A multi-level sealing method wherein an electrical element to be sealed is heated. Once heated, a quick curing first seal is applied to the electrical element thereby preventing the introduction of moisture, and other elements, into an electrical element as the seal cures. A second seal is applied, over the first seal and the element, that is high temperature and slow curing. As a result, the second seal has a different grain structure and the likelihood of a common moisture path into the electrical element is substantially eliminated. Therefore, once the high temperature, slow curing, second seal has cured, the reintroduction of moisture into the electrical element is prohibited. Additional layers of sealant may be added, if desired.

BACKGROUND OF THE INVENTION 
This invention relates to an improved multi-level sealing method for 
effectively sealing electronic elements from moisture. 
A variety of means and methods have been known in the art for preventing 
moisture from entering electrical elements. For example, preformed solid 
sealants have been designed, which melt, or are liquefiable upon the 
application of heat, and are hardenable again to a solid state. Such a 
sealant is disclosed in Siner, U.S. Pat. No. 3,077,639, which is shaped so 
as to allow air, caused to expand in the casing or electrical element 
being sealed upon the application of heat, to escape through the area 
being sealed prior to the sealing thereof. The air, therefore, escapes 
before the sealant covers the opening and formation of bubbles in the 
sealant is presumably prevented. 
Other inventions have been patented which are designed to protect the 
electrical element from direct contact from a mold while sealant is being 
applied to the electrical element. Such patents are exemplified by 
Sternbeck, U.S. Pat. No. 3,165,568, which uses a "free form foil" and 
Schroeder, U.S. Pat. No. 4,374,080, which utilizes a "silicon mold", 
which, because of the design of the mold, allows the molds to be stacked. 
Because the mold is made of silicon rubber, no mold release agents are 
needed and, therefore, the metal electrical leads of the electrical device 
to be sealed are free to bond with the encapsulation or sealant material. 
Other inventions utilize a combination of pressure and vacuum to ensure a 
proper seal, free of bubbles. Such an invention is disclosed in Oldham, 
U.S. Pat. No. 4,681,718. 
A drawback to the methods of sealing electrical elements, known in the art, 
is that water is drawn into the electrical element, through the seal, as 
the sealant cools. Additionally, with only one layer of sealant applied in 
a single application, water typically can seep through minute pathways in 
the sealant into the electrical element. Thus, there is a need in the art 
for providing a method of sealing electrical elements and the like, so 
that moisture is prevented from being drawn into the electrical element as 
the seal cools and that, once the seal is cooled, that moisture is 
prevented from seeping through the seal into the element. It, therefore, 
is an object of this invention to provide an improved multi-level sealing 
method for preventing moisture from being drawn into the element as the 
seal cools and for preventing seepage of moisture into the element after 
the seal has cooled. 
SHORT STATEMENT OF THE INVENTION 
Accordingly, the multi-level sealing method of the present invention 
includes heating the element to be sealed and applying a quick curing 
first sealant to the element. The quick curing first sealant is then 
allowed to cure and, once cured, an application of a high temperature, 
slow curing, second sealant is applied over the first sealant. The element 
with the first seal and the second sealant in place, is then reheated and 
the second sealant is then allowed to slowly cure so that a multi-level 
seal is provided. 
The first sealant is comprised of an epoxy selected for its ability to 
adhere to metals, plastics, and ceramics. This first epoxy is mixed with a 
curing catalyst so that the resultant first sealant cures quickly at room 
temperature and even more quickly if heated. Once the first sealant is 
prepared, it is applied to the heated element so that the element is 
sealed quickly before moisture is drawn past the first sealant into the 
element. This first sealant can be allowed to cure at room temperature or, 
once again, the first sealant may be cured more quickly if heated, 
preferably to temperature above 200.degree. F. 
Once the first sealant is cured and in place, a second, high temperature 
sealant is prepared that is comprised of a high temperature epoxy that 
adheres to metals, plastics, and ceramics. The high temperature epoxy is 
mixed with a curing catalyst so that the resultant second sealant cures 
slowly at high temperatures. Once the second sealant is prepared, it is 
applied over the first sealant so that two sealant boundaries are formed 
and any moisture that happens to pass the second sealant is prevented from 
entering the element by the first sealant. Because the grains of the two 
epoxys are substantially different, no common path into the element is 
likely to exist when utilizing this procedure. 
The reheating of the element is preferably done in two stages so that the 
second sealant is heated to temperature above 200.degree. F. for curing 
for a first period of time, followed by heating to temperatures above 
300.degree. F. for curing for an additional, second, period of time. In 
the preferred embodiment, utilizing the preferred epoxy and catalyst, the 
first period of time is at least 3 hours and curing for the second period 
of time is also at least 3 hours. 
As a result, a multi-level sealing method is provided that initially 
prevents the introduction of moisture into the electronic element by means 
of the application of a first quick curing sealant while the element is 
hot. Further, the application of a second, slow curing high temperature 
sealant creates a multi-level boundary that effectively prevents the 
introduction of moisture into the electronic element once the second seal 
has cured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention is disclosed hereafter 
and illustrated by way of example in FIGS. 1 and 2. Referring to FIG. 1, 
the first step is to heat the element to be sealed to a temperature high 
enough to drive moisture out. The utilization of the method of this 
invention can be with any of a variety of electronic devices that require 
sealing against the introduction of foreign matter such as moisture. The 
use of this method has been specifically utilized to solve the problem of 
moisture being introduced to resistance temperature detectors (RTD's). 
Sealant methods known in the art, as previously disclosed, have the 
disadvantage that, as the seal is allowed to cure on the electrical 
element, moisture, in fact, is introduced and drawn back into the element 
itself. As a result, moisture is actually sealed within the element and 
the life expectancy of the element is greatly decreased. Further, the 
accuracy of readings obtained from these devices are decreased and waste 
and scrapage of prior art electrical devices is significant. 
The second step is to verify that moisture has been driven out. This is 
accomplished by applying voltage to the electrical element and measuring 
resistance if the resistance is low, then moisture is present. If 
resistance is high, then the element is dry. 
The third step of the method of this invention is to apply a quick curing 
first sealant to the heated element. The quick curing first sealant can be 
any sealant known in the art. A preferred sealant is created by use of the 
Emerson and Cuming STYCAST epoxy 2651-40. This epoxy is a casting resin 
with excellent adhesion to metals, plastics, and ceramics. It is a very 
versatile epoxy resin which will cure very easily in a variety of 
different ways, including a room temperature cure when mixed with a 
specific catalyst. This catalyst is known as "Catalyst 9". The proper 
mixture of the epoxy with the catalyst 9 is 8% by weight of catalyst 9 
added to the 2651-40 epoxy. This mixture will have a "pot life" of about 
30 minutes after it is mixed. This mixture will cure by itself at room 
temperature within eight hours. It can also be cured in 5 minutes at 
250.degree. F., which is the preferred fourth step. 
The epoxy 2651-40, of the preferred embodiment, can also be mixed with 
catalyst 11. In this case, 10 to 11 parts of catalyst 11 by weight are 
added for each 100 parts of the 2651-40 epoxy. This mixture will have a 
pot life of at least four hours after it is mixed. The sealant produced 
will then cure at 210.degree. F. to 220.degree. F. for two hours, at a 
minimum. For the best high temperature properties, the post cure time 
should be four to eight hours at 250.degree. F. The advantage of catalyst 
11 are that it provides a long pot life, enhanced thermo shock and 
improved high temperature properties. 
The fourth step allows the first sealant means to cure. Once the first 
sealant has cured, the fifth step is for a second, high temperature, slow 
curing sealant is applied over the first sealant. There are many such 
second sealants available that are known in the prior art. A preferred 
second sealant is Emerson and Cuming STYCAST 2762 epoxy. The 2762 epoxy is 
an excellent, very high temperature, epoxy casting resin and sealing 
compound. It exhibits an excellent adhesion to metals, plastics, and 
ceramics. Further it is a very versatile epoxy which can be cured easily 
and in a variety of ways with a variety of different kinds of catalysts. A 
preferred embodiment is to utilize catalyst 17. When epoxy 2762 is to be 
used with catalyst 17, 10% by weight of catalyst 17 is added to the 
STYCAST 2762 epoxy. Catalyst 17 may be solid at room temperature, and, 
therefore, it may require slight warming (66.degree. C.) if it is desired 
to improve the flow. The 2762 epoxy resin itself may also be warmed 
slightly (66.degree. C.) if it is desired to improve the flow of the 
epoxy as well. The resultant sealant mixture will have a pot life of 30 
minutes. Once application of this sealant is accomplished, it should cure 
for a minimum of 3 hours at a minimum of 93.degree. C. (200.degree. F.) 
followed by a minimum of 3 hours at a minimum of 149.degree. C. 
(300.degree. F.), as the preferred sixth and seventh steps. 
The heating of the element and the first and second sealants, and the step 
of allowing the second sealant to slowly cure at high temperatures ensures 
that a multi-level seal is provided. The multi-level seal is comprised of 
two seals with different grain structures. As a result, it is unlikely 
that a common moisture path into the electrical element will exist. The 
application of the second, multi-level, seal ensures that seepage into the 
electrical element is prevented once the second seal has cured. The eight 
and final step is to perform a final, acceptance test of resistance. If 
measured resistance is high, the element is dry and very likely to stay 
so. 
Referring to FIG. 2, a cross-sectional view of an electrical element 10 is 
shown. Electrical element 10 in this preferred embodiment is a resistance 
temperature detector comprised of insulated stranded lead wires 12 and 
solid lead wires 14. Solid lead wires 14 are encapsulated in an 
encapsulation case 16 of some impervious material known in the art and not 
discussed further hereafter. Additionally, the interior of encapsulation 
case 16 is filled with hygroscopic insulation material 18 also known in 
the art. First sealant 20 comprised of fast curing epoxy, as discussed 
above, forms the first seal of electrical element 10 after the moisture 
has been driven out and cures quickly enough so that moisture is prevented 
from being drawn back in as the first sealant 20 cures. Second sealant 22 
is applied on top of first sealant 20 after first sealant 20 has cured. 
Second sealant 22, as discussed above, is comprised of a high temperature 
epoxy that is cured in two stages in the preferred embodiment. Once second 
sealant 22 is in place, the electrical element is once again tested for 
resistance to determine if the interior of the element is, in fact, dry. 
In summary, by means of the method of the present invention, a method of 
sealing is provided that accomplishes two vital functions. First, the 
application of a quick curing, first sealant, while the electrical element 
is hot, ensures that the moisture content is driven out completely prior 
to application of the sealant and that the moisture is not drawn back into 
the electrical element as the seal is applied and cured. Secondly, the 
application of a second, slow curing sealant, ensures that first, two 
different grain structures exist so that a similar leakage path is 
precluded and second, a higher temperature application is achieved. As a 
result, once the second sealant is in place and cured, the reintroduction 
of moisture to the device is prevented. Therefore, the accuracy of RTD's, 
in particular, and electrical elements, in general, is significantly 
improved and the scrapage rate for electrical elements, due to moisture 
damage, is sigificantly decreased. 
It is important to know that by use of the method of this invention, not 
only is moisture dealt with, but the introduction of gases, and other 
elements into sensitive electrical elements is likewise prohibited. 
Applicants have not found it necessary to add a third level, although one 
or more could be added. It is entirely possible that some electrical 
elements will require more than the single multi-level application 
disclosed herein. 
Therefore, while the present invention has been disclosed in connection 
with the preferred embodiment thereof, it should be understood that there 
may be other embodiments which fall within the spirit and scope of the 
invention as defined by the following claims.