Liquified nitrogen thermal checking of electronic circuitry

Electronic circuitry 22, 24, 26 is thermally checked by cooling with liquified nitrogen from a cryogenic delivery unit 19, directly by means of a stream 10 or a spray 28 delivered through an aperture 11, 30 of the delivery unit 19 spaced from the surface of the circuitry.

TECHNICAL FIELD 
This invention relates to thermal checking of electronic circuitry, and 
more particularly, to checking electronic circuitry by means of liquified 
nitrogen. 
BACKGROUND ART 
It is common practice to utilize thermal checking to identify specific 
failed components in electronic circuitry, and to ensure operation at 
specific, cold temperatures. A typical example of large complex circuitry 
may include a plurality of individual components and integrated circuits 
mounted on a "mother" board (typically a printed circuit board which 
physically and electrically integrates the components into a subsystem). 
When such circuitry is intended for military, aerospace or other critical 
end use, it is common to perform many tests thereon prior to assemblage 
into an overall system, including thermal checks to ensure operation at 
cold temperatures. Such circuitry may be interconnected with very complex 
test equipment, which performs series of tests, the results of which 
indicate probable causes for certain malfunctions, but which cannot 
isolate faults in all cases. In such cases, it has also been known to 
perform thermal checks on the circuitry while it is connected to test 
equipment, to see if the thermal checking will locate the fault. Although 
the phenomenon is not entirely understood, it is believed that one type of 
fault which is overcome through thermal checking is minor cracks in 
conductors which become reconnected when cooled to temperatures in the 
range of minus tens of degrees centigrade. In such cases, the temporary 
correction of the fault will provide an indication of proper operation 
within the test equipment, thus indicating a component or circuit area 
which is probably at fault. Further testing and/or replacement of 
components or portions of circuitry is then undertaken to cure the defect. 
Heretofore, it has been common to use coolants which are generally referred 
to herein as chlorofluorocarbons (CFCs) of which there are a large number 
of varieties. A most common variety is dichlorodifluoromethane, which is 
also known as Freon 12 and Halon 22. The CFC is typically applied from an 
aerosol can (much like a hairspray can) which can be carried by workers in 
a tool pouch. 
For some years, scientists have been concerned about the effects of CFCs on 
the atmosphere. First, CFC molecules themselves trap 20,000 times more 
heat than a molecule of carbon dioxide, thereby increasing the greenhouse 
effect far out of proportion to its concentration in air. More 
importantly, chlorine released when CFC molecules break up combines with 
and destroys ozone molecules. And each chlorine atom can eventually be 
re-released and combined with yet other ozone molecules so that their 
destructive effect is repetitive, perpetually. And, as is known, it is the 
ozone molecules which absorb most of the ultraviolet radiation from the 
sun, which is known to be extremely harmful to all forms of animal life, 
from humans down to the simplest of forms. For that reason, many 
governments of the world are now restricting, with the ultimate aim at 
totally banning, the production and use of CFCs. 
Some attempts have been made to provide alternative methods of performing 
thermal checks on circuitry. Heat pumps have such minor cooling as to be 
unable to reach the desired temperatures (on the order of -30.degree. to 
-60.degree. C.) in even five or six minutes. Expansion of high pressure 
gases can produce temperatures as low as -30.degree. C., but the high 
pressure gas causes physical damage to the circuitry under test. Thus far, 
no reasonable substitute seems to be available. 
DISCLOSURE OF INVENTION 
An object of the invention is to eliminate the need to use CFCs in thermal 
checking of electronic circuitry. Another object of the invention is to 
provide improved thermal checking of electronic circuitry. 
According to the present invention, liquified nitrogen is sprayed directly 
on the surface of circuitry which is to be thermally checked. 
In accordance with more specific aspects of the invention, the liquified 
nitrogen sprayed onto a surface in the process of thermal checking of 
integrated circuits contains a significant fraction of nitrogen in the 
liquid phase, which may be on the order of 30% to 90% liquid by molecular 
weight. Liquified nitrogen may be sprayed directly on surfaces of 
electronic circuitry with a relatively collimated stream (which may be 
less than a tenth of an inch in diameter), or in broad, fan-like sprays 
which may be on the order of 3/8 of an inch to an inch or more in length 
and one or a few tenths of an inch in width. According to the invention, 
the relative amount of nitrogen in the liquid phase being applied, the 
size (volume) of the stream, and the shape of the apertures can all be 
adjusted so as to best suit the particular needs of any thermal check to 
be performed. 
The invention uses a gas which occurs in nature (not man-made), which is 
readily available throughout industry, and is therefore easy to obtain and 
inexpensive to use. The invention provides increased accuracy and 
discrimination in applying the coolant, reaches circuit-responsive 
temperatures more quickly, is totally inert to the atmosphere, and 
extremely safe for use by humans.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, liquified nitrogen is propelled in a substantially 
collimated stream 10 from an aperture 11 formed in a tip 12 which may be 
threaded onto a fitting 13 of a delivery tube 14 from a valve 15 which is 
operated by a handle 16. The valve 15 is mounted on a cap 17 which in turn 
is threaded onto a dewar 18, with a feed pipe (not shown) extending from 
the valve 15 into the liquid within the dewar 18. The apparatus 11-18 
comprises a cryogenic delivery unit 19 which may be of the type disclosed 
in U.S. Pat. No. 4,116,199 and 4,269,390. The dewar 18 is typically a 
double walled, stainless steel dewar having a high vacuum between the 
walls, so as to insulate the nitrogen contained therein from environmental 
heat. The dewar may be on the order of a third of a liter to a liter in 
capacity, although about 1/2 liter is found to be a good balance between 
weight and cumbersomeness (on the one hand) versus capacity, static 
holding time, and the like (on the other hand). The aperture 11 may range 
from about 20 mils (0.5 mm) to one the order of 50 mils (1.2 mm). The 
cryogenic delivery unit 19 may of course take a wide variety of forms, so 
long as it provides the practical application of a proper stream of 
liquified nitrogen through an aperture, having a substantial fraction of 
nitrogen in the liquid phase. 
In FIG. 1, the nitrogen stream 10 is depicted as being propelled onto the 
surface of an electronic component 22 mounted on a circuit board 24. The 
circuit board 24 is also shown as having a much larger component 26 (which 
might be a complete integrated circuit, or otherwise) mounted thereon. The 
depiction in FIG. 1 is of course supersimplified, simply to illustrate the 
precepts of the present invention. 
For treating a much larger component (particularly one that is elongated 
such as the component 26 of FIG. 1), a broad, fan-like spray 28 may be 
utilized as shown in FIG. 2. The spray 28 is emerging from a relatively 
narrow, elongated aperture 30 which may be formed by carefully flattening 
one end of a tubular structure 32 which has a suitable fitting 34 to 
facilitate being fastened to the fitting 13 of the cryogenic delivery unit 
19 (FIG. 1). Such a tube may be on the order of 1/4 to 3/4 inch in 
diameter, yielding an aperture 30 having a length on the order of 1/2 to 
11/4 inches, or the like. Depending on the volume of nitrogen which is 
capable of being delivered, much larger fan-like apertures 30 can be 
provided. 
The analysis of liquified nitrogen as a coolant for thermal checking 
reveals some spectacular advantages in comparison with the use of CFCs. 
The aerosol-type cans, utilized to spray CFCs on circuitry, produce a very 
broad, comb-like spray, which makes it impossible to confine the 
application thereof to specific small portions of a circuit. On the other 
hand, the characteristics of liquified nitrogen (which may have something 
to do with its high surface tension) allow delivering the liquified 
nitrogen in the form of a substantially collimated stream (that is, a 
parallel, non-diverging stream). Therefore, the application of the 
nitrogen can, in the extreme, be confined essentially to the diameter of 
the emerging stream, which is slightly more than the diameter of the 
nozzle aperture through which the stream is propelled. This can be as 
small as fractions of a millimeter. 
CFCs tend to wet the surface, and tend to stand on the surface as 
evaporation occurs. The CFCs are also likely to be propelled along 
surfaces by the force of the propellant (whether the CFC is used as its 
own propellant or another propellant is used). Thus, a much larger surface 
than that which is desired to be tested is frequently wetted and cooled 
down by the CFCs. On the other hand, a stream of liquified nitrogen having 
a high liquid content has a tendency, when it impinges on the surface, to 
form a defined wetted surface area of liquid within which beads or 
droplets of liquid nitrogen are flowing radially outward and gasifying as 
they flow. The diameter of such a wetted area is relatively confined: for 
instance, a typical wetted area might be of a size on the order of a dime 
or a nickel. For small surfaces, the nitrogen stream, propelled radially 
outward from the point of impingement of the stream on the surface, will 
simply continue to propel outwardly, and vaporize into the atmosphere. On 
the other hand, the wetting characteristics of CFCs tend to cause them to 
flow around corners and wet other surfaces of the component. 
Because CFCs have a maximum low temperature of -65.degree. C. (and 
typically deliver the CFC to the surface at a higher temperature), it may 
take half a minute or more to cool an extremely small component (such as 
on the order of a quarter of an inch cube) sufficiently to cause the 
desired effect of locating the fault. On the other hand, the nitrogen can 
be delivered in the liquid phase, which is at -196.degree. C., thus giving 
the capability for cooling components much more quickly. In fact, only on 
the order of 5 to 15 seconds is required for cooling most components using 
liquified nitrogen. 
CFCs typically require the use of cardboard dams or other tools to tend to 
confine the CFCs to the portion which is desired to be cooled, thereby 
causing that portion to be cooled more quickly while at the same time 
avoiding cooling of other components. It is to be noted that the accuracy 
of which part of the circuitry is cooled, versus which part of the 
circuitry is not cooled, is very important in the diagnostic determination 
of where the fault lies. The more of the circuitry which is cooled, the 
less pinpointing there is of the precise area of defect. 
The CFCs tend to splash around and frequently may impinge on the skin of an 
operator, particularly if he or she is using one hand to manipulate a tool 
or hold a unit in a particular relationship. The CFCs tend to adhere to, 
and wet, the skin and thereby cool it. A precaution required when 
utilizing liquid nitrogen is to avoid other than fleeting contact with the 
skin. Of course, in any given application of the present invention, simple 
precautions, such as gloves of any sort, can be utilized, if desired. 
CFCs can react chemically with some of the materials which may be found in 
electronic circuits. On the other hand, nitrogen is known to be totally 
inert to any materials utilized in electronic circuitry. 
CFCs are known to be toxic and a health hazard in the workplace. Effects on 
operators can include dizziness, involuntary trembling, unconsciousness, 
irregular heart beat, and even death. It has a greater tendency to promote 
frostbite on the skin or in the eyes. On the other hand, nitrogen (in any 
quantities which possibly could be utilized in the present invention) is 
totally innocuous and of no hazard whatsoever to humans (other than its 
heat extraction). All that occurs to humans is that the atmosphere (having 
approximately 89% nitrogen to begin with) has a slightly increased 
nitrogen content. In other than nearly total occlusion of the oxygen in an 
operator's environment, the nitrogen will not affect humans at all. And, 
as described hereinbefore, the characteristics of nitrogen as the liquid 
dances around and gasifies, render it even safer on the skin that CFCs. 
CFCs are though to be relatively inexpensive however, the amount of 
nitrogen which is utilized in place of them, its general availability and 
the like, result in costs for the coolant itself which may range from 20% 
to 50% of the cost of the CFCs. 
It has been known to use liquid nitrogen as the coolant for cryogenic high 
speed supercomputers. In such cases, the circuits are designed to be 
immersed in a liquified cryogen; the materials and other design factors 
are chosen so that such materials can all be chilled to the temperature of 
the cryogen without structural damage of alteration of the electronic 
phenomenon of the materials, other than the desired result of increased 
circuit speed. Immersion in cryogenic liquids is not practical for thermal 
checking of circuitry for several reasons: when initially immersed in the 
liquid, the warm circuitry structure will cause violent boiling 
(gasification) of the liquid nitrogen, the nitrogen gas tends to form an 
insulating sheath between the liquid and the surfaces of the structure, 
and thus precludes cooling the circuitry within a few seconds, as is 
required in thermal checking. And, naturally, cooling a substantial 
portion of a circuitry structure does not sufficiently pinpoint the fault 
as required in thermal checking. 
Additionally, it is thought that extreme cooling (below -100.degree. C.) of 
a structure comprised of a variety of materials (epoxies, and the like) 
would tend to crystallize some of the materials, rendering them brittle 
and causing spontaneous cracking from shrinking. On the other hand, the 
use of a controlled, liquified nitrogen stream, as described herein, will 
cool an extremely small portion of a circuitry structure, usually being 
able to be confined to a surface of a single material, so that 
crystallization, brittleness and cracking can be avoided. Additionally, 
the desired temperature (of on the order of -20.degree. to -60.degree. C.) 
can be concentrated in a sufficiently small area so as to pinpoint the 
fault in a relatively short time. 
As described in the embodiments of FIGS. 1 and 2, liquified nitrogen is 
propelled directly on a surface of the circuitry to be tested. At the 
present time, it is believed that such practice is to be preferred in most 
procedures which are now being performed with the use of CFCs. However, it 
is foreseen herein that with a broader application of liquified nitrogen 
in thermal checking processes, specific applications may be found which 
suggest the use of a closed probe to cool the circuitry under test. 
The problem with a probe is that surface to surface contact is found to be 
less effective for thermal extraction from a surface than the direct 
application of the nitrogen as described hereinbefore with respect to 
FIGS. 1 and 2. This is believed to be due in part to the fact that such 
surfaces do not actually join molecularly, and therefore there are 
relatively lesser paths for heat flow from the surface to be cooled to the 
cooling surface. This is also believed to be due in part to the fact that 
the direct application of flowing liquid nitrogen across the surface has 
been found to be the most effective way to cool the surface. This is 
because of extracting heat of vaporization at the low boiling temperature 
rather than merely conducting heat into a mass at a low temperature. 
Another problem with probe tips is that it is extremely difficult to get 
the surface of the probe to conform well to the surface of the device 
being cooled, thereby to have maximum heat transfer therebetween. 
Therefore, it is believed that the use of liquified nitrogen properly 
applied directly to a surface to be cooled is to be preferred to use of 
probes. 
The present invention has the additional advantage of nitrogen being inert 
to the materials, as well as being capable of application with a highly 
collimated stream. Thus, it can be used to cool surfaces which are not 
very accessible. In addition, if a circuit is connected with test 
equipment utilizing a lot of clips, probes and the like, the nitrogen can 
be applied through all of such connections either by means of an elongated 
delivery tube 14, or directly in the form of the collimated stream, 
because such collimated stream is so readily controlled. 
Thus, although the invention has been shown and described with respect to 
exemplary embodiments thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions and 
additions may be made therein and thereto without departing from the 
spirit and the scope of the invention.