Fluid-tight protective system for an electromechanical component

A fluid-tight protection system is provided for an electromechanical component in a casing. An insulating absorbent covering layer which has a high capillary absorption capacity is disposed over joints or apertures in the casing requiring sealing. The covering layer is impregnated with a sealing compound which preferably has a thin liquid form during the impregnation. An impermeable film may also be provided to cover the absorbent covering layer.

BACKGROUND OF THE INVENTION 
The invention relates to a fluid-tight protective system for an 
electromechanical component, for example a relay, in which an insulating 
covering is disposed over casing joints or apertures to be sealed, and to 
methods of sealing such a component. 
Where electrical components are used on circuit boards, modern soldering 
techniques require that not only the circuit boards themselves but also 
the individual components are at least partially immersed in soldering and 
cleaning baths. This involves the risk of liquid and gaseous substances 
penetrating through holes and apertures into the inside of a component and 
there contaminating contact surfaces, for example. Components such as 
relays, for example, which for the most part admittedly have a dust cap 
but are not fitted with a hermetically sealed casing, would become much 
more expensive if they had to be made with a hermetically sealed casing 
just for the soldering process although such a casing would be unnecessary 
for subsequent operations. Consequently, many attempts have been made to 
make such components fluid-tight for soldering on circuit boards using 
simple and inexpensive protective devices and thereby provide at the same 
time a certain amount of protection against the damaging effects of 
industrial atmosphere. 
In the case of relays, the problem is mainly one of sealing the joints 
between a base and a protective cap and the apertures provided in the base 
for the contact connections. Various possibilities have already been 
suggested such as thermoplastic distortion of the casing parts using ultra 
sound or a hot die, or again casting the casing in a plastic compound. In 
many cases these methods provide the desired success but only at 
considerable expense. Casting components frequently cannot be used because 
this causes the casting compound itself to flow inside the casing and 
provoke contamination. In addition, German Published Specification No. 21 
29 918 proposed fitting a thermoplastic insulating element in the form of 
a film or shell over the connecting pins with close-fitting holes. Even 
this method does not provide adequate sealing in every case as such films 
frequently are affected by temperature and become distorted and then 
possibly no longer fit snugly enough around the connecting pins of the 
component. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a protective device in the form 
of an additional insulating covering such that reliable sealing of the 
component is guaranteed without high production costs. This is achieved in 
accordance with the invention in that the covering layer is made of a 
material with high capillary absorption power and is impregnated with a 
sealing compound that can be processed as a thin liquid. 
The absorbent material used in accordance with the invention, a so-called 
fleece, spreads the sealing compound very uniformly over the entire 
surface of the component to be sealed so that a uniformly thin covering 
layer is produced with the assistance of the adhesive power. This layer 
only produces a minimal increase in the overall height of the component. 
In addition, this absorbent material holds the sealing compound securely 
in the covering layer so that the compound neither contaminates projecting 
connecting pins nor flows through apertures into the interior of the 
component in undesirably large quantities, a problem previously feared 
when using adhesives and similar sealing structures. Preferably the 
absorbent material is cut exactly to size at the edge of the surface to be 
sealed so that even peripheral apertures, for example between a base and a 
protective cap in a relay, are reliably sealed through adhesion of the 
layer to the surface of the component together with the capillary action. 
With irregular fleeces, the sealing compound spreads uniformly on a 
concentric basis, but with longitudinally orientated fleeces it spreads 
more rapidly in the favored direction. To ensure that the covering layer 
is completely impregnated in the shortest possible time, it is 
advantageous to seal elongated components with longitudinally orientated 
fleeces. The absorbent covering layer can be provided with holes 
beforehand to accommodate projecting parts of the component before it is 
fitted on the component. In many applications, however, it may also be 
advantageous from a production viewpoint not to provide the covering layer 
with holes beforehand but to perforate it with the projecting connecting 
pins of the component when fitting the two together. Differences in level 
or parts lying on the surface of the component can also be sealed off with 
the absorbent covering layer. Preferably the thickness of the layer will 
then be greater than the differences in level in the surface to be sealed. 
To adapt the covering layer to differences in level in the surface of the 
component, the layer can be pressed down with a die. Preferably this die 
has a surface that cannot be wetted by the sealing compound, 
polytetrafluorethylene, for instance. The sealing compound absorbed by the 
covering layer can solidify by cooling or can even be hardened depending 
upon the application in point. 
In one form of the invention, the absorbent covering layer is additionally 
coated with an impermeable film on the side facing away from the 
component. Such a film is easy to stamp or provide with lettering and can 
also be deep-drawn to accommodate fairly large differences in level in the 
surface of the component, for example. Naturally this impermeable film 
ensures better sealing because its adhesive forces can also be utilized. 
In this way, fairly large apertures can be sealed using relatively thin 
fleeces. In addition, it is also possible to reduce large apertures first 
with an impermeable film or even cover them altogether and then apply the 
absorbent covering layer in the manner described. 
In one useful embodiment of the invention the impermeable film acts as a 
carrier for the absorbent covering layer which can be applied to the 
deep-drawn film electrostatically, for example. With suitable measures the 
absorbent covering layer could also be applied on the component directly 
by electrostatic means. In both production methods it is expedient to 
provide a thin coat of adhesive priming to fix the covering layer. When 
impregnating the absorbent covering layer it is preferable to provide the 
impermeable film with a filling aperture for the liquid sealing compound. 
In certain cases it is also advantageous if the component exhibits a 
depression in the area of the filling aperture. Channels emanating from 
the depression may also be useful in many sealing operations. Instead of 
carrying out the impregnation of the covering layer through a filling 
aperture, the component can also be partially dipped in a thin liquid 
casting resin compound. In this case the sealing compound is again 
uniformly spread through the absorbent fleece. When removing the component 
from the casting resin, it is necessary to insure that the thin liquid 
casting resin compound is allowed to run off at an angle at one corner of 
the component.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an electrical component 1 the surface of which is broken by a 
projecting pin 2 and an aperture 3. A gap 4 also exists alongside the pin 
2. The entire surface of the component is sealed with a covering layer 5 
which is made of an absorbent material, a so-called fleece, and 
impregnated with a thin liquid casting resin compound. This sealing 
compound is uniformly spread in the covering layer because of the 
capillary action and held securely at the same time so that it fits snugly 
around the pin 2 but does not run too far into the apertures 3 or 4. To 
avoid contamination of the connecting pin 2, for example, the fleece is 
applied in a dry state and then impregnated using a metering device. 
However one can use a pre-impregnated material which is also dry when 
fitted and in which the sealing compound is re-liquified by warming after 
the fitting operation. 
FIG. 2 shows the invention applied to an electromagnetic relay 11 with a 
protective cap 12 and a base 13 in which the connecting pins 14 and the 
aperture 15 between a base and a protective cap are to be sealed. For this 
purpose an absorbent fleece 16 is provided which reliably seals not only 
the connecting pins 14 but also partly unused apertures 17 (see FIG. 3). 
The fleece is cut as in FIG. 4 so that it can either be laid over the edge 
18 of the cap 12 as in FIG. 2 or pressed into the cap 12 as in FIG. 5. At 
the points corresponding with the contact pins 14 the fleece 16 is 
provided with slots 19. At a suitable point sufficiently far removed from 
the connecting pins so that they are not contaminated, a metered quantity 
of thin liquid adhesive 20 (see FIG. 2) which does not need to be 
accurately measured is dripped onto the pressed-on fleece. With a fleece 
thickness of only a few tenths of a millimeter, the height of the relay is 
only minimally increased by the sealing. 
The following figures show further embodiments of the invention. FIGS. 6 
and 7 illustrate a component 21, the surface of which exhibits differences 
in level as a result of components 22 and 23 lying thereon. Here again, 
sealing with an absorbent covering layer 24 is possible if the impregnated 
fleece is pressed down with a suitable die which will not stick to the 
impregnating compound. Similarly with this method, recesses in the faces 
to be sealed do not have any negative effect on the quality of the seal. 
However, differences in level h1 in the area to be sealed should not be 
greater than the thickness h2 of the impregnated fleece 24 (FIG. 7). 
Another embodiment of the invention is shown in FIG. 8 where a component 31 
with an aperture 32, a connecting pin 33, and a protective cap 34 is 
sealed with an impregnatable covering layer 35. This impregnatable fleece 
35 is provided with a coating of impermeable film 36 so that when applied 
with a stamping die, for example, no adhesive sealing compound appears on 
the surface. A filling aperture 37 is provided for impregnating the 
fleece. In addition, a filling depression 38 is provided in the component 
to be sealed in the area of the filling aperture so that the fleece can 
suck up the sealing compound from its underside 39 over a larger area. 
Moreover the plastic film 36 can easily be stamped and provided with 
lettering, which is advantageous for many components. 
FIG. 9 shows a variance from the structure of FIG. 8. Here the plastic film 
41 is deep drawn so as to seal even greater differences in level in the 
surface of the component. The film 41 is coated with an absorbent fleece 
42 and has a filling aperture 43 as in FIG. 8. Adhesive priming 44 is 
provided between the impermeable film and the absorbent fleece for fixing 
the two together. The coating operation itself can be carried out 
electrostatically, for example, or with other known methods. 
Instead of impregnating the fleece through a filling aperture as in FIGS. 8 
and 9, one can also use other methods. One of these is shown by way of 
example in FIG. 10. Here a component 51 is dipped in a thin liquid casting 
resin compound 52 to such an extent that the fleece 54 coated with an 
impermeable film 53 can absorb the sealing compound 52. Through the 
capillary action of the fleece 54 the casting resin compound spreads 
uniformly over the surface of the component and provides a seal after 
setting. When the component is removed from the liquid sealing compound 
52, the thin liquid casting resin compound is drained off over a corner by 
holding the component at an angle. 
As already mentioned, pre-impregnated fleeces can also be used to provide 
the covering layer in accordance with the invention. These are impregnated 
with a resin which is still not cross-linked. The resin is only softened 
again when thermally treated after the covering layer has been applied to 
the component, and only then does the cross-linking begin. The liquified 
resin spreads in the desired manner by virtue of the capillary action of 
the fleece and by virtue of the adhesive action of the surface of the 
component and seals the component reliably when it sets. Naturally these 
pre-impregnated covering layers can also be stamped in the manner 
described beforehand and made to adapt to differences in level in the 
surface of the component, i.e. pressed down with a stamping die. 
Although various minor modifications may be suggested by those versed in 
the art, it should be understood that we wish to embody within the scope 
of the patent warranted hereon, all such embodiments as reasonably and 
properly come within the scope of our contribution to the art.