Charge plate with conductive pads and method of manufacture

Connection of electrical leads to individual charge electrodes in a charge plate structure is facilitated by the formation of conductive pads extending from each charge electrode to the corresponding electrical lead. The conductive pads are formed by applying a mask, containing open areas corresponding to areas between individual charge electrodes and electrical leads, to the charge plate structure and spraying a conductive epoxy resin onto the mask. Electrical connections between the leads and charge electrodes are rapidly and reliably made in this manner.

BACKGROUND OF THE INVENTION 
This invention relates to charge plates for use in a laminated coating head 
of the general type described in Beam et al. U.S. Pat. No. 3,586,907, and 
more particularly to an improved method of connecting electrical leads to 
charge electrodes in such charge plates. 
As explained in copending application Ser. No. 912,495, coating heads of 
the type described by Beam et al. are used in ink jet printing systems, 
which create printed matter by selective charging, deflecting, and 
catching of drops produced by one or more rows of continuously flowing ink 
jets. The jets themselves are produced by forcing ink under pressure 
through a series of orifices in an orifice plate, which is one component 
of the laminated head. 
A stimulation arrangement stimulates the jets to break the ink up into 
uniformly sized and regularly spaced drops, with drop formation occurring 
in all jets at more or less fixed positions, all located approximately the 
same distance from the orifice plate. The charge plate is positioned 
within the coating head so that electrical charging of selected ones of 
the drops being generated is achieved. 
A charge plate of the type used by the Beam et al. patent utilizes a plate 
of dielectric material provided with a series of charging tunnels located 
equidistantly along a straight line. Each charging tunnel is coated with 
an electrically conductive material which defines a cylindrical charging 
electrode. Electrical leads must be connected to each such charge 
electrode, and the electrical leads in turn are selectively activated by 
an appropriate data processing system. 
Typical prior art charge plates including such electrodes are disclosed in 
Solyst, U.S. Pat. No. 3,975,741, in Kuhn, U.S. Pat. No. 3,984,843, and in 
Bassous et al., U.S. Pat. No. 4,047,184. The prior art also includes 
charge plates having charging electrodes formed in notches along the edges 
of the plate, as disclosed in the above mentioned Solyst patent, and also 
in Robertson, U.S. Pat. No. 3,604,980, Culp, U.S. Pat. No. 3,618,858, and 
in Van Breemen et al., U.S. Pat. No. 4,035,812. 
In addition to the difficulties arising in the fabrication of charge 
plates, described in detail in copending application Ser. No. 912,495, 
difficulties have also arisen in reliably and rapidly connecting 
electrical leads to each individual charge electrode on the charge plate. 
Previous methods for connecting the electrical leads to each charge 
electrode used laborious and slow hand painting, with conductive paint, of 
a connecting line between each lead and charge electrode. The difficulty 
of such an operation will be appreciated when it is remembered that the 
center-to-center spacing of each charge electrode is only about 0.423 mm 
and each charge electrode has an internal diameter of only about 0.355 mm, 
leaving a spacing between charge electrodes of only about 0.068 mm. 
Depending upon the size of the charge plate and the area to be printed, 
anywhere from several hundred to over one thousand connections per charge 
plate must be made. The previous methods suffered not only from the length 
of time required to complete the operation, but also from globbing of the 
conductive paint over into charge electrode tunnels and partially blocking 
them and irregularities in the conductive connecting lines being painted. 
This resulted in either a poor or no connection on the one hand to two or 
more connections flowing into one another on the other. 
Accordingly, the need exists in the art for a rapid and reliable method for 
connecting electrical leads to charge electrodes on a charge plate. 
SUMMARY OF THE INVENTION 
Electrically conductive pads are formed on a charge plate extending from 
each individual charge electrode to the corresponding electrical lead of a 
multiple lead cable by applying an electrically conductive coating of 
material through open areas in a mask. The electrically conductive coating 
can be an epoxy resin containing silver particles which will permanently 
adhere to the charge plate structure and can be sprayed onto the charge 
plate. 
The mask through which the conductive coating is applied can be made by 
either of two alternative methods. The first method is to form the mask 
from a thin plate of copper or copper alloy. Such a thin plate can be 
etched, using photofabrication and etching techniques well-known in the 
art, to provide slots which correspond to the intended size and relative 
placement of the electrically conductive pads on the charge plate 
structure. One side of the mask is lightly sprayed with an adhesive, and 
that side of the mask is then laid down onto the charge plate and aligned. 
With proper alignment, each slot in the mask will extend from a charge 
tunnel to an electrical lead from a cable which has been attached to the 
charge plate structure. 
The second method of making the mask is to form it from photoresist 
material. Photoresist is applied to the charge plate and is then exposed, 
using well-known photofabrication techniques, through a positive working 
master which contains openings corresponding to the areas where the 
conductive pads are to be placed. After exposure, the photoresist material 
is developed and removed from the areas corresponding to where the 
conductive pads are to be placed. The photoresist mask is now ready to be 
sprayed with the conductive resin. 
Conductive epoxy resin is oversprayed onto the mask (either of copper or 
photoresist material) to completely cover the individual areas between the 
electrical leads and charge electrodes. To insure a good electrical 
connection, the conductive resin is sprayed into the charge electrode 
tunnels to overlap with the conductive material in such tunnels. After 
spraying, the mask is removed and the charge plate is ready for use. In 
the case where a photoresist mask has been used, it must be stripped away 
using known photoresist removal methods. Any extraneous conductive resin 
will be removed upon removal of the mask. In the case where electrical 
leads are to be attached to both sides of the charge plate structure, the 
masking and spraying procedure is repeated for the opposite side of the 
charge plate. 
Accordingly, it is a primary object of this invention to produce a more 
rapid and reliable method of attaching electrical leads to charge 
electrodes in a charge plate structure. This and other objects and 
advantages of the invention will be apparent from the following 
description, accompanying drawings, and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIGS. 1, 2, and 3 and as described more fully in copending 
application Ser. No. 912,495, herein incorporated by reference, charge 
plate 10 is formed by casting a nonconductive electrode support structure 
14 containing charge electrodes 16 within support plate 12. Charge 
electrodes 16 are formed during the casting operation and when finished 
are on the order of at least about 1.0 mm thick and have an hour glass 
configuration best shown in FIG. 2. At the time of casting, electrodes 16 
are formed when a conductive epoxy coating, such as epoxy resin containing 
silver particles available under the name ECR 4100 from Formulated Resins, 
Inc., Greenville, R.I., transfers from the casting mold to the sides of 
electrode support structure 14. After completion of the fabrication of the 
charge plate, it is ready for attachment of flexible printed circuit 
leads. As illustrated in FIG. 3, cables 18 containing a multiplicity of 
leads 20 are attached to charge plate 10. Preferably, the cables are 
attached to both sides of the charge plate. 
As illustrated in FIG. 4, cables 18 are attached on either side of charge 
plate 10 with leads 20 aligned with individual charge electrodes 16. Leads 
20 are now ready to be electrically connected to individual charge 
electrodes 16. A mask is prepared, and a conductive epoxy resin is sprayed 
onto the mask to form conductive pads 22 illustrated in FIG. 5. This 
conductive epoxy resin is preferably the same resin containing silver 
particles which has been used previously to form the charge electrodes. 
The resin may be thinned for easier spraying by mixing it with a small 
amount of toluol. It is important that the spray overlaps the conductive 
coating on the charge electrodes to assure a good electrical connection. 
After removal of the mask with accompanying removal of excess resin, the 
conductive pads which now connect leads 20 with charge electrodes 16 are 
allowed to cure for several hours to assure their adherence to the 
electrode support structure. The masking and spraying procedure is then 
repeated for the leads on the opposite side of the charge plate. Although 
the rows of charge electrodes have been illustrated as having the 
individual electrodes aligned directly opposite each other, it is to be 
understood that in practice the rows of electrodes may be offset from each 
other by various degrees depending upon the intended alignment of the 
charge plate in the ink jet printer. If the rows of electrodes are offset 
from each other, corresponding changes in the masks utilized in forming 
the conductive pads would have to be made. 
To form the mask, two different methods have been found to produce 
satisfactory results. The first method makes use of a mask formed from a 
thin copper plate. Preferably, the mask is formed from an alloy of 
beryllium and copper and has a thickness of about 0.006 inches. Using a 
negative master of the areas which correspond to where the conductive pads 
are to be laid down, the copper alloy mask is etched to provide slots for 
those areas. Just prior to masking, one side of the mask is lightly 
sprayed with an adhesive and placed, adhesive side down, onto the charge 
plate. The adhesive helps to prevent movement of the mask after proper 
alignment. Once the mask has been properly aligned, conductive epoxy resin 
is sprayed onto it and into the charge tunnels. Upon removal of the mask, 
sharply defined conductive pads connecting the electrical leads to the 
charge electrodes have been formed. 
The second masking method involves use of photoresist material. Photoresist 
material is applied to the surface of the charge plate covering the areas 
where the conductive pads are to be formed. The photoresist material is 
then exposed through a positive working master which has open areas 
corresponding to those areas on the charge plate onto which the conductive 
pads will be formed. After exposure, the photoresist material is developed 
and removed from areas which were exposed. Conductive epoxy resin is then 
sprayed onto the charge plate and into the individual charge electrodes. 
The photoresist material is then stripped away taking any excess resin 
along with it and leaving behind well defined conductive pads connecting 
the electrical leads to the charge electrodes. 
It will be appreciated that once a master copper mask or positive working 
master for the photoresist method has been fabricated, the hundreds of 
individual connections to be made on a charge plate can be rapidly and 
reliably made. Such masks can be used repeatedly with great time savings 
as compared to the prior art hand painting methods. 
While the apparatus and methods herein described constitute preferred 
embodiments of the invention, it is to be understood that the invention is 
not limited to these precise methods or apparatus, and that changes may be 
made in either without departing from the scope of the invention.