Partial immersion flow coating method and apparatus

Apparatus and method for applying a coating material to a workpiece. The apparatus includes a coating material flow surface capable of supporting the workpiece. A coating material flow source provides substantially even flow of the coating material across the flow surface. The flow volume may be controlled by suitable means such as a variable speed pump. After flowing over the flow surface, the coating material drains into a drain vat. Portions of the workpiece to which coating should not be applied can be prevented from contacting the coating material by placing those portions over a drain edge of the flow surface.

FIELD OF THE INVENTION 
This invention relates to an apparatus and method for applying a coating of 
material to a workpiece, such as an assembled printed circuit board, by 
partially immersing the workpiece in a flow of the coating substance. In 
addition, the invention relates to a method whereby selected perimeter 
portions of a workpiece may be excluded from the coating process. 
BACKGROUND 
It is often desirable to place a partial coating of protective material on 
workpieces such as assembled printed circuit ("PC") boards. Such coatings 
generally provide several functions, such as preventing short circuits 
caused by stray conductive debris and providing insulation against extreme 
temperatures and vibration. 
One objective when applying such coatings is to avoid the introduction of 
the coating material into electronic components which may be adversely 
affected by the coating. Such components include, for example, 
potentiometers, variable capacitors, and other components with moving 
parts, as well as heat sinks and the like whose heat dissipation 
characteristics are hindered by coating material. Thus, it is generally 
desirable to coat the bottom of a PC board where soldering connections are 
made, and to also apply a thin or "shallow" coating to the top of the 
board where the components are attached to the board. However, the coating 
should not be allowed to "wick up" the components. In addition, coating 
should be excluded from any component, wherever located, which may be 
adversely affected by the coating. 
Many techniques are known to accomplish such an application of a coating, 
but none are completely satisfactory. For example, coating may be sprayed 
on a PC board, but it is difficult, even with stencils and shields 
covering the board, to prevent the coating from getting on components 
which should not receive coating. In addition, spraying does not allow a 
thin layer of coating material to be placed on the upper surface of a 
board without also entirely coating the components. Another method is to 
partially immerse the board in a bath of coating material by placing it on 
supports in the bath. The supports allow the board to be submerged to a 
depth sufficient to coat the bottom and a slight amount of the top of the 
board. However, air bubbles can prevent the complete coating of bottom 
portions of a board, the coating tends to "wick up" the components on the 
board, the boards are subject to being accidentally immersed in the 
coating material when the board is being inserted or removed from the bath 
of coating material, and this submersion process is generally 
time-consuming. Other coating methods and apparatuses must be designed for 
a workpiece of a particular shape, but these may not allow convenient 
substitution of differently sized workpieces. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a method and 
device for partially coating workpieces which allows selected portions of 
the workpiece to be excluded from the coating process. 
It is another object of the invention to provide a method and device for 
partially coating workpieces which reduces waste of coating material. 
It is another object of the invention to provide a method and device for 
partially coating workpieces which can be easily adapted for differently 
shaped workpieces. 
It is another object of the invention to provide a method and device for 
partially coating workpieces which allows quick and economical coating of 
workpieces.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a perspective view of the apparatus of the invention. Vat 10 
holds coating material 11. For coating PC boards, the coating may be 
1-2577 conformal coating, manufactured by Dow Corning of Midland, Mich. 
Three of the side walls 40 of vat 10 are higher than the fourth wall 12. 
Fourth wall 12 has a cut away portion 13 which acts as a flow source to 
introduce coating material 11 onto flow surface 14. Alternatively, the 
entire length of fourth wall 40 may be lower than the remaining side walls 
40, and the guide walls 20 of flow surface 14 may perpendicularly extend 
above flow surface 14 of fourth wall 12 to control the width of the flow 
source area. Flow surface 14 is of a sufficient size and shape to support 
article (not shown) to be partially coated. Flowing coating material 17 
flows from vat 10 across flow surface 14 and drains into drain tank 18, 
which is generally larger in surface area than flow surface 14 so that all 
flowing coating material flowing from the flow surface 14 flows into drain 
tank 18. Drain tank 18 is connected via pipes 42 and 43, and pump 19 to 
vat 10 to provide constant recirculation of coating material 11, which 
helps to prevent coagulation of the coating material. A mesh screen (not 
shown) may be placed over the pipe outlet of drain tank 18 to prevent 
debris from entering pipe 42. Also, pipe 42 may include an enclosed 
storage tank (not shown) to hold the coating material when it is not in 
use. 
The pumping volume of pump 19 may be adjusted via controls (not shown) to 
accurately control the thickness of the flowing coating material 17 
generated by the flow source. The depth should be adjusted to be equal to 
the desired thickness of coating on the article to be coated. 
Guide walls 20 prevent flowing coating material 17 from prematurely flowing 
into drain tank 18 before coating an article placed on flow surface 14, 
and thus provide an even depth of flowing coating material 17 over the 
entire flow surface 14. 
FIG. 2 is a vertical sectional view of the apparatus of the invention. Flow 
surface 14 is attached to vat 10 by pair of horizontally space flanges 21 
and 22 which fit over the fourth wall 12 of vat 10. Secure attachment 
thereto is provided by a pair of thumbscrews 23, which also allow quick 
replacement of flow surface 14. Thus, the apparatus of the invention 
allows the size and shape of the flow surface to be easily replaced to 
accommodate articles of different sizes and shapes. 
The depth of flowing coating material 17 may be adjusted to give the 
desired thickness. For example, printed circuit board 30 may be placed on 
flow surface 14 and the flow volume is adjusted so that flowing coating 
material 17 is thick enough to coat the bottom of board 30 and a thin 
layer on the top of the board. 
The speed of the flow of coating material along flow surface 14 may also be 
controlled by inclining the flow surface so that gravity provides an 
additional method of drawing coating material toward drain tank 18. Such 
inclination may be accomplished by either incorporating it into the design 
of the flow surface, or by loosening thumbscrews 23 securing the flow 
surface 14 to vat 10, and allowing the flow surface to slightly tilt 
downwards. 
FIG. 3 is a perspective view of a printed circuit board with a peripheral 
edge which should not be coated, situated in the apparatus of the 
invention. A primary feature of the invention is that it allows peripheral 
edges of articles to be excluded from the coating process at will. This is 
particularly convenient for coating articles such as printed circuit 
boards which often have components with moving parts such as 
potentiometers which can be damaged if they come into contact with coating 
material. 
Printed circuit board 30 has components which may receive coating material, 
and components 32 which are to remain uncoated. Flow surface 14 is 
designed so that components 32 extends beyond the edge 33 of flow surface 
14. Thus, flowing coating material 17 drains into drain tank 18 without 
coating components 32. It may thus be appreciated that the flow surface 14 
and guide walls 20 may be manufactured in any desired shape to accommodate 
boards of various sizes and shapes, and having peripheral edge portions 
that should not receive coating in various locations. 
FIG. 4 is a top view of the apparatus of the invention showing the relative 
dimensions of the coating material vat 10, flow surface 14 and drain tank 
18. The width of vat 10 should be at least as wide as flow surface 14 to 
provide a flow of coating material along the entire width of flow surface 
14. However, it should be understood that the apparatus of the invention 
does not require the use of an overflow vat as the source for the flow of 
coating material. For example, a slotted feed line (not shown) such as 
that disclosed in U.S. Pat. No. 3,605,689 may also be placed at the front 
edge 41 of flow surface 14. 
Ideally, the sides of drain tank 18 should extend beyond the sides of flow 
surface 14 so that all coating material flowing off the flow surface 
enters drain tank 18. 
It may also be appreciated from FIG. 4 that components 32 on printed 
circuit board 30 that should not be coated extend beyond the edge of flow 
surface 14, and that guide walls 20 serve to maintain an even coating 
depth along the entire width of flow surface 14. 
To apply a partial coating of material to an article in accordance with the 
invention, it is first necessary to determine the size of the article to 
be coated and thickness of coating. A flow surface as described above 
should be manufactured using techniques well-known in the art to 
accommodate the size and shape of the article. If any peripheral edges of 
the article are not to be coated, then the portion of the flow surface 
that would otherwise support such portions should be cut away to allow 
such portions to extend beyond the edge of the flow surface. The flow 
surface may be attached to a suitable coating material flow source. The 
flow volume should then be adjusted so that the depth of the flow across 
the flow surface is equal to the thickness of coating desired on the 
article to be coated. The article is then placed on the flow surface, and 
is coated by the flow of coating material. The article is removed from the 
flow surface, and may be hung for drying or other desired processing. 
It may be appreciated that the foregoing description of the preferred 
embodiment is only an example of the invention claimed, and should not be 
construed as limiting in any way other variations which may be employed 
without departing from the spirit or scope of the invention.