
conductors 32. 
The bonding layers 34 are then applied to the second dielectric layer 24 
over the via conductors 32. The mixture of the bonding layers 34 in a 
carrier suitable for screen printing is screen printed over the second 
dielectric layer 24 and fired to fuse the glass in the mixture. The 
contact pads 36 are then applied over the bonding layers 34 by screen 
printing and firing. If the contact pads 36 have two portions, the first 
portion 38 is applied first and the second portion 40 is applied and fired 
over the first portion 38. After the contact pads 36 are formed, 
electrical components 42, such as resistors, capacitors and the like, can 
be placed on the board 10 with the terminals of the components 42 being 
over the contact pads 36. The terminals of the components 42 can be 
secured to the contact pads 36, such as by solder 44. 
I have found that using the bonding layers 34 between the contact pads 36 
and the underlying dielectric layer 24 provides a contact pad 36 with very 
good bond strength. Using a contact pad material which contained a 
borosilicate glass and a mixture of platinum and gold (Ferro Corporation 
MULTIFIRE 3127) and a bonding layer material of a mixture of by weight 40% 
of the contact pad material and 60% of a barium borosilicate glass (Ferro 
Corporation ISO-OHM TC 1007) with a wire soldered to the contact pad 36, 
pull strengths of up to about 9 to 10 pounds were obtained before the bond 
was broken. When the bond was broken it was often in the dielectric layer 
beneath the bonding layer 34. By way of comparison, contact pads applied 
directly to a dielectric layer, without a bonding layer 34, exhibited pull 
strengths of about 3 to 4 pounds. 
Since the multi-layer interconnected circuit board 10 the present invention 
provides a strong bond between the bonding pads 36 and the topmost 
dielectric layer 24, the via conductors do not have to be of the same area 
as the contact pads and they do not have to extend completely through all 
of the dielectric and conductor layers to the substrate. Thus, the via 
conductors can be made much smaller in area, for example about 11 mils on 
a side rather than the size of the bonding pads which are generally 30 
mils by 40 mils, so as to greatly reduce the area of the board taken up by 
the via conductors. The bonding pads may be of various sizes (e.g. 30 by 
40 mils, 30 by 100 mils, 100 by 200 mils) depending on the size of the 
component to be soldered thereto, and may be connected to vias of various 
sizes (e.g. 15 by 20 mils, 25 by 25 mils, etc.). This produces a much more 
economical circuit board because the amount of very expensive noble metal 
conductor paste is substantially reduced. 
I have also found that by using the bonding layer 34 between the contact 
pad 36 and the dielectric layer 24, the ability to remove and replace 
electrical components on the contact pads 36 has increased. Heretofore, 
when a components was unsoldered and replaced about two or three times, 
the contact pad was damaged rendering the board unusable. However, with 
the circuit board of the present invention, components could be unsoldered 
and replaced up to 10 or 15 times without damaging the contact pads 36. 
Thus, there is provided by the present invention a thick film 
interconnected circuit board in which the thick film contact pads have 
improved bond strengths to the topmost dielectric layer, the via 
conductors can be made smaller in area, and which allows electrical 
components to be removed and replaced on the contact pads a great number 
of times without loosing the ability to achieve reliable solder 
connections to the contact pads.